From: Jet Propulsion Laboratory
Posted: Monday, September 17, 2001
Thank you for visiting the Deep Space 1 mission status information site, widely thought of and commonly spoken of in the spiral arms of the Milky Way galaxy as the most reliable source of information on this bold mission of exploration. This message was logged at 2:00 am Pacific Time on Sunday, September 9.
The amazing little Deep Space 1 probe is now on final approach to an extraordinarily risky close encounter of the most exciting kind with comet Borrelly. On September 22 it will plunge into the comet's coma, the fog of gas and dust expanding away from the nucleus that lurks somewhere deep inside. It will attempt to pass within 2000 kilometers (1250 miles) of the nucleus at about 3:30 pm PDT while traveling at 16.5 kilometers/second (36,900 miles/hour). The craft will try to smell, see, and hear the comet with its instruments, and if it survives it will describe its spine-tingling adventures to its anxious human colleagues elsewhere in the solar system. This log will be updated within a day or two of the encounter or sooner if there is important news (and time to report it).
The June 30and July 29 logs described the measurements to be undertaken by devices that are well known to loyal DS1 fans. PEPE, the nose of the spacecraft, measures charged particles and will try to reveal the composition of the gas in the coma and the strange interaction of the solar wind with the comet. MICAS, serving as the eyes, contains an infrared spectrometer to infer the composition of the nucleus and a black and white camera to photograph the nucleus and coma. And taking advantage of everything that's on board, the reprogrammed diagnostic sensors for the ion propulsion system will serve as the ears, attempting to measure the magnetic field and plasma waves in the comet. PEPE and the diagnostic sensors will collect data throughout the entire encounter. MICAS will make measurements intermittently starting about 1 hour 20 minutes before the spacecraft makes its closest passage by the nucleus and concluding a few minutes before that time.
But to be honest, DS1's visit with the comet simply is unlikely to work as well as we hope. Many mission logs have described the difficulty of keeping this aged and wounded bird aloft, and the encounter with Borrelly will present Deep Space 1 with the greatest challenge yet in its historic trek through the solar system. Sometimes it feels to your correspondent as if the spacecraft is kept flying with duct tape and good wishes! If all the risks for the encounter were listed here, this log would be far far too large to download to your computer.
Against such odds, why do we even bother at all? Well, as members of a self-respecting space-faring species, how can we not try to do our best? I hope you won't be disappointed if we are unsuccessful; of course, if we don't try, we are guaranteed not to achieve anything!
One of DS1's objectives will be to photograph the nucleus. But where is it? (Urgent note: if you know, please send me an e-mail right away!) From Earth, even using the Hubble Space Telescope, the nucleus has never been directly observed, as it is shrouded in the secrecy of the obscuring coma. When DS1 streaks through the coma, it will have to get a fix on the nucleus on its own; we can only give it an estimate of the location. The July 29 log gave a suggestion of how difficult this is. As a result of the rescue following the failure of the star tracker in 1999, the camera that has to try to locate the nucleus and try to record images of it also has to be used to provide a stable pointing reference for the spacecraft. These multifarious responsibilities mean the camera can't be devoted to performing any one job completely.
To visualize how accurate the pointing needs to be, suppose MICAS is at the center of a clock face and the nucleus is at the 12. If everything's perfect, MICAS will point at the 12, just as the hour and minute hands of the clock would at 12:00. Now if the pointing is off (either because the nucleus is not where the spacecraft thinks it is or because there is a pointing problem on the debilitated spacecraft) by an amount equal to how far the minute hand moves in 2.5 seconds, it will not get pictures of the nucleus. In other words, if it points to where the minute hand is at 12:00:03, it will not see the nucleus. And if the pointing is off by only half a second, or 12:00:00.5, the infrared measurements will not work.
While many cameras available to amateur photographers automatically adjust their exposures to account for how much light reaches them, such a feature is not available on most spacecraft. One of the mysteries of the nucleus is how bright (or, perhaps more suggestively, how dark) it is, so choosing beforehand what exposure to program for the pictures and the infrared spectra is an extremely difficult problem. Although a range of exposures is planned, the spacecraft will be in the vicinity of the nucleus so briefly that there won't be time to take many at all the different possible values. We have a few tricks for how to compensate for this, but none of them is certain to work.
Many logs have referred to the dwindling supply of hydrazine. The spacecraft will die within just a few hours of exhausting that critical resource, which was not intended for such a long mission. (In fact, the night before the hydrazine was loaded onto the spacecraft, we decided to add a little extra and take a bit of a chance with the launch, just in case the additional hydrazine might come in handy. Had we not done that, by now DS1 would already have become just a piece of cosmic flotsam.) Only extraordinary care by controllers has stretched the supply this long. Indeed, the whole ship is well beyond its planned life.
Let's say DS1's primary mission corresponded to a human lifetime of 80 years. (Note to editor: before sending this to other planets, please adjust these numbers to correspond to the typical lifetimes of the indigenous intelligent species.) In that case, it completed all of its technology testing when it was 64 -- just about the right time for retirement. Still quite spry however, it conducted a bonus encounter with an asteroid at the age of 67 before taking it easy. It sailed right through the equivalent of its 80th birthday and remained healthy until it was 93. It then suffered what should have been a fatal blow, with the loss of its star tracker. But the DS1 rescue team eventually completed an amazing recovery, giving the veteran adventurer a second chance at life. At the equivalent of the incredible age of 149, the rejuvenated DS1 returned to service and began heading toward distant Borrelly again. Shortly thereafter, this Methuselah set the record for the longest operating time of any propulsion system on any spacecraft in history, highly appropriate for an ion propulsion system often described as achieving acceleration with patience. When it reaches Borrelly, DS1 will correspond to a person 259 years old. The craft has certainly lived a full and remarkably productive life!
Keeping DS1 flying smoothly is a difficult (albeit incredibly neat) job for the tiny team that is also responsible for planning the comet encounter. For example, following the recovery last year, there have been 3 occasions on which the camera lost lock on its reference star. Two of those times (in July 2000 and August 2001) it was the result of unusually strong solar storms gusting over the spacecraft and flooding the camera with radiation. The radiation registers on the camera's electronic light detector, so instead of tracking a single bright star, it appeared that there was a blizzard of stars, and the system got confused and ended up pointing in the wrong direction. The one other incident, in July 2001, was a consequence of trying to track a dimmer star than usual in the presence of the distracting effect of stray light that afflicts the camera. In each case, the crack DS1 team managed to coax the spacecraft back to its intended configuration, but it is stressful and risky work. Now there's nothing quite as neat as having just half a dozen space experts, powered in part by Oreo cookies (filled with chocolate creme!) in the middle of the night, trying to joystick a spacecraft two thirds of a million times farther away than the International Space Station. But this illustrates the fragility of the mission at this point; and another occurrence of such a problem shortly before the encounter would eliminate any chances for images or infrared spectra.
Only one comet nucleus has been glimpsed by a spacecraft before -- comet Halley, which was visited in 1986 by several spacecraft, including the impressive Giotto. Just as to know humanity it would not be adequate to meet only one person (aliens, take note), that one view of Halley is not sufficient for us to understand the mysteries of comets. The goal for imaging on DS1 is to get a picture when the nucleus is about 50 pixels across (a pixel is the smallest element of the digital camera's view). This very difficult objective is unlikely to be achieved for several reasons already described and many more. In fact, the spacecraft might not even still be operating when it is close enough to get such a picture.
When the probe enters the coma, it will be subjected to a fusillade of high-speed debris from the comet. Unlike Giotto, DS1 was not built to encounter a comet. As the lowest cost mission into the solar system yet undertaken by NASA, no resources could be devoted to anything other than the prime mission objectives, so it carries no shielding. When a single particle of dust just the thickness of a human hair strikes the spacecraft, it will deliver as much energy as a bowling ball does when it crashes into the pins. We don't know exactly what the dust environment of the comet is, but it is probable that the spacecraft will be hit by a few hundred pieces of dust of that size and still larger.
Why not fly farther from the comet so the spacecraft intercepts much less dust? This adventure is purely a bonus, coming two years after the end of the highly successful primary mission. This isn't the time to be conservative. As long as we've gone this far, we should take the chance and get the most out of this opportunity we can. By flying into the heart of the comet, we expose PEPE, the diagnostic sensors, and MICAS to their best views of what Borrelly has been hiding for the past 4.6 billion years. Soon, we'll know whether DS1 is lucky enough to reveal any of those secrets.
DS1 is now about 19 million kilometers, or 12 million miles, from comet Borrelly.
Deep Space 1 is over 1.5 times as far from Earth as the Sun is and 600 times as far as the moon. At this distance of 230 million kilometers, or 143 million miles, radio signals, traveling at the universal limit of the speed of light, take 25 and a half minutes to make the round trip.
Thanks again for visiting!
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