Deep Space 1 Mission Log 08-06-2000

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!