Dawn’s fourth anniversary of being in space is very different from its previous ones. Indeed, those days all were devoted to reaching the distant destination the ship is now exploring. Celebrating its anniversary of leaving Earth, Dawn is in orbit around a kindred terrestrial-type world, the ancient protoplanet Vesta.
The adventurer spent August on Vesta’s shores and now it's ready to dive in. Dawn devoted most of this month to working its way down from the 2,700-kilometer (1,700-mile) survey orbit to its current altitude of about 680 kilometers (420 miles) and changing the orientation of the orbit. (For a more detailed discussion of the altitude, go here or continue reading patiently for another six paragraphs.) The sensationally successful observing campaign in survey orbit produced captivating views, revealing a complex, fascinating landscape. Now four times closer to the surface, the probe is nearly ready for an even more comprehensive exploration from the high altitude mapping orbit (HAMO). The plans for HAMO have changed very little since it was described on the third anniversary of Dawn’s launch.
Dawn’s spiral descent went extremely well. We have seen before that bodies travel at higher velocities in lower altitude orbits, where the force of gravity is greater. For example, Mercury hurtles around the sun faster than Earth in order to balance the stronger pull of gravity, and Earth’s speed is greater than that of more remote Vesta. Similarly, satellites in close orbits around Earth, such as the International Space Station, race around faster than the much more distant moon. When it began its spiral on August 31, Dawn’s orbital speed high above Vesta was 76 meters per second (170 mph), and each revolution took nearly 69 hours. Under the gentle thrust of its ion propulsion system, the spacecraft completed 18 revolutions of Vesta, the loops getting tighter and faster as the orbital altitude gradually decreased, until it arrived at its new orbit on schedule on Sept. 18. In HAMO, Dawn orbits at 135 meters per second (302 mph), circling the world beneath it every 12.3 hours.
When Dawn’s itinerary called for it to stop thrusting, it was very close to HAMO but not quite there yet. As mission planners had recognized long beforehand, small differences between the planned and the actual flight profiles were inevitable. Extensive and sophisticated analysis has been undertaken in recent years to estimate the size of such discrepancies so the intricate plans for completing all the work at Vesta could account for the time and the work needed to deliver the robotic explorer to the intended destination. In order to accomplish the intensive program of observations with its scientific instruments, the spacecraft must follow an orbital path carefully matched to the sequences of commands already developed with painstaking attention to detail. The beauty of Dawn’s artistically choreographed pas de deux with Vesta depends on the music and the movements being well synchronized.
During its descent, Dawn paused frequently to allow controllers to update the flight profile, accounting for some of the variances in its course along the way. Following the completion of thrusting, navigators tracked the ship more extensively as it sailed around Vesta, measuring its orbit with great accuracy. This revealed not only the details of the orbital parameters (such as size, shape, and orientation) but also more about the character of Vesta’s gravity field than could be detected at higher altitudes. With the new information, the team designed two short maneuvers to adjust the orbit. The first, lasting four hours, was executed last night, and the second, half an hour shorter, will be completed tonight. After further measurements to verify the final orbit, the month of HAMO observations will begin on Sept. 29.
When the main portion of the thrusting was finished on Sept. 18, there was still more for Dawn to do than let navigators determine its exact orbit. It trained its sensors on Vesta, acquiring more exciting and valuable data. Although these observations are not part of the meticulously orchestrated and systematic mapping in HAMO, they contribute to the overall effort to squeeze as much as possible out of the precious time at Vesta. Engineers also performed more tests with the visible and infrared mapping spectrometer. In addition, they acquired images with the backup science camera, confirming that it is still fully functional and ready for action should its perfectly healthy twin ever become infirm.
Following instructions loaded earlier, on Sept. 21 the spacecraft reconfigured its memory to prepare for the great volume of data it will collect in HAMO. One of the software functions took longer than expected, causing the main computer to reset. The robot is designed to enter safe mode after a reboot, so it dutifully powered off nonessential systems, turned to point at the sun (the only celestial reference easily detectable anywhere along Dawn’s long route through the solar system), and waited for further instructions. Controllers detected the condition late that night and quickly identified the cause. With calm professionalism they swiftly executed all the steps necessary to return Dawn to its normal flight configuration less than two days later, and operations have continued smoothly.
As the spacecraft flies around Vesta, its altitude is constantly changing. Indeed, it would vary even if the orbit were a perfect circle, because Vesta is not a perfect sphere. This is similar to flying in a plane over Earth. If the plane maintains a constant altitude above sea level, the distance above the ground can change because the elevation of the ground itself varies, coming closer to the aircraft on mountains and farther in valleys. The topography on Vesta is even more pronounced, reflecting the tortured history it has experienced during 4.5 billion years in the rough and tumble asteroid belt.
The event that created the huge gouge centered near the south pole, now officially known as Rheasilvia (after the vestal virgin who was the mythical mother of Romulus and Remus, a weird story unlikely to be clarified by Dawn’s investigations), has left Vesta not only with astounding and jumbled terrain but also with an overall shape that is very peculiar. Indeed, although this world is smaller than Earth, it displays some of the most extreme topography in the solar system. The tremendous mountain in the center of Rheasilvia towers about twice as high above the surrounding plains as Mt. Everest does above sea level. Despite their being widely separated, the difference in elevation between the highest features near the equator and the lowest points deep in craters punched into Rheasilvia is more than 60 kilometers (37 miles).
Even if we imagined Dawn as being stationary while Vesta rotated underneath it, the altitude would change as the misshapen surface surges and subsides. In addition, the craft’s path is not a perfect circle, and the lower the orbit, the more it will deviate, as the irregular gravity field tugs on it with changing strength depending on where in that complex field the spacecraft is. When Dawn pushes down to closer orbits, we will discuss more about the actual height above the surface. In the meantime, for simplicity, these logs will continue to present the altitude as an average value, measured with respect to the average distance from the center of Vesta to its rocky surface. This is analogous to using sea level on Earth for the reference to describe altitude for aircraft and satellites. It is on that basis that the altitude in HAMO is given as 680 kilometers (420 miles).
On the last three September 27s, we have summarized Dawn’s progress on its journey. Now that it is at its first destination, the best measure of its progress is the stunning images and other scientific results it has transmitted to Earth. In addition to special announcements at press conferences in the coming months, beautiful and intriguing views continue to be posted here every day.
For those who would like to track the probe’s progress in the same terms used on previous (and, we boldly predict, subsequent) anniversaries, we present here the fourth annual summary, reusing the text from last year with updates where appropriate. Readers who wish to cogitate about the extraordinary nature of this deep-space expedition may find it helpful to compare this material with the first parts of the logs from its first, second, and third anniversaries. (On this special day, members of the operations team will further reflect upon the mission with the help of cupcakes decorated with the Dawn/Vesta logo.)
In its four years of interplanetary travels, the spacecraft has thrust for a total of about 988 days, or 68% of the time (and about 0.000000020% 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 254 kilograms (561 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 6.85 kilometers per second (15,300 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 barely half of the thrust time planned for its entire mission, Dawn has already far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.)
Since launch, our readers who have remained on or near Earth have completed four revolutions around the sun, covering about 25.1 AU (3.76 billion kilometers or 2.34 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 19.4 AU (2.91 billion kilometers or 1.81 billion miles). As it climbed away from the sun to match its orbit to that of Vesta, it continued to slow down to Vesta’s speed. Since Dawn’s launch, Vesta has traveled only 16.4 AU (2.46 billion kilometers or 1.53 billion miles).
Another way to investigate the progress of the mission is to chart how Dawn’s orbit around the sun has changed. This discussion will culminate with a few more numbers than we usually include, and readers who prefer not to indulge may skip this material, leaving that much more for the grateful Numerivores. In order to make the table below comprehensible (and to fulfill our commitment of environmental responsibility), we recycle some more text here on the nature of orbits.
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 ventured as far out of that plane in order to achieve orbit around another body as Dawn has.)
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.
|Minimum distance from the Sun (AU)||Maximum distance from the Sun (AU)||Inclination|
|Dawn's orbit on Sept. 27, 2007 (before launch)||0.98||1.02||0.0°|
|Dawn's orbit on Sept. 27, 2007 (after launch)||1.00||1.62||0.6°|
|Dawn's orbit on Sept. 27, 2008||1.21||1.68||1.4°|
|Dawn's orbit on Sept. 27, 2009||1.42||1.87||6.2°|
|Dawn's orbit on Sept. 27, 2010||1.89||2.13||6.8°|
|Dawn's orbit on Sept. 27, 2011||2.15||2.57||7.1°|
For readers who are not overwhelmed by the number of numbers, the table may help to demonstrate how Dawn has transformed its orbit during the course of its mission. Note that now the spacecraft's orbit around the sun is the same as Vesta’s. Achieving that was, of course, the objective of the long flight that started in the same solar orbit as Earth. While simply flying by Vesta would have been far easier, matching orbits with it has required the extraordinary capability of the ion propulsion system. Without it, NASA’s Discovery Program would not have been able to afford a mission to explore this fascinating world, and a mission to both Vesta and Ceres would be impossible.
Amazing and inspiring as its extraordinary trek has been, climbing the solar system hill atop a blue-green pillar of xenon ions, gently reshaping its orbit with the finesse of a sculptor creating a cosmic masterpiece, traveling far from its home planet through the forbidding and lonely depths of interplanetary space, it is the destination, and not the journey, that provides the grand prize. For most of the two centuries prior to Dawn’s arrival, Vesta was known as little more than a small fuzzy patch of light amidst the stars. The sharply focused picture that we are developing now of a complex alien world, with a dramatic history and a truly unique character, is the great reward for the long years and the billions of kilometers (miles) to get there. As the expedition continues, how can anyone not thrill to the experience of Vesta simultaneously becoming both more familiar and yet more mysterious?
Now as the operations team completes preparations for the next phase of its scrutiny of Vesta, Dawn embarks on its fifth year of spaceflight doing what it was designed to do. At the limits of human ingenuity, powered by the zeal of those who seek to perceive and to understand the beauty of the cosmos, the stalwart ship forges ahead with its exploration of a relict from the dawn of the solar system.
Dawn is 680 kilometers (420 miles) from Vesta. It is also 1.59 AU (238 million kilometers or 148 million miles) from Earth, or 665 times as far as the moon and 1.59 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 26 minutes to make the round trip.
Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2011