Deep Space 1 Mission Encounters Major Problems
According to memos sent around the space science community, the DS1 mission is currently engaged in contingency operations after the DS-1 spacecraft encountered a series of problems. The most problematic issue is the spacecraft’s loss of attitude lock. Taken together, the problems described below, if not resolved, threaten the success of the DS-1 encounter with Comet Borrelly.
Update:
Date: Fri, 24 Aug 2001
To: keith@spaceref.com
From: mrayman@jpl.nasa.gov
Subject: DS1 status
Dear Mr. Cowing,
I thought you might be interested to know that DS1 is no longer in
contingency operations. The spacecraft is operating normally.
Recoveries from anomalies such as this, although quite taxing, are
part of the reward of controlling a spacecraft far out in the solar
system. The DS1 team has overcome significant challenges on a number
of occasions, including the one that led to the successful rescue of
this bonus, extended mission well over a year ago following the loss
of the spacecraft’s star tracker. This shows how well the small DS1
team can do in keeping this aged and wounded bird aloft.
At the same time, the risks of this bold extended mission have been
described extensively in my mission logs (available at
http://nmp.jpl.nasa.gov/ds1), and this is simply another example of
those risks. The encounter with comet Borrelly will be a very
challenging and exciting undertaking, and following 2 years after the
success of the primary mission, regardless of the outcome it will be
wonderful way to conclude Deep Space 1’s historic trek through the
solar system.
Sincerely yours,
Marc Rayman
Date: Thu, 23 Aug 2001
From: “Marc D. Rayman” mrayman@jpl.nasa.gov
Subject: DS1 contingency operations
Dear DS1 Enthusiast:
Deep Space 1 is involved in contingency operations as a result of the
spacecraft’s loss of attitude lock. Rather than wait until tomorrow’s
regular status report to inform you of the situation, I am forwarding now
messages that I sent to Code S and the Planetary Flight Projects
Directorate over the last two days.
Marc Rayman
Date: Wed, 22 Aug 2001
From: “Marc D. Rayman” mrayman@jpl.nasa.gov
Subject: DS1 loss of attitude lock
Dear DS1ophile
When the scheduled DSN pass began at 7:00 am on August 21, a few members
of the team were already here waiting for it so that we could conduct a
hydrazine trajectory correction maneuver test. We discovered however
that the spacecraft had lost lock on its attitude reference star and thus
developed an attitude error. It was correctly relying on gyros and Sun
sensor to hold attitude.
The scheduled DSN coverage ended shortly after 3:00 pm, and we were able
to get some additional coverage without affecting any other flight
projects. The extra coverage is with 34-m stations, thus limiting our
ability to return images with which to establish the exact attitude. We
have spent much of the day (it is now after 2:00 am on August 22) trying
to determine the attitude accurately enough that we can regain lock to a
reference star, but we have not been able to do so. We will resume in
the morning when our small team has had a chance to get some sleep.
We already have a likely explanation for the loss of attitude lock. I
will wait until we’ve had some more time to think about it before
describing the suspected cause. In the meantime, in case you are
curious, I will point out that the previous time the spacecraft lost lock
was one month ago, and the only time before that was in July 2000.
We also had unexpected behavior of the ion propulsion system (IPS)
today. The IPS switched to an alternate mode of operation that is its
internal response to a fault condition. (If you are familiar with the
details of the IPS, it had switched to single plenum operation.) This
mode maintains the desired thrust but operates at different xenon
pressures. We only partially understand why this occurred, and it is
related, at least in part, to the loss of attitude lock. This evening we
took corrective action and subsequently verified the resumption of normal
thrusting. One of the reasons for attempting this fix late at night was
so that we could return to having a spacecraft that is in a completely
normal configuration except for the absence of an attitude lock, thus
allowing us to focus on only one problem on Wednesday.
I will send you more information on Wednesday (probably not until the
afternoon) as we continue our attempt to establish an accurate attitude
and relock to a reference star. I will be more than happy to provide any
additional information you would like, but I will be exceedingly busy
tomorrow, and it will be a very difficult day.
Marc Rayman
Date: Wed, 22 Aug 2001
From: “Marc D. Rayman” mrayman@jpl.nasa.gov
Subject: DS1 attitude status
Dear DS1ian:
This message is to keep you informed of DS1’s situation. Right now, I do
not believe the continuing problem presents a critical threat to the mission.
The DS1 operations team devoted an exceedingly intensive effort today to
trying to establish the spacecraft attitude with sufficient accuracy, but
we were unsuccessful.
We maintained the high gain antenna (HGA) near Earth point all day, but
most of the coverage we had was on 34-m stations of the DSN. To
determine the attitude, we need to return an image and identify the
stars. A typical image has 0 stars, so this procedure can be time
consuming. For most of the day, it took ~ 100 minutes to return each
image. (We also have a scheme to process the images on board and
downlink only the locations of stars, but this has limited utility
because of the noise in the camera.)
The Galileo project manager was very helpful in giving up some coverage
on a 70-m station this afternoon and evening. We made good use of it but
still were not able to determine the attitude.
As the team grew more fatigued and the DSN coverage was running out, we
elected, after weighing the potential benefits and the risks, to turn on
the system to lock to a star in hopes that it would lock to one which
showed up in some of the images processed on-board. This did not work,
and we subsequently disabled the locking to conserve hydrazine (the
system *might* expend excessive hydrazine locking to noise, and that was
a risk I did not want to take). The spacecraft now will continue to rely
on the gyros and Sun sensor; it will drift because of differences between
its estimate of the gyro drift (actually gyro bias, for those who know
the meaning of this term) and the actual gyro drift. The drift can occur
only around the Sun-spacecraft line. The spacecraft is not in danger.
Our preliminary analysis is that the effect of thrusting in this
unplanned attitude (that is, thrusting with the HGA on Earth) will not
cause an unrecoverable comet targeting error. The two biggest costs that
I see in what has occurred so far are the loss of valuable comet
encounter planning and the loss of hydrazine last week during the initial
attitude problems. Neither appears critical so far.
This problem does mean that we will miss our first planned attempt to
observe comet Borrelly with the spacecraft, which was scheduled for
August 25. The purpose of distant (i.e., prior to about September 21)
comet observations is for optical navigation (exclusively with
ground-based analysis), but the first planned observations were as much
to validate the sequences as to obtain navigation data. At this point,
this loss appears minor.
I think it is very important for the team to get some rest, so after a
pep talk this evening everyone left. I think everyone believes that once
we get some rest we will be ready to resume. Indeed, there is good
reason at this point to believe that our established procedures will
result in relocking the spacecraft to an attitude reference star,
although I prefer not to predict how many days this will take. We are
meeting tomorrow to prepare for our next steps. We will continue to try
to get additional DSN coverage with minimal impact on other projects.
I will continue to keep you informed with daily e-mails.
Marc Rayman
