Dawn Spacecraft Creeping Up on Vesta

Dawn Spacecraft Creeping Up on Vesta: "

By Marc Rayman

Artist’s concept of NASA’s Dawn spacecraft at the large asteroid Vesta. The mission is less than five months away from getting into orbit around the large asteroid, its first target.

NASA’s Dawn spacecraft is less than five months away from getting into orbit around its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.






Dear Pleasant Dawnversions,



Deep in the asteroid belt, Dawn continues thrusting with its ion propulsion system. The spacecraft is making excellent progress in reshaping its orbit around the sun to match that of its destination, the unexplored world Vesta, with arrival now less than five months away.



We have considered before the extraordinary differences between Dawn’s method of entering orbit and that of planetary missions employing conventional propulsion. This explorer will creep up on Vesta, gradually spiraling closer and closer. Because the probe and its target already are following such similar routes around the sun, Dawn is now approaching Vesta relatively slowly compared to most solar system velocities. The benefit of the more than two years of gentle ion thrusting the spacecraft has completed so far is that now it is closing in at only 0.7 kilometers per second (1600 mph). Each day of powered flight causes that speed to decrease by about 7 meters per second (16 mph) as their orbital paths become still more similar. Of course, both are hurtling around the sun much faster, traveling at more than 21 kilometers per second (47,000 mph), but for Dawn to achieve orbit around Vesta, what matters is their relative velocity.



It may be tempting to think of that difference from other missions as somehow being a result of the destination being different, but that is not the case. The spiral course Dawn will take is a direct consequence of its method of propelling itself. If this spacecraft were entering orbit around any other planetary body, it would follow the same type of flight plan. This unfamiliar kind of trajectory ensues from the long periods of thrusting (enabled by the uniquely high fuel efficiency of the ion propulsion system) with an extremely gentle force.



Designing the spiral trajectories is a complex and sophisticated process. It is not sufficient simply to turn the thrust on and expect to arrive at the desired destination, any more than it is sufficient to press the accelerator pedal on your car and expect to reach your goal. You have to steer carefully (and if you don’t, please don’t drive near me), and so does Dawn. As the ship revolves around Vesta in the giant asteroid’s gravitational grip, it has to change the pointing of the xenon beam constantly to stay on precisely the desired winding route to the intended science orbits.



Dawn will scrutinize Vesta from three different orbits, known somewhat inconveniently as survey orbit, high altitude mapping orbit (HAMO), and low altitude mapping orbit (LAMO). Upon concluding its measurements in each phase, it will resume operating its ion propulsion system, using the mission control team’s instructions for pointing its thruster to fly along the planned spiral to the next orbit.



› Continue reading Dawn Spacecraft Creeping Up on Vesta
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Dawn Spacecraft Getting Ready for Vesta

Dawn Spacecraft Getting Ready for Vesta: "

By Marc Rayman

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

NASA’s Dawn spacecraft is less than four months away from getting into orbit around its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.



Dear Conndawnsseurs,


Three and a half years after launch, Dawn continues its travels around the sun, maneuvering to take the same orbital path as Vesta. Following its usual pattern, the spacecraft has spent most of the past month gently thrusting with its ion propulsion system. Some of the thrusting this month, however, was not designed to propel Dawn to Vesta. In addition, mission controllers stopped the thrusting to conduct other planned activities.


Spacecraft that use conventional propulsion coast through space most of the time, just as the moon coasts around Earth, and the planets and asteroids coast around the sun. In contrast, Dawn is in powered flight most of the time, using its ion propulsion system to change its orbit. The flight plan requires pointing the ion thruster in just the right direction to deliver the adventurer to its destination. The spacecraft orientation needed to aim the thruster ends up pointing the main antenna in an arbitrary direction. We have seen before that the robotic craft interrupts thrusting for about eight hours each week to direct the antenna toward Earth for communications.


Ever since Dawn’s trajectory was first being designed, long before launch, it has included coast periods for activities that require orientations incompatible with routine thrusting. One such period was the week of March 14; the previous was in July 2010.


Engineers and scientists operate the science instruments about twice each year to ensure they remain in good condition. This time was the last scheduled use of the sensors prior to their observations of Vesta. All tests showed they are in excellent condition and ready to expose the mysteries of the world they are about to visit.


Controllers transmitted upgraded software to each of the two identical science cameras, containing a few improvements over the version installed in July. The procedure went as smoothly as it had for previous software updates, including the first time such an operation was performed. After each camera received its new software, it performed its standard routine of exercises, just as it did only three weeks after reaching space. The tests confirmed that each camera’s electronics, optics, detector, cover, and filter wheel are in perfect condition.


Sometimes the spacecraft is turned to aim the cameras at carefully selected astronomical targets for their tests; other times, they take pictures of whatever stars happen to be in their field of view. This month’s tests were of the latter type, in which the orientation of the spacecraft was set to keep the antenna pointed at Earth. That put stars from a region near the border between Pisces and Cetus in the grasp of the cameras, quite appropriate for a ship voyaging across the cosmic ocean on its way to a distant and unfamiliar land.


Continue reading this entry from Marc Rayman’s Dawn journal …

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Dawn Begins its Vesta Phase

Dawn Begins its Vesta Phase: "

By Marc Rayman

NASA’s Dawn spacecraft is less than three months away from getting into orbit around its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.

Artist’s concept of NASA’s Dawn spacecraft. The giant asteroid Vesta, Dawn’s next destination, is on the lower left. The largest body in the asteroid belt and Dawn’s second destination, dwarf planet Ceres, is on the upper right. Image credit: NASA/JPL-Caltech

Dear Dawntalizingly Close Readers,

Dawn is on the threshold of a new world. After more than three and a half years of interplanetary travel covering in excess of 2.6 billion kilometers (1.6 billion miles), we are closing in on our first destination. Dawn is starting its approach to Vesta.



The interplanetary cruise phase of the mission ends today and the 15-month Vesta phase begins. The first three months are the “approach phase,” during which the spacecraft maneuvers to its first science orbit. Many of the activities during approach were discussed in detail in March and April last year, and now we are about to see those plans put into action.



The beginning of the phase is marked by the first images of the alien world Dawn has been pursuing since it left Earth. Vesta will appear as little more than a smudge, a small fuzzy blob in the science camera’s first pictures. But navigators will analyze where it shows up against the background stars to help pin down the location of the spacecraft relative to its target. To imagine how this works, suppose that distant trees are visible through a window in your house. If someone gave you a photo that had been taken through that window, you could determine where the photographer (Dawn) had been standing by lining up the edge of the window (Vesta) with the pattern of the background trees (stars). Because navigators know the exact position of each star, they can calculate where Dawn and Vesta are relative to each other. This process will be repeated as the craft closes in on Vesta, which ultimately will provide a window to the dawn of the solar system.



Even though the mysterious orb is still too far away to reveal new features, it will be exciting to receive these first pictures. During the approach phase, images will be released in periodic batches, with priority viewing for residents of Earth. The flow will be more frequent thereafter. For most of the two centuries that Vesta has been studied, it has been little more than a pinpoint of light. Interrupting thrusting once a week this month to glimpse its protoplanetary destination, Dawn will watch it grow from about five pixels across to 12. By June, the images should be comparable to the tantalizing views obtained by the Hubble Space Telescope. As the approach phase continues and the distance diminishes, the focus will grow still sharper and new details will appear in each subsequent set of images.



› Continue reading Marc Rayman’s May Dawn Journal

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Dawn Longs for Vesta’s Gravitational Pull

Dawn Longs for Vesta’s Gravitational Pull: "

By Marc Rayman



NASA’s Dawn spacecraft



NASA’s Dawn spacecraft is less than two months away from getting into orbit around its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.

Artist’s concept of the Dawn spacecraft using its ion propulsion system during the approach to Vesta. Image credit: NASA/JPL-Caltech

Dear Dependawnble Readers,


Dawn remains healthy and on course as it continues to approach Vesta. Thrusting with its ion propulsion system, as it has for most of its interplanetary journey so far, the spacecraft is gradually matching its solar orbit to that of the protoplanet just ahead.


As these two residents of the asteroid belt, one very new and one quite ancient, travel around the sun, they draw ever closer. Vesta follows its own familiar path, repeating it over and over, just as Earth and many other solar system bodies do. Dawn has been taking a spiral route, climbing away from the sun atop a blue-green pillar of xenon ions. With an accumulated total in excess of two and a half years of ion thrusting, providing an effective change in velocity of more than 6.5 kilometers per second (14,500 mph), the probe is close to the end of the first leg of its interplanetary trek. On July 16, Vesta’s gravity will capture the ship as it smoothly transitions from spiraling around the sun to spiraling around Vesta, aiming for survey orbit in August. For several reasons, the date for the beginning of the intensive observations there has not yet been set exactly.


Astronomers have estimated Vesta’s mass, principally by measuring how it occasionally perturbs the orbits of some of its neighbors in the asteroid belt and even the orbit of Mars, but this method yields only an approximate value. Because the mass is not well known, there is some uncertainty in the precise time that Dawn will become gravitationally bound to the colossal asteroid. As we have seen before, entry into orbit is quite unlike the highly suspenseful and stressful event of missions that rely on conventional chemical propulsion. Dawn simply will be thrusting, just as it has for 70 percent of its time in space. Orbit entry will be much like a typical day of quiet cruise. That Vesta will take hold at some point will matter only to the many Dawnophiles throughout the cosmos following the mission. The ship will continue to sail along a gently curving arc to survey orbit.

› Continue reading Marc Rayman’s May 27, 2011 Dawn Journal

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Dawn Sets its Sights, and Lens, on Vesta

Dawn Sets its Sights, and Lens, on Vesta: "

By Marc Rayman

NASA’s Dawn spacecraft obtained this image on its approach to the protoplanet Vesta, the second-most massive object in the main asteroid belt. Image Credit: NASA/JPL-Caltech/UCLA/MPS/DLR/PSI. › See more images

NASA’s Dawn spacecraft is less than one month away from getting into orbit around its first target, the giant asteroid Vesta. Each month, Marc Rayman, Dawn’s chief engineer, shares an update on the mission’s progress.

Dear Dawnstinations,

Vesta beckons, and Dawn responds. Now more than halfway through its approach to Vesta, Dawn continues creeping up on the destination it has been pursuing since it began its interplanetary travels. The separation between them gradually shrinks as the probe’s ion thrusting brings its orbit around the sun into a closer and closer match with Vesta’s. At the same time, the giant protoplanet’s gravity tugs gently on the approaching ship, luring it into orbit.


Starting at the beginning of the approach phase on May 3, Dawn interrupted thrusting once a week to photograph Vesta against the background stars. These images help navigators determine exactly where the probe is relative to its target. This technique does not replace other means of navigation but rather supplements them. One of the principal methods of establishing the spacecraft’s trajectory relies on accurately timing how long it takes radio signals, traveling, as all readers know, at the universal limit of the speed of light, to make the round trip between Earth and Dawn. Another uses the Doppler shift of the radio waves, or the slight change in pitch caused by the craft’s motion. These sensitive measurements remain essential to navigating the faraway ship as it sails the interplanetary seas.


Despite the very slow approach, the distance is small enough now that observing Vesta weekly is no longer sufficient. To achieve the navigational accuracy required to reach the intended orbit in early August, last week the frequency of imaging was increased to twice per week. In each session, half of the pictures are taken with long exposures to ensure many stars are detectable, thus overexposing the much brighter disc of the nearby Vesta. The other half use short exposures to ensure that the rocky world shows up correctly so its precise location can be measured. The visible and infrared mapping spectrometer has been commanded to observe Vesta during three of these sessions, each time providing valuable information that will help scientists select instrument settings for when Dawn is close enough to begin its detailed scientific measurements.


In addition to the regular campaign of imaging for navigation, mission controllers have other plans in store for the approach phase that were laid out more than a year ago. Twice in the next few weeks, the spacecraft will watch Vesta throughout its complete 5.3-hour rotation on its axis, revealing exciting new perspectives on this uncharted body. The explorer also will search for moons of the alien world.


› Continue reading Marc Rayman’s June 23, 2011 Dawn Journal
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