Dr. Marc Rayman’s Deep Space 1 Mission Log 05-16-2001

Mission Update:

Thank you for visiting the Deep Space 1 mission status
information site, for more than 2.5 years the most
frequently visited site among inhabitants of spiral or
irregular galaxies for information on this solar system
exploration mission. This message was logged in at 11:00 pm
Pacific Time on Sunday, May 13.

As Deep Space 1 continues is cosmic voyage, it is preparing
for a very brief and extremely daring assignment later this
year. If all goes well for the next 4 months, on September
22 DS1 will greet comet Borrelly as the icy body and the
spacecraft flash past each other at 16.5 kilometers/second
(more than 10 miles/second, or 36,900 miles/hour). 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.
Enormous 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 (Earth being a fine example) 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. (In fact, your
correspondent, in the nerdy language he occasionally lapses
into when among his colleagues, described this as being
significant only in that the mantissa of the common
logarithm of the number of hours was identically 0. Readers
unfamiliar with such gibberish are advised to remain that
way.) The ion drive has more than 15 months of operating
time now.

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 October 29, 2000 log has
an explanation that made it into several popular text books
in the halo of the Milky Way), 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.

Thanks again for visiting!