While this is a great bonus opportunity to try to gather some unique and wondrous information about comets, it is also a very very challenging and risky undertaking.

But with a marvelously successful primary mission to its credit as well as a remarkably exciting and rewarding extension, the bold challenge of the comet encounter is a worthwhile adventure. Comets are believed to be remnants from the formation of the solar system, and studying them may shed light on the origin and evolution of our solar system and perhaps even on the evolution of Earth.

With its motto of "If it isn't impossible, it isn't worth doing" always in mind, the very small Deep Space 1 team has been preparing for the event.

The measurements DS1 will attempt at the comet will be described in detail in future logs. In brief, however, the probe will attempt to fly through the coma, the cloud of gas and dust surrounding the nucleus, and measure its composition.

Then as it closes in to near the center of the coma, it will be faced with its greatest challenge -- to obtain pictures and infrared spectra of the diminutive nucleus, invisible from Earth because of its size and the obscuration by the coma.

The craft will have to locate the nucleus on its own and point the camera at it as it streaks by. That would be difficult enough, given that we can't tell DS1 exactly where the nucleus is nor what it will look like. But the little robot's assignment will be still more challenging because in the absence of its star tracker, which failed in November 1999, it normally has to stay locked to a reference star to remain stable.

It can't point its camera at a star while it is trying to find and photograph the nucleus, so it will have to rely on its gyros, which provide approximate measurements of the spacecraft's turns. These gyros however were not meant for such a job, and they are not accurate enough to provide a stable platform throughout the encounter period.

To get an inkling of just one facet of the problem, suppose someone were holding a pair of high-power binoculars for you while you tried to look through them. Her hands would not be perfectly steady, and you would have a hard time seeing what you wanted. In fact, unless you told her how to position the binoculars, she might even move them around enough that the object of interest would completely leave your field of view.

DS1 is faced with a similar situation, with the binoculars being like the camera, and the gyros being the assistant's hands. But now if you could tell your friend how to move the binoculars ("a little to the right, now lower them -- no, that's too much") you might be able to guide her well enough for you to get a good view.

Some of the new software that was installed in DS1 in March is designed to analyze the pictures, look for what might be the nucleus, and decide how to move the spacecraft to keep it in the camera's sights.

During the spacecraft's encounter with the comet, it will rely on the software and an extremely complex set of carefully timed commands to execute the myriad steps necessary to collect its measurements. But how do we test all of this?

Of course, we have ground-based simulators of the spacecraft, but they are of only limited fidelity. So to make sure we are on the right track in developing the commands that will give the probe its best chance to point its camera at the comet as it closes in on it, the DS1 control team conducted some clever experiments with the spacecraft on May 1 and May 8.

Such tests involve some risk and a great deal of work to prepare and execute. The very long hours of hard (but, frankly, incredibly cool!) work by the team keep paying off however. In addition, because the Deep Space 1 project's resources are quite limited, the team's careful decisions in how it deals with risky undertakings have been an important ingredient in the success of such difficult operations.

After much planning, on May 1 DS1 took advantage of a coincidental alignment of itself with two planets to conduct a valuable test of the new software. On that date, when DS1 pointed its main antenna to distant Earth, its camera ended up pointing to still-more-distant Jupiter.

With controllers thus able to monitor data (of course delayed by the long wait for signals to travel from the probe to the second floor of JPL's Space Flight Operations Facility on Earth), DS1 used this new software to keep Jupiter in the view of its camera for the duration of the test -- over 2 hours. This provided the spacecraft with a rare opportunity to try to track a target other than a star, which would have appeared only as a pinpoint.

Jupiter is around 30,000 times larger than the nucleus of the comet (whose actual size is very poorly known) DS1 will meet in September. So although it was over 820 million kilometers (510 million miles) from the craft, the planet, the largest in our solar system, looked to DS1 about the same size that the comet will appear when DS1 is on its final approach, only about half an hour before the moment of closest encounter.

This also illustrates part of the difficulty of the encounter -- this comet nucleus is going to be very tiny and thus difficult to locate! The software successfully detected Jupiter (appearing as just a little fuzzy ball) in the picture frame and correctly computed compensations for the gyros to hold Jupiter in about the right spot.

Jupiter was so far away that its position did not vary during the test, but when the spacecraft gets to the vicinity of comet Borrelly, it will have to keep turning to keep its camera pointed at the moving target. In addition, it will execute many other commands to control its scientific instruments, to move and record data in its computer system, to set various operating modes of the spacecraft systems, etc.

To rehearse all of that, on May 8 DS1 executed a practice encounter with comet Spoof.

This comet exists only in the virtual universe of software (as well as, of course, the hearts and minds of the mission operations team), but DS1 did not know the difference (and don't tell the impressionable probe!).

It dutifully followed the sequence of commands, all the while recording its own performance for later analysis by engineers. Each time it took a picture, the computer file containing the image was intercepted by a special routine on board that "painted" a comet nucleus on it.

The software determined how big Spoof should be at that point in the encounter, and how much of the portion visible to the spacecraft would be illuminated by the Sun. The image file was subsequently sent back on its electronic way, and nothing else on board knew that the nucleus in the picture was synthetic.

The spacecraft then processed each of these pictures and exercised the systems that will be used to try to follow the nucleus during the encounter. By using the actual camera on the actual spacecraft, the test included such phenomena as unwanted stray light, camera flaws, and cosmic rays (which can show up in some pictures and confuse the software); this made the rehearsal much more realistic.

The test proved very successful, giving the DS1 team important information on the detailed performance of the spacecraft using the software and the commands that have been formulated thus far.

This will be important in helping guide our work in designing the comet encounter, as we now have a new comparison of the operation of the genuine spacecraft with that of the Earth-based simulator. An encore performance rehearsal will take place near the end of June.

The Sun, now at the peak of its 11-year cycle of activity, is spewing forth much more radiation than usual. Any readers in the vicinity of Earth are protected from this by our planet's vast magnetic field, and those near the surface have the extra protection of the thick (and mostly breathable) atmosphere.

Those of you on several of our solar system's planets may still be treated to some lovely auroras these days triggered by the solar activity, and observers who are very careful can see Sun spots, some large enough to be visible without magnification.

But lonely DS1 does not have a planet's magnetic field or atmosphere to shield it from the buffeting of the raging storms on the Sun. Nevertheless, much to the relief of the busy and fatigued operations team, it is managing to fly smoothly and happily; solar radiation does not appear to be causing problems.

As DS1 continues its flight, the thrusting with the ion propulsion system has passed several milestones. On March 21, DS1 had accumulated 10,000 hours of thrusting. This number is not inherently special, but it certainly does illustrate the system's fantastic longevity.

On May 1 DS1 had completed enough firing of its ion engine to coast to the comet -- we're on target! But as several mission logs have described, the spacecraft is so low on its supply of the conventional rocket fuel known as hydrazine that it must keep the ion engine thrusting at a low throttle level to control its orientation in space. So it will remain at "impulse power" for most of the time until shortly before the spacecraft reaches Borrelly.

DS1 is now about 157 million kilometers, or 97 million miles, from comet Borrelly.

Deep Space 1 is 1.9 times as far from Earth as the Sun is and more than 750 times as far as the moon. At this distance of 290 million kilometers, or 180 million miles, radio signals, traveling at the universal limit of the speed of light, take over 32 minutes to make the round trip.

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