Posts Tagged ‘asteroid’

So Close, Yet So Far Away: Dawn’s Trajectory Explained

Monday, March 31st, 2014

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft
Artist’s concept depicting the Dawn spacecraft thrusting with its ion propulsion system as it travels from Vesta (lower right) to Ceres (upper left). The galaxies in the background are part of the Virgo supercluster. Dawn, Vesta and Ceres are currently in the constellation Virgo from the perspective of viewers on Earth. Image credit: NASA/JPL
› Larger image

Dear Correspondawnts,

Powering its way through deep space, Dawn draws ever closer to dwarf planet Ceres. To reach its destination, the interplanetary spaceship gently reshapes its path around the sun with its extraordinary ion propulsion system. In about a year, the spacecraft will gracefully slip into orbit so it can begin to unveil the nature of the mysterious world of rock and ice, an intriguing protoplanetary remnant from the dawn of the solar system.

Even as Dawn ascends the solar system hill, climbing farther and farther from the sun, penetrating deeper into the main asteroid belt between Mars and Jupiter, its distance to Earth is shrinking. This behavior may be perplexing for readers with a geocentric bias, but to understand it, we can take a broader perspective.

The sun is the conductor of the solar system symphony. Its gravity dictates the movements of everything that orbits it: Earth as well as the other planets, Vesta, Ceres, and myriad smaller objects, including asteroids and Dawn. (Actually, the gravity of every single body affects how all of the others move, but with more than 99 percent of the entire solar system’s mass concentrated in the gargantuan sun, it dominates the gravitational landscape.)

Whether it is for a planet or Dawn orbiting the sun, a spacecraft or moon orbiting a planet, the sun or other stars orbiting the Milky Way (the Milky Way galaxy, that is, not your correspondent’s cat Milky Way), or the Milky Way galaxy orbiting the Virgo supercluster of galaxies (home to an appreciable fraction of our readership), any orbit is the perfect balance between the inward tug of gravity and the inexorable tendency of objects to travel in a straight line. If you attach a weight to a string and swing it around in a circle, the force you use to pull on the string mimics the gravitational force the sun exerts on the bodies that orbit it. The effort you expend in keeping the weight circling serves constantly to redirect its course, forcing it to curve; if you release the string, the weight’s natural motion would take it away in a straight line (we are ignoring here the effect of Earth’s gravity on the weight).

The force of gravity dwindles as the distance increases, so the sun pulls harder on a nearby body than on a farther one. Therefore, to remain in orbit, to balance the relentless gravitational lure, the closer object must travel at higher speed, resisting the stronger attraction. The same effect applies at Earth. Satellites that orbit very close (including, for example, the International Space Station, 250 miles, or 400 kilometers, above the surface) must streak around the planet at about 17,000 mph (7.6 kilometers per second) to avoid being drawn down. The moon, orbiting almost a thousand times farther above, needs only to travel at less than 2300 mph (about 1.0 kilometers per second) to balance Earth’s weaker hold at its remote location.

For that reason, Mercury zips around the sun faster than any of the other planets. Mars travels more slowly than Earth, and the still more distant residents of the asteroid belt, whether natural (all of them but one) or a product of human ingenuity (one: Dawn), proceed at an even more leisurely pace. As Earth makes its relatively rapid annual trip around the sun, the distance to the spacecraft that left it behind in 2007 alternately shrinks and grows.

We can visualize this with one of the popular models of clocks available in the Dawn gift shop on your planet, in which the hour hand is longer than the minute hand. Imagine the sun as being at the center of the clock. The tip of the short minute hand represents Earth, and the end of the hour hand represents Dawn. Some of the time (such as between noon and shortly after 12:30), the distance between the ends of the hands increases. Then the situation reverses as the faster minute hand begins moving closer and closer to the hour hand as the time approaches about 1:05.

The Dawn spacecraft's trajectory
This graphic shows the Dawn spacecraft’s interplanetary trajectory from launch through its arrival at Ceres next year. The positions of the spacecraft and Earth are shown on April 10, 2014, when their independent orbits bring them relatively close together. Image credit: NASA/JPL-Caltech

› Larger image

Earth and Dawn are exhibiting the same repetitive behavior. Of course, their relative motion is more complicated than that of the clock hands, because Dawn’s ion thrusting is constantly changing its solar orbit (and so the distance and speed at which it loops around the sun), but the principle is the same. They have been drawing closer since August 2013. Earth, coming from behind, is now about to pass Dawn and move ahead. The stalwart probe will not even take note however, as its sights remain firmly set on an unexplored alien world.

On April 10, the separation will be 1.56 AU (1.56 times the average distance between Earth and the sun, which means 145 million miles, or 233 million kilometers), an almost inconceivably large distance (well in excess of half a million times farther than the International Space Station, which orbits Earth, not the sun) but less than it has been since September 2011. (The skeptical reader may verify this by reviewing the concluding paragraph of each log in the intervening months.) Enjoy the upcoming propinquity while you can! As the ship sails outward from the sun toward Ceres, it will never again be this close to its planet of origin. The next time Earth, taking an inside track, overtakes it, in July 2015 (by which time Dawn will be orbiting Ceres), they will only come within 1.94 AU (180 million miles, or 290 million kilometers) of each other.

By the way, Vesta, the endlessly fascinating protoplanet Dawn unveiled in 2011-2012, will be at its smallest separation from Earth of 1.23 AU (114 million miles, or 183 million km) on April 18. Ceres, still awaiting a visitor from Earth, despite having first been glimpsed from there in 1801, will attain its minimum distance on April 15, when it will be 1.64 AU (153 million miles, or 246 million km) away. It should not be a surprise that Dawn’s distance is intermediate; it is between them as it journeys from one to the other.

Finder chart showing the locations of Vesta and Ceres
This finder chart can help you locate Vesta and Ceres (and even Dawn, although it is too small to see) in the constellation Virgo. Click it for a larger version. Image credit: Sky & Telescope Magazine
› Larger image

Not only is each one nearly at its shortest geocentric range, but from Earth’s point of view, they all appear to be near each other in the constellation Virgo. In fact, they also look close to Mars, so you can locate these exotic worlds (and even the undetectably small spacecraft) in the evening sky by using the salient red planet as a signpost. In June, the coincidental celestial alignment will make Vesta and Ceres appear to be separated by only one third the diameter of the full Moon, although these behemoths of the asteroid belt will be 0.57 AU (52 million miles, or 85 million kilometers) from each other.

We mentioned above that by constantly modifying its orbit under the persistent pressure of its ion engine, Dawn complicates the simple clock-like behavior of its motion relative to Earth. On Halloween 2012, we were treated to the startling fact that to rendezvous with Ceres, at a greater distance from the sun, Dawn had to come in toward the sun for a portion of its journey; quite a trick! In that memorable log (which is here, for those readers who didn’t find every detail to be so memorable), we observed that it would not be until May 2014 that Dawn would be as far from the sun as it was on Nov. 1, 2012. Sure enough, having faithfully performed all of the complex and intricate choreography since then, it will fly to more than 2.57 AU from the solar system’s star in May, and it will continue heading outward.

With the sun behind it and without regard to where Earth or most other residents of the solar system are in their orbits, Dawn rises to ever greater heights on its extraordinary expedition. Distant though it is, the celestial ambassador is propelled by the burning passion for knowledge, the powerful yearning to reach beyond the horizon, and the noble spirit of adventure of the inhabitants of faraway Earth. The journey ahead presents many unknowns, promising both great challenges and great rewards. That, after all, is the reason for undertaking it, for such voyages enrich the lives of all who share in the grand quest to understand more about the cosmos and our humble place in it.

Dawn is 11 million miles (18 million kilometers) from Ceres. It is also 1.57 AU (146 million miles, or 235 million kilometers) from Earth, or 625 times as far as the moon and 1.57 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 26 minutes to make the round trip.

› Read more from Marc Rayman’s Dawn Journal


It’s All About Grace Under Pressure for Dawn’s Drop Into Orbit

Friday, January 31st, 2014

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft at the dwarf planet Ceres
Artist’s concept of NASA’s Dawn spacecraft thrusting with its ion propulsion system as it approaches the dwarf planet Ceres. Image credit: NASA/JPL-Caltech

Dear Rendawnvous,

Dawn is continuing its trek through the main asteroid belt between Mars and Jupiter. Leaving behind a blue-green wake of xenon from its ion propulsion system, its sights are set on dwarf planet Ceres ahead. The journey has been long, but the veteran space traveler (and its support team on distant Earth) is making good progress for its rendezvous early next year.

Last month, we had a preview of many of the activities the probe will execute during the three months that culminate in settling into the first observational orbit at Ceres in April 2015. At that orbit, about 8,400 miles (13,500 kilometers) above the alien landscapes of rock and ice, Dawn will begin its intensive investigations. Nevertheless, even during the “approach phase,” it will often observe Ceres with its camera and one of its spectrometers to gain a better fix on its trajectory and to perform some preliminary characterizations of the mysterious world prior to initiating its in-depth studies. The discussion in December did not cover the principal activity, however, which is one very familiar not only to the spacecraft but also to readers of these logs. The majority of the time in the approach phase will be devoted to continuing the ion-powered flight. We described this before Vesta, but for those few readers who don’t have perfect recall (we know who you are), let’s take another look at how this remarkable technology is used to deliver the adventurer to the desired orbit around Ceres.

Thrusting is not necessary for a spacecraft to remain in orbit, just as the moon remains in orbit around Earth and Earth and other planets remain in orbit around the sun without the benefit of propulsion. All but a very few spacecraft spend most of their time in space coasting, following the same orbit over and over unless redirected by a gravitational encounter with another body. In contrast, with its extraordinarily efficient ion propulsion system, Dawn’s near-continuous thrusting gradually changes its orbit. Thrusting since December 2007 has propelled Dawn from the orbit in which the Delta rocket deposited it after launch to orbits of still greater distance from the sun. The flight profile was carefully designed to send the craft by Mars in February 2009, so our celestial explorer could appropriate some of the planet’s orbital energy for the journey to the more distant asteroid belt, of which it is now a permanent resident. In exchange for Mars raising Dawn’s heliocentric orbit, Dawn lowered Mars’s orbit, ensuring the solar system’s energy account remained balanced.

While spacecraft have flown past a few asteroids in the main belt (although none as large as the gargantuan Vesta or Ceres, the two most massive objects in the belt), no prior mission has ever attempted to orbit one, much less two. For that matter, this is the first mission ever undertaken to orbit any two extraterrestrial destinations. Dawn’s exclusive assignment would be quite impossible without its uniquely capable ion propulsion system. But with its light touch on the accelerator, taking nearly four years to travel from Earth past Mars to Vesta, and more than two and a half years from Vesta to Ceres, how will it enter orbit around Ceres? As we review this topic in preparation for Ceres, bear in mind that this is more than just a cool concept or neat notion. This is real. The remarkable adventurer actually accomplished the extraordinary feats at Vesta of getting into and out of orbit using the delicate thrust of its ion engines.

Whether conventional spacecraft propulsion or ion propulsion is employed, entering orbit requires accompanying the destination on its own orbit around the sun. This intriguing challenge was addressed in part in February 2007. In February 2013, we considered another aspect of what is involved in climbing the solar system hill, with the sun at the bottom, Earth partway up, and the asteroid belt even higher. We saw that Dawn needs to ascend that hill, but it is not sufficient simply to reach the elevation of each target nor even to travel at the same speed as each target; the explorer also needs to travel in the same direction. Probes that leave Earth to orbit other solar system bodies traverse outward from (or inward toward) the sun, but then need to turn in order to move along with the body they will orbit, and that is difficult.

Those of you who have traveled around the solar system before are familiar with the routine of dropping into orbit. The spacecraft approaches its destination at very high velocity and fires its powerful engine for some minutes or perhaps even about an hour, by the end of which it is traveling slowly enough that the planet’s gravity can hold it in orbit and carry it around the sun. These exciting events may range from around 1,300 to 3,400 mph (0.6 to 1.5 kilometers per second). With ten thousand times less thrust than a typical propulsion system on an interplanetary spacecraft, Dawn could never accomplish such a rapid maneuver. As it turns out, however, it doesn’t have to.

Dawn’s method of getting into orbit is quite different, and the key is expressed in an attribute of ion propulsion that has been referred to 63 times (trust or verify; it’s your choice) before in these logs: it is gentle. (This example shows just how gentle the acceleration is.) With the gradual trajectory modifications inherent in ion propulsion, sharp changes in direction and speed are replaced by smooth, gentle curves. The thrust profiles for Dawn’s long interplanetary flights are devoted to the gradual reshaping of its orbit around the sun so that by the time it is in the vicinity of its target, its orbit is nearly the same as that of the target. Rather than hurtling toward Vesta or Ceres, Dawn approaches with grace and elegance. Only a small trajectory adjustment is needed to let its new partner’s gravity capture it, so even that gentle ion thrust will be quite sufficient to let the craft slip into orbit. With only a nudge, it transitions from its large, slow spiral away from the sun to an inward spiral centered around its new gravitational master.

illustration of Dawn's orbit
This graphic shows the planned trek of NASA’s Dawn spacecraft from its launch in 2007 through its arrival at the dwarf planet Ceres in early 2015. Note how Dawn spirals outward to Vesta and then still more to Ceres. Image credit: NASA/JPL-Caltech

To get into orbit, a spacecraft has to match speed, direction and location with its target. A mission with conventional propulsion first gets to the location and then, using the planet’s gravity and its own fuel-guzzling propulsion system, very rapidly achieves the required speed and direction. By spiraling outward from the sun, first to the orbit of Vesta and now to Ceres, Dawn works on its speed, direction and location all at the same time, so they all gradually reach the needed values at just the right time.

To illustrate this facet of the difference between how the different systems are applied to arrive in orbit, let’s imagine you want to drive your car next to another traveling west at 60 mph (100 kilometers per hour). The analogy with the conventional technology would be similar to speeding north toward an intersection where you know the other car will be. You arrive there at the same time and then execute a screeching, whiplash-inducing left turn at the last moment using the brakes, steering wheel, accelerator and adrenaline. When you drive an ion propelled car (with 10 times higher fuel efficiency), you take an entirely different path from the start, one more like a long, curving entrance ramp to a highway. As you enter the ramp, you slowly (perhaps even gently) build speed. You approach the highway gradually, and by the time you have reached the far end of the ramp, your car is traveling at the same speed and in the same direction as the other car. Of course, to ensure you are there when the other car is, the timing is very different from the first method, but the sophisticated techniques of orbital navigation are up to the task.

› Continue reading Marc Rayman’s January 2014 Dawn Journal


NASA’s Dawn Plans for Planetary Shores Ahead

Tuesday, December 31st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

NASA Dawn spacecraft between its targets, Vesta and Ceres
Artist’s concept of NASA’s Dawn spacecraft between the giant asteroid Vesta and the dwarf planet Ceres. Image credit: NASA/JPL-Caltech

Dear Clairvoydawnts,

Now more than halfway through its journey from protoplanet Vesta to dwarf planet Ceres, Dawn is continuing to use its advanced ion propulsion system to reshape its orbit around the sun. Now that the ship is closer to the uncharted shores ahead than the lands it unveiled astern, we will begin looking at the plans for exploring another alien world. In seven logs from now through August, we will discuss how the veteran adventurer will accomplish its exciting mission at Ceres. By the time it arrives early in 2015 at the largest object between Mars and Jupiter, readers will be ready to share not only in the drama of discovery but also in the thrill of an ambitious undertaking far, far from Earth.

Mission planners separate this deep-space expedition into phases. Following the “launch phase” was the 80-day “checkout phase”. The “interplanetary cruise phase” is the longest. It began on December 17, 2007, and continued to the “Vesta phase,” which extended from May 3, 2011, to Sept. 4, 2012. We are back in the interplanetary cruise phase again and will be until the “Ceres phase” begins in 2015. (Other phases may occur simultaneously with those phases, such as the “oh man, this is so cool phase,” the “we should devise a clever name for this phase phase,” and the “lunch phase.”) Because the tasks at Vesta and Ceres are so complex and diverse, they are further divided into sub-phases. The phases at Ceres will be very similar to those at Vesta, even though the two bodies are entirely different.

In this log, we will describe the Ceres “approach phase.” The objectives of approach are to get the explorer into orbit and to attain a preliminary look at the mysterious orb, both to satisfy our eagerness for a glimpse of a new and exotic world and to obtain data that will be helpful in refining details of the subsequent in-depth investigations. The phase will start in January 2015 when Dawn is about 400,000 miles (640,000 kilometers) from Ceres. It will conclude in April when the spacecraft has completed the ion thrusting necessary to maneuver into the first orbit from which it will conduct intensive observations, at an altitude of about 8,400 miles (13,500 kilometers). For a reason to be revealed below, that orbit is known by the catchy cognomen RC3.

(Previews for the Vesta approach phase were presented in March 2010 and May 2011, and the accounts of its actual execution are in logs from June, July, and August 2011. Future space historians should note that the differing phase boundaries at Vesta are no more than a matter of semantics. At Vesta, RC3 was described as being part of the approach phase. For Ceres, RC3 is its own distinct phase. The reasons for the difference in terminology are not only unimportant, they aren’t even interesting.)

The tremendous maneuverability provided by Dawn’s uniquely capable ion propulsion system means that the exact dates for events in the approach phase likely will change between now and then. So for those of you in 2015 following a link back to this log to see what the approach plan has been, we offer both the reminder that the estimated dates here might shift by a week or so and a welcome as you visit us here in the past. We look forward to meeting you (or even being you) when we arrive in the future.

Most of the approach phase will be devoted to ion thrusting, making the final adjustments to Dawn’s orbit around the sun so that Ceres’s gravity will gently take hold of the emissary from distant Earth. Next month we will explain more about the unusual nature of the gradual entry into orbit, which will occur on about March 25, 2015.

Starting in early February 2015, Dawn will suspend thrusting occasionally to point its camera at Ceres. The first time will be on Feb. 2, when they are 260,000 miles (420,000 kilometers) apart. To the camera’s eye, designed principally for mapping from a close orbit and not for long-range observations, Ceres will appear quite small, only about 24 pixels across. But these pictures of a fuzzy little patch will be invaluable for our celestial navigators. Such “optical navigation” images will show the location of Ceres with respect to background stars, thereby helping to pin down where it and the approaching robot are relative to each other. This provides a powerful enhancement to the navigation, which generally relies on radio signals exchanged between Dawn and Earth. Each of the 10 times Dawn observes Ceres during the approach phase will help navigators refine the probe’s course, so they can update the ion thrust profile to pilot the ship smoothly to its intended orbit.

Whenever the spacecraft stops to acquire images with the camera, it also will train the visible and infrared mapping spectrometer on Ceres. These early measurements will be helpful for finalizing the instrument parameters to be used for the extensive observations at closer range in subsequent mission phases.

Dawn obtained images more often during the Vesta approach phase than it will on approach to Ceres, and the reason is simple. It has lost two of its four reaction wheels, devices used to help turn or stabilize the craft in the zero-gravity, frictionless conditions of spaceflight. (In full disclosure, the units aren’t actually lost. We know precisely where they are. But given that they stopped functioning, they might as well be elsewhere in the universe; they don’t do Dawn any good.) Dawn’s hominin colleagues at JPL, along with excellent support from Orbital Sciences Corporation, have applied their remarkable creativity, tenacity, and technical acumen to devise a plan that should allow all the original objectives of exploring Ceres to be met regardless of the health of the wheels. One of the many methods that contributed to this surprising resilience was a substantial reduction in the number of turns during all remaining phases of the mission, thus conserving the precious hydrazine propellant used by the small jets of the reaction control system.

When Dawn next peers at Ceres, nine days after the first time, it will be around 180,000 miles (290,000 kilometers) away, and the pictures will be marginally better than the sharpest views ever captured by the Hubble Space Telescope. By the third optical navigation session, on Feb. 21, Ceres will show noticeably more detail.

At the end of February, Dawn will take images and spectra throughout a complete Ceres rotation of just over nine hours, or one Cerean day. During that period, while about 100,000 miles (160,000 kilometers) distant, Dawn’s position will not change significantly, so it will be almost as if the spacecraft hovers in place as the dwarf planet pirouettes beneath its watchful eye. Dawn will see most of the surface with a resolution twice as good as what has been achieved with Hubble. (At that point in the curving approach trajectory, the probe will be south of Ceres’s equator, so it will not be able to see the high northern latitudes.) This first “rotation characterization,” or RC1, not only provides the first (near-complete) look at the surface, but it may also suggest to insightful readers what will occur during the RC3 orbit phase.

There will be six more imaging sessions before the end of the approach phase, with Ceres growing larger in the camera’s view each time. When the second complete rotation characterization, RC2, is conducted on March 16, the resolution will be four times better than Hubble’s pictures. The last photos, to be collected on March 24, will reveal features seven times smaller than could be discerned with the powerful space observatory.

The approach imaging sessions will be used to accomplish even more than navigating, providing initial characterizations of the mysterious world, and whetting our appetites for more. Six of the opportunities also will include searches for moons of Ceres. Astronomers have not found moons of this dwarf planet in previous attempts, but Dawn’s unique vantage point would allow it to discover smaller ones than would have been detectable in previous attempts.

When the approach phase ends, Dawn will be circling its new home, held in orbit by the massive body’s gravitational grip and ready to begin more detailed studies. By then, however, the pictures and other data it will have returned will already have taught Earthlings a great deal about that enigmatic place. Ceres has been observed from Earth for more than two centuries, having first been spotted on January 1, 1801, but it has never appeared as much more than an indistinct blob amidst the stars. Soon a probe dispatched by the insatiably curious creatures on that faraway planet will take up residence there to uncover some of the secrets it has held since the dawn of the solar system. We don’t have long to wait!

Dawn is 20 million miles (32 million kilometers) from Vesta and 19 million miles (31 million kilometers) from Ceres. It is also 2.42 AU (225 million miles, or 362 million kilometers) from Earth, or 1,015 times as far as the moon and 2.46 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 40 minutes to make the round trip.

› Read more entries from Marc Rayman’s Dawn Journal


For Dawn, a Time to Thrust and and a Time to Coast

Thursday, October 31st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear All Hallows’ Dawns,

Deep in the main asteroid belt between Mars and Jupiter, Dawn is continuing its smooth, silent flight toward dwarf planet Ceres. Far behind it now is the giant protoplanet Vesta, which the spacecraft transformed from a tiny splotch in the night sky to an exotic and richly detailed world.

The voyage from Vesta to Ceres will take the pertinacious probe 2.5 years. The great majority of spacecraft coast most of the time (just as planets and moons do), each one following a trajectory determined principally by whatever momentum they started with (usually following release from a rocket) and the gravitational fields of the sun and other nearby, massive bodies. In contrast, Dawn spends most of its time thrusting with its ion propulsion system. The gentle but efficient push from the high velocity xenon ions gradually reshapes its orbit around the sun. In September 2012, as it departed Vesta after 14 months of scrutinizing the second most massive resident of the asteroid belt, Dawn’s heliocentric orbit was the same as the rocky behemoth’s. Now they are very far apart, and by early 2015, the robotic explorer’s path will be close enough to Ceres’s that they will become locked in a gravitational embrace.

Without ion propulsion, Dawn’s unique mission to orbit two extraterrestrial destinations would be impossible. No other spacecraft has attempted such a feat. To accomplish its interplanetary journey, the spaceship has thrust more than 96 percent of the time since propelling itself away from Vesta last year. Whenever it points its ion engine in the direction needed to rendezvous with Ceres, its main antenna cannot also be aimed at Earth. Dawn functions very well on its own, however, communicating only occasionally with its terrestrial colleagues. Once every four weeks, it interrupts thrusting to rotate so it can use its 5-foot (1.52-meter) antenna to establish contact with NASA’s Deep Space Network, receiving new instructions from the Dawn operations team at JPL and transmitting a comprehensive report on all its subsystems. Then it turns back to the orientation needed for thrusting and resumes its powered flight.

During its years of interplanetary travel, Dawn has reliably followed a carefully formulated flight plan from Earth past Mars to Vesta and now from Vesta to Ceres. We discussed some of the principles underlying the development of the complex itinerary in a log written when Dawn was still gravitationally anchored to Earth. To carry out its ambitious adventure, Dawn should thrust most of the time, but not all of the time. Indeed, at some times, thrusting would be unproductive.

We will not delve into the details here, but remember that Dawn is doing more than ascending the solar system hill, climbing away from the sun. More challenging than that is making its orbit match the orbit of its targets so that it does not fly past them for a brief encounter as some other missions do. Performing its intricate interplanetary choreography requires exquisite timing with the grace and delicacy of the subtly powerful ion propulsion.

Of course Dawn does not thrust much of the time it is in orbit at Vesta and Ceres; rather, its focus there is on acquiring the precious pictures and other measurements that reveal the detailed nature of these mysterious protoplanets. But even during the interplanetary flight, there are two periods in the mission in which it is preferable to coast. Sophisticated analysis is required to compute the thrusting direction and schedule, based on factors ranging from the physical characteristics of the solar system (e.g., the mass of the sun and the masses and orbits of Earth, Mars, Vesta, Ceres and myriad other bodies that tug, even weakly, on Dawn) to the capabilities of the spacecraft (e.g., electrical power available to the ion thrusters) to constraints on when mission planners will not allow thrusting (e.g., during spacecraft maintenance periods).

The first interval that interplanetary trajectory designers designated as “optimal coast” was well over four years and 1.8 billion miles (2.8 billion kilometers) ago. Dawn coasted from October 31, 2008, to June 8, 2009. During that time, the ship took some of Mars’s orbital energy to help propel itself toward Vesta. (In exchange for boosting Dawn, Mars slowed down by an amount equivalent to about 1 inch, or 2.5 centimeters, in 180 million years.)

The second and final interval when coasting is better than thrusting begins next month. From Nov. 11 to Dec. 9, Dawn will glide along in its orbit around the sun without modifying it. The timing of this coast period is nearly as important to keeping the appointment with Ceres as is the timing of the thrusting. In next month’s log, we will describe some of the special assignments the sophisticated robot will perform instead of its usual quiet cruise routine of accelerating and emitting xenon ions. We also will look ahead to some interesting celestial milestones and alignments in December.

While the spacecraft courses through the asteroid belt, the flight team continues refining the plans for Ceres. In logs in December and several months in 2014, we will present extensive details of those plans so that by the time Dawn begins its mission there, you will be ready to ride along and share in the experience.

In the meantime, as the stalwart ship sails on, it is propelled not only by ions but also by the promise of exciting new knowledge and the prospects of a thrilling new adventure in exploring an uncharted alien world.

Dawn is 16 million miles (26 million kilometers) from Vesta and 25 million miles (39 million kilometers) from Ceres. It is also 3.07 AU (286 million miles, or 460 million kilometers) from Earth, or 1,200 times as far as the moon and 3.10 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 51 minutes to make the round trip.

– Dr. Marc D. Rayman

P.S. This log is posted early enough to allow time for your correspondent to don his Halloween costume. In contrast to last year’s simple (yet outlandish) costume, this year’s will be more complex. He is going in double costume, disguised as someone who is only pretending to be passionate about the exploration of the cosmos and the rewards of scientific insight.

› Read more entries from Marc Rayman’s Dawn Journal


NASA’s Dawn Spacecraft Celebrates Six Years in Space

Friday, September 27th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft
Artist’s concept of NASA’s Dawn spacecraft between the giant asteroid Vesta and the dwarf planet Ceres. Image credit: NASA/JPL-Caltech

Dear Dawnniversaries,

On the sixth anniversary of leaving Earth to embark on a daring deep-space expedition, Dawn is very, very far from its erstwhile planetary residence. Now humankind’s only permanent resident of the main asteroid belt between Mars and Jupiter, the seasoned explorer is making good progress toward the largest object in that part of the solar system, the mysterious dwarf planet Ceres. The voyage is long, and the intrepid but patient traveler will not reach its next destination until half a year after its seventh anniversary of departing Earth.

On its fifth anniversary, Dawn was still relatively close to Vesta, the giant protoplanet that had so recently held the craft in its gravitational grip. The only probe ever to orbit a main belt asteroid, Dawn spent 14 months (including its fourth anniversary) accompanying Vesta on its way around the sun. After more than two centuries of appearing to astronomers as little more than a fuzzy blob of light among the stars, the second most massive body in the asteroid belt has been revealed as a fascinating, complex, alien world more closely related to terrestrial planets (including Earth) than to typical asteroids.

Most of the ship’s first four years of spaceflight were devoted to using its ion propulsion system to spiral away from the sun, ascending the solar system hill from Earth to Vesta. Now it is working to climb still higher up that hill to Ceres.

For those who would like to track the probe’s progress in the same terms used on previous (and, we boldly predict, subsequent) anniversaries, we present here the sixth annual summary, reusing the text from last year with updates where appropriate. Readers who wish to cogitate about the extraordinary nature of this deep-space expedition may find it helpful to compare this material with the logs from its first, second, third, fourth, and fifth anniversaries.

In its six years of interplanetary travels, the spacecraft has thrust for a total of 1,410 days, or 64 percent of the time (and about 0.000000028 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 318 kilograms (701 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 8.7 kilometers per second (19,500 mph). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Having accomplished about three-quarters of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)

Since launch, our readers who have remained on or near Earth have completed six revolutions around the sun, covering about 37.7 AU (5.6 billion kilometers or 3.5 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 27.4 AU (4.1 billion kilometers or 2.5 billion miles). As it climbed away from the sun to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. It will have to slow down still more to rendezvous with Ceres. Since Dawn’s launch, Vesta has traveled only 24.2 AU (3.6 billion kilometers or 2.2 billion miles), and the even more sedate Ceres has gone 22.8 AU (3.4 billion kilometers or 2.1 billion miles).

Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.

› Continue reading Marc Rayman’s Dawn Journal


Earth and Dawn on Opposite Sides Now

Friday, August 30th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear Antecedawnts,

Traveling confidently and alone, Dawn continues to make its way through the silent depths of the main asteroid belt. The only spacecraft ever to have orbited a resident of the vast territory between Mars and Jupiter, Dawn conducted a spectacular exploration of gigantic Vesta, revealing a complex place that resembles the terrestrial planets more than typical asteroids. Now the interplanetary adventurer is on its long journey to the uncharted dwarf planet Ceres, by far the largest of all asteroids (975 kilometers, or more than 600 miles, in equatorial diameter). In 2015, the mysterious world of rock and ice will begin to give up its ancient secrets to the immigrant from distant Earth.

Earth, Vesta, Dawn, and Ceres are following their own separate paths around the sun. The spacecraft is patiently reshaping its orbit, using its uniquely efficient ion propulsion system to accomplish a deep-space expedition that would be impossible with conventional propulsion.

As we have seen in many previous logs (including, for example, here), the higher an object’s orbit, the slower it needs to move in order to balance the gravitational pull, which diminishes with distance. Blistering Mercury orbits the sun faster than Venus, Venus goes faster than Earth, Earth goes faster than Mars, and Mars goes faster than the residents of the asteroid belt and the cold planets of the outer solar system. In the same way, satellites that orbit close to Earth, including the International Space Station, move faster than those at greater altitudes, and the moon travels even more slowly in its very high orbit.

Dawn is now a permanent inhabitant of the main asteroid belt. Therefore, the massive sun, the gravitational master of the solar system, has a weaker grip on it than on Earth. So as Dawn maneuvers from Vesta to Ceres, Earth revolves more rapidly around the sun. This month, their independent motions have taken them to their greatest separation of the year, as they are on opposite sides of the sun. How truly remarkable that humankind can accomplish such a feat!

On August 4, the planet and its robotic ambassador to the cosmos were an extraordinary 3.47 AU (519 million kilometers, or 322 million miles) apart. (To recapture the feeling of your position in the universe then, it may be helpful to know that the maximum range was attained at 4:16 a.m. PDT.) From the perspective of terrestrial observers, had they possessed the superhuman (and even supertelescopic) vision needed to descry the tiny ship far beyond the blindingly bright star, Dawn would have appeared to be very close to the sun but not directly behind it. To rendezvous with Vesta and then with Ceres, the spacecraft has tilted the plane of its solar orbit. Some of the time it is north of Earth’s orbital plane, sometimes it is south. August 4 was during the northern segment, so Dawn would have been a little north of the sun.

It’s time to refer to one of those novel clocks available in the Dawn gift shop on your planet (although if you already have such a clock, it probably doesn’t tell you that it’s time — we stand by our policy of full refunds within 24 hours, as measured by our Dawn clocks). With the sun at the center of the clock, Earth’s motion would be like that of a short minute hand. Dawn, both farther from the sun and moving more slowly, would be following the path of a longer hour hand. If we ignore the effect of the ion thrust, which is constantly changing the orbit, and the slight misalignment of the hour hand representing Dawn’s being in a different plane, the conditions on August 4 were like those at 6:00.

As time progresses and Earth continues circling the sun, it will come closer to Dawn until April 2014 (like 12:00). Even then, however, they will be over 1.55 AU (232 million kilometers, or 144 million miles) apart, and they will never be that close again. The spacecraft will continue climbing higher and higher from the sun toward Ceres, so by the time Earth loops around once more, Dawn will be even farther from it. In the meantime, when next the arrangement is like 6:00, in December 2014, the separation will be more than 3.78 AU (565 million kilometers, or 351 million miles), even greater than the remarkable range a few weeks ago.

› Continue reading Marc Rayman’s Dawn Journal


To Be in the Right Place, Dawn Catches Up With Time

Wednesday, May 1st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The Dawn spacecraft's orbits
In this graphic of Dawn’s interplanetary trajectory, the thin solid lines represent the orbits of Earth, Mars, Vesta and Ceres. After leaving Vesta, Dawn’s orbit temporarily takes it closer to the Sun than Vesta, although in this view they are so close together the difference is not visible because of the thickness of the lines. Dawn will remain in orbit around Ceres at the end of its primary mission. Image credit: NASA/JPL-Caltech

Dear Dawnscerning Readers,

Nearly three times as far from Earth as the sun is, the Dawn spacecraft is making very good progress on its ambitious trek from Vesta to Ceres. After a spectacular adventure at the second most massive resident of the main asteroid belt between Mars and Jupiter, Dawn used its extraordinary ion propulsion system to leave it behind and undertake the long journey to a dwarf planet.

Ceres orbits the sun outside Vesta’s orbit, yet Dawn is now closer to the sun than both of these alien worlds. How can it be that as the probe climbs from one to the other, it seems to be falling inward? Perhaps the answer lies in the text below; let’s venture on and find out!

On Halloween we discussed why Dawn is heading in toward the sun, but this question is different. Vesta also is getting closer to the sun, but what’s of interest now is that Dawn, despite its more remote destination, has been approaching the sun more quickly. That earlier log stands out as the best one ever written on this exciting mission in the entire history of October 2012, but if you prefer not to visit it now, we can summarize here the explanation for the spacecraft moving toward the sun. Like all members of the sun’s entourage, Vesta and Ceres follow elliptical orbits, their distances from the master of the solar system growing and shrinking as they loop around it. Even Earth’s orbit, although nearly round, certainly is not perfectly circular. Our planet is a little closer to the sun in the northern hemisphere winter (southern hemisphere summer) than it is in the summer (southern hemisphere winter). Dawn’s orbit is elliptical as well, so it naturally moves nearer to the sun sometimes, and now is such a time. But that does not address why it is currently closer to the sun than Vesta, even though it is seeking out the more distant Ceres.

Because it will orbit Ceres, and not simply fly past it (which would be significantly easier but less valuable), Dawn must make its own orbit around the sun be identical to its target’s. But that is not the entire story. After spending 14 months orbiting Vesta, Dawn’s challenge is more than to change the shape of its orbit to match Ceres’s. The spacecraft also must be at the same place in Ceres’s heliocentric orbit that Ceres itself is.

It would not be very rewarding to follow the same looping path around the sun but always be somewhere else on that path. You can visualize this if you have one of the many defective — er, exotic clocks from the Dawn gift shop on your planet that have two minute hands. If the clock starts with one hand pointed at 12 and another pointed at 1, they will take the same repetitive route, but neither hand will ever catch up with the other. For Dawn’s goal of exploring Ceres, this would not prove satisfying. Therefore, part of the objective of the ion thrusting is to ensure the spacecraft arrives not only on the same heliocentric course as Ceres but is there when Ceres is also.

This is a problem familiar to all readers who have maneuvered in orbit, where the principles of orbital mechanics are the rules of the road. To solve it, we rely on one of the laws that we have addressed many times in these logs: objects in a lower orbit travel faster. We described this in more detail in February, and we can recall the essential idea here. The gravitational attraction of any body, whether it is the sun, Earth, a black hole, or anything else, is greater at shorter ranges. So to balance that strong inward pull, an orbiter is compelled to race around quickly. At higher orbits, where gravity is weaker, a more leisurely orbital pace suffices.

We can take advantage of this characteristic of orbits. If we drop to a slightly lower orbit, we travel along more swiftly. That is precisely what Dawn needs to do in order to ensure that when it finishes expanding and tilting its orbit in 2015 so that it is the same as Ceres’s, it winds up at the same location as its target. This would be like speeding up the minute hand that had begun at the 12, allowing it to catch up with the hand that would otherwise always be leading it.

› Continue reading Marc Rayman’s Dawn Journal


While Dawn Keeps Cruising, Engineers Carry On

Friday, March 29th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Indawnstrious Readers,

In the depths of the main asteroid belt between Mars and Jupiter, far from Earth, far even from any human-made object, Dawn remains in silent pursuit of dwarf planet Ceres. It has been more than six months since it slipped gracefully away from the giant protoplanet Vesta. The spacecraft has spent 95 percent of the time since then gently thrusting with its ion propulsion system, using that blue-green beam of high velocity xenon ions to propel itself from one alien world to another.

The ship set sail from Earth more than two thousand days ago, and its voyage on the celestial seas has been wonderfully rewarding. Its extensive exploration of Vesta introduced humankind to a complex and fascinating place that had only been tantalizingly glimpsed from afar with telescopes beginning with its discovery 206 years ago today. Thanks to the extraordinary capability of ion propulsion, Dawn was able to spend 14 months orbiting Vesta, observing dramatic landscapes and exotic features and collecting a wealth of measurements that scientists will continue to analyze for many years.

When it was operating close to Vesta, the spacecraft was in frequent contact with Earth. It took Dawn quite a bit of time to beam the 31,000 photos and other precious data to mission control. In addition, engineers needed to send a great many instructions to the distant adventurer to ensure it remained healthy and productive in carrying out its demanding work in the unforgiving depths of space.

Dawn is now more than 20 times farther from Vesta than the moon is from Earth. Alone again and on its long trek to Ceres, it is not necessary for the ship to be in radio contact as often. As we saw in November, the spacecraft now stops ion thrusting only once every four weeks to point its main antenna to Earth. This schedule conserves the invaluable hydrazine propellant the explorer will need at Ceres. But communicating less frequently does not mean the mission operations team is any less busy. Indeed, as we have explained before, “quiet cruise” consists of a considerable amount of activity.

Each time Dawn communicates with Earth, controllers transmit a second-by-second schedule for the subsequent four weeks. They also load a detailed flight profile with the ion throttle levels and directions for that period. It takes about three weeks to calculate and formulate these plans and to analyze, check, double check, and triple check them to ensure they are flawless before they can be radioed to Dawn.

In addition to all the usual information Dawn needs to keep flying smoothly, operators occasionally include some special instructions. As one example, over the last few months, they have gradually lowered the temperatures of some components slightly in order to reduce heater power. When Dawn stretched out its solar array wings shortly after separating from the Delta rocket on September 27, 2007, its nearly 65-foot wingspan was the longest of any NASA interplanetary probe. The large area of solar cells is needed to collect enough light from the distant sun to power the ion propulsion system and all other spacecraft systems. Devoting a little less power to heaters allows more power to be applied to ionizing and accelerating xenon, yielding greater thrust. With two and a half years of powered flight required to travel from Vesta to Ceres, even a little extra power can make a worthwhile difference to a mission that craves power.

Most temperature adjustments are only two degrees Celsius (3.8 degrees Fahrenheit) at a time, but even that requires careful analysis and investigation, because lowering the temperature of one component may affect another. Xenon and hydrazine propellants need to be maintained in certain ranges, and the lines they flow through follow complicated paths around the spacecraft, so the temperatures all along the way matter. Most of the hardware onboard, from valves and switches to electronics to structural mounts for sensitively aligned units, needs to be thermally regulated to keep Dawn shipshape.

It can take hours for a component to cool down and stabilize at a new setting, and sometimes the change won’t even occur until the spacecraft has turned away to resume thrusting, when the faint warmth of the sun and the deep cold of black space affect different parts of the complex robot. Then it will be another four weeks until engineers will receive a comprehensive report on all the temperatures, so they need to be cautious with each change.

› Continue reading Marc Rayman’s Dawn Journal


A Hard Day’s Flight: Dawn Achieves Orbital Velocity

Friday, March 1st, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Mosaic of Dawn's images of asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Impordawnt Readers,

The indefatigable Dawn spacecraft is continuing to forge through the main asteroid belt, gently thrusting with its ion propulsion system. As it gradually changes its orbit around the sun, the distance to dwarf planet Ceres slowly shrinks. The pertinacious probe will arrive there in 2015 to explore the largest body between the sun and Neptune that has not yet been glimpsed by a visitor from Earth. Meanwhile, Vesta, the fascinating alien world Dawn revealed in 2011 and 2012, grows ever more distant. The mini-planet it orbited and studied in such detail now appears only as a pinpoint of light 15 times farther from Dawn than the moon is from Earth.

Climbing through the solar system atop a column of blue-green xenon ions, Dawn has a great deal of powered flight ahead in order to match orbits with faraway Ceres. Nevertheless, it has shown quite admirably that it is up to the task. The craft has spent more time thrusting and has changed its orbit under its own power more than any other ship from Earth. While most of the next two years will be devoted to still more thrusting, the ambitious adventurer has already accomplished much more than it has left to do. And now it is passing an interesting milestone on its interplanetary trek.

With all of the thrusting Dawn has completed, it has now changed its speed by 7.74 kilometers per second (17,300 mph), and the value grows as the ion thrusting continues. For space enthusiasts from Earth, that is a special speed, known as “orbital velocity.” Many satellites, including the International Space Station, travel at about that velocity in their orbits. So does this mean that Dawn has only now achieved the velocity necessary to orbit Earth? The short answer is no. The longer answer constitutes the remainder of this log.

We have discussed some of these principles before, but they are counterintuitive and questions continue to arise. Rather than send our readers on a trajectory through the history of these logs even more complicated than Dawn’s flight through the asteroid belt, we will revisit a few of the ideas here. (After substantial introspection, your correspondent granted and was granted permission to reuse not only past text but also future text.)

While marking Dawn’s progress in terms of its speed is a convenient description of the effectiveness of its maneuvering, it is not truly a measure of how fast it is moving. Rather, it is a measure of how fast it would be moving under very special (and unrealistic) circumstances. To understand this, we need to look at the nature of orbits in general and Dawn’s interplanetary trajectory in particular.

The overwhelming majority of craft humans have sent into space have remained in the vicinity of Earth, accompanying that planet on its annual revolutions around the sun. All satellites of Earth (including the moon) remain bound to it by its gravity. (Similarly, Dawn spent much of 2011 and 2012 as a satellite of distant Vesta, locked in the massive body’s gravitational grip.) As fast as satellites seem to travel compared to terrestrial residents, from the larger solar system perspective, their incessant circling of Earth means their paths through space are not very different from Earth’s itself. Consider the path of a car racing around a long track. If a fly buzzes around inside the car, to the driver it may seem to be moving fast, but if someone watching the car from a distance plotted the fly’s path, on average it would be pretty much like the car’s.

Everything on the planet and orbiting it travels around the sun at an average of 30 kilometers per second (67,000 mph), completing one full solar orbit every year. To undertake its interplanetary journey and travel elsewhere in the solar system, Dawn needed to break free of Earth’s grasp, and that was accomplished by the rocket that carried it to space more than five years ago. Dawn and its erstwhile home went their separate ways, and the sun became the natural reference for the spacecraft’s position and speed on its voyage in deep space.

Despite the enormous push the Delta II rocket delivered (with affection!) to Dawn, the spacecraft still did not have nearly enough energy to escape from the powerful sun. So, being a responsible resident of the solar system, Dawn has remained faithfully in orbit around the sun, just as Earth and the rest of the planets, asteroids, comets, and other members of the star’s entourage have.

Whether it is for a spacecraft or moon orbiting a planet, a planet or Dawn orbiting the sun, the sun orbiting the Milky Way galaxy, or the Milky Way galaxy orbiting the Virgo supercluster of galaxies (home to a sizeable fraction of our readership), any orbit is the perfect balance between the inward tug of gravity and the inexorable tendency of objects to travel in a straight path. If you attach a weight to a string and swing it around in a circle, the force you use to pull on the string mimics the gravitational force the sun exerts on the bodies that orbit it. The effort you expend in keeping the weight circling serves constantly to redirect its path; if you let go of the string, the weight’s natural motion would carry it away in a straight line (ignoring the effect of Earth’s gravity).

The force of gravity diminishes with distance, so the sun’s pull on a nearby body is greater than on a more distant one. Therefore, to remain in orbit, to balance the relentless tug of gravity, the closer object must travel faster, fighting the stronger pull. The same effect applies at Earth. Satellites that orbit very close (including, for example, the International Space Station, around 400 kilometers, or 250 miles, from the surface) must streak around the planet at about 7.7 kilometers per second (17,000 mph) to keep from being pulled down. The moon, orbiting almost 1000 times farther above, needs only to travel at about 1.0 kilometers per second (less than 2300 mph) to balance Earth’s weaker hold at that distance.

Notice that this means that for an astronaut to travel from the surface of Earth to the International Space Station, it would be necessary to accelerate to quite a high speed to rendezvous with the orbital facility. But then once in orbit, to journey to the much more remote moon, the astronaut’s speed eventually would have to decline dramatically. Perhaps speed tells an incomplete story in describing the travels of a spacecraft, just as it does with another example of countering gravity.

A person throwing a ball is not that different from a rocket launching a satellite (although the former is usually somewhat less expensive and often involves fewer toxic chemicals). Both represent struggles against Earth’s gravitational pull. To throw a ball higher, you have to give it a harder push, imparting more energy to make it climb away from Earth, but as soon as it leaves your hand, it begins slowing. For a harder (faster) throw, it will take longer for Earth’s gravity to stop the ball and bring it back, so it will travel higher. But from the moment it leaves your hand until it reaches the top of its arc, its speed constantly dwindles as it gradually yields to Earth’s tug. The astronaut’s trip from the space station to the moon would be accomplished by starting with a high speed “throw” from the low starting orbit, and then slowing down until reaching the moon.

The rocket that launched Dawn threw it hard enough to escape from Earth, sending it well beyond the International Space Station and even the moon. Dawn’s maximum speed relative to Earth on launch day was so high that Earth could not pull it back. As we saw in the explanation of the launch profile, Dawn was propelled to 11.46 kilometers per second (25,640 mph), well in excess of the space station’s orbital speed given three paragraphs above. But it has remained under the sun’s control.

Now we can think of the general problem of flying elsewhere in space as similar to climbing a hill. For terrestrial hikers, the rewards of ascent come only after doing the work of pushing against Earth’s gravity to reach a higher elevation. Similarly, Dawn is climbing a solar system hill with the sun at the bottom. It started part way up the hill at Earth; and its first rewards were found at a higher elevation, where Vesta, traveling around the sun at only about two thirds of Earth’s speed, revealed its fascinating secrets to the visiting ship. The ion thrusting now is propelling it still higher up the hill toward Ceres, which moves even more slowly to balance the still-weaker pull of the sun.

› Continue reading Marc Rayman’s Dawn Journal for more on how Dawn achieved orbital velocity


The Giant Asteroid, Near and Far

Thursday, January 10th, 2013

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft departing asteroid Vesta
Artist’s concept of NASA’s Dawn spacecraft departing the giant asteroid Vesta. Image credit: NASA/JPL-Caltech

Dawn concluded 2012 almost 13,000 times farther from Vesta than it began the year. At that time, it was in its lowest orbit, circling the alien world at an average altitude of only 210 kilometers (130 miles), scrutinizing the mysterious protoplanet to tease out its secrets about the dawn of the solar system.

To conduct its richly detailed exploration, Dawn spent nearly 14 months in orbit around Vesta, bound by the behemoth’s gravitational grip. In September they bid farewell, as the adventurer gently escaped from the long embrace and slipped back into orbit around the sun. The spaceship is on its own again in the main asteroid belt, its sights set on a 2015 rendezvous with dwarf planet Ceres. Its extensive ion thrusting is gradually enlarging its orbit and taking it ever farther from its erstwhile companion as their solar system paths diverge.

Meanwhile, on faraway Earth (and all the other locations throughout the cosmos where Dawnophiles reside), the trove of pictures and other precious measurements continue to be examined, analyzed, and admired by scientists and everyone else who yearns to glimpse distant celestial sights. And Earth itself, just as Vesta, Ceres, Dawn, and so many other members of the solar system family, continues to follow its own orbit around the sun.

Thanks to a coincidence of their independent trajectories, Earth and Dawn recently reached their smallest separation in well over a year, just as the tips of the hour hand and minute hand on a clock are relatively near every 65 minutes, 27 seconds. On Dec. 9, they were only 236 million kilometers (147 million miles) apart. Only? In human terms, this is not particularly close. Take a moment to let the immensity of their separation register. The International Space Station, for example, firmly in orbit around Earth, was 411 kilometers (255 miles) high that day, so our remote robotic explorer was 575 thousand times farther. If Earth were a soccer ball, the occupants of the orbiting outpost would have been a mere seven millimeters (less than a third of an inch) away. Our deep-space traveler would have been more than four kilometers (2.5 miles) from the ball. So although the planet and its extraterrestrial emissary were closer than usual, they were not in close proximity. Dawn remains extraordinarily far from all of its human friends and colleagues and the world they inhabit.

As the craft reshapes its solar orbit to match Ceres’s, it will wind up farther from the sun than it was while at Vesta. (As a reminder, see the table here that illustrates Dawn’s progress to each destination on its long interplanetary voyage.) We saw recently, however, that the route is complex, and the spacecraft is temporarily approaching the sun. Before the ship has had time to swing back out to a greater heliocentric range, Earth will have looped around again, and the two will briefly be even a little bit closer early in 2014. After that, however, they will never be so near each other again, as Dawn will climb higher and higher up the solar system hill, its quest for new and exciting knowledge of distant worlds taking it farther from the sun and hence from Earth.

› Continue reading Marc Rayman’s Dawn Journal to learn how to approximate Dawn’s position in the sky on Jan. 21 and 22


Short Puffs Keep Dawn Chugging Along

Tuesday, December 4th, 2012

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft at Ceres
Artist’s concept of NASA’s Dawn spacecraft at its next target, the protoplanet Ceres. Image credit: NASA/JPL-Caltech

Dear Dawndroids,

Dawn is continuing to gently and patiently change its orbit around the sun. In September, it left Vesta, a complex and fascinating world it had accompanied for 14 months, and now the bold explorer is traveling to the largest world in the main asteroid belt, dwarf planet Ceres.

Dawn has spent most of its time since leaving Earth powering its way through the solar system atop a column of blue-green xenon ions emitted by its advanced ion propulsion system. Mission controllers have made some changes to Dawn’s operating profile in order to conserve its supply of a conventional rocket propellant known as hydrazine. Firing it through the small jets of the reaction control system helps the ship rotate or maintain its orientation in the zero-gravity of spaceflight. The flight team had already taken some special steps to preserve this precious propellant, and now they have taken further measures. If you remain awake after the description of what the changes are, you can read about the motivation for such frugality.

Dawn’s typical week of interplanetary travel used to include ion thrusting for almost six and two-thirds days. Then it would stop and slowly pirouette to point its main antenna to Earth for about eight hours. That would allow it to send to the giant antennas of NASA’s Deep Space Network a full report on its health from the preceding week, including currents, voltages, temperatures, pressures, instructions it had executed, decisions it had made, and almost everything else save its wonderment at operating in the forbidding depths of space so fantastically far from its planet of origin. Engineers also used these communications sessions to radio updated commands to the craft before it turned once again to fire its ion thruster in the required direction.

Now operators have changed the pace of activities. Every turn consumes hydrazine, as the spacecraft expels a few puffs of propellant through some of its jets to start rotating and through opposing jets to stop. Instead of turning weekly, Dawn has been maintaining thrust for two weeks at a time, and beginning in January it will only turn to Earth once every four weeks. After more than five years of reliable performance, controllers have sufficient confidence in the ship to let it sail longer on its own. They have refined the number and frequency of measurements it records so that even with longer intervals of independence, the spacecraft can store the information engineers deem the most important to monitor.

Although contact is established through the main antenna less often, Dawn uses one of its three auxiliary antennas twice a week. Each of these smaller antennas produces a much broader signal so that even when one cannot be aimed directly at Earth, the Deep Space Network can detect its weak transmission. Only brief messages can be communicated this way, but they are sufficient to confirm that the distant ship remains healthy.

In addition to turning less often, Dawn now turns more slowly. Its standard used to be the same blinding pace at which the minute hand races around a clock (fasten your seat belt!). Engineers cut that in half two years ago but returned to the original value at the beginning of the Vesta approach phase. Now they have lowered it to one quarter of a minute hand’s rate. Dawn is patient, however. There’s no hurry, and the leisurely turns are much more hydrazine-efficient.

With these two changes, the robotic adventurer will arrive at Ceres in 2015 with about half of the 45.6-kilogram (101-pound) hydrazine supply it had when it rocketed away from Cape Canaveral on a lovely September dawn in 2007. Mission planners will be able to make excellent use of it as they guide the probe through its exploration of the giant of the main asteroid belt.

Any limited resource should be consumed responsibly, whether on a planet or on a spaceship. Hydrazine is not the only resource that Dawn’s controllers manage carefully, but let’s recall why this one has grown in importance recently.

› Continue reading Marc Rayman’s Dawn Journal


Dawn Comes Closer to Go Farther

Thursday, November 1st, 2012

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft at Ceres
Artist’s concept of NASA’s Dawn spacecraft at its next target, the protoplanet Ceres. Image credit: NASA/JPL-Caltech

Dear Indawnspensable Readers,

Dawn is making good progress on the second segment of its cosmic travels. Following more than a year of arduous but sensationally productive and exciting work revealing the fascinating character of the giant protoplanet Vesta, it is now patiently pursuing its next target, the mysterious dwarf planet Ceres, which resides farther from the sun. For the second (and final) time in its interplanetary journey, however, Dawn is about to turn around, going closer to the sun rather than farther away.

In August 2008, we saw in detail how it could be that even as the bold explorer travels outward in the solar system from Earth, past Mars, to Vesta, and then on to Ceres, it could occasionally appear to reverse course temporarily. We present here a shorter explanation for those readers who did not memorize the log explaining this perplexing behavior (you know who you are, and we do as well, but your secret remains safe under the terms of our reader privacy agreement).

Dawn orbits the sun, as do Vesta, Ceres, the other residents of the main asteroid belt, and the planets. All orbits, whether of these objects around the star at the center of our solar system, artificial satellites or the moon in orbit around Earth, or even Dawn when it was in orbit around Vesta, are ellipses (like flattened circles). Earth, for example, orbits the sun at an average distance of 150 million kilometers (93.0 million miles), which astronomers call one astronomical unit (AU). During its year-long revolution, however, our planet comes in to 0.98 AU from the sun and goes out to 1.02 AU. Earthlings manage quite nicely with these small variations. (Note that the seasons are not caused by the changes in distance but instead are a result of the tilt of Earth’s axis and thus the differing angles at which the warming rays of the sun arrive during the year. If the sun’s distance were all that mattered, the northern and southern hemispheres would have the same seasons.) So, orbiting bodies move smoothly between a minimum and a maximum range from their gravitational masters rather than remaining at a constant distance.

When Dawn was in orbit around Vesta, it accompanied that world on its regular journey around the sun. The table last month showing the probe’s progress over the five years of its deep space trek reminds us that Vesta’s path brings it as close to the sun as 2.15 AU and takes it out to 2.57 AU.

If Dawn had remained in orbit around Vesta, it would have continued to follow the same elliptical course as its host in the asteroid belt. The pair would have reached their maximum solar distance next month and then would have fallen back to 2.15 AU in September 2014. While visiting Vesta was extremely gratifying, this explorer’s ambitions are greater. It broke free of Vesta’s grip, its sights set on a new and distant alien destination.

Now the spacecraft is in its own independent orbit around the sun, and the persistent but gentle pressure of its advanced ion propulsion system gradually reshapes that orbit. At any moment, the orbit is an ellipse, and an instant later, it is a slightly different ellipse, courtesy of the thrust. As Dawn departed from Vesta only last month, its orbit is not yet dramatically different, but over the course of the coming years, the effect of the thrusting will be to change the orbit tremendously. To reach Ceres in 2015, the ship will enlarge and tip its elliptical course to match the motion of the dwarf planet around the sun. (Some of the parameters characterizing each object’s orbit are shown here.)

Although the ship’s orbit is growing, it will reach the current high point on Nov. 1. It will then be 2.57 AU from the sun and, just as in 2008 (albeit at a smaller distance), it will begin moving closer, even as it continues to thrust.

If Dawn stopped thrusting on Nov. 1, its elliptical orbit would carry it down to 2.19 AU from the sun in September 2014. That’s a higher orbit than Vesta’s but still well below what it needs to be for the rendezvous with Ceres. Astute readers have already anticipated that the plan is not to stop thrusting but to continue reworking the trajectory, just as a ceramicist gradually achieves a desired shape to create the envisioned artistic result. The ongoing thrusting will raise the low point of the orbit, so if the ship follows the flight plan, it will descend only to 2.45 AU in October 2013 before sailing outward again. By May 2014 it will have risen to the same solar altitude as it is now. All the thrusting in the interim will have altered its course so much, however, that it will not turn around then; rather, it will continue ascending to keep its 2015 appointment with Ceres.

› Continue reading Marc Rayman’s Dawn Journal


Dawn’s Stellar Anniversary

Thursday, September 27th, 2012

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Artist's concept of the Dawn spacecraft
Artist’s concept of NASA’s Dawn spacecraft. Image credit: NASA/JPL-Caltech

Dear Dawnniversaries,

On the fifth anniversary of the beginning of its ambitious interplanetary adventure, Dawn can look back with great satisfaction on its spectacular exploration of the giant protoplanet Vesta and forward with great eagerness to reaching dwarf planet Ceres. Today Earth’s robotic ambassador to the main asteroid belt is in quiet cruise, gradually reshaping its orbit around the sun so it can keep its appointment in 2015 with the mysterious alien world that lies ahead.

This anniversary resembles the first three more than the fourth. Its first years in space were devoted to spiraling away from the sun, ascending the solar system hill so it could gracefully slip into orbit around Vesta in time for its fourth anniversary. One year ago, Dawn was in the behemoth’s gravitational grip and preparing to map its surface in stereo and make other measurements. The subsequent year yielded stunning treasures as Dawn unveiled the wondrous secrets of a world that had only been glimpsed from afar for over two centuries. While at Vesta, it spiraled around the massive orb to position itself for the best possible perspectives. Its final spiral culminated in its departure from Vesta earlier this month. Now for its fifth anniversary, it is spiraling around the sun again, climbing beyond Vesta so that it can reach Ceres.

For those who would like to track the probe’s progress in the same terms used on previous (and, we boldly predict, subsequent) anniversaries, we present here the fifth annual summary, reusing the text from last year with updates where appropriate. Readers who wish to cogitate about the extraordinary nature of this deep-space expedition may find it helpful to compare this material with the logs from its first, second, third, and fourth anniversaries.

In its five years of interplanetary travels, the spacecraft has thrust for a total of 1060 days, or 58 percent of the time (and about 0.000000021 percent of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the craft only 267 kilograms (587 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.

The fraction of time the ship has spent in powered flight is lower than last year (when it was 68 percent), because Dawn devoted relatively little of the past year to thrusting. Although it did change orbits extensively at Vesta, most of the time it was focused on exactly what it was designed and built to do: scrutinize the ancient world for clues about the dawn of the solar system.

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 7.14 kilometers per second (16,000 miles per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft’s propulsive work. Having accomplished slightly more than half of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)

Since launch, our readers who have remained on or near Earth have completed five revolutions around the sun, covering about 31.4 AU (4.70 billion kilometers or 2.92 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 23.4 AU (3.50 billion kilometers or 2.18 billion miles). As it climbed away from the sun to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. Since Dawn’s launch, Vesta has traveled only 20.4 AU (3.05 billion kilometers or 1.90 billion miles) and the even more sedate Ceres has gone 18.9 AU (2.82 billion kilometers or 1.75 billion miles).

› Continue reading Marc Rayman’s Dawn Journal


Dawn’s Split from Asteroid Vesta - Mission Insider Explains

Wednesday, September 5th, 2012

By Marc Rayman
As NASA’s Dawn spacecraft makes its journey to its second target, the dwarf planet Ceres, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

The dwarf planet Ceres as imaged by the Keck Observatory
NASA’s Dawn spacecraft departed the giant asteroid Vesta on Sept. 04, 2012 PDT to begin its journey to a second destination, the dwarf planet Ceres, which is seen in this image from the Keck Observatory on Mauna Kea, Hawaii. Image credit: NASA/JPL-Caltech, Keck Observatory, C. Dumas

Dear Marvestalous Readers,

An interplanetary spaceship left Earth in 2007. Propelling itself gently and patiently through the solar system with a blue-green beam of xenon ions, it gradually spiraled away from the sun. It sailed past Mars in 2009, its sights set on more distant and exotic destinations. In July 2011, it gracefully and elegantly entered orbit around the second most massive resident of the main asteroid belt, Vesta. It spent more than 13 months there scrutinizing the gigantic protoplanet with all of its sensors and maneuvering to different orbits to optimize its investigations, making myriad marvelous discoveries. After they traveled together around the sun for 685 million kilometers (426 million miles), the ship left orbit in September 2012 and is now headed for dwarf planet Ceres, the largest body between the sun and Neptune not yet visited by a spacecraft. No other probe has ever been capable of the amazing feats Dawn is performing, exploring two of the largest uncharted worlds in the inner solar system.

The population of the main asteroid belt numbers in the millions. Vesta is such a behemoth that Dawn has now single-handedly examined about eight percent of the mass of the entire belt. And by the time it finishes at the colossus Ceres, it will have investigated around 40 percent.

The expedition to Vesta has produced riches beyond everyone’s hopes. With 31,000 photos, 20 million visible and infrared spectra, and thousands of hours of neutron spectra, gamma ray spectra, and gravity measurements, Dawn has revealed to humankind a unique and fascinating member of the solar system family. More akin to Earth and the other terrestrial planets than to typical asteroids, Vesta is not just another chunk of rock. It displays complex geology and even has a dense iron-nickel core, a mantle, and a crust. Its heavily cratered northern hemisphere tells the story of more than 4.5 billion years of battering in the rough and tumble asteroid belt. Its southern hemisphere was wiped clean, resurfaced by an enormous impact at least two billion years ago and an even greater collision one billion years ago. These events excavated the 400-kilometer (250-mile) Veneneia and 500-kilometer (310-mile) Rheasilvia basins. The larger basin has a mountain at the center that towers more than twice the height of Mt. Everest; indeed, it soars higher than all but one of the mountains known in the solar system. The impacts were so forceful, they nearly destroyed Vesta. The fierce shock reverberated through the entire body and left as scars an extraordinary network of vast troughs near the equator, some hundreds of kilometers (miles) long and 15 kilometers (10 miles) wide.

The powerful impacts liberated tremendous amounts of material, flinging rocks far out into space, some of which eventually made it all the way to Earth. It is astonishing that more than one thousand meteorites found here came from Vesta. We have some meteorites from Mars, and we have some meteorites from the moon, but we have far, far more that originated in those impacts at Vesta, so distant in time and space. Vesta, Mars, and the moon are the only celestial bodies identified as the source of specific meteorites.

Scientists will spend years productively poring through Dawn’s fabulous findings and learning what secrets they hold about the dawn of the solar system, and many more people will continue to marvel at the spectacular sights of this alien world. But the emissary from Earth has completed its assignment there and moved on. It has spent most of its time since the previous log using its ion propulsion system to climb higher and higher above Vesta. This departure spiral is the mirror image of the approach spiral the robotic adventurer followed last year. The unique method of entering and leaving orbit is one of the many intriguing characteristics of a mission that uses ion propulsion. Without that advanced technology, this ambitious deep space adventure would be impossible.

As Dawn ascended, Vesta’s gravitational grip grew weaker and weaker. At some point along its spiral, the explorer was far enough and moving fast enough that Vesta could no longer hold it in orbit. As smoothly and tenderly as Vesta had taken Dawn in its embrace last year, it released its erstwhile companion, each to go its own way around the sun. The bond was severed at about 11:26 p.m. PDT yesterday, when they were 17,200 kilometers (10,700 miles) apart, separating at the remarkably leisurely speed of less than 33 meters per second (73 miles per hour). Many of our readers drove their cars that fast today (although we hope it was not in school zones).

Unlike missions that use conventional chemical propulsion, there was no sudden change on the spacecraft and no nail-biting on Earth. If you had been in space watching the action, you probably would have been hungry, cold, and hypoxic, but you would not have noticed anything unusual about the scene. Apart from a possible hint of self-satisfaction, Dawn would have looked just as it had for most of its interplanetary flight, a monument to humankind’s ingenuity and passionate drive to know the cosmos perched atop a blue-green pillar of xenon ions. If, instead, you had been in Dawn mission control watching the action, you would have been in the dark and all alone (until JPL Security arrived). There was no need to have radio contact with the reliable spaceship. It had already thrust for almost 2.9 years, or 58 percent of its time in space. Thrusting during escape was no different. No one was tense or anxious; rather, all the drama is in the spectacular results of the bold mission at Vesta and the promise of what is to come at Ceres. When Dawn entered orbit, your correspondent was dancing. When Dawn left orbit, he was sleeping serenely.

A month earlier, on August 8, with the craft more than 2,100 kilometers (1,300 miles) above the surface, patiently powering its way up through Vesta’s gravity field, one of the reaction wheels experienced an increase in internal friction. Reaction wheels are used to control a spacecraft’s orientation in the frictionless, zero-gravity conditions of spaceflight. By electrically changing a wheel’s spin rate, Dawn can rotate or stabilize itself. Protective software quickly detected the event and correctly responded by deactivating that wheel and the other two that were operating, switching to the small jets that are available for the same function, and reconfiguring other systems, including powering off the ion thrust and turning to point the main antenna to Earth.

A routine communications session the next day revealed to mission controllers what had occurred. They had planned long ago to turn the wheels off for the flight from Vesta to Ceres, so having them off a few weeks early was not a significant change. The team soon restored the spacecraft to normal operations and reformulated the departure plan, and on August 17 Dawn resumed its ascent. Because of the hiatus in thrusting, escape shifted from August 26 to September 4. The flexibility in the mission timeline provided by ion propulsion made this delay easy to accommodate.

In order to conserve the hydrazine propellant that the jets use, the bonus departure observations described before were curtailed, as they were not a high priority for the mission. Nevertheless, on August 25 and 26, at an altitude of around 6,000 kilometers (3,700 miles), the explorer did peer at Vesta once more with its camera and visible and infrared mapping spectrometer. The last time it had been this far away was July 21, 2011, during its descent to an unfamiliar destination. This time, 13 months later, the spacecraft turned back for a final gaze at the magnificent world it had unveiled during its remarkable time there, a world that prior to last year had appeared as little more than a tiny smudge among the stars for the two centuries it had been observed.

The delay in the departure schedule provided a convenient benefit. Vesta has seasons, just as Earth does, although they progress more slowly on that distant orb. August 20 was the equinox, when northern hemisphere spring began. Until then, the sun had been in Vesta’s southern hemisphere throughout Dawn’s residence there. While most of the northern hemisphere was revealed during the second high-altitude mapping orbit, the illumination of the landscape immediately around the north pole was even better for this last look. After radioing its parting shots to wistful mission controllers, the ship commenced its climb again.

And then, with an stunningly successful mission behind it, a newly explored world below it, and a mysterious dwarf planet ahead of it, the indomitable and indefatigable adventurer left Vesta forever.

Dawn is 18,500 kilometers (11,500 miles) from Vesta and 64 million kilometers (40 million miles) from Ceres. It is also 2.45 AU (367 million kilometers or 228 million miles) from Earth, or 910 times as far as the moon and 2.43 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.

Dr. Marc D. Rayman
10:00 a.m. PDT September 5, 2012

› Read previous Dawn Journals by Marc Rayman


Dawn Sets Its Sights on Ceres

Monday, July 30th, 2012

By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

near-true color image of the remarkable snowman feature on asteroid Vesta's surface
Three impact craters of different sizes, arranged in the shape of a snowman, make up one of the most striking features on Vesta, as seen in this view from NASA’s Dawn mission. In this view the three “snowballs” are upside down, so that the shadows make the features easily recognizable. North is to the lower right in the image, which has a resolution of 230 feet (70 meters) per pixel. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA

Dear Dawnpartures,

Dawn has completed the final intensive phase of its extraordinary exploration of Vesta, and it has now begun its gradual departure. Propelled by its uniquely efficient ion propulsion system, the probe is spiraling ever higher, reversing the winding path it followed into orbit last year.

In the previous log (which gained prominence last month by making it into the list of the top 78 logs ever written on this ambitious interplanetary adventure), we saw the plan for mapping Vesta from an altitude of 680 kilometers (420 miles). In this second high-altitude mapping orbit (HAMO2), the spacecraft circled the alien world beneath it every 12.3 hours. On the half of each orbit that it was on the day side, it photographed the dramatic scenery. As it passed over the night side, it beamed the precious pictures to the distant planet where its human controllers (and many of our readers) reside. Tirelessly repeating this strategy while Vesta rotated allowed Dawn’s camera to observe the entirety of the illuminated land every five days.

The robot carried out its complex itinerary flawlessly, completely mapping the surface six times. Four of the maps were made not by pointing the camera straight down at the rocky, battered ground but rather at an angle. Combining the different perspectives of each map, scientists have a rich set of stereo images, allowing a full three dimensional view of the terrain that bears the scars of more than 4.5 billion years in the main asteroid belt between Mars and Jupiter.

Dawn also mapped Vesta six times during the first high-altitude mapping orbit (HAMO1) in September and October 2011. The reason for mapping it again is that Vesta has seasons, and they progress more slowly than on Earth. Now it is almost northern hemisphere spring, so sunlight is finally reaching the high latitudes, which were under an impenetrable cloak of darkness throughout most of Dawn’s residence here.

For most of the two centuries this mysterious orb had been studied from Earth, it was perceived as little more than a small fuzzy blob in the night sky. With the extensive imaging from HAMO1 and HAMO2, as well as from the low-altitude mapping orbit (LAMO, earthlings now know virtually all of the protoplanet’s landscape in exquisite detail.

Among the prizes for the outstanding performance in HAMO2 are more than 4,700 pictures. In addition to the comprehensive mapping, Dawn collected nearly nine million spectra with its visible and infrared mapping spectrometer (VIR) to help scientists determine more about the nature of the minerals. This phenomenal yield is well over twice that of HAMO1, illustrating the great benefit of dedicating valuable observation time in HAMO2 to VIR before the mapping.

Dawn’s measurements of the peaks and valleys, twists and turns of Vesta’s gravity field, from which scientists can map the distribution of material in the interior of the behemoth, were at their best in LAMO. That low altitude also was where the gamma ray and neutron detector (GRaND) obtained its finest data, revealing the atomic constituents of the surface and subsurface. Indeed, the motivation for undertaking the challenging descent to LAMO was for those investigations, although the bonus pictures and spectra greatly enhanced the reward. Even in HAMO2, however, gravity and GRaND studies continued, adding to an already fabulous bounty.

Mission controllers have continued to keep the distant spacecraft very busy, making the most of its limited time at Vesta. Pausing neither to rest nor to marvel or delight in its own spectacular accomplishments, when the robot finished radioing the last of its HAMO2 data to Earth, it promptly devoted its attention to the next task: ion thrusting.

Missions that use conventional propulsion coast almost all of the time, but long-time readers know that Dawn has spent most of its nearly five years in deep space thrusting with its advanced ion propulsion system, the exotic and impressive technology it inherited from NASA’s Deep Space 1. Without ion propulsion, the exploration already accomplished would have been unaffordable for NASA’s Discovery Program and the unique exploit to orbit both Vesta and dwarf planet Ceres would have been quite impossible. Ion propulsion not only enables the spacecraft to orbit residents of the main asteroid belt, something no other probe has attempted, but it also allows the interplanetary spaceship to maneuver extensively while at each destination, thus tailoring the orbits for the different investigations.

On July 25 at 9:45 a.m. PDT, as it has well over 500 times before, the sophisticated craft began emitting a beam of high-velocity xenon ions. In powered flight once again, it is now raising its orbital altitude. On August 26, the ship will be too far and traveling too fast for Vesta’s gravity to maintain its hold. Dawn will slip back into orbit around the sun with its sights set on Ceres.

Although HAMO2 is complete, the spacecraft will suspend thrusting four times to direct its instruments at Vesta during the departure phase, much as it did in the approach phase. The approach pictures aided in navigation and provided tantalizing views of the quarry we had been seeking for so long. This time, however, we will see a familiar world receding rather than an unfamiliar one approaching. But as the sun creeps north, advancing by about three quarters of a degree of latitude per week, the changing illumination around the north pole will continue to expose new features.

› Continue reading Marc Rayman’s Dawn Journal


Shedding Light on the Scarred Face of Asteroid Vesta

Thursday, July 5th, 2012

By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Image of the giant asteroid Vesta taken by NASA's Dawn spacecraft
This image, from NASA’s Dawn spacecraft, shows rock material that has moved across the surface and flowed into a low area in the ridged floor of the Rheasilvia basin on Vesta. The image shows how impacts and their aftermath constantly reshape the landscape. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI

Dear Upside Dawn Readers,

Dawn is now seeing Vesta in a new light. Once again the probe is diligently mapping the ancient protoplanet it has been orbiting for nearly a year. Circling the alien world about twice a day, the ardent adventurer is observing the signatures of Vesta’s tortured history, including the scars accumulated during more than 4.5 billion years in the main asteroid belt between Mars and Jupiter.

Having successfully completed its orbital raising maneuvers to ascend to its second high-altitude mapping orbit (HAMO2), Dawn looks down from about 680 kilometers (420 miles). This is the same height from which it mapped Vesta at the end of September and October 2011. The lifeless rocky landscape has not changed since then, but its appearance to the spacecraft’s sensors has. The first high-altitude mapping orbit (HAMO1) was conducted shortly after southern hemisphere summer began on Vesta, so the sun was well south of the equator. That left the high northern latitudes in the deep darkness of winter night. With its slower progression around the sun than Earth, seasons on Vesta last correspondingly longer. Thanks to Dawn’s capability to linger in orbit, rather than simply conduct a brief reconnaissance as it speeds by on its way to its next destination, the probe now can examine the surface with different lighting.

Much of the terrain that was hidden from the sun, and thus the camera, during HAMO1 is now illuminated. Even the scenery that was visible then is lit from a different angle now, so new observations will reveal many new details. In addition to the seasonal northward shift in the position of the sun, Dawn’s orbit is oriented differently in HAMO2, as described last month, so that makes the opportunity for new insights and discoveries even greater.

The strategy for mapping Vesta is the same in HAMO2 now as it was in HAMO1. Dawn’s orbital path takes it nearly over the north pole. (As we saw last month, the orbit does not go exactly over the poles but rather reaches to 86 degrees latitude. That slight difference is not important for this discussion.) During the ship’s southward passage over the sunlit side, the camera and the visible and infrared mapping spectrometer (VIR) acquire their precious data. After passing (almost) above the south pole, Dawn sails north over the night side. Instead of pointing its sensors at the deep black of the ground below, the probe aims its main antenna to the extremely distant Earth and radios its findings to the exquisitely sensitive receivers of the Deep Space Network. The pattern repeats as the indefatigable spacecraft completes loop after loop after loop around the gigantic asteroid every 12.3 hours.

As Dawn revolves, Vesta rotates on its axis beneath it, turning once every 5.3 hours. Just as in HAMO1, mission planners artfully choreographed this celestial pas de deux so that over the course of 10 orbits, lasting just over five days, the camera would be able to view nearly all of the lit surface. A set of 10 orbits is known to Dawn team members (and to you, loyal readers) as a mapping cycle.

Until a few months ago, HAMO2 was planned to be four cycles. Thanks to the determination in April that Dawn could extend its residence at Vesta and still meet its 2015 appointment with dwarf planet Ceres, HAMO2 has been increased to six mapping cycles (plus even a little more, as we shall see below), promising a yet greater scientific return.

In cycle 1, which began on June 23, the camera was pointed at the surface directly underneath the spacecraft. The same view will be obtained in cycle 6. In cycles 2 through 5, images are acquired at other angles, providing different perspectives on the complex and dramatic landscape. Scientists combine the pictures to formulate topographical maps, revealing Vesta’s full three-dimensional character from precipitous cliffs and towering peaks of enormous mountains to gently rolling plains and areas with mysterious ridges and grooves to vast troughs and craters punched deep into the crust. Knowing the elevations of the myriad features and the angles of slopes is essential to understanding the geological processes and forces that shaped this exotic mini-planet. In addition to the exceptional scientific value, the stereo imagery provides realistic, exciting views for anyone who wants to visualize this faraway world. If you have not traveled there yourself, be sure to visit the Image of the Day regularly and the video gallery occasionally to see what you and the rest of humankind had been missing during the two centuries of Vesta’s appearance being only that of a faint, tiny blob in the night sky.

› Continue reading Marc Rayman’s Dawn Journal


Getting the Lowdown on Asteroid Vesta

Monday, December 5th, 2011

By Marc Rayman

As NASA’s Dawn spacecraft investigates its first target, the giant asteroid Vesta, Marc Rayman, Dawn’s chief engineer, shares a monthly update on the mission’s progress.

Still from a 3-D video incorporating images from NASA's Dawn spacecraft
This 3-D video incorporates images from the framing camera instrument aboard NASA’s Dawn spacecraft from July to August 2011. The images were obtained as Dawn approached Vesta and circled the giant asteroid during the mission’s survey orbit phase. Survey orbit took place at an altitude of about 1,700 miles (2,700 kilometers). To view this video in 3-D use red-green, or red-blue, glasses (left eye: red; right eye: green/blue). Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA
› See video

Dear Dawnward Spirals,

Continuing its ambitious campaign of exploration deep in the asteroid belt, Dawn has spent most of the past month spiraling ever closer to Vesta. Fresh from the phenomenal success of mapping the alien world in detail in October, the spacecraft and its human team members are engaged in one of the most complicated parts of the mission. The reward will be the capability to scrutinize this fascinating protoplanet further.

Thanks to the extraordinary performance of its ion propulsion system, Dawn can maneuver to different orbits that are best suited for conducting each of its scientific observations. The probe is now headed for its low altitude mapping orbit (LAMO), where the focus of its investigations will be on making a census of the atomic constituents with its gamma ray and neutron sensors and on mapping the gravity field in order to determine the interior structure of this protoplanet.

As secondary objectives, Dawn will acquire more images with its camera and more spectra with its visible and infrared mapping spectrometer. As we will see in a future log, these measurements will receive a smaller share of the resources than the high priority studies. The spectacular pictures obtained already will keep scientists happy for years, and you can continue to share in the experience of marveling at the astonishing discoveries by seeing some of the best views here, including scenes captured during the spiral to LAMO.

Planning the low altitude mapping orbit around massive Vesta, with its complicated gravity field, required a great deal of sophisticated analysis. Before Dawn arrived, mission designers studied a range of possible gravitational characteristics and honed the methods they would use for plotting the actual orbit once the details of the protoplanet’s properties were ascertained. In the meantime, the team used a tentative orbit at an altitude over the equator of 180 kilometers (110 miles). As explained in a previous log, the altitude varies both because the orbit is not perfectly circular and because Vesta displays such exceptional topography. The highest elevations turn out to be at the equator, and the average altitude of that orbit would be 200 kilometers (125 miles).

Now that navigators have measured Vesta’s gravity, they have the knowledge to refine the design for LAMO, and they decided to raise it by 10 kilometers (6 miles). The target then is an average altitude of 210 kilometers (130 miles). But there is more to the specification of the orbit than simply its height. To meet all of the scientific objectives, the orientation of this orbit needs to be different from the orientation of the previous orbits, the high altitude mapping orbit (HAMO) and survey orbit.

› Continue reading Marc Rayman’s Dawn Journal