Posts Tagged ‘ion propulsion’

How Dawn Will Get the Low-Down on the First Dwarf Planet Ever Discovered

Tuesday, September 2nd, 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.

Illustration of Dawn's spiral transfer from HAMO to LAMO
This image illustrates Dawn’s spiral transfer from high altitude mapping orbit (HAMO) to low altitude mapping orbit (LAMO). The trajectory turns from blue to red as time progresses over two months. Red dashed sections are where ion thrusting is stopped so the spacecraft can point its main antenna toward Earth. Image credit: NASA/JPL-Caltech

Dear Omnipodawnt Readers,

Dawn draws ever closer to the mysterious Ceres, the largest body between the sun and Pluto not yet visited by a probe from Earth. The spacecraft is continuing to climb outward from the sun atop a blue-green beam of xenon ions from its uniquely efficient ion propulsion system. The constant, gentle thrust is reshaping its solar orbit so that by March 2015, it will arrive at the first dwarf planet ever discovered. Once in orbit, it will undertake an ambitious exploration of the exotic world of ice and rock that has been glimpsed only from afar for more than two centuries.

An important characteristic of this interplanetary expedition is that Dawn can linger at its destinations, conducting extensive observations. Since December, we have presented overviews of all the phases of the mission at Ceres save one. (In addition, questions posted by readers each month, occasionally combined with an answer, have helped elucidate some of the interesting features of the mission.) We have described how Dawn will approach its gargantuan new home (with an equatorial diameter of more than 600 miles, or 975 kilometers) and slip into orbit with the elegance of a celestial dancer. The spacecraft will unveil the previously unseen sights with its suite of sophisticated sensors from progressively lower altitude orbits, starting at 8,400 miles (13,500 kilometers), then from survey orbit at 2,730 miles (4,400 kilometers), and then from the misleadingly named high altitude mapping orbit (HAMO) only 910 miles (1,470 kilometers) away. To travel from one orbit to another, it will use its extraordinary ion propulsion system to spiral lower and lower and lower. This month, we look at the final phase of the long mission, as Dawn dives down to the low altitude mapping orbit (LAMO) at 230 miles (375 kilometers). We will also consider what future awaits our intrepid adventurer after it has accomplished the daring plans at Ceres.

It will take the patient and tireless robot two months to descend from HAMO to LAMO, winding in tighter and tighter loops as it goes. By the time it has completed the 160 revolutions needed to reach LAMO, Dawn will be circling Ceres every 5.5 hours. (Ceres rotates on its own axis in 9.1 hours.) The spacecraft will be so close that Ceres will appear as large as a soccer ball seen from less than seven inches (17 centimeters) away. In contrast, Earth will be so remote that the dwarf planet would look to terrestrial observers no larger than a soccer ball from as far as 170 miles (270 kilometers). Dawn will have a uniquely fabulous view.

As in the higher orbits, Dawn will scrutinize Ceres with all of its scientific instruments, returning pictures and other information to eager Earthlings. The camera and visible and infrared mapping spectrometer (VIR) will reveal greater detail than ever on the appearance and the mineralogical composition of the strange landscape. Indeed, the photos will be four times sharper than those from HAMO (and well over 800 times better than the best we have now from Hubble Space Telescope). But just as in LAMO at Vesta, the priority will be on three other sets of measurements which probe even beneath the surface.

All of the mass within Ceres combines to hold Dawn in orbit, exerting a powerful gravitational grip on the ship. But as the spacecraft moves through its orbit, any variations in the internal structure of Ceres from one place to another will lead to slight perturbations of the orbit. If, for example, there is a large region of unusually dense material, even if deep underground, the craft will speed up slightly as it travels toward it. After Dawn passes overhead, the same massive feature will slightly retard its progress, slowing it down just a little.

Dawn will be in almost constant radio contact with Earth during LAMO. When it is pointing its payload of sensors at the surface, it will broadcast a faint radio signal through one of its small auxiliary antennas so exquisitely sensitive receivers on a planet far, far away can detect it. At other times, in order to transmit its findings from LAMO, it will aim its main antenna directly at Earth. In both cases, the slightest changes in speed toward or away from Earth will be revealed in the Doppler shift, in which the frequency of the radio waves changes, much as the pitch of a siren goes up and then down as an ambulance approaches and then recedes. Using this and other remarkably powerful techniques mastered for traveling throughout the solar system, navigators will carefully plot the tiny variations in Dawn’s orbit and from that determine the distribution of mass throughout the interior of the dwarf planet.

The spacecraft will use its sophisticated gamma ray and neutron detector (GRaND) to determine the atomic constituents of the material on the surface and to a depth of up to about a yard (a meter). Gamma rays are a very, very high frequency form of electromagnetic radiation, beyond visible light, beyond ultraviolet, beyond even X-rays. Neutrons are very different from gamma rays. They are the electrically neutral particles in the nuclei of atoms, slightly more massive than protons, and in most elements, neutrons outnumber them too. It would be impressive enough if GRaND only detected these two kinds of nuclear radiation, but it also measures the energy of each kind. (Unfortunately, that description doesn’t lend itself to such a delightful acronym).

Most of the gamma rays and neutrons are byproducts of the collisions between cosmic rays (radiation from elsewhere in space) and the nuclei of atoms in the ground. (Cosmic rays don’t do this very much at Earth; rather, most are diverted by the magnetic field or stopped by atoms in the upper atmosphere.) In addition, some gamma rays are emitted by radioactive elements near the surface. Regardless of the source, the neutrons and the gamma rays that escape from Ceres and travel out into space carry a signature of the type of nucleus they came from. When GRaND intercepts the radiation, it records the energy, and scientists can translate those signatures into the identities of the atoms.

The radiation reaching GRaND, high in space above the surface, is extremely faint. Just as a camera needs a long exposure in very low light, GRaND needs a long exposure to turn Ceres’ dim nuclear glow into a bright picture. Fortunately, GRaND’s pictures do not depend on sunlight; regions in the dark of night are no fainter than those illuminated by the sun.

For most of its time in LAMO, Dawn will point GRaND at the surface beneath it. The typical pattern will be to make 15 orbital revolutions, lasting about 3.5 days, staring down, measuring each neutron and each gamma ray that encounters the instrument. Simultaneously, the craft will transmit its broad radio signal to reveal the gentle buffeting by the variations in the gravitational field. On portions of its flights over the lit terrain, it will take photos and will collect spectra with VIR. Then the spacecraft will rotate to point its main antenna to distant Earth, and while it makes five more circuits in a little more than a day, it will beam its precious discoveries to the 230-foot (70-meter) antennas at NASA’s Deep Space Network.

Illustration of Dawn's low altitude mapping orbit.
This image illustrates Dawn’s low altitude mapping orbit (LAMO) and how it naturally shifts slightly (relative to the sun) over three months, starting in blue and ending in red. The Dawn spacecraft completes each revolution in 5.5 hours, and Ceres rotates in 9.1 hours, so Dawn will be able to view the entire surface of the dwarf planet. Image credit: NASA/JPL-Caltech

Dawn will spend more time in each successive observational phase at Ceres than the ones before. After two months in HAMO, during which it will complete about 80 orbits, the probe will devote about three months to LAMO, looping around more than 400 times. That is more than enough time to collect the desired data. Taxpayers have allocated sufficient funds to operate Dawn until June 2016, allowing some extra time for the flight team to grapple with the inevitable glitches that arise in such a challenging undertaking. As in all phases, mission planners recognize that complex operations in that remote and hostile environment probably will not go exactly according to plan, but even if some of the measurements are not completed, enough should be to satisfy all the scientific objectives.

The indefatigable explorer will work hard in LAMO. Aiming its sensors at the surface beneath it throughout its 5.5-hour orbits does not happen naturally. Dawn needs to keep turning to point them down. When it is transmitting its scientific bounty, it needs to hold steady enough to maintain Earth in the sights of its radio antenna. An essential element of the design of the spacecraft to achieve these and related capabilities was the use of three reaction wheels. By electrically changing the speed at which these gyroscope-like devices rotate, the probe can turn or stabilize itself. Because they are so important, four were included, ensuring that if any one encountered difficulty, the ambitious mission could continue with the other three.

As long-time readers know, one did falter in August 2012. The failure of two such vital devices could have proven fatal for a mission, but thanks to the expertise, creativity, swiftness, and persistence of the members of the Dawn flight team, the prospects for completing the exploration of Ceres are bright.

› Continue reading Marc Rayman’s September 2014 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


Dawn’s Journey: A Power Trip

Tuesday, July 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.

Dawn's solar arrays are folded to fit inside the nose cone in preparation for launch
The Dawn spacecraft’s solar array wings — pictured here in a folded position in preparation for launch — span 19.7 meters (nearly 65 feet) and are designed to keep the spacecraft powered even as it ventures further from the sun into the remote asteroid belt. Image credit: NASA/JPL-Caltech

Dear Megalodawniacs,

Powering its way through the main asteroid belt between Mars and Jupiter, Dawn continues on course and on schedule for its 2015 appointment with dwarf planet Ceres. After spending more than a year orbiting and scrutinizing Vesta, the second most massive object in the asteroid belt, the robotic explorer has its sights set on the largest object between the sun and Neptune that a spacecraft has not yet visited. This exotic expedition to unveil mysterious alien worlds would be impossible without the probe’s ion propulsion system.

Ion propulsion is not a source of power for this interplanetary spaceship. Rather, the craft needs a great deal of power to operate its ion propulsion system and all other systems. It needs so much that…

We crave power!!

The ion propulsion system is power-hungry. The process of ionizing xenon and then accelerating it to high velocity consumes a significant amount of electrical power, all of which is provided by the spacecraft’s huge solar arrays. With these two wings and its ion tail, Dawn resembles a celestial dragonfly. But this extraterrestrial odonate is a giant, with a wingspan of 19.7 meters (nearly 65 feet). When it was launched in 2007, this was the greatest tip-to-tip length of any probe NASA had ever dispatched on an interplanetary voyage. (Some such spacecraft have had flexible wire-like antennas that reach to greater lengths.) The large area of solar cells is needed to capture feeble sunlight in the remote asteroid belt to meet all of the electrical needs. Each solar array wing is the width of a singles tennis court, and the entire structure would extend from a pitcher’s mound to home plate on a professional baseball field, although Dawn is engaged in activities considerably more inspiring and rewarding than competitive sports.

To sail the ship to its intended destination, navigators plot a complex course on the solar system sea. The thrust delivered by the ion engine depends on the power level; higher power translates into higher (but still ever so gentle) thrust. The farther Dawn is from the luminous sun, the less power is available, so the thrust is lower. Therefore, to keep it on its itinerary, mission planners need to know the thrust at all times in the future. It would not be a recipe for success to propel the spacecraft to a position in space from which it could not achieve enough thrust to accomplish the rest of the carefully designed journey to Ceres.

To formulate the flight plan then requires knowing how much power will be available even as the probe ventures farther from the sun. Engineers make mathematical predictions of the power the solar arrays will generate, but these calculations are surprisingly difficult. Well, perhaps some readers would not be surprised, but it is more complicated than simply reducing the power in proportion to the intensity of the sunlight. As one example, at greater distances from the sun, the temperature of the arrays in the cold depths of space would be even lower, and the efficiency of the solar cells depends on their temperature. In 2008, the operations team devised and implemented a method to refine their estimates of the solar array performance, and that work enabled the deep-space traveler to arrive at Vesta earlier and depart later. Now they have developed a related but superior technique, which the faithful spacecraft executed flawlessly on June 24.

The only way to measure the power generation capability of the arrays is to draw power from them. With the ion thrust off, even with all other systems turned on, the spacecraft cannot consume as much power as the arrays can provide, so no meaningful measurement would be possible.

In typical operations, Dawn keeps its solar arrays pointed directly at the sun. For this special calibration, it rotated them so the incident sunlight came at a different angle. This reduced the total amount of light falling on the cells, effectively creating the conditions the spacecraft will experience when it has receded from the sun. As the angle increased, corresponding to greater distances from the brilliant star, the arrays produced less power, so the ion engine had to be throttled down. (The engines can be operated at 112 different throttle levels, each with a different input power and different thrust level.)

Engineers estimated what the maximum throttle level would be at each of the angles as well as the total power all other systems would consume during the test and then programmed it so the ion propulsion system would throttle down appropriately as the solar array angle increased. Of course, they could not know exactly what the highest throttle level at each angle would be; if they did, then they would already know the solar array characteristics well enough that the calibration would be unnecessary. Fortunately, however, they did not need to determine the perfect levels in advance. The sophisticated robot is smart enough to reduce by a few throttle levels if it detects that all systems combined are drawing more power than the solar arrays generate.

Under normal circumstances, the spacecraft doesn’t need to adjust the ion throttle level on its own. Engineers know the solar array performance well enough that they can predict the correct setting with high accuracy for a typical four-week sequence of commands stored onboard. It is only for the much greater distances from the sun in the years ahead that the uncertainty becomes important. In addition, during regular operations, if the spacecraft temporarily needs to use more heaters than usual (more than 140 heaters are distributed around the ship, each turning on and off as needed), thereby increasing the power demand, its battery can make up for the difference. That avoids unnecessary throttle changes.

› Continue reading Marc Rayman’s Dawn Journal


For Dawn, a Little Push Goes a Long Way

Monday, July 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 Dawnamic Readers,

The indefatigable Dawn spacecraft is continuing its extraordinary interplanetary flight on behalf of inquisitive creatures on distant Earth. Progressing ever farther from Vesta, the rocky and rugged world it so recently explored, the ship is making good progress toward its second port of call, dwarf planet Ceres.

We have seen in many logs that this adventure would be quite impossible without its advanced ion propulsion system. Even a mission only to orbit Vesta, which Dawn has accomplished with such stunning success, would have been unaffordable in NASA’s Discovery Program without ion propulsion. This is the only probe ever to orbit an object in the main asteroid belt between Mars and Jupiter. But now, thanks to this sophisticated technology, it is going beyond even that accomplishment to do something no other spacecraft has attempted. Dawn is the only mission ever targeted to orbit two extraterrestrial destinations, making it truly an interplanetary spaceship.

Ion Propulsion System Hot Fire Test for Deep Space 1
Ion Propulsion System Hot Fire Test for the Deep Space 1 spacecraft. Image credit: NASA/JPL-Caltech

Ion propulsion is 10 times more efficient than conventional chemical propulsion, so it enables much more ambitious missions. It uses its xenon propellant so parsimoniously, however, that the thrust is also exceptionally gentle. Indeed, the ion engine exerts about as much force on the spacecraft as you would feel if you held a single sheet of paper in your hand. At today’s thrust level, it would take more than five days to accelerate from zero to 60 mph. While that won’t rattle your bones, in the frictionless, zero-gravity conditions of spaceflight, the effect of the thrust gradually accumulates. Instead of thrusting for five days, Dawn thrusts for years. Ion propulsion delivers acceleration with patience, and patience is among this explorer’s many virtues.

To accomplish its mission, Dawn is outfitted with three ion engines. In the irreverent spirit with which this project has always been conducted, the units are fancifully known as #1, #2, and #3. (The locations of the thrusters were disclosed in a log shortly after launch, once the spacecraft was too far from Earth for the information to be exploited for tawdry sensationalism.) For comparison, the Star Wars TIE fighters were Twin Ion Engine ships, so now science fact does one better than science fiction. On the other hand, the TIE fighters employed a design that did seem to provide greater agility, perhaps at the expense of fuel efficiency. Your correspondent would concur that when you are trying to destroy your enemy while dodging blasts from his laser cannons, economy of propellant consumption probably isn’t the most important consideration.

At any rate, Dawn only uses one ion engine at a time. Since August 31, 2011, it has accomplished all of its thrusting with thruster #3. That thruster propelled Dawn along its complex spiral path down from an altitude of 2,700 kilometers (1,700 miles) to 210 kilometers (130 miles) above Vesta’s dramatic landscape and then back up again. Eventually, the engine pushed Dawn out of orbit, and it has continued to work to reshape the spacecraft’s heliocentric course so that it ultimately will match Ceres’s orbit around the sun.

Although any of the thrusters can accomplish the needed propulsion, and all three are still healthy, engineers consider many factors in deciding which to use at different times in the mission. Now they have decided to put #2 back to work. So on June 24, after its regular monthly hiatus in thrusting to point the main antenna to Earth for a communications session, the robotic explorer turned to aim that thruster, rather than thruster #3, in the direction needed to continue the journey to Ceres. Despite not being operated in nearly two years, #2 came to life as smoothly as ever. It is now emitting a blue-green beam of xenon ions as the craft has its sights set on the mysterious alien world ahead.

› Continue reading Marc Rayman’s Dawn Journal


Smooth Sailing: Dawn Spacecraft Passes Endurance Test

Monday, June 3rd, 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 Confidawnts,

Traveling from one alien world to another, Dawn is reliably powering its way through the main asteroid belt with its ion propulsion system. Vesta, the fascinating and complex protoplanet it explored in 2011 and 2012, falls farther and farther behind as the spacecraft gently and patiently reshapes its orbit around the sun, aiming for a 2015 rendezvous with dwarf planet Ceres.

The stalwart adventurer has recently completed its longest uninterrupted ion thrust period yet. As part of the campaign to conserve precious hydrazine propellant, Dawn now suspends thrusting once every four weeks to point its main antenna to Earth. (In contrast, spacecraft with conventional chemical propulsion spend the vast majority of time coasting.) Because of details of the mission operations schedule and the schedule for NASA’s Deep Space Network, the thrust durations can vary by a few days. As a result, the spacecraft spent 31.2 days thrusting without a hiatus. This exceeds Deep Space 1’s longest sustained powered flight of 29.2 days. While there currently are no plans to thrust for longer times, the unique craft certainly is capable of doing so. The principal limitation is how much data it can store on the performance of all subsystems (pressures, temperatures, currents, voltages, valve positions, etc.) for subsequent reporting to its terrestrial colleagues.

Thanks to the ship’s dependability, the operations team has been able to devote much of its energies recently to developing and refining the complex plans for the exploration of Ceres. You might be among the privileged readers who will get a preview when we begin describing the plans later this year.

Controllers also have devised some special activities for the spacecraft to perform in the near future, accounts of which are predicted to be in the next two logs.

In addition, team members have had time to maintain their skills for when the spacecraft needs more attention. Earlier this month, they conducted an operational readiness test (ORT). One diabolical engineer carefully configured the Dawn spacecraft simulator at JPL to behave as if a pebble one-half of a centimeter (one-fifth of an inch) in diameter shooting through the asteroid belt collided with the probe at well over twice the velocity of a high-performance rifle bullet.

When the explorer entered this region of space, we discussed that it was not as risky as residents of other parts of the solar system might assume. Dawn does not require Han Solo’s piloting skills to avoid most of the dangerous rocky debris.

The robot could tolerate such a wound, but it would require some help from operators to resume normal operations. This exercise presented the spacecraft team with an opportunity to spend several days working through the diagnosis and performing the steps necessary to continue the mission (using some of the ship’s backup systems). While the specific problem is extremely unlikely to occur, the ORT provided valuable training for new members of the project and served to keep others sharp.

One more benefit of the smooth operations is the time that it enables your correspondent to write his third shortest log ever. (Feel free to do the implied research.) Frequent readers can only hope he strives to achieve such a gratifying feat again!

Dawn is 13 million kilometers (7.9 million miles) from Vesta and 54 million kilometers (34 million miles) from Ceres. It is also 3.25 AU (486 million kilometers or 302 million miles) from Earth, or 1,275 times as far as the moon and 3.20 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 54 minutes to make the round trip.

› Read previous Dawn Journals by Marc Rayman


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 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