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Comet Borrelly: The Data So Far

Deep Space 1 is the lowest cost interplanetary mission NASA has ever conducted
by Marc D. Rayman
DS1 Mission Manager
Pasadena - Nov 13, 2001
After introducing another member of the solar system family to Earth, Deep Space 1, the little spacecraft that could -- and did! -- continues flying contentedly in its orbit around the Sun. Meanwhile, scientists are analyzing the fantastically rich harvest of data returned from the historic encounter with comet Borrelly. More than two years after the end of its 11-month primary mission, on September 22 DS1 stepped up to its greatest challenge of all with the elegance and skill of a true master. The encounter certainly did not go the way I expected -- instead, everything went perfectly!

The scientific analysis of the visible images, infrared spectra, ion and electron energy and angle spectra, ion composition measurements, magnetic field measurements, and plasma wave burst data will go on for quite some time. After just the initial impressions of a subset of these data were described in a press conference a few days after the encounter, the real analysis began. DS1's images are the only ones in existence that are detailed enough to allow geological analysis of the nucleus of a comet. These images are still being processed to bring out additional details not discernible in the raw images releases so far.

It literally will take years to mine everything from these data, but preliminary results will be announced in press releases at the end of November. They will contain some fascinating and exciting news, but in order not to steal any of the impending thunder, let's focus instead on what happened in the time between that last two logs (available as a special discounted gift set at fine establishments throughout the halo of the Milky Way), as the spacecraft closed in on its quarry. Following that admittedly somewhat dry material, we'll turn back to more of the human experience. As all corporeal readers know, DS1 has had to thrust with its ion propulsion system even when we did not want it to change its course. This served to reduce the consumption of its conventional chemical propellant. As a result, DS1 thrusted at impulse power back and forth for many months, tacking its way to Borrelly.

The push of the ion engine, as delicate as it is, adds up to problems for the navigators who try to predict DS1's course with the accuracy needed for the encounter. (During its normal interplanetary travels, the subtle uncertainties that arise from the continuous acceleration are too small to be of concern.)

As planned for many months, DS1 stopped firing its ion engine on September 15 and coasted most of the rest of the way to the comet. Following almost 15 months of being in powered flight nearly 100% of the time, the ship was silently drawing near its destination.

A total of 11 times from August 25 until about 10 hours before the closest approach on September 22 (still at a range of over 600,000 kilometers (around 375,000 miles), or more than 1.5 times the distance between Earth and the moon), DS1 took images of where it expected Borrelly to be. These distant views were used to improve estimates of the location of the comet.

Although it has been observed many times from Earth since its discovery in 1904, as with all astronomical bodies there are significant limitations in astronomers' ability to pin down the orbit. But by combining data from Earth-based observations of Borrelly with DS1's views of the comet as they raced toward their eagerly awaited appointment, it was possible to get a better estimate of the comet's location.

These observations were complex, but, to our great relief, all of them worked just the way they were supposed to. At first, the comet was so distant that many images had to be electronically combined for the comet even to be detected. What appeared in the images was the coma (the vast cloud that cloaks the nucleus in gas and dust) and the tail.

As DS1 closed in on Borrelly, these images were used by navigators to compute course corrections. Before routine thrusting stopped on September 15, the trajectory was altered by changing the planned direction and throttle level of the ion drive. After September 15, the corrections were accomplished by firing the engine only at specially selected times and throttle levels.

Nearly every activity on the spacecraft represents an opportunity for a problem to arise. As just two examples, when the probe turns, it might have trouble locking to a new reference star, or any commands sent from Earth might contain an error. (The extremely small mission control team, despite the excellence shown during three years of complex and successful flight, could -- gasp! -- make a mistake.) The closer the spacecraft was to the comet, the more important it was to avoid actions that, if they did not go as planned, could compromise the encounter. Less time to recover from problems meant it was more important than ever to avoid them.

We devised a clever strategy over the summer that made it likely (but not guaranteed) that most course corrections would require the spacecraft to thrust while it was in nearly the same orientation needed for communicating with Earth. In fact, this worked so well that all but one of the course corrections required thrusting in exactly that orientation.

That meant that the spacecraft did not have to execute extra turns and did not have to expend extra hydrazine. When it came time to modify the trajectory, we simply told the spacecraft the duration and power level, and it dutifully and calmly executed the thrusting as needed. We could communicate with it at the same time, uploading more of the many files it would need for the encounter and monitoring its health to be sure no problems were brewing that might interfere with its chances of collecting at least some of the data we sought at Borrelly.

One of the reasons that so much effort had been devoted to saving hydrazine was that during the last day before the encounter, we expected not to be able to use the ion engine for course corrections. The ion engine delivers what I've often described as "acceleration with patience," but with only hours before closest approach, patience was not a virtue: the spacecraft needed to point its antenna to Earth most of the time.

But our strategy paid off handsomely, and we were able to make the final corrections by firing the engine with no turns at all. As a result, we did not have to use any extra hydrazine, and no time was lost in using the reliable and efficient, if leisurely, ion engine.

A few days before the encounter, our old friend PEPE was activated, its software loaded, and its operation verified. Its job would be to try to measure the composition, energies, and directions of the charged particles in the coma as well as how the comet and the solar wind affect each other.

Myriad other preparations were conducted in the remaining few days, including finalizing plans for what to do in a variety of unplanned circumstances too nerve-wracking to try to recall if I want to sleep well tonight. In brief, however, we filled up the available time working as hard as we could to give the spacecraft its best chances for success.

Everything went surprisingly smoothly in the days leading up to Saturday, September 22. I had had myriad concerns before the encounter. Even casual readers of these logs know that I did not have high confidence in this daring undertaking, and many logs over the past few months have described different aspects of the risk.

As I ate breakfast well before sunrise on Saturday, one of my fears was that everything would continue to go well but for one mistake, one oversight, one simple little thing that we should have done or not done. The encounter would be conducted with 685 stored instructions, containing nearly 4000 parameters, relying on complex software that had been used for tests but never for visiting a comet.

That represented an extraordinary number of opportunities for a mistake. I prepared myself for coming home dejected that night, expecting to scold myself for missing that one lurking error. It would be no worse that the entire encounter going poorly, but just one simple error would make it easier to devote excessive energy to repeating "If only..." for years to come.

It was an almost eerily calm day in DS1's mission control room, as the spacecraft continued to be well behaved. Most members of the team had little to do but make sure the spacecraft was healthy. We did have several critical decisions to make, based on plans we had worked out carefully during the preceding year.

When the final pre-encounter images were obtained, we analyzed them and our earlier ones with a mathematical model of how the brightness of the scene formed by the combination of the vast coma plus the tiny nucleus should change as the distance to the comet diminished. This allowed us to make our selection for the camera exposure times and to formulate our final estimate of where the spacecraft should begin looking for the nucleus.

Some team members suggested we make other, unplanned changes, but that is always dangerous. It is easy for late anxiety (augmented with the burritos we had for lunch) to foster new ideas that, in the absence of calm reflection, may seem meritorious.

But making changes to such an intricate plan in the final hours requires very careful consideration, and, by definition, there probably is not time for that. We did discuss some ideas but opted instead to trust the more considered judgment we had exercised earlier and stayed the course. Shortly before 1:30 pm PDT, signals confirmed the spacecraft had begun turning. The main antenna would not point to Earth again until after the encounter (if the spacecraft survived), and we had only very limited signals with which to infer its progress.

The spacecraft had a tremendously complicated plan to follow, including locking to a reference star for a while, then trying to obtain some views of the nucleus while still more than 85,000 kilometers (53,000 miles) away, then trying to point its very narrow-view infrared spectrometer at the nucleus. Next it had to lock to another reference star in a special location that would provide it information it would need later in the final encounter.

Finally, about 35 minutes before its closest passage by the nucleus, still 35,000 kilometers (22,000 miles) away, it turned to the 8-kilometer (5-mile) long nucleus for the final time. It began taking two images per minute in order to try to find it and lock on so it could track the mysterious core of the comet.

The elaborate choreography continued with many changes in spacecraft modes and constant measurements by PEPE and by the reprogrammed ion engine sensors, smelling and hearing phenomena in the coma as the camera tried to record the sights.

The indications we had on Earth were that everything was going well, but that could have been deceptive. If the signals indicated the spacecraft were having problems, we could have trusted that. But its belief that it was tracking the nucleus was not proof that indeed it was; there were many ways it could fail and not realize it.

Still, it was reassuring that no problems were evident. We later determined that of the 53 pictures the spacecraft took, it managed to identify the nucleus in 52 of them. In one of the pictures, a cosmic ray that struck the electronic detector in the camera left a track that got most of the way through the various software guards meant to eliminate spurious signals.

Although it prevented the system from finding the nucleus in that one image, it did not disrupt or confuse the attempt to track the nucleus; rather, the software ultimately discarded the picture, refusing to be fooled by the deceptive information it contained.

There was tentative applause at JPL when signals showed that the spacecraft had traveled half-way through the coma, completing its closest approach to the nucleus. But it still had to survive its trip back out of the coma, with potentially fatal dust impacts and more complex maneuvers. Finally the spacecraft turned to point its main antenna back to Earth, and we waited like expectant children listening as a masterful story teller begins to unfold a tale of daring, mystery, and adventure.

As the spacecraft regaled us with its spine-tingling exploits, we gathered around a few of the monitors on which the pictures would first be displayed. We already had good reason to believe that the tremendously important PEPE and ion propulsion system diagnostics sensor data had been acquired. They would reveal much of great interest about the comet.

But, regardless of our technical or scientific interests, the roughly 100% human controllers are visual creatures, and we frankly hoped for a cool picture. Our goal had been to get a picture in which the nucleus spanned 50 pixels (a pixel is the smallest element of the digital camera's view).

This would be of great scientific value and would be good enough to give us a feel for what this completely unknown body looked like. The images were returned in a special order, but not in the order in which they were taken.

Still, the first images we saw had been taken from so far away that the nucleus was still small, and the scene was dominated by a powerful jet of dust. These showed that the comet was an unfamiliar and strange place indeed.

Why didn't the dust destroy the spacecraft
It appears that our prediction of a few hundred dust impacts, based on analysis of Earth-based images of Borrelly, was tricked by this powerful jet. Earth was too distant for the jet to show up; all that could be inferred was the total amount of dust in the vicinity of the comet. But by being concentrated in a jet that DS1 did not fly through, it left other regions less dangerous. While we know DS1 was hit, it did not experience enough blows to suffer damage. Indeed, the only effect of the encounter we have been able to identify is the appearance on the spacecraft now of a big grin!

After a few additional distant images were returned, the first image at appeared on the monitors at about 5:30 pm, and the real celebration of the Borrelly encounter began.

The spacecraft had managed to track the nucleus better than we had hoped and took a picture when it was close enough that the body was about 170 pixels across -- more than 3 times better than our goal. The purity of the human joy that I shared with my colleagues there, as cheers and applause erupted in mission control, is something I will never forget.

On behalf of our curious and noble species, we beheld the first detailed views not only of a place, but of a kind of place, never seen before. Our spirits soared to heights unachievable even with ion propulsion! Your ever-devoted correspondent, normally of at least average eloquence, found himself unable to say little more than "I just can't believe how incredibly cool this is" every 30 seconds for the next few hours.

In the two years following the end of the primary mission, we had made many thousands of difficult decisions, particularly, but by no means exclusively, because of the failure of the craft's star tracker.

With the very small budget for Deep Space 1 (indeed, it is the lowest cost interplanetary mission NASA has ever conducted), many times we simply did not have the resources to analyze problems in as much detail as we might have liked. With a small team and a very complex mission, too often we found ourselves having to choose which problems we would penetrate.

For the others, it generally became necessary to go with our best estimate through a combination of specific and limited technical information and a strong dose of human judgment. But what if we had made a wrong choice in which areas to focus our greatest attention, or what if the less well considered decisions proved to be wrong in an important way? Well, in that case, I wouldn't be writing about the jubilation that followed a truly flawless encounter.

Coming at a time when so many of us were witness to the most shocking human actions and were forced to confront our greatest fears, we felt that we were taking humanity's highest ideals to its greatest reaches. More than just an incremental step, in that one day we made an astronomical jump forward in our cosmic view. While our grand news may have been largely lost in the midst of these other terrestrial events, we were proud to accomplish something beautiful, surprising, and inspiring on behalf of everyone who has ever wondered about the universe.

Deep Space 1 completed its primary mission in 1999 and its extended mission this autumn. So what is left? The hyperextended mission, of course. Beginning in October, our attention shifted to retesting many of the technologies that were the reason DS1 was built and launched.

Nine of the 12 technologies on board are hardware (three were autonomous software systems), and each is being exercised more during this phase of the flight, as we return DS1 to its roots.

With a mission that was intended to last 11 months, the opportunity to test these systems after three years in space (celebrated a few weeks ago with a yummy cake displaying the gleeful proclamation "3 sweet years!"), with greater exposure to radiation and other hazards of the space environment, many large swings in temperature, and other possible sources of wear, this is an opportunity to add still more to our understanding of these systems that are important for reducing the cost and risk of ambitious space and Earth science missions of the future.

The focus of the hyperextended mission is on the ion propulsion system, and we are performing many tests to quantify the effects of its having operated for so long. We are also testing it in various modes that would have been too risky or otherwise inappropriate earlier in the mission.

This amazing system has provided the equivalent of about 4.2 kilometers/second (9400 miles/hour) to the spacecraft, while consuming less than 70 kilograms (157 pounds) of xenon propellant. The system has accumulated more than 640 days of thrust time. (The requirement for "minimum mission success" for DS1 included operating the ion propulsion system for 200 hours.

We have only exceeded that by a factor of 77; but don't despair, still more hours of operation are ahead.) The results of these tests will represent still greater bonus from the mission as it continues blazing trails in space exploration.

The hyperextended mission will conclude in December, and the next log will describe what fate awaits the aged, wounded, intrepid, and very very happy explorer.

DS1 is now over 68 million kilometers, or 42 million miles, from its new friend comet Borrelly. As they continue on their separate ways, we can be sure each will retain a fond memory of their brief meeting, a special moment of shared discovery in their very different solar system journeys.

Meanwhile, DS1 and Earth are continuing to get closer in their individual orbits. Since launch in October 1998, DS1 has completed two orbits of the Sun while Earth has completed three. By lapping the craft, Earth is now catching up again. (See the June 30, 2001 log for more on DS1's orbit.) Deep Space 1 is almost 1.2 times as far from Earth as the Sun is and over 460 times as far as the moon. At this distance of 177 million kilometers, or 110 million miles, radio signals, traveling at the universal limit of the speed of light, take over 19 and a half minutes to make the round trip.

Related Links
Collection of Comet Borrelly Images
Deep Space 1
Deep Space 1 Post Comet Webcast
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NASA Spacecraft Captures Best-Ever View of Comet's Core
Pasadena - September 25, 2001
In a risky flyby, NASA's ailing Deep Space 1 spacecraft successfully navigated past a comet, giving researchers the best look ever inside the glowing core of icy dust and gas. The space probe's close encounter with comet Borrelly provided the best-resolution pictures of the comet to date. The already-successful Deep Space 1, without protection from the little-known comet environment, whizzed by just 2,200 kilometers (1,400 miles) from the rocky, icy nucleus of the 10-kilometer-long (more than 6-mile-long) comet.

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