Deep Space Mission Log 17 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 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!