Dawn concluded 2012 almost 13,000 times farther from Vesta than it began the year. At that time, it was in its lowest orbit, circling the alien world at an average altitude of only 210 kilometers (130 miles), scrutinizing the mysterious protoplanet to tease out its secrets about the dawn of the solar system.
To conduct its richly detailed exploration, Dawn spent nearly 14 months in orbit around Vesta, bound by the behemoth's gravitational grip. In September they bid farewell, as the adventurer gently escaped from the long embrace and slipped back into orbit around the sun. The spaceship is on its own again in the main asteroid belt, its sights set on a 2015 rendezvous with dwarf planet Ceres. Its extensive ion thrusting is gradually enlarging its orbit and taking it ever farther from its erstwhile companion as their solar system paths diverge.
Meanwhile, on faraway Earth (and all the other locations throughout the cosmos where Dawnophiles reside), the trove of pictures and other precious measurements continue to be examined, analyzed, and admired by scientists and everyone else who yearns to glimpse distant celestial sights. And Earth itself, just as Vesta, Ceres, Dawn, and so many other members of the solar system family, continues to follow its own orbit around the sun.
Thanks to a coincidence of their independent trajectories, Earth and Dawn recently reached their smallest separation in well over a year, just as the tips of the hour hand and minute hand on a clock are relatively near every 65 minutes, 27 seconds. On Dec. 9, they were only 236 million kilometers (147 million miles) apart. Only? In human terms, this is not particularly close. Take a moment to let the immensity of their separation register. The International Space Station, for example, firmly in orbit around Earth, was 411 kilometers (255 miles) high that day, so our remote robotic explorer was 575 thousand times farther. If Earth were a soccer ball, the occupants of the orbiting outpost would have been a mere seven millimeters (less than a third of an inch) away. Our deep-space traveler would have been more than four kilometers (2.5 miles) from the ball. So although the planet and its extraterrestrial emissary were closer than usual, they were not in close proximity. Dawn remains extraordinarily far from all of its human friends and colleagues and the world they inhabit.
As the craft reshapes its solar orbit to match Ceres's, it will wind up farther from the sun than it was while at Vesta. (As a reminder, see the table here that illustrates Dawn's progress to each destination on its long interplanetary voyage.) We saw recently, however, that the route is complex, and the spacecraft is temporarily approaching the sun. Before the ship has had time to swing back out to a greater heliocentric range, Earth will have looped around again, and the two will briefly be even a little bit closer early in 2014. After that, however, they will never be so near each other again, as Dawn will climb higher and higher up the solar system hill, its quest for new and exciting knowledge of distant worlds taking it farther from the sun and hence from Earth.
This interactive computer-based stereo viewing system was used to analyze Mars topography images generated by the cameras on NASA's Viking 1 Mars lander. Two 17-inch video monitors faced a scanning stereoscope mounted between them on a table. Left and right lander camera image data were sent to the left and right monitors. Panning controls on the stereoscope helped align one image with the other to create a stereo image, 640 by 512 pixels in size. A mouse was used for finely controlled rotation of the monitors. An article about the system described a prototype mouse, used before this photo was taken in 1976. "The track ball is a baseball-sized sphere protruding from the top of a retaining box and capable of being rotated freely and indefinitely about its center ..."
The resulting images could be displayed on additional monitors and were used to create contour maps and other images that aided lander surface operations. The system was developed by Stanford University and NASA's Jet Propulsion Laboratory in Pasadena, Calif.
Dawn is continuing to gently and patiently change its orbit around the sun. In September, it left Vesta, a complex and fascinating world it had accompanied for 14 months, and now the bold explorer is traveling to the largest world in the main asteroid belt, dwarf planet Ceres.
Dawn has spent most of its time since leaving Earth powering its way through the solar system atop a column of blue-green xenon ions emitted by its advanced ion propulsion system. Mission controllers have made some changes to Dawn's operating profile in order to conserve its supply of a conventional rocket propellant known as hydrazine. Firing it through the small jets of the reaction control system helps the ship rotate or maintain its orientation in the zero-gravity of spaceflight. The flight team had already taken some special steps to preserve this precious propellant, and now they have taken further measures. If you remain awake after the description of what the changes are, you can read about the motivation for such frugality.
Dawn's typical week of interplanetary travel used to include ion thrusting for almost six and two-thirds days. Then it would stop and slowly pirouette to point its main antenna to Earth for about eight hours. That would allow it to send to the giant antennas of NASA's Deep Space Network a full report on its health from the preceding week, including currents, voltages, temperatures, pressures, instructions it had executed, decisions it had made, and almost everything else save its wonderment at operating in the forbidding depths of space so fantastically far from its planet of origin. Engineers also used these communications sessions to radio updated commands to the craft before it turned once again to fire its ion thruster in the required direction.
Now operators have changed the pace of activities. Every turn consumes hydrazine, as the spacecraft expels a few puffs of propellant through some of its jets to start rotating and through opposing jets to stop. Instead of turning weekly, Dawn has been maintaining thrust for two weeks at a time, and beginning in January it will only turn to Earth once every four weeks. After more than five years of reliable performance, controllers have sufficient confidence in the ship to let it sail longer on its own. They have refined the number and frequency of measurements it records so that even with longer intervals of independence, the spacecraft can store the information engineers deem the most important to monitor.
Although contact is established through the main antenna less often, Dawn uses one of its three auxiliary antennas twice a week. Each of these smaller antennas produces a much broader signal so that even when one cannot be aimed directly at Earth, the Deep Space Network can detect its weak transmission. Only brief messages can be communicated this way, but they are sufficient to confirm that the distant ship remains healthy.
In addition to turning less often, Dawn now turns more slowly. Its standard used to be the same blinding pace at which the minute hand races around a clock (fasten your seat belt!). Engineers cut that in half two years ago but returned to the original value at the beginning of the Vesta approach phase. Now they have lowered it to one quarter of a minute hand's rate. Dawn is patient, however. There's no hurry, and the leisurely turns are much more hydrazine-efficient.
With these two changes, the robotic adventurer will arrive at Ceres in 2015 with about half of the 45.6-kilogram (101-pound) hydrazine supply it had when it rocketed away from Cape Canaveral on a lovely September dawn in 2007. Mission planners will be able to make excellent use of it as they guide the probe through its exploration of the giant of the main asteroid belt.
Any limited resource should be consumed responsibly, whether on a planet or on a spaceship. Hydrazine is not the only resource that Dawn's controllers manage carefully, but let's recall why this one has grown in importance recently.
The spacecraft can stabilize or change its orientation using the hydrazine powered jets or reaction wheels. By electrically changing a wheel's spin rate, Dawn can start or stop rotating. When it is relying principally on these gyroscope-like devices, it still occasionally has to expend a little hydrazine to keep them from spinning too fast, as explained nearly four years ago. While thrusting (which is most of the time), the ion thruster works in concert with one of those other actuators to control the orientation.
For an ambitious and complex eight-year interplanetary expedition, Dawn's builders equipped it with backup systems. The craft was designed to use three reaction wheels at a time for normal operations, so it is outfitted with four. One of them encountered increased friction in June 2010. To preserve the life of the remaining wheels, engineers flew the spacecraft with all the wheels turned off from August 2010 until the Vesta approach phase began in May 2011, and they are doing the same during the flight from Vesta to Ceres.
As soon as the wheel had difficulty in 2010, Orbital Sciences Corporation and JPL began working on a method to operate with fewer than three, in case another one faltered. They developed software to operate in a "hybrid" mode with two wheels plus the hydrazine jets and installed it in the robot's main flight computer in April 2011 so it would be available at Vesta if needed.
The exploration of that alien orb, which exceeded all expectations not only for productivity but also for pure awesomeness, went very smoothly with the three operational wheels. As Dawn was spiraling away from the rocky behemoth in August 2012, however, another one experienced the same peculiar friction. Because the wheels had already been scheduled to be powered off shortly thereafter, the flight team continued the departure with them turned off, and it proceeded without further interruptions. With their typical swift professionalism, they immediately began working on the long-term ramifications of two wheels being unavailable in case the devices could not be recovered.
Because the hybrid control scheme uses more hydrazine than three wheels would, and using the hydrazine jets by themselves with no wheels consumes still more, operators undertook the new campaign to conserve the propellant during the journey to Ceres. Ever resourceful, engineers now anticipate that regardless of how healthy the wheels are, the probe will be able to conduct an exciting and rewarding exploration there.
Dawn will arrive at the distant and mysterious Ceres in 2015, and that allows plenty of time for the terrestrial members of the team to complete the exquisitely detailed plans for its adventures there. While that work is underway, the intrepid ship continues forging silently through the vast emptiness of space, distant and alone, patient and persistent. Despite its remoteness, the robot remains tightly bound to its human colleagues, for it is on their behalf and under the power of their ingenuity, thirst for knowledge, and hunger for adventure that it sails deeper into uncharted cosmic seas.
Dawn is 1.5 million kilometers (960 thousand miles) from Vesta and 57 million kilometers (36 million miles) from Ceres. It is also 1.59 AU (238 million kilometers or 148 million miles) from Earth, or 590 times as far as the moon and 1.61 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
11:00 p.m. PST November 30, 2012
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.
If not for its ion propulsion system, this extraordinary interplanetary expedition would be impossible. Conventional chemical propulsion does not have the requisite capability. The key to taking advantage of the unique performance of ion propulsion is patience, which Dawn demonstrates exemplarily. Whereas the vast majority of spacecraft spend almost all of their time coasting, Dawn devotes the preponderance of its time to powered flight, emitting a tenuous but very high velocity beam of blue-green xenon ions to propel itself. On Nov. 2, coincidentally around the same time it begins approaching the sun, the indefatigable robot will exceed three years of total thrust. By then, the gentle but persistent flow of ions will have imparted the equivalent of 7.28 kilometers per second (16,300 miles per hour) to the spacecraft. (As we have seen in many previous logs, such as here, this is not actually a measure of Dawn's speed. It is a convenient description of the effectiveness of a propulsion system that avoids the complications of orbital mechanics. The effect of the propulsion is not to increase velocity but rather to climb the solar system hill, just as pressing a car's accelerator while on a steep slope may not gain speed compared to driving on a level road, but it will gain elevation.)
As the ambitious adventurer forges through the asteroid belt, pushing tirelessly against the sun's incessant pull, the temporary reduction in heliocentric distance is just one element of the complex plan for a grand adventure to explore two of the last uncharted worlds in the inner solar system. Next month, we will see some of the changes the mission control team is making in how Dawn operates in order to ensure its work at Ceres is as richly productive as it was at Vesta.
Dawn is 708 thousand kilometers (440 thousand miles) from Vesta and 59 million kilometers (37 million miles) from Ceres. It is also 1.78 AU (267 million kilometers or 166 million miles) from Earth, or 660 times as far as the moon and 1.80 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 30 minutes to make the round trip.
Dr. Marc D. Rayman
7:00 a.m. PDT October 31, 2012
On the fifth anniversary of the beginning of its ambitious interplanetary adventure, Dawn can look back with great satisfaction on its spectacular exploration of the giant protoplanet Vesta and forward with great eagerness to reaching dwarf planet Ceres. Today Earth's robotic ambassador to the main asteroid belt is in quiet cruise, gradually reshaping its orbit around the sun so it can keep its appointment in 2015 with the mysterious alien world that lies ahead.
This anniversary resembles the first three more than the fourth. Its first years in space were devoted to spiraling away from the sun, ascending the solar system hill so it could gracefully slip into orbit around Vesta in time for its fourth anniversary. One year ago, Dawn was in the behemoth's gravitational grip and preparing to map its surface in stereo and make other measurements. The subsequent year yielded stunning treasures as Dawn unveiled the wondrous secrets of a world that had only been glimpsed from afar for over two centuries. While at Vesta, it spiraled around the massive orb to position itself for the best possible perspectives. Its final spiral culminated in its departure from Vesta earlier this month. Now for its fifth anniversary, it is spiraling around the sun again, climbing beyond Vesta so that it can reach Ceres.
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 fifth 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 logs from its first, second, third, and fourth anniversaries.
In its five years of interplanetary travels, the spacecraft has thrust for a total of 1060 days, or 58 percent of the time (and about 0.000000021 percent 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 267 kilograms (587 pounds) of its supply of xenon propellant, which was 425 kilograms (937 pounds) on September 27, 2007.
The fraction of time the ship has spent in powered flight is lower than last year (when it was 68 percent), because Dawn devoted relatively little of the past year to thrusting. Although it did change orbits extensively at Vesta, most of the time it was focused on exactly what it was designed and built to do: scrutinize the ancient world for clues about the dawn of the solar system.
The thrusting so far in the mission has achieved the equivalent of accelerating the probe by 7.14 kilometers per second (16,000 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 slightly more than 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 five revolutions around the sun, covering about 31.4 AU (4.70 billion kilometers or 2.92 billion miles). Orbiting farther from the sun, and thus moving at a more leisurely pace, Dawn has traveled 23.4 AU (3.50 billion kilometers or 2.18 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 20.4 AU (3.05 billion kilometers or 1.90 billion miles) and the even more sedate Ceres has gone 18.9 AU (2.82 billion kilometers or 1.75 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 may 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 part of the arduousness of the journey is changing the inclination of its orbit, an energetically expensive task.)
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°|
|Dawn's orbit on Sept. 27, 2012||2.17||2.57||7.3°|
For readers who are not overwhelmed by the number of numbers, the table may help to demonstrate how Dawn has patiently transformed its orbit during the course of its mission. Note that last year, the spacecraft's path around the sun was exactly the same as Vesta's. Achieving that perfect match was, of course, the objective of the long flight that started in the same solar orbit as Earth, and that is how Dawn managed to get into orbit around Vesta. While simply flying by Vesta would have been far easier, matching orbits with it required the unique 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 have been impossible.
Although the probe left Vesta only three weeks ago, the effect of the ion thrusting is already evident. Dawn is no longer in the same orbit as Vesta. It is propelling itself along a different path, forging its own course through the asteroid belt. The journey will be long, and the exploration of Ceres will not commence until well after Dawn's seventh anniversary of venturing into space. Many exciting discoveries and many daunting challenges lie ahead, and some of them have yet even to be recognized. But this stalwart ship (supported by its crew on distant Earth) has proven itself capable of accomplishing remarkable feats in its quest to expand frontiers and reap the great rewards of new knowledge and exciting new perspectives on the solar system for the bold creatures whose passions and insightful creativity fuel its extraordinary cosmic adventure.
Dawn is 160 thousand kilometers (99 thousand miles) from Vesta and 62 million kilometers (38 million miles) from Ceres. It is also 2.18 AU (325 million kilometers or 202 million miles) from Earth, or 840 times as far as the moon and 2.17 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 36 minutes to make the round trip.
Dr. Marc D. Rayman
4:34 a.m. PDT September 27, 2012
Dear Marvestalous Readers,
An interplanetary spaceship left Earth in 2007. Propelling itself gently and patiently through the solar system with a blue-green beam of xenon ions, it gradually spiraled away from the sun. It sailed past Mars in 2009, its sights set on more distant and exotic destinations. In July 2011, it gracefully and elegantly entered orbit around the second most massive resident of the main asteroid belt, Vesta. It spent more than 13 months there scrutinizing the gigantic protoplanet with all of its sensors and maneuvering to different orbits to optimize its investigations, making myriad marvelous discoveries. After they traveled together around the sun for 685 million kilometers (426 million miles), the ship left orbit in September 2012 and is now headed for dwarf planet Ceres, the largest body between the sun and Neptune not yet visited by a spacecraft. No other probe has ever been capable of the amazing feats Dawn is performing, exploring two of the largest uncharted worlds in the inner solar system.
The population of the main asteroid belt numbers in the millions. Vesta is such a behemoth that Dawn has now single-handedly examined about eight percent of the mass of the entire belt. And by the time it finishes at the colossus Ceres, it will have investigated around 40 percent.
The expedition to Vesta has produced riches beyond everyone's hopes. With 31,000 photos, 20 million visible and infrared spectra, and thousands of hours of neutron spectra, gamma ray spectra, and gravity measurements, Dawn has revealed to humankind a unique and fascinating member of the solar system family. More akin to Earth and the other terrestrial planets than to typical asteroids, Vesta is not just another chunk of rock. It displays complex geology and even has a dense iron-nickel core, a mantle, and a crust. Its heavily cratered northern hemisphere tells the story of more than 4.5 billion years of battering in the rough and tumble asteroid belt. Its southern hemisphere was wiped clean, resurfaced by an enormous impact at least two billion years ago and an even greater collision one billion years ago. These events excavated the 400-kilometer (250-mile) Veneneia and 500-kilometer (310-mile) Rheasilvia basins. The larger basin has a mountain at the center that towers more than twice the height of Mt. Everest; indeed, it soars higher than all but one of the mountains known in the solar system. The impacts were so forceful, they nearly destroyed Vesta. The fierce shock reverberated through the entire body and left as scars an extraordinary network of vast troughs near the equator, some hundreds of kilometers (miles) long and 15 kilometers (10 miles) wide.
The powerful impacts liberated tremendous amounts of material, flinging rocks far out into space, some of which eventually made it all the way to Earth. It is astonishing that more than one thousand meteorites found here came from Vesta. We have some meteorites from Mars, and we have some meteorites from the moon, but we have far, far more that originated in those impacts at Vesta, so distant in time and space. Vesta, Mars, and the moon are the only celestial bodies identified as the source of specific meteorites.
Scientists will spend years productively poring through Dawn's fabulous findings and learning what secrets they hold about the dawn of the solar system, and many more people will continue to marvel at the spectacular sights of this alien world. But the emissary from Earth has completed its assignment there and moved on. It has spent most of its time since the previous log using its ion propulsion system to climb higher and higher above Vesta. This departure spiral is the mirror image of the approach spiral the robotic adventurer followed last year. The unique method of entering and leaving orbit is one of the many intriguing characteristics of a mission that uses ion propulsion. Without that advanced technology, this ambitious deep space adventure would be impossible.
As Dawn ascended, Vesta's gravitational grip grew weaker and weaker. At some point along its spiral, the explorer was far enough and moving fast enough that Vesta could no longer hold it in orbit. As smoothly and tenderly as Vesta had taken Dawn in its embrace last year, it released its erstwhile companion, each to go its own way around the sun. The bond was severed at about 11:26 p.m. PDT yesterday, when they were 17,200 kilometers (10,700 miles) apart, separating at the remarkably leisurely speed of less than 33 meters per second (73 miles per hour). Many of our readers drove their cars that fast today (although we hope it was not in school zones).
Unlike missions that use conventional chemical propulsion, there was no sudden change on the spacecraft and no nail-biting on Earth. If you had been in space watching the action, you probably would have been hungry, cold, and hypoxic, but you would not have noticed anything unusual about the scene. Apart from a possible hint of self-satisfaction, Dawn would have looked just as it had for most of its interplanetary flight, a monument to humankind's ingenuity and passionate drive to know the cosmos perched atop a blue-green pillar of xenon ions. If, instead, you had been in Dawn mission control watching the action, you would have been in the dark and all alone (until JPL Security arrived). There was no need to have radio contact with the reliable spaceship. It had already thrust for almost 2.9 years, or 58 percent of its time in space. Thrusting during escape was no different. No one was tense or anxious; rather, all the drama is in the spectacular results of the bold mission at Vesta and the promise of what is to come at Ceres. When Dawn entered orbit, your correspondent was dancing. When Dawn left orbit, he was sleeping serenely.
A month earlier, on August 8, with the craft more than 2,100 kilometers (1,300 miles) above the surface, patiently powering its way up through Vesta's gravity field, one of the reaction wheels experienced an increase in internal friction. Reaction wheels are used to control a spacecraft's orientation in the frictionless, zero-gravity conditions of spaceflight. By electrically changing a wheel's spin rate, Dawn can rotate or stabilize itself. Protective software quickly detected the event and correctly responded by deactivating that wheel and the other two that were operating, switching to the small jets that are available for the same function, and reconfiguring other systems, including powering off the ion thrust and turning to point the main antenna to Earth.
A routine communications session the next day revealed to mission controllers what had occurred. They had planned long ago to turn the wheels off for the flight from Vesta to Ceres, so having them off a few weeks early was not a significant change. The team soon restored the spacecraft to normal operations and reformulated the departure plan, and on August 17 Dawn resumed its ascent. Because of the hiatus in thrusting, escape shifted from August 26 to September 4. The flexibility in the mission timeline provided by ion propulsion made this delay easy to accommodate.
In order to conserve the hydrazine propellant that the jets use, the bonus departure observations described before were curtailed, as they were not a high priority for the mission. Nevertheless, on August 25 and 26, at an altitude of around 6,000 kilometers (3,700 miles), the explorer did peer at Vesta once more with its camera and visible and infrared mapping spectrometer. The last time it had been this far away was July 21, 2011, during its descent to an unfamiliar destination. This time, 13 months later, the spacecraft turned back for a final gaze at the magnificent world it had unveiled during its remarkable time there, a world that prior to last year had appeared as little more than a tiny smudge among the stars for the two centuries it had been observed.
The delay in the departure schedule provided a convenient benefit. Vesta has seasons, just as Earth does, although they progress more slowly on that distant orb. August 20 was the equinox, when northern hemisphere spring began. Until then, the sun had been in Vesta's southern hemisphere throughout Dawn's residence there. While most of the northern hemisphere was revealed during the second high-altitude mapping orbit, the illumination of the landscape immediately around the north pole was even better for this last look. After radioing its parting shots to wistful mission controllers, the ship commenced its climb again.
And then, with an stunningly successful mission behind it, a newly explored world below it, and a mysterious dwarf planet ahead of it, the indomitable and indefatigable adventurer left Vesta forever.
Dawn is 18,500 kilometers (11,500 miles) from Vesta and 64 million kilometers (40 million miles) from Ceres. It is also 2.45 AU (367 million kilometers or 228 million miles) from Earth, or 910 times as far as the moon and 2.43 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 41 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 a.m. PDT September 5, 2012
Monday, August 6, 2012 1:13:26 AM
Welcome to Gale Crater. "Adam...you're a genius!" I shout to Adam Steltzner. He pauses. Stops. Turns around. "I'm not a genius -- I just work with a team of them."
Sunday, August 5, 2012 10:04:10 PM
The EDL Phase Lead, Adam Steltzner, has just thanked the cruise team for their 350-million-mile ride. "Curiosity is in fantastic shape, she's here because you guys got her here. See you on Mars."
Go Curiosity. And break out the peanuts.
Mars really has us now.
Sunday, August 5, 2012 10:03:56 PM
Ten thousand and sixty three. Sixty four. Sixty five. As quick as you can count it, our speed towards Mars is accelerating.
Mars is about half the diameter of Earth, but only about 10 percent as heavy as Earth. Even so -- on its surface, gravity is about 38 percent that of Earth. In the next 28 minutes, we will gain another 3,000 miles per hour until Curiosity, heatshield ready, slams into the top of the Martian atmosphere.
40 billion to 1
Sunday, August 5, 2012 9:15:28 PM
A quiet approach to Mars as we watch a tiny plot of a graph. The X-band frequency that Curiosity is currently transmitting is a frequency of more than 8 Gigahertz -- 8 billion cycles per second. As it rotates, that tiny little graph shows that frequency moving up and down, by about 0.2 Hz. One part in 40 billion. That little bounce up and down is the rotation of the spacecraft, two revolutions per minute. We have that accuracy because we're bouncing a radio signal from the ground, up to spacecraft and back again. But that signal, after a final poll, will be going away.
Systems Go. Power Go. Thermal Go. Propulsion Go. Nav Go. Uplink Go. Avionics Go. Flight. Software Go. Fault Protection Go. Chief Engineer Go. EDL FLight System Go. Data Management Go. GDS Go. Telecom Go. ACS Go. EDL Activity Lead Go. ACE Go.
"You are clear to bring down the uplink." So in just over 13 minutes time, Curiosity will no longer have that amazing signal to bounce back - and our little squiggly 1-in-40-billion line will be gone. We will just hear the spacecraft's own transmitter from more than 150 million miles.
Curiosity is truly on her own.
A Final Check
Sunday, August 5, 2012 8:44:21 PM
This full poll of the flight team is a lengthy and exhaustive tour of the rover, the cruise stage and all the systems. My favorite call is from the chief engineer:
"We are green across the board"
That's the word from Rob Manning -- a veteran of four successful Mars landings. When Rob says things are green, you know you're in good shape. If you were hoping to spend some time exploring the martian moon Deimos on your way to Gale Crater -- please alight the rover now, we just crossed its orbit. Now there are 16,000 miles to go.
Calm before the Storm
Sunday, August 5, 2012 8:32:58 PM
Things got a little quiet in the control room. People heading out for some food before we get down to the business of landing on Mars. It takes huge team to watch over a spacecraft as complex, and activites and intricate as a Mars landing. As they get back to their consoles, they do a comm check to make sure they can all hear each other. Systems. Power. Thermal. Prop. Nav. Uplink. Flight Software. Fault Protection. EO Team Chief. GDS. Telecom. EDL Comm. ACS ... the calls, and acronyms, go on and on. Now they are all back on console, the whole team is about to do a full system poll.
Can you hear me?
Sunday, August 5, 2012 7:59:37 PM
Between now and landing, Curiosity will use a total of eight antennas. The Deep Space Network is now listening to a medium-gain antenna transmitting on X-Band on the cruise stage. During entry, two low gain antennas on the back of the spacecraft continue that signal of "tones." There are also low-gain antennas on the descent stage and the rover. However, Earth will have set at this time.
Meanwhile, a UHF antenna on the backshell, followed by another on the descent stage and finally one on the rover, will continue to transmit telemetry during landing. This data will be received by Mars Odyssey and Mars Reconnaissance Orbiter. Odyssey will relay it straight to Earth so we can track landing. Mars Reconnaissance Orbiter records everything it hears and sends it back a few hours later. Mars Express will also record just the pitch of this signal as a final backup.
The ground stations at the Canberra, Australia Deep Space Communications Complex will follow us the whole way -- direct from the rover 'til Earth sets behind it -- and from Odyssey and Mars Reconnaissance Orbiter as well. All the way to the ground, a complex system of systems will be trying to keep that tenuous link between Earth and Mars alive.
Sunday, August 5, 2012 5:58:00 PM
"Nominal" sounds like a very boring word, but in the world of spaceflight, nominal is engineer for "awesome." Thanks to the Deep Space Network, we know just how nominal everything is. Deep Space Station 43, a 70-meter-diameter antenna in Tidbindilla, Austraila is currently receiving a steady stream of data at 2,000 bits per second that informs the engineers how all their subsystems are doing. Attitude control, thermal performance, power systems, avionics, propulsion, communication, the list is long. The flight team (meet them all here: www.gigapan.com/gigapans/110926) just took a poll, and all subsystems are nominal. The MEDLI instrument is now powered up, and healthy. It's talking to the flight computer, and the power system can see it drawing just 300 milliamps. It will record first-of-its-kind data on temperature, pressure and other readings through Curiosity's heatshield during entry. This data will help us understand how the heatshield behaves and can help us make them better for the future. As MEDLI lives on the inside of the heatshield, it is thrown overboard when the heatshield is separated about six miles above the surface. Its data will be safely stored on the rover to be downlinked after landing.
Sunday, August 5, 2012 1:15:54 PM
When you're a spacecraft it's important to know which way you're facing. If you know which way you're facing, you know which way Earth is, so you can talk to home; which way the sun is, so you can get power on a solar array; and if you're Curiosity, you know which way Mars is. There are two ways spacecraft typically orient themselves. One is called "three-axis stabilized," which means the spacecraft uses thrusters and reaction wheels to keep itself pointed the right way. You may have heard about trouble with reaction wheels on the Mars Odyssey orbiter recently (it carries a spare just in case, and we're now using it). Curiosity (as well as its older sisters Spirit and Opportunity, and Juno right now on its way to Jupiter) just spin their way through deep space. They point in one direction and spin, like a top. That spin stops the spacecraft wandering off and pointing somewhere else. Curiosity, all the way till after we wave goodbye to its cruise stage about 17 minutes before landing, spins at 2 rpm. During its 253-day cruise, Curiosity will have spun more than 720,000 times. It's enough to give a rover a headache.
Sunday, August 5, 2012 1:05:01 PM
I've arrived "on lab" (JPL-speak for "at the office") to check up on our computer running Eyes on the Solar System (http://eyes.nasa.gov) that will be fed to NASA Television tonight. Looking up in the control room -- I see we've just crossed 80,000 miles to go. Less than four- times the distance from Earth to our geostationary communication satellites. Mars is about 4,200 miles in diameter - so with a little high school trig, we can calculate that Mars would appear 3 degrees across to Curiosity. That's six times larger than the size of the full moon from Earth. This time yesterday, Curiosity was only 170 mph slower than it is now. In the next 10 hours as it falls to Mars it gains another 5,000. As an astronaut onboard Apollo 13 said to mission control on their way home, "The world's getting awful big in the window."
The Runners Up
Friday, August 3, 2012 11:15:00 AM
Adam Steltzner (MSL EDL phase lead) is a great speaker and real highlight of today's NASA Social event. A fantastic question from the audience asked what ideas for landing Curiosity were rejected.
The runner-up: airbags. There isn't a fabric that we know of strong enough to handle the impact loads that a 899-kg rover would create. Good enough for the 180-kg of Spirit and Opportunity, but it just can't get scaled up to something as big as Curiosity.
Third place: Put the rover on top of the rockets. The problem there is that the rover is so heavy, and the propellant tanks so large, that you would have a very tall vehicle prone to toppling over on touchdown.
It may look a little crazy -- but the skycrane actually makes a lot of sense.
Speed Up, Slow Down
Thursday, August 2, 2012 5:12:47 PM
The art of flying between the planets is a balancing act of gravity, velocity, trajectory and timing. These variables come to a thrilling climax on Sunday evening as Curiosity reaches the Red Planet.
Launched into a trajectory around the sun in November 2011, Curiosity is currently in a solar orbit that just reaches the orbit of Mars. That trajectory means that, from the perspective of the sun, by noon Pacific time on August 1 Curiosity was travelling at 47,500 miles per hour. Yet Mars is travelling at more than 53,000 mph -- some 5,500 mph faster than Curiosity. Left alone, Curiosity would soon begin a slow cruise back towards the orbit of Earth, while Mars would carry on along its own, faster trajectory.
But breathtaking accuracy by the navigation team guiding Curiosity means that Mars will be at the right place Sunday to pick up Curiosity. The planet's gravity will speed up the spacecraft by 13,000 mph (as viewed from the sun) until their speeds match and Curiosity is safely on the surface. On the freeway of interplanetary navigation, Curiosity is the bug, and Mars is the windshield. To get ready for a martian year of exploration, you've got to take a big hit.
Welcome to the Landing Blog
Thursday, August 2, 2012 5:12:16 PM
Welcome to the Curiosity landing blog. I'm Doug Ellison, a visualization producer here at JPL. Our group is responsible for many of the graphics you will see that show how Curiosity has made its way to Mars, and what it will do when it gets there.
The landing animation was a nine-month-long project of obsessing over details of every piece of the spacecraft and its adventure. We've launched a special version of Eyes on the Solar System at http://eyes.nasa.gov that lets you ride with Curiosity all the way to the surface. We've become so familiar with the spacecraft and what it does that we even surprise the mission team themselves sometimes!
On landing night, I'll be in our mission control (the "Dark Room") keeping you up to date with some of the goings-on as Curiosity approaches Mars. Until then I'll post a few little factoids about Curiosity, its trip to Mars, and its epic landing at Gale Crater.
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.
On August 15, the craft will interrupt its ascent for four and a half days. By then, Dawn will be at an altitude of about 5,000 kilometers (3,100 miles), but it will still be in orbit. Before it resumes thrusting, it will coast to as high as 6,400 kilometers (4,000 miles) and then descend again. Meanwhile, four times during this period it will photograph the giant asteroid throughout a full Vestan day of 5 hours, 20 minutes. This is a familiar activity for the spacecraft, as it watched Vesta rotate beneath it from a similar vantage point during its spiral descent in July 2011. With Vesta's weak gravitational grip at this distance, Dawn would take more than a week to complete one revolution, so it will be almost as if the probe hovers in place as Vesta pirouettes before its camera. The itinerary is planned so the explorer will begin its observations while flying over the highest northern latitudes, and subsequently it will take the opportunity to observe lower latitudes as it sails down to the equator. The ship will circle so slowly that there will be time between acquiring each set of rotation images to point its main antenna to Earth to transmit its findings. After the third session, while waiting for the orbit to carry it to the latitude needed for the final one, mission planners are squeezing in a routine calibration of the camera and VIR. Dawn will turn to aim them at Jupiter. It is much too far away to reveal any new or interesting details, because the sensors are designed for mapping from close orbit. The planet will appear to be little more than a speck. (Terrestrial observers can gain a better view with binoculars.) But Jupiter is bright and easily seen from there, and it is so well studied that it is a useful reference source to verify that the instruments are still performing in top condition as they continue their discoveries at Vesta.
On August 22, nearly 6,000 kilometers (3,700 miles) over the night side, the probe will halt thrusting again. With the sun on the other side of the protoplanet, Dawn will see only a thin glowing crescent against the deep blackness of space, like a new moon. This is a perspective we have not yet had for Vesta, and although not much of the terrain will be visible, a few pictures to measure the strength of the sunlight's reflection at this extreme angle will be useful for understanding certain properties of the surface material. As a bonus, the view may prove to be quite aesthetically appealing.
Dawn will be patiently and gently thrusting at the moment of escape from Vesta on August 26 and will not even notice a change. It will be as serene and uneventful for the spacecraft (and operations team) as the moment of capture was. Shortly after, when it is around 17,000 kilometers (over 10,000 miles) away, it will watch Vesta rotate once again. On September 1, at a distance of 38,000 kilometers (almost 24,000 miles), it will gaze upon Vesta for the last time. By then, the world it has scrutinized for more than a year will be shrinking rapidly and few details will be visible. Although scientists will spend many years delving into the data the probe has returned, learning more and more not only about Vesta but also what it reveals about the dawn of the solar system, Dawn will leave it behind as it journeys deeper into the main asteroid belt in search of another uncharted world to explore.
Dawn is 740 kilometers (460 miles) from Vesta. It is also 2.94 AU (439 million kilometers or 273 million miles) from Earth, or 1,185 times as far as the moon and 2.89 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 49 minutes to make the round trip.
Dr. Marc D. Rayman
10:00 p.m. PDT July 25, 2012
For the past 18 months, NASA's Cassini spacecraft has been orbiting Saturn in practically the same plane as the one that slices through the planet's equator. Beginning with the Titan flyby on May 22, navigators started to tilt Cassini's orbit in order to obtain a different view of the Saturnian system. The measure of the spacecraft orbit's tilt relative to Saturn's equator is referred to as its inclination. The recent Titan flyby raised Cassini's inclination to nearly 16 degrees. Seven more Titan flybys will ultimately raise Cassini's inclination to nearly 62 degrees by April 2013. On Earth, an orbit with a 62-degree inclination would pass as far north as Alaska and, at its southernmost point, skirt the latitude containing the tip of the Antarctic Peninsula.
You may wonder why this change has been planned and how this feat is achieved. The "why" is to allow scientists to observe Saturn and the rings from different geometries in order to obtain a more comprehensive three-dimensional understanding of the Saturnian system. For instance, because Saturn's rings lie within Saturn's equatorial plane, they appear as a thin line when viewed by Cassini in a near-zero-degree orbit inclination. From higher inclinations, however, Cassini can view the broad expanse of the rings, making out details within individual ringlets and helping to unlock the secrets of ring origin and formation. Some of those images have already started to come in.
At higher inclinations, Cassini can also obtain excellent views of Saturn's poles, and measure Saturn's atmosphere at higher latitudes via occultation observations, where radio signals, sunlight or starlight received after passing through the atmosphere help to determine its composition and density.
The "how" is by using the gravity of Titan -- Saturn's largest moon by far -- to change the spacecraft's trajectory. Like the rings and Cassini's previous orbit, Titan revolves around Saturn within a plane very close to Saturn's equatorial plane. As Cassini flies past Titan, Titan's gravity bends the spacecraft's path by pulling it towards the moon's center -- similar to a ball bearing rolling on a smooth horizontal surface past a magnet. Near Titan, the motion is confined to a plane containing the spacecraft's path and Titan's center of mass. If this "local" plane coincides with Cassini's orbital plane about Saturn, the trajectory's inclination will remain unchanged. However, if this plane differs from Cassini's orbital plane about Saturn, then the bending from Titan's gravity will have a component out of Cassini's orbital plane with Saturn, and this will change the tilt of the spacecraft's orbit. Repeated Titan flybys will raise Cassini's orbit inclination to nearly 62 degrees by April of next year and then lower it back to the Saturn equatorial plane in March 2015.
Gravity assists are key to Cassini's ever-changing orbital geometries. Onboard propellant alone would quickly become depleted attempting to accomplish these same changes. A gravity assist can be characterized by the amount of "delta-v," or change in the velocity vector, it imparts to a spacecraft. Delta-v may of course also be imparted to the spacecraft via rocket engines and, either way, alters the spacecraft's orbit. The eight Titan gravity assists responsible for raising Cassini's inclination to 62 degrees will provide a delta-v of 15,000 mph (6.6 kilometers per second). For comparison, Cassini's rocket engines had only enough propellant after initially achieving orbit around Saturn to deliver about 2,700 mph (1.2 kilometers per second) of delta-v. That's 15,000 mph of capability spread over 11 months via gravity assists versus a modest 2,700 mph of capability spread over more than 13 years via rocket engines! Because delta-v is a vector, it may change both the speed and direction of Cassini at a point along its orbit, so the speed of Cassini is not changing by 15,000 mph, but mostly all of the directional changes sum to 15,000 mph. To give these values some context, Cassini's speed typically varies between as low as 2,500 mph (1.1 kilometers per second) and as high as 79,000 mph (35 kilometers per second) relative to Saturn between apokrone and perikrone, the farthest and closest points from Saturn along its orbit. Gravity assists from the initial prime mission Titan flyby in 2004 to the final Solstice Mission Titan flyby in 2017 will provide nearly 200,000 mph (90 kilometers per second) of delta-v, leveraging the onboard propellant by a ratio of 75 to 1. The bulk of the Saturn tour trajectory is shaped by gravity assists, while the role of onboard propellant is to fine-tune the trajectory.
At the end of year 2015, Cassini will again begin climbing out of Saturn's equatorial plane in preparation for its grand finale. After reaching an inclination of nearly 64 degrees, a Titan gravity assist in April 2017 will change Cassini's perikrone so that Cassini will pass through the narrow 2,000-mile (3,000-kilometer) gap between Saturn's atmosphere and innermost ring. Twenty-two spectacular orbits later, one final distant Titan gravity assist will alter Cassini's course for a fiery entry into Saturn's atmosphere to end the mission.
Dear Upside Dawn Readers,
Dawn is now seeing Vesta in a new light. Once again the probe is diligently mapping the ancient protoplanet it has been orbiting for nearly a year. Circling the alien world about twice a day, the ardent adventurer is observing the signatures of Vesta's tortured history, including the scars accumulated during more than 4.5 billion years in the main asteroid belt between Mars and Jupiter.
Having successfully completed its orbital raising maneuvers to ascend to its second high-altitude mapping orbit (HAMO2), Dawn looks down from about 680 kilometers (420 miles). This is the same height from which it mapped Vesta at the end of September and October 2011. The lifeless rocky landscape has not changed since then, but its appearance to the spacecraft's sensors has. The first high-altitude mapping orbit (HAMO1) was conducted shortly after southern hemisphere summer began on Vesta, so the sun was well south of the equator. That left the high northern latitudes in the deep darkness of winter night. With its slower progression around the sun than Earth, seasons on Vesta last correspondingly longer. Thanks to Dawn's capability to linger in orbit, rather than simply conduct a brief reconnaissance as it speeds by on its way to its next destination, the probe now can examine the surface with different lighting.
Much of the terrain that was hidden from the sun, and thus the camera, during HAMO1 is now illuminated. Even the scenery that was visible then is lit from a different angle now, so new observations will reveal many new details. In addition to the seasonal northward shift in the position of the sun, Dawn's orbit is oriented differently in HAMO2, as described last month, so that makes the opportunity for new insights and discoveries even greater.
The strategy for mapping Vesta is the same in HAMO2 now as it was in HAMO1. Dawn's orbital path takes it nearly over the north pole. (As we saw last month, the orbit does not go exactly over the poles but rather reaches to 86 degrees latitude. That slight difference is not important for this discussion.) During the ship's southward passage over the sunlit side, the camera and the visible and infrared mapping spectrometer (VIR) acquire their precious data. After passing (almost) above the south pole, Dawn sails north over the night side. Instead of pointing its sensors at the deep black of the ground below, the probe aims its main antenna to the extremely distant Earth and radios its findings to the exquisitely sensitive receivers of the Deep Space Network. The pattern repeats as the indefatigable spacecraft completes loop after loop after loop around the gigantic asteroid every 12.3 hours.
As Dawn revolves, Vesta rotates on its axis beneath it, turning once every 5.3 hours. Just as in HAMO1, mission planners artfully choreographed this celestial pas de deux so that over the course of 10 orbits, lasting just over five days, the camera would be able to view nearly all of the lit surface. A set of 10 orbits is known to Dawn team members (and to you, loyal readers) as a mapping cycle.
Until a few months ago, HAMO2 was planned to be four cycles. Thanks to the determination in April that Dawn could extend its residence at Vesta and still meet its 2015 appointment with dwarf planet Ceres, HAMO2 has been increased to six mapping cycles (plus even a little more, as we shall see below), promising a yet greater scientific return.
In cycle 1, which began on June 23, the camera was pointed at the surface directly underneath the spacecraft. The same view will be obtained in cycle 6. In cycles 2 through 5, images are acquired at other angles, providing different perspectives on the complex and dramatic landscape. Scientists combine the pictures to formulate topographical maps, revealing Vesta's full three-dimensional character from precipitous cliffs and towering peaks of enormous mountains to gently rolling plains and areas with mysterious ridges and grooves to vast troughs and craters punched deep into the crust. Knowing the elevations of the myriad features and the angles of slopes is essential to understanding the geological processes and forces that shaped this exotic mini-planet. In addition to the exceptional scientific value, the stereo imagery provides realistic, exciting views for anyone who wants to visualize this faraway world. If you have not traveled there yourself, be sure to visit the Image of the Day regularly and the video gallery occasionally to see what you and the rest of humankind had been missing during the two centuries of Vesta's appearance being only that of a faint, tiny blob in the night sky.
With 3-D movies and other familiar stereo pictures, only two angles are needed. That's sufficient to reproduce what our two eyes would perceive, but it does not tell the entire story. A left-right pair reveals nothing about the up-down dimension. Scientists chose the directions to point Dawn's camera that yield the best combinations of perspective and illumination to construct a complete contour map.
In cycle 2, the craft soars over the sunlit side with its camera pointed both ahead and to the left of the ground directly below. In cycle 3, the instrument will be targeted behind and slightly to the left. Cycle 4 will observe the surface farther back and to the right. Cycle 5 will look slightly ahead and to the right. Together these pictures will yield a fabulous sense of the detailed shape of Vesta, and combining them with the HAMO1 images will afford an extraordinarily comprehensive 3-D view.
The camera and VIR are mounted on the spacecraft so that they point in the same direction. During these six cycles, the direction is determined by what's needed for the topographic mapping, but VIR collects valuable spectra as well wherever it is aimed. A spectrum is a measure of the intensity of light at different wavelengths and is reminiscent of the rainbow you see when a glass prism or droplets of water separate white light into its constituent colors. The material on Vesta imprints its signature on the light it reflects from the sun, so VIR's measurements reveal the nature of the minerals. The sensor has already found that Vesta displays a highly varied composition, attesting to its complex geological history. VIR records light from ultraviolet through the entire visible range and into the infrared. Indeed, the instrument operates so far into the infrared that it can detect the meager heat emitted from the surface, thereby also functioning as a remote thermometer. Each VIR snapshot consists of the spectrum at 256 locations on the surface, providing a great richness of information.
Compared to the camera, VIR trades greater spectral coverage for smaller spatial coverage. VIR was the prime instrument in survey orbit, where it was high enough that even with its narrow view, it could observe most of the surface. At the lower altitude of HAMO1 and HAMO2, VIR cannot map all of Vesta in a single mapping cycle or even in six cycles. (And even with all the bonus data it collected during months of operation in the low-altitude mapping orbit (LAMO), the proximity to the surface allowed it to obtain excellent close-up views but only of small regions.) HAMO1 was so outstandingly productive that VIR did see much of the surface, and now the coverage is being increased significantly with HAMO2.
Because the mission has been going so well, mission planners decided to devote some extra time in HAMO2 to additional VIR measurements. From June 15 through 23, before the six mapping cycles commenced, VIR was the star of the celestial show again. Every orbit was dedicated exclusively to collecting as many spectra as could be transmitted to Earth. The telecommunications link that stretches across the solar system is very limited. By not splitting it between the camera's images and VIR's spectra, controllers could maximize the latter's coverage of Vesta.
Dawn's exceedingly productive exploration may make its accomplishments appear easy, but as with all such undertakings, the success is enabled by a group of people applying their collective expertise, discipline, creativity, and powerful drive to reveal the unknown. It is thanks to their extraordinary investment of time and energy that the distant probe is able to execute such an ambitious mission, unveiling an ancient world that previously had only been glimpsed from afar by telescopes.
When the previous log was unleashed upon readers of all dawnominations, Dawn was partway through its long spiral route from LAMO to HAMO2. (You can see the weekly progress in altitude by checking the May mission status reports.) Complex and challenging though it was, the flight went precisely as intended. Because maneuvering the spacecraft exactly to its targeted destination is so difficult, mission planners had scheduled a window to fine tune the orbit on June 9 and 10 after the main phase of ion thrusting was complete. This is very similar to the trajectory correction maneuvers planned before the swing past Mars. Nevertheless, upon carefully measuring the actual orbit following the end of thrusting on June 4, navigators determined that it was so good that no adjustments were needed.
Before the resumption of Vesta observations on June 15, engineers reversed some reconfigurations of the spacecraft they had made for operation at lower altitude. They also took advantage of the time to perform a routine verification of the health of the back-up camera, ensuring that it remained ready to take over if the primary camera encountered problems. Both instruments are in excellent condition.
As Dawn continues tirelessly to scrutinize Vesta and report its fascinating findings, the mission control team is putting the finishing touches on the plans for its departure. On July 25, the ship will begin climbing out of HAMO2, its sights set on Ceres. Just as during the approach phase, however, it will pause occasionally for some additional observations. As Vesta grows farther and smaller but sunlight touches more of the high northern latitudes, the instruments will take some parting shots. We will describe those plans in the next log. As we shall see, even as Dawn says goodbye to its companion of more than a year deep in the main asteroid belt, it will continue to discover new secrets to thrill and delight all the passionately curious and bold creatures who champion the eager explorer on its interplanetary voyage. Through this robot, they are transported far, far into space to behold sights and gain knowledge that otherwise would remain forever beyond their reach.
Dawn is 680 kilometers (420 miles) from Vesta. It is also 3.17 AU (474 million kilometers or 294 million miles) from Earth, or 1305 times as far as the moon and 3.12 times as far as the sun today. Radio signals, traveling at the universal limit of the speed of light, take 53 minutes to make the round trip.
Dr. Marc D. Rayman
10:30 p.m. PDT June 30, 2012