Status Report

Dr. Marc Rayman’s Deep Space 1 Mission Log 6 July 2000

By SpaceRef Editor
July 6, 2000
Filed under

Mission Update:

Thank you for visiting the Deep Space 1 mission status
information site, now in its 21st month on the list of most
frequently visited sites in the solar system for
information on this exciting mission of exploration. This
message was logged in at 6:00 pm Pacific Time on Tuesday,
July 4. This log is an edited transcript of a telephone
recording. If you would like to access the same information
from any place with a telephone, please call 1-800-391-6654
and select option 3.

Following a rescue mission of cosmic proportions, Deep
Space 1 is in powered flight again. Its advanced ion
propulsion system is gently but persistently pushing the
craft along as DS1 orbits the Sun.

The whole story leading up to the resumption of thrusting
deserves special mention in the history of space
exploration. Indeed, in the admittedly highly biased but
reasonably well informed opinion of your faithful
correspondent, the entire undertaking has been one of the
most challenging yet one of the most successful and
impressive robotic space rescues ever accomplished. A great
deal of difficult work lies ahead, but providing DS1 with a
new lease on life represents an outstanding achievement in
NASA’s efforts to extend humankind’s reach into the cosmos.

First, a quick recap for the seven species new to this
popular source of information: Two months after the end of
its extremely successful primary mission, Deep Space 1’s
star tracker, which was responsible for determining the
probe’s orientation in the zero-gravity of space, ceased
operating. By that time, DS1 had already achieved more than
it set out to do, so it could have been retired to rest on
its many laurels. But engineers devised a way to restore
the craft’s sense of direction by writing new computer
programs to use the camera instead of the star tracker.
Previous mission logs, still the focus of admiration in two
of the spiral arms of the Milky Way Galaxy, have described
the extraordinary difficulty of this job. The challenge was
made still greater when the team elected to aim for the
grand prize of being ready to resume thrusting in July in
time to give DS1 a chance to encounter a comet in September

The new system that has allowed Deep Space 1 to regain full
three-dimensional control and knowledge of its orientation
works in part by taking pictures of a reference star. As
the spacecraft rotates, the reference star appears to drift
away from the center of the camera’s view, so the computer
programs analyze the pictures and determine how to reorient
the craft to bring the star back to its intended location.
This provides the means to keep the spacecraft stable. You
can imagine a similar situation in a more terrestrial
setting. Suppose you were in a boat in the middle of the
ocean, with no references such as land or the Sun. Because
of Earth’s gravity you can distinguish up from down, but
you cannot find any of the points of the compass. In the
same way, once the star tracker failed, DS1’s Sun sensor
allowed it to know the direction toward and away from the
Sun, but it could not sense other directions in the cosmic
ocean. Now suppose you wanted to travel a certain
direction: you would not know how to steer your craft. But
if you had a telescope mounted on your boat and you could
locate a star in the direction you wanted to go, you could
keep looking through the telescope as you journeyed along
to make sure you maintained your desired heading. If you
cruised for a while without checking it, you might find
that you had begun drifting in a slightly different
direction, but you could correct your course when you
checked on the star. DS1 uses its camera in the same way,
by holding a star in its view.

Following the rapid pace of ambitious and successful
testing reported in the June 18 mission log, the team
continued by evaluating how well the new system could work
to provide a stable platform for thrusting with the ion
propulsion system. On June 21, with the spacecraft locked
on a star in the view of the camera, after more than 7
months of dormancy the ion propulsion system obediently
began thrusting. The attitude control system, whose
on-board task of controlling the spacecraft’s orientation
was made so difficult when the star tracker failed, not
only has to keep the spacecraft steady while the ion
propulsion system is firing, but it actually uses the ion
drive to stabilize the spacecraft’s rotation whenever it is
thrusting. After testing this in the Deep Space 1
simulators located on Earth, controllers were eager to see
how well it would work on the real spacecraft. They were
rewarded when data transmitted from the tremendously
distant probe revealed that it was operating exactly as it
was supposed to.

Engineers have devised a new way to chart a course from
where DS1 is now to where it will reach the comet by
thrusting with just a few selected stars in the camera’s
view during the next year. I like to call these celestial
references “thrustars”. When we want it to aim the main
antenna at Earth, we choose a reference star that is in
just the right position so that when the camera is pointed
at it, the antenna faces Earth. This is known as an
Earthstar. The Earthstar that is used changes as the
spacecraft and Earth gracefully perform their separately
choreographed orbital ballets around the Sun.

On Friday June 23, DS1 turned through a large angle to
point to the first thrustar. During the few minutes that
the spacecraft is turning, it can’t hold a star in the
camera’s view, so it relies on its gyros. (Instead of
old-fashioned rotating mechanical devices, DS1’s gyros use
laser light traveling through loops of optical fiber.) When
it turned away from the Earthstar, the antenna was not
pointing at Earth, so it could not communicate with the
home planet of its controllers. But following instructions
stored on board, it found the thrustar, locked on, and
remained there for about an hour. Then it turned back to
the Earthstar and returned the data it had accumulated
while it was away. This allowed engineers to verify that it
could indeed find the thrustar, and yielded an early
opportunity to determine whether any adjustments were
needed in the myriad settings used to operate the new
system before it would be on its own for a longer duration.
Following further tests over the next few days, on
Wednesday, June 28, DS1 turned back to the thrustar,
brought its ion engine to the maximum throttle level it
could support at this distance from the Sun, and began a
week of thrusting. It is scheduled to turn back to a new
Earthstar on July 5 and report on its week of thrusting. If
the data look good, a few days after that it will resume
thrusting on the same thrustar.

The extremely ambitious schedule the operations team had
set early in June did not have DS1 thrusting toward the
comet until July. Given the difficult and complex work that
had to be completed in such a short time, it is remarkable
that everything went so smoothly that the team could get
ahead of schedule, accomplishing such a great deal in the
face of such a great challenge. Truly, this rescue is one
for the history books.

Deep Space 1 is now 2.1 times as far from Earth as the Sun
is and more than 820 times as far as the moon. At this
distance of nearly 315 million kilometers, or over 195
million miles, radio signals, traveling at the universal
limit of the speed of light, take more than 35 minutes to
make the round trip.

Thanks again for logging in!

SpaceRef staff editor.