Posts Tagged ‘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


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: A Retrospective

Thursday, January 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.

Mosaic of Dawn's images of asteroid Vesta
As NASA’s Dawn spacecraft takes off for its next destination, this mosaic synthesizes some of the best views the spacecraft had of the giant asteroid Vesta. Image credit: NASA/JPL-Caltech/UCAL/MPS/DLR/IDA
› full image and caption

Dear Dawnt Look Backs,

Its long and daring interplanetary journey continuing smoothly, Dawn is making good progress in gradually reshaping its orbit around the sun. Its uniquely efficient ion propulsion system is gently bringing it closer to its next destination, dwarf planet Ceres, and ever farther from its previous one, Vesta. Although the robotic explorer’s sights are set firmly ahead, let’s take one last look back at the fascinating alien world it unveiled during its 14 months in orbit there.

Vesta, the second most massive resident of the main asteroid belt between Mars and Jupiter, was discovered in 1807. For more than two centuries thereafter, the mysterious object appeared as little more than a fuzzy patch of light among the stars. The only one of the millions of main belt asteroids to be bright enough to be visible to the naked eye, Vesta beckoned, but its invitation was not answered until Dawn arrived in July 2011, nearly four years after it left distant Earth. The cosmic ambassador is the only spacecraft ever to have orbited an object in the main asteroid belt, and its ambitious mission would have been impossible without ion propulsion.

Dawn found a complex and exotic place, and it returned a fabulously rich collection of pictures and other measurements that will continue to be analyzed for many, many years. For now, we will simply touch on a very few of the many insights that already have been illuminated by the light of Dawn.

Scientists recognize Vesta as being more like a mini-planet than like the chips of rock most people think of as asteroids. The behemoth is 565 kilometers (351 miles) wide at the equator and has a surface area more than twice that of California (although it is populated by far fewer eccentrics, billionaires, and other colorful characters found in that state). Dawn’s measurements of the gravity field provide good evidence that Vesta separated into layers, much like Earth did as the planet was forming. Vesta’s dense core, composed principally of iron and nickel, may be 200 to 250 kilometers (125 to 150 miles) across. Surrounding that is the mantle, which in turn is covered by the veneer of the crust, about 20 kilometers (12 miles) thick. The once-molten core is now solid (in contrast to Earth’s, which remains hot enough to be liquid), but the differentiation into layers gives Vesta a key distinction from most asteroids. Because it was likely still in the process of accumulating material to become a full-sized planet when Jupiter’s immense gravity terminated its growth, scientists often refer to Vesta as a protoplanet.

Among the most prominent features of the alien landscape is a huge gouge out of the southern hemisphere so large that its presence was inferred from observations with the Hubble Space Telescope. Dawn found this gigantic crater to be even deeper and wider than expected, penetrating about 19 kilometers (12 miles) and spanning more than 500 kilometers (310 miles), or nearly 90 percent of the protoplanet’s equatorial diameter.

The yawning hole is now known as Rheasilvia, after the Vestal Virgin who not only was the mythical mother of Romulus and Remus, but also surely would have been astounded by the spectacular sights on Vesta as well as the spacecraft’s capability to point any user-defined body vector in a time-varying inertial direction defined by Chebyshev polynomials. As Dawn has brought Vesta into focus, cartographers have needed labels for the myriad features it has discovered. The International Astronomical Union names Vestan craters for Vestal Virgins and other famous Roman women; mountains, canyons, and other structures are named for towns and festivals associated with the Vestal Virgins.

Vesta dates to the dawn of the solar system, more than 4.5 billion years ago, and its age shows. Myriad craters tell the story of a timeworn surface that has been subjected to the rough and tumble conditions of life in the asteroid belt ever since. A virtual rain of space rocks has fallen upon it. While Rheasilvia records the most powerful punch, from an object as much as 50 kilometers (30 miles) across, there are at least seven craters, some quite ancient indeed, more than 150 kilometers (nearly 100 miles) in diameter. As the eons pass, craters degrade and become more difficult to discern, their crisp shapes eroded by subsequent impacts large and small.

The long history of cratering is particularly evident in the startling difference between the northern and southern hemispheres. The north is very densely cratered, but the south is not. Why? The titanic blow that carved out Rheasilvia is estimated to have occurred over one billion years ago. It excavated a tremendous volume of material. Much of it fell back to the surface, wiping it clean, so the cratering record had to start all over again. Recall that the crater itself is 500 kilometers (310 miles) in diameter, and scientists estimate that 50 kilometers (30 miles) outside the rim, the debris may have piled about 5 kilometers (3 miles) high. Even at greater distances, preexisting features would have been partially or completely erased by the thick accumulation. The effect did not reach to the northern hemisphere, however, so it retained the craters than had formed before this enormous impact.

Some of the rocks were ejected with so much energy that they broke free of Vesta’s gravitational grip, going into orbit around the sun. They then went their own way as they were yanked around by the gravitational forces of Jupiter and other bodies, and many of them eventually made it to the part of the solar system where your correspondent and some of his readers spend most of their time: Earth. When our planet’s gravity takes hold of one of these Vesta escapees, it pulls the rock into its atmosphere. Some lucky witness might even observe it as a meteor. Its blazing flight to the ground is not the end of its glory, however, for these rocks are prized by planetary geologists and other enthusiasts who want a souvenir from that impact.

Scientists now know that about 6 percent of the meteorites seen to fall to Earth originated on Vesta. Six percent! One of every 16 meteorites! This is an astonishingly large fraction. Apart from Mars and the moon, Vesta is the only known source of specific meteorites. Although rocks from Vesta had to travel much farther, they far outnumber meteorites from these other two more familiar celestial bodies.

Combining laboratory studies of the numerous samples of Vesta with Dawn’s measurements at the source provides an extraordinary opportunity to gain insights into the nature of that remote world. Meteorites from Vesta are so common that they are often displayed in museums (occasionally even without the curators’ awareness of their special history) and can be obtained from many vendors. Anyone who has seen or held one surely must be moved by contemplating its origin, so distant in space and time, from well beyond Mars and long before animal or plant life arose on Earth.

› Continue reading Marc Rayman’s Dawn Journal for more Vesta history


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 Goes Over ‘n’ Out

Monday, June 4th, 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.

Images of the giant asteroid Vesta taken by NASA's Dawn spacecraft in 2011 and 2012
On May 3, 2011, the mapping camera on NASA’s Dawn spacecraft captured its first image (left) of the giant asteroid Vesta. Only 5 pixels across, the image didn’t provide any new information about the asteroid, but it was important for navigation purposes and provided an exciting first look at Dawn’s eventual target. About five months later, Dawn snapped the much more detailed image on the right from only 700 kilometers (435 miles) from the surface of Vesta and has since provided unparalleled views of the mysterious world. Image credit: NASA/JPL-Caltech

Dear Readers of all Dawnominations,

Far from Earth, on the opposite side of the sun, deep in the asteroid belt, Dawn is gradually spiraling around the giant protoplanet Vesta. Under the gentle pressure of its uniquely efficient ion propulsion system, the explorer is scaling the gravitational mountain from its low-altitude mapping orbit (LAMO) to its second high-altitude mapping orbit (HAMO2).

Dawn spent nearly five months in LAMO, circling the rocky world at an average altitude of 210 kilometers (130 miles) as it acquired a fabulous bounty of pictures; visible, infrared, neutron, and gamma ray spectra; and measurements of the gravity field. As we saw last month, the probe was far more productive in each investigation than the ambitious team members had expected or had ever dared hope it would be. With that outstanding success behind it, it is looking ahead and up to its work in HAMO2, about 680 kilometers (420 miles) high.

Dawn is the first spacecraft to explore Vesta, the second most massive resident of the main asteroid belt between Mars and Jupiter. Indeed, this is the only craft ever to orbit a body in the asteroid belt. No other missions are currently on the books to visit this remote, exotic world, which is now appreciated to be more closely related to the terrestrial planets (including Earth) than to typical asteroids. And now Dawn is receding from it. On May 1, it began the slow ascent to its next observation orbit. It may well be decades before another robotic ambassador from Earth comes as close to Vesta as this bold traveler has.

Humankind’s first exploration of Vesta has been exceptionally rewarding. A simple measure of that can be seen with just two photographs. More than two centuries after its discovery, this giant asteroid was first glimpsed by the approaching spaceship from Earth on May 3, 2011. From a distance of 1.2 million kilometers (750 thousand miles), or more than three times the separation between Earth and the moon, Dawn’s mapping camera perceived Vesta as only five pixels across. Each pixel spanned more than 110 kilometers (70 miles), revealing nothing new compared to what astronomers’ most powerful telescopes had shown (but the image was of importance for navigation purposes). Nevertheless, at the time, it was tremendously exciting to obtain the first views of a distant, unfamiliar shore after a voyage of more than 2.6 billion kilometers (1.6 billion miles) on the interplanetary ocean. Sighting our first celestial port of call more than three and a half years after this cosmic adventure began was thrilling indeed. But now, with more than 25 thousand spectacular photos in hand from much smaller distances, it is even more gratifying to acknowledge that first picture as one of the worst ever taken of Vesta. The Image of the Day from one year later
was acquired in October 2011 from 1,700 times closer; and most of the images have been obtained from LAMO, about 5,700 times nearer than that first one. Dawn has rapidly transformed Vesta from a mere fleck among the stars into a fascinating, complex and splendidly detailed world.

Keeping the remote vessel on the planned spiraling course from one mapping orbit to another presents the crew with a set of formidable challenges, but this team has accomplished the maneuvers to successively reach survey orbit, the first high-altitude mapping orbit (HAMO1) and LAMO. The current orbital transfer is complex and demanding, but it is proceeding very well. Controllers update the flight profile every few days to ensure the probe stays close to the carefully designed trajectory to HAMO2. To gain a sense of the progress, go here for your correspondent’s atypically succinct weekly summaries of the spiral status.

› Continue reading Marc Rayman’s Dawn Journal


Dawn Ascends Over Asteroid Vesta

Wednesday, May 2nd, 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.

Artist's concept of the Dawn spacecraft at asteroid Vesta
This artist’s concept shows NASA’s Dawn spacecraft orbiting the giant asteroid Vesta. The depiction of Vesta is based on images obtained by Dawn’s framing cameras. Image credit: NASA/JPL-Caltech |
› Full image and caption

Dear Dawnright Spectacular Readers,

Dawn is wrapping up a spectacularly rewarding phase of its mission of exploration. Since descending to its low-altitude mapping orbit (LAMO) in December, the stalwart probe has circled Vesta about 800 times and collected a truly outstanding trove of precious observations of the protoplanet. Having far exceeded the plans, expectations, and even hopes for what it would accomplish when LAMO began, the ambitious explorer is now ready to begin its ascent. On May 1, atop its familiar blue-green pillar of xenon ions, the craft will embark upon the six-week spiral to its second high-altitude mapping orbit.

When the intricate plans for Dawn’s one-year orbital residence at Vesta were developed, LAMO was to be 70 days, longer than any other phase. Because of the many daunting challenges of exploring an uncharted, alien world in the forbidding depths of the asteroid belt so far from home, mission planners could not be confident of staying on a rigid schedule, and yet they wanted to make the most of the precious time at the giant asteroid. They set aside 40 days (with no committed activities) to use as needed in overcoming problems during the unique approach and entry into orbit as well as the intensive observation campaigns in survey orbit and the first high-altitude mapping orbit plus the complex spiral flights from each science orbit to the next. To no one’s surprise, unexpected problems did indeed arise on occasion, and yet in every case, the dedicated professionalism and expertise of the team (occasionally augmented with cortisol, caffeine, and carbohydrates) allowed the expedition to remain on track without needing to draw on that reserve. To everyone’s surprise and great delight, by the beginning of LAMO on December 12, the entirety of the 40 days remained available. Therefore, all of it was used to extend the time the spacecraft would spend at low altitude studying the fascinating world beneath it.

Dawn’s mission at Vesta, exciting and successful though it is, is not the craft’s sole objective. Thanks to the extraordinary capability of its ion propulsion system, this is the first vessel ever planned to orbit two extraterrestrial destinations. After it completes its scrutiny of the behemoth it now orbits, the second most massive resident of the main asteroid belt, Dawn will set sail for dwarf planet Ceres, the largest body between the orbits of Mars and Jupiter.

Since 2009, the interplanetary itinerary has included breaking out of Vesta orbit in July 2012 in order to arrive at Ceres on schedule in February 2015. Taking advantage of additional information they have gained on the spacecraft’s generation and consumption of electrical power, the performance of the ion propulsion system, and other technical issues, engineers have refined their analyses for how long the journey through the asteroid belt to Ceres will take. Their latest assessment is that they can shave 40 days off the previous plan, once again demonstrating the valuable flexibility of ion propulsion, and that translates into being able to stay that much longer at the current celestial residence. (This extension is different from the 40 days described above, because that was designed to ensure Dawn could complete its studies and still leave on schedule in July. For this new extension, the departure date is being changed.) Even though a larger operations team is required at Vesta than during the cruise to Ceres, the Dawn project has the wherewithal to cover the cost. Because operations at Vesta have been so smooth, no new funds from NASA are needed; rather, the project can use the money it had held in reserve in case of problems. In this new schedule, Dawn will gently free itself of Vesta’s gravitational hold on August 26.

Most of the bonus time has been devoted to extending LAMO by a month, allowing the already richly productive investigations there to be even better. (Future logs will describe how the rest of the additional time at Vesta will be spent.) With all sensors fully operational, the robotic explorer has been making the best possible use of its precious time at Vesta, revealing more and more thrilling details of an exotic world deep in the asteroid belt.

› Continue reading Marc Rayman’s Dawn Journal


All Eyes on Asteroid Vesta

Friday, March 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.

Layered young crater as imaged by NASA's Dawn spacecraft
This image from NASA’s Dawn spacecraft shows a young crater on Vesta that is 9 miles (15 kilometers) in diameter. Layering is visible in the crater walls, as are large boulders that were thrown out in the material ejected from the impact. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA |
› Full image and caption

Dear Dawnscoverers,

On March 29, Vesta spent the 205th anniversary of its discovery by treating Dawn to more spectacular vistas, as it does so often these days. When Heinrich Wilhelm Matthäus Olbers first spotted Vesta, he could hardly have imagined that the power of the noble human spirit for adventure and the insatiable hunger for knowledge would propel a ship from Earth to that mysterious point of light among the stars. And yet today our spacecraft is conducting a detailed and richly rewarding exploration of the world that Olbers found.

Dawn is continuing its intensive low-altitude mapping orbit (LAMO) campaign, scrutinizing the protoplanet 210 kilometers (130 miles) beneath it with all instruments. The primary objectives of the craft’s work here are to measure the atomic composition and the interior distribution of mass in this geologically complex world. In addition, this low orbit provides the best vantage point for high resolution pictures and visible and infrared spectra to reveal the nature of the minerals on the surface.

Ever since it left its home planet behind in September 2007, the robotic adventurer has pursued its own independent course through the solar system. As Earth and its orbiting retinue (including the moon and many artificial satellites) followed their repetitive annual loop around the sun, Dawn used its ion propulsion system to spiral outward to rendezvous with Vesta in July 2011. When the gigantic asteroid’s gravity gently took hold of the visiting craft, the two began traveling together around the sun, taking the same route Vesta has since long before humans gazed in wonder at the nighttime sky.

As we have discussed before, the speed of an object in orbit, whether around Earth, the sun, the Milky Way (either my cat or the galaxy of the same name) or anything else, decreases as its orbital altitude increases. Farther from the sun than Earth is, and hence bound to it by a weaker gravitational grip, Vesta moves at a more leisurely pace, taking more than 3.6 years per revolution. When Dawn travels to the more remote Ceres, it will orbit the sun even more slowly, eventually matching Ceres’ rate of 4.6 years for each loop.

Just as the hour hand and minute hand of a clock occasionally are near each other and at other times are on opposite sides of the clock face, Earth and Dawn sometimes are relatively close and other times are much farther apart. Now their orbits are taking them to opposite sides of the sun, and the distance is staggering. They have been on opposite sides of the sun twice before (albeit not as far apart as this time), in November 2008 and November 2010. We used both occasions to explain more about the nature of the alignment as well as to contemplate the profundity of such grand adventures.

On April 18, Dawn will attain its greatest separation yet from Earth, nearly 520 million kilometers (323 million miles) or more than 3.47 astronomical units (AU). Well beyond Mars, fewer than a dozen spacecraft have ever operated so far from Earth. Those interested in the history of space exploration (such as your correspondent) will enumerate them, but what should be more rewarding is marveling at the extent of humanity’s reach. At this extraordinary range, Dawn will be nearly 1,400 times farther than the average distance to the moon (and 1,300 times farther than the greatest distance attained by Apollo astronauts 42 years ago). The deep-space ship will be well over one million times farther from Earth than the International Space Station and Tiangong-1.

Vesta does not orbit the sun in the same plane that Earth does. Indeed, a significant part of the challenge in matching Dawn’s orbit to Vesta’s was tipping the plane of its orbit from Earth’s, where it began its journey, to Vesta’s, where it is now. As a result, when they are on opposite sides of the sun this time, Dawn will not appear to go directly behind the sun but rather will pass a little south of it. In addition, because the orbits are not perfectly circular, the greatest separation does not quite coincide with the time that Dawn and the sun appear to be most closely aligned. The angular separation will be at its minimum of less than five degrees (about 10 times the angular size of the sun itself) on April 9, but the sun and Dawn appear to be within ten degrees of each other from March 23 until April 27. For our human readers, that small angle is comparable to the width of your palm at arm’s length, providing a handy way to find the approximate position of the spacecraft in the sky. Earth’s robotic ambassador to the cosmos began east of the salient celestial signpost and progresses slowly to the west over the course of those five weeks. Readers are encouraged to step outside and join your correspondent in raising a saluting hand to the sun, Dawn, and what we jointly accomplish in our efforts to gain a perspective on our place in the universe.

For those awestruck observers who lack the requisite superhuman visual acuity to discern the faraway spacecraft amidst the dazzling light of the sun, this alignment provides a convenient occasion to reflect once again upon missions deep into space. Formed at the dawn of the solar system, Vesta, arguably the smallest of the terrestrial planets, has waited mostly in patient inconspicuousness for a visit from the largest terrestrial planet. For the entire history of life on Earth, the inhabitants remained confined to the world on which they have lived. Yet finally, one of the millions upon millions of species, inspired by the splendor of the universe, applied its extraordinary talents and collective knowledge to overcome the limitations of planetary life and strove to venture outward. Dawn is the product of creatures fortunate enough to be able to combine their powerful curiosity about the workings of the cosmos with their impressive abilities to explore, investigate and ultimately understand. While its builders remain in the vicinity of the planet upon which they evolved, their emissary now is passing on the far side of the sun! This is the same sun that is more than 100 times the diameter of Earth and a third of a million times its mass. This is the same sun that has been the unchallenged master of our solar system for more than 4.5 billion years. This is the same sun that has shone down on Earth throughout that time and has been the ultimate source of so much of the heat, light and other energy upon which the planet’s residents have been so dependent. This is the same sun that has so influenced human expression in art, literature, mythology and religion for uncounted millennia. This is the same sun that has motivated scientific studies for centuries. This is the same sun that is our signpost in the Milky Way galaxy. And humans have a spacecraft on the far side of it. We may be humbled by our own insignificance in the universe, yet we still undertake the most valiant adventures in our attempts to comprehend its majesty.

Dawn is 210 kilometers (130 miles) from Vesta. It is also 3.45 AU (516 million kilometers or 321 million miles) from Earth, or 1,290 times as far as the moon and 3.45 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 57 minutes to make the round trip.


Highs and Lows of Exploring the Giant Asteroid

Friday, March 2nd, 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.

Artist's concept of the Dawn spacecraft soaring over the giant asteroid Vesta.
This artist’s concept shows NASA’s Dawn spacecraft orbiting the giant asteroid Vesta. The depiction of Vesta is based on images obtained by Dawn’s framing cameras. Image credit: NASA/JPL-Caltech |
› Full image and caption

Dear Ups and Dawns,

Dawn is continuing its exploits at Vesta, performing detailed studies of the colossal asteroid from its low altitude mapping orbit (LAMO). The robotic ambassador is operating extremely well on behalf of the creatures it represents on a distant planet. On this second intercalary day of its ambitious adventure, the spacecraft is doing exactly what it was designed to do: exploring a previously uncharted alien world.

Although we usually describe LAMO as being at an average altitude of 210 kilometers (130 miles), that does not mean it is at a constant altitude. As we saw on the fourth anniversary of Dawn’s departure from Earth, there are two reasons the spacecraft’s height changes. One is that the elevation of the surface itself changes, so if the probe flew in a perfect circle around Vesta, its altitude would vary according to the topography. Like the planet from which Dawn embarked upon its deep space journey in 2007 (and even some of the residents there), Vesta is broadest near its equator, and that is where the ground generally reaches its greatest distance from the center. In addition, the ancient surface, battered over billions of years in the rough and tumble of the asteroid belt, displays remarkable variations in shape. The giant Rheasilvia basin is a scar from an extraordinary impact that excavated a region encompassing the south pole more than 500 kilometers (over 300 miles) in diameter. This immense gouge has left that part of Vesta at a much lower elevation than elsewhere. In the center of the enormous depression is the second tallest mountain known in the solar system, soaring to well over twice the height of Mt. Everest. The vertical range from the highest locations near the equator to the bottoms of the deepest craters within Rheasilvia is more than 60 kilometers (37 miles). So as Dawn loops around in just over four hours, the surface underneath it rises and falls dramatically.

The second reason is that the orbit itself is not exactly a circle. Let’s ignore for a moment the effect of the topography and focus solely on the shape of the craft’s path around Vesta. As Vesta rotates and Dawn revolves, the gravitational forces acting on the orbiter are always changing because of the irregular distribution of material inside the geologically complex protoplanet. This effect occurred at the higher altitudes as well, but it was much less pronounced there. Now that the adventurer is deep in the gravity field, the peaks and valleys of its own motion are magnified.

Navigators were very careful in choosing the parameters for LAMO, recognizing that the orbital waters were turbulent. Nevertheless, their mapping of the gravitational currents proved quite accurate, and the spacecraft has followed the planned course quite well. The lengthy and relatively technical discussions in the two previous logs described why the ship drifts off a little, but operators occasionally nudge it back with the ion propulsion system.

Orbits usually are best described by ellipses, like flattened circles. Now Vesta’s bumpy gravity field does not allow perfectly smooth, regular orbits at low altitude. Moreover, the variations in the strength of the gravitational attraction transform the orbits. Sometimes, the difference between the high point of a loop and the low point is less than 16 kilometers (10 miles). As the changing forces reshape the orbit, the ellipse gets more exaggerated, with the low points going lower and the high points going higher. The differences within one revolution grow to be more than 75 kilometers (47 miles). Thanks to the ingenious design of the orbital trajectory however, those same forces then will gradually attenuate the profile, causing it to become more round again. This pattern repeats every 11.5 days in LAMO. It is almost as if the orbit breathes slowly, its envelope expanding and contracting.

› Continue reading Marc Rayman’s Dawn Journal


A Look Inside Dawn’s Grand Asteroid Adventure

Wednesday, February 1st, 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 asteroid Vesta taken by NASA's Dawn spacecraft from low altitude mapping orbit, or LAMO
The south pole of the giant asteroid Vesta, as imaged by the framing camera on NASA’s Dawn spacecraft in September 2011. Image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA |
› Full image and caption

Dear Asdawnished Readers,

Dawn is scrutinizing Vesta from its low-altitude mapping orbit (LAMO), circling the rocky world five and a half times a day. The spacecraft is healthy and continuing its intensive campaign to reveal the astonishing nature of this body in the mysterious depths of the main asteroid belt.

Since the last log, the robotic explorer has devoted most of its time to its two primary scientific objectives in this phase of the mission. With its gamma ray and neutron detector (GRaND), it has been patiently measuring Vesta’s very faint nuclear emanations. These signals reveal the atomic constituents of the material near the surface. Dawn also broadcasts a radio beacon with which navigators on distant Earth can track its orbital motion with exquisite accuracy. That allows them to measure Vesta’s gravity field and thereby infer the interior structure of this complex world. In addition to these top priorities, the spacecraft is using its camera and its visible and infrared mapping spectrometer (VIR) to obtain more detailed views than they could in the higher orbits.

As we have delved into these activities in detail in past logs, let’s consider here some more aspects of controlling this extremely remote probe as it peers down at the exotic colossus 210 kilometers (130 miles) beneath it.

Well, the first aspect that is worth noting is that it is incredibly cool! Continuing to bring this fascinating extraterrestrial orb into sharper focus is thrilling, and everyone who is moved by humankind’s bold efforts to reach into the cosmos shares in the experience. As a reminder, you can see the extraordinary sights Dawn has by going here for a new image every weekday, each revealing another intriguing aspect of the diverse landscape.

The data sent back are providing exciting and important new insights into Vesta, and those findings will continue to be announced in press releases. Therefore, we will turn our attention to a second aspect of operating in LAMO. Last month, we saw that various forces contribute to Dawn moving slightly off its planned orbital path. (That material may be worth reviewing, either to enhance appreciation of what follows or as an efficacious soporific, should the need for one ever arise.) Now let’s investigate some of the consequences. This will involve a few more technical points than most logs, but each will be explained, and together they will help illustrate one of the multitudinous complexities that must be overcome to make such a grand adventure successful.

› 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