Deep Space 1 Mission Log Mission Update 10 Sep 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 30 and 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.