From: Jet Propulsion Laboratory
Posted: Monday, August 6, 2001
Dr. Marc Rayman's Deep Space 1 Mission Log Mission Update:
Thank you for visiting the Deep Space 1 mission status information site, for more than 1000 days the most popular site on any habitable planet in or near the plane of the Milky Way galaxy for information on this daring mission of discovery. This message was logged at 10:30 pm Pacific Time on Sunday, July 29.
As Deep Space 1 continues its silent and calm flight through the solar system, its terrestrial colleagues continue to plan a risky adventure for the probe. DS1's assignment: to peek into a time capsule called comet Borrelly on September 22. The aged and debilitated spacecraft will face myriad challenges as it attempts to bring its sensors to bear on this relic from the formation of the solar system.
The comet may hold fascinating clues to the distant past. What was the composition of the materials from which the Sun and planets formed? What were the conditions under which the materials coalesced? The answers to these and similar questions have been erased on Earth and most other large solar system bodies, which have changed a great deal in the nearly 4.6 billion years since their formation. But comets may guide scientists who are trying to piece together the puzzle of our cosmic origins.
Last month's log described how PEPE and the reprogrammed ion propulsion system diagnostic sensors will help to unravel some of Borrelly's mysteries, including a direct sampling of the materials in the coma, the expanding cloud of gas and dust surrounding the diminutive nucleus. In addition to those instruments, DS1 carries a black and white camera and an infrared spectrometer, both contained in an innovative device known as the miniature integrated camera/spectrometer (or "MICAS" to the less patient). All of these instruments were included on the flight to contribute to the testing of advanced technologies for future science missions. Following the conclusion of that highly successful primary mission in September 1999, the craft embarked on an ambitious bonus mission, the end of which is now only about two months away, and these instruments will now play a key role in this final drama.
As DS1 plunges into the coma, PEPE and the diagnostic sensors need only to be pointed in the general direction that scientists specify in order to make their measurements, but MICAS needs to be pointed quite accurately, just as looking through a telescope requires careful aiming. When comets are viewed from a distance, all that we see is the coma and tail, but a tiny nucleus is shrouded deep in the coma, and that is what MICAS will attempt to spot. PEPE and MICAS are like the nose and the eyes of DS1. (For species unfamiliar with such organs, simply ignore these feeble attempts at creative writing.) Together they will sense the comet on behalf of scientists and space enthusiasts.
MICAS' infrared spectrometer senses light completely imperceptible with the limited vision of humans, just as there are kinds of light invisible to people but that bees can see. But more than just detecting the light, MICAS records a spectrum, in which the light is broken into its individual components, much as looking through a prism, or like a rainbow in which white light is separated into its various colors. Such a spectrum is very valuable to scientists because often the character of infrared light depends upon the chemical composition of the material that is reflecting it. While many rocks, for example, may simply look gray when viewed with human eyes, MICAS' infrared vision can help distinguish among different types. An infrared spectrum contains the unique signature of the material whose light is being analyzed, like a fingerprint.
MICAS' black and white camera will try to capture images of the nucleus, to show its shape, size, and perhaps something of its topography. It will also aim for images of the coma to allow scientists to understand the nature of that complex cloud.
As DS1 hurtles through the coma at 16.5 kilometers/second (36,900 miles/hour), it will try to locate the nucleus and keep MICAS pointed at it as it closes in. There are far far too many obstacles to the collection of the desired data to describe in a single mission log, but let's take a brief look at a few...
The nucleus is believed to be less than 10 kilometers (6 miles) in diameter, thousands of times smaller than the coma. Where in the coma will the nucleus be? While we presume it is somewhere near the center, we do not know accurately enough to supply DS1 with the information it will need in order to point MICAS. On Borrelly's last passage through the inner solar system (in 1994), the Hubble Space Telescope peered at it, but the nucleus was too small even for that remarkably powerful observatory to find. And by the time DS1 is close enough that the nucleus would show up, it would be too late for its tremendously distant controllers to offer guidance. The craft's course will be set by the time it enters the coma. There won't be time during the last few hours for it to alter its trajectory, so the spacecraft will be speeding through the coma like a train hurtling along its tracks in a dense fog, and somewhere in that fog is a small object, perhaps the size of a basketball, we want to see.
Although the odds remain against the spacecraft, the software that was loaded in March is designed to give DS1 a chance of pointing MICAS at the nucleus by analyzing pictures, looking for what might be the nucleus, and deciding how to orient the spacecraft to keep it in the camera's sights. After MICAS takes a picture, the resulting electronic image file is delivered to a software system known as the blobber (not to be confused with a professional wrestler of the same name in the galaxy NGC 5195). The blobber tries to locate the nucleus in each image, but, of course, with the appearance of the nucleus being highly uncertain, this is significant challenge in itself. The problem is further complicated by unwanted stray light, which compromises MICAS' vision; you see something similar when looking in the general direction of the Sun through a dirty window. So the blobber has to find the nucleus in a picture that contains an irregular diffuse glow, cosmic rays (which register in MICAS' electronic detector), variations in the coma, and other phenomena. Of course, the nucleus is only in the image if the camera is pointed in approximately the correct direction. If it's not there, the blobber will be unable to find it, and there is only a very very limited capability for the spacecraft to "look around" for it. So if you are riding on the speeding train, this is like looking through binoculars, with their very narrow field of view; and the window you are looking through is dirty, creating distracting patterns of light.
As devoted readers know, lacking the star tracker that failed in November 1999, the craft's ability to point its camera accurately is seriously degraded. While the rescue of the spacecraft was unexpectedly successful, DS1 still cannot provide a highly stable platform for observations. Normally it stabilizes itself by fixing MICAS' gaze on a reference star, but while MICAS is attempting to image the nucleus, it will have to stop tracking a star. Then the probe will rely on gyros, which simply cannot keep the craft as steady; even if the nucleus were detected, it would be difficult for the spacecraft to keep pointing MICAS at it. So now you have a companion on the train who is holding the binoculars for you. Her hands are not perfectly steady, so the binoculars could move around enough that they wouldn't pointed where you want. You have to keep telling your friend how to move the binoculars ("a little to the right, now lower them -- no, that's too much") in order to guide her well enough for you to get a good view.
A sequence of events 30 seconds long forms the basic pattern for the attempts to point MICAS at the nucleus. That is how long it takes to expose an image, transfer the data from MICAS to the main spacecraft computer, process it through the blobber, then process it with the software that does the computation of where the nucleus is in real space (as opposed to in the picture) in relation to DS1, and finally deliver that information to the system that is responsible for pointing the craft. The spacecraft travels so fast, and the nucleus is so small, that the target will be large enough for interesting views only for about 5 minutes or so, thus significantly limiting the number of pictures that might be taken. But to get the images during the few minutes that it is close enough, DS1 must have been tracking it already. If it loses sight of the nucleus earlier, it will not be able to find it when it gets closer. Furthermore, no single view of the nucleus can reveal exactly where it is. If the nucleus does show up in a picture, the cyclopean view of the spacecraft, lacking stereo vision, cannot determine whether the nucleus is large and distant or small and nearby. It has to use several views as it closes in on the nucleus to estimate where it really is to help in point for subsequent pictures.
There are other, equally important obstacles to the successful acquisition of the MICAS data. But considering only the ones mentioned, we are left with you riding on a train speeding through a thick fog. You want to see something of unknown shape and uncertain size, but probably roughly the size of a basketball. To do so, you look through binoculars held up to a dirty window by an assistant with shaky hands. You take just a very quick look and then close your eyes for 30 seconds while you describe how to reorient the binoculars so that when you next look they will be pointed where you want. Of course, this is simply an analogy -- the challenges DS1 faces are far more daunting!
Regardless of what occurs with this risky, bonus adventure, the great successes of Deep Space 1's primary mission will always be remembered as an important part of humankind's early cosmic steps. A documentary on the primary mission is scheduled for cable broadcast in the United States in an episode of Science Frontiers on The Learning Channel (TLC) on August 22. (The program also will be shown in some European and Asian countries and in most large, permanently shadowed craters throughout the solar system, but we do not know when.) As the date nears, viewers in the US should verify the schedule by clicking here or here. In the Eastern, Central, and Mountain time zones, it will be on at 10:00 pm EDT and again at 1:00 am EDT on August 23. For viewers in the Pacific time zone, it will be shown at 10:00 pm PDT, and it will be on at either 7:00 pm PDT or 1:00 am PDT depending upon your local cable company. According to sources in an unnamed globular cluster where the documentary was previewed, it is a chance to see some of the faces behind the great successes of Deep Space 1 as well as to learn a bit about a few of the exotic technologies that were tested on this incredible mission. The show incorrectly treats the low-priority, bonus encounter with an asteroid as if it were a focus of the mission, which it was not, and it exaggerates or even creates problems for the purposes of drama; but it also will remind you how much was accomplished by a small, enthusiastic, and capable team, and it probably contains the only close-up view, albeit an extremely brief one, that will be televised that whole day of the license plate and bumper sticker on your correspondent's car. No faithful Deep Space 1 enthusiast will want to miss this program!
Both Earth and comet Borrelly are closing in on DS1, as the 3 captives of the Sun follow their separate courses. Of course, DS1's orbit was designed so that the probe and Borrelly would approach each other; it is entirely coincidental that Earth is now getting closer to the spacecraft. As the previous log described, Earth and the spacecraft will not meet again, as DS1 now inhabits a part of the solar system beyond the limited range of travels of its home world. Still, DS1 and Earth are now closer than at any time since February 2000, and the distance is diminishing at about 523,000 kilometers (325,000 miles) per day. Meanwhile, the separation between the spacecraft and the comet is shrinking at more than 1.2 million kilometers (750,000 miles) per day
DS1 is now about 74 million kilometers, or 46 million miles, from comet Borrelly.
Deep Space 1 is 1.7 times as far from Earth as the Sun is and over 660 times as far as the moon. At this distance of 254 million kilometers, or 158 million miles, radio signals, traveling at the universal limit of the speed of light, take over 28 minutes to make the round trip.
Thanks again for visiting!
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