Dawn Journal: Third Anniversary in Space, and How to Map Vesta

Dear Dawnniversaries,

On the third anniversary of traveling through the solar system on its own since dispatching Dawn on a separate journey, Earth continues to orbit the sun in much the same way it has been. Meanwhile, the spacecraft is thrusting with its ion propulsion system, making steady progress in reshaping its orbit to rendezvous with Vesta in July 2011.

In its three years of interplanetary travels, the spacecraft has thrust for a total of about 715 days, or 65% of the time (and about 0.000000014% of the time since the Big Bang). While for most spacecraft, firing a thruster to change course is a special event, it is Dawn's wont. All this thrusting has cost the craft only 189 kilograms (417 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.

The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 5.01 kilometers per second (11,200 miles per hour). As previous logs have described (see here for one of the more extensive discussions), because of the principles of motion for orbital flight, whether around the Sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. But the effective change in speed remains a useful measure of the effect of any spacecraft's propulsive work. Having accomplished only one-third of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)

Since launch, our readers who have remained on or near Earth have completed three revolutions around the sun, covering about 18.9 AU (2.82 billion kilometers or 1.75 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 15.1 AU (2.26 billion kilometers or 1.40 billion miles). As it climbs away from the sun to match its orbit to that of Vesta, it will continue to slow down to Vesta's speed. Since Dawn's launch, Vesta has traveled only 12.0 AU (1.80 billion kilometers or 1.12 billion miles).

Readers with eidetic memory have already noticed that much of the text in the three preceding paragraphs is taken nearly verbatim from the logs that commemorated Dawn's first and second anniversaries of being in space, with the principal changes being that the numbers are updated here and we have generously expunged more (but not all!) humor each time. For those who wish to cogitate about the extraordinary nature of this interplanetary journey, comparing the first half of this log with those others may be helpful. In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we reuse some more of the text here.

Another way to investigate the progress of the mission is to chart how Dawn's orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores.

Orbits are ellipses (like flattened circles, or ovals in which the ends are of equal size). So as members of the solar system family follow their paths around the sun, they sometimes move closer and sometimes move farther from it.

In addition to orbits being characterized by shape, or equivalently by the amount of flattening (that is, the deviation from being a perfect circle), and by size, they may be described in part by how they are oriented in space. Using the bias of terrestrial astronomers, the plane of Earth's orbit around the sun (known as the ecliptic) is a good reference. Other planets and interplanetary spacecraft travel in orbits that are tipped at some angle to that. The angle between the ecliptic and the plane of another body's orbit around the sun is the inclination of that orbit. Vesta and Ceres do not orbit the sun in the same plane that Earth does, and Dawn must match its orbit to that of its targets. (The major planets orbit closer to the ecliptic, and no spacecraft has had to venture as far out of that plane in order to achieve orbit around another body as Dawn will.)

Now we can see how Dawn has been doing by considering the size and shape (together expressed by the minimum and maximum distances from the sun) and inclination of its orbit on each of its anniversaries. (Experts readily recognize that there is more to describing an orbit than these parameters. Our policy remains that we link to the experts' websites when their readership extends to one more elliptical galaxy than ours does.)

The table below shows what the orbit would have been if the spacecraft had terminated thrusting on its anniversaries; the orbits of its destinations, Vesta and Ceres, are included for comparison. Of course, when Dawn was on the launch pad on September 27, 2007, its orbit around the sun was exactly Earth's orbit. After launch, it was in its own solar orbit.

Readers may disregard the table or gaze into it for insight or inspiration for as long as they like. The point of it, however, is to illustrate that Dawn has come a long way since the launch pad, and while considerably more work remains to climb to Vesta, the ascent ahead is not as daunting as what has already been accomplished. On its next anniversary, the probe will be in the same orbit around the sun that Vesta is in; Dawn will be orbiting that distant world, where much of the mission's scientific destiny lies.

During the intervening year, there is a great deal more to look forward to than further travels through interplanetary space. In recent logs, we have had an overview of the activities during the "approach phase," which begins in less than eight months, and how the ship will slip into orbit around this mysterious protoplanet in July, becoming the first spacecraft to circle a body in the main asteroid belt. We also summarized the plans for the first intensive science phase, known as "survey orbit," which will be conducted in August.

Dawn is a mapping mission. In survey orbit it will use the science camera and the visible and infrared mapping spectrometer (VIR) to map most of the surface. (The gamma-ray and neutron detector, GRaND, will record radiation, but it will not be until a much lower altitude that the full power of its measurements will be achieved.) See here for a description of the instruments.

Dawn will not be able to observe all of the surface from survey orbit, because some of it will not be illuminated. The reason is simple: Vesta has seasons. This is no different from Earth and most of the other planets. Of course, the seasons don't refer to weather on this airless world but rather to the Sun's apparent motion relative to Vesta's equator, a consequence of the tilt of Vesta's pole. Each of the seasons lasts about 11 Earth months. When Dawn begins surveying Vesta, it will be summer in the southern hemisphere; the high northern latitudes will be in the darkness of winter and hence concealed from the camera and VIR. By the latter phases of our mission at the protoplanet, in 2012, the seasons will have progressed, the sun will illume more of the northern hemisphere, and the instruments will see previously invisible terrain. We will consider the effect of the seasons and the implications for observing Vesta in a future log. For now, we will focus on the science phase that follows survey orbit.

While survey orbit affords the robotic explorer a broad overview of the uncharted world, subsequent observations from a lower orbit will reveal more details. This second mapping orbit is known to insiders on the Dawn project, now including you, as the high altitude mapping orbit (HAMO). (Most team members have avoided the disquietude of trying to understand why that name is not applied to survey orbit, and readers are encouraged to do the same.)

With its survey from an altitude of about 2700 kilometers (1700 miles) complete, the ship will set sail again with the gentle touch of its ion propulsion system, gradually spiraling down for a month until it arrives at the HAMO altitude of about 660 kilometers (410 miles). As with survey orbit, the final selection of the parameters of the orbit will be possible only when Dawn is in the vicinity of Vesta and the massive asteroid's gravity field has been measured accurately.

awn will follow a polar orbit again, affording it the opportunity to pass over all latitudes, from the north pole to the equator to the south pole and then back, as Vesta spins on its axis. Unlike the nearly three days to complete one revolution in survey orbit, in HAMO the spacecraft will loop around the world beneath it twice a day. Between Dawn's orbital motion and Vesta's rotation, ten orbits will be required to present most of the illuminated surface to the camera's eye. Although they will take a brief break today to celebrate with some cake decorated with an image of Dawn's launch (and with real xenon in the frosting), mission controllers are now developing the detailed sequences of commands for the spacecraft in six groups, each corresponding to one of these 10-orbit/five-day cycles in HAMO.

As before, the probe will devote most its time over the day side of Vesta to acquiring a wealth of information with its sensors and most of the time over the night side beaming those precious data back to eager scientists on Earth. There is a whole new world to explore, and the instruments will be commanded to gather more data during passage over the lit side than the communications system can transmit to the Deep Space Network during the half of the orbit over the dark side. As a result, Dawn's computer memory will fill faster than it can be emptied, and the spacecraft will leave HAMO with some of its treasure trove still onboard. In the course of its flight to the next science orbit (suggestively and even appropriately named the low altitude mapping orbit) it will radio the stored data during the regular hiatuses in thrusting to point its main antenna to Earth.

In two of the HAMO cycles, Dawn will peer straight down at the exotic landscape below it, taking pictures with the camera and recording spectra with VIR. By performing these mapping observations twice, there will be a ready opportunity to see most of the visible surface even if some observations are not completed because of the occasional glitches that are inevitable in such a complex undertaking in an unknown environment. The process of designing, testing, verifying, and executing the intricate sequences for mapping Vesta is far too complicated for the mission control team to wait until any such minor problem occurs and then formulate plans to acquire the missed data.

During the other four HAMO cycles, instead of pointing its instruments at the nadir, the spacecraft will aim them off at an angle, providing a different view in each cycle, effectively acquiring stereo imagery. Scientists will combine the pictures taken in all the directions to create topographic maps, revealing the heights of mountains, the depths of craters, the slopes of plains, etc. This will be of great value in inferring the nature of the geological processes that shaped this protoplanet. In addition, it will yield exciting perspectives for everyone who wants to visualize this alien world.

As this journey of exploration enters its fourth year, we continue to be exceedingly grateful for the many readers who travel with us on this voyage. An adventure of humankind, Dawn raises our collective sights and our spirits as it strives on behalf of everyone who longs to see far past mere terrestrial horizons. While physically we remain confined to the vicinity of our planet, we do not allow that to limit our reach. Powered by our ambition, our imagination, our curiosity, and our determination, missions like Dawn take all of us along to real places that lie beyond what our imaginations could construct. Although readers throughout the cosmos have participated in this experience, it is ironic that some very nearby have not been able to. Thanks to the generous efforts of Pablo Gutiérrez-Marqués, the operations manager for Dawn's science camera at the Max-Planck-Institut für Sonnensystemforschung (Max Planck Institute for Solar System Research) in Katlenburg-Lindau, Germany, it is with great delight that we welcome our hispanophone friends onboard. Small as the spacecraft is, there is plenty of room for everyone who shares in the wonderment of the cosmos, the fulfillment of gaining new knowledge and new insights, the exhilaration of discovery, and the thrill of exploration.

Dawn is 0.15 AU (23 million kilometers or 14 million miles) from Vesta, its next destination. It is also 2.96 AU (443 million kilometers or 275 million miles) from Earth, or 1120 times as far as the moon and 2.95 times as far as the sun. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.

Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2010