|
If it isn't impossible, it isn't worth doing...
-- anon. Deep Space 1 engineer
 July
11, 2000 -- On June 28, 2000, NASA's Deep Space 1 (DS1) probe
revved its experimental ion engine
to full throttle and began accelerating toward a planned encounter
with comet Borrelly in September 2001. If the spacecraft completes
this challenging journey, it will reach the comet when Borrelly
will be near its closest approach to the Sun and, thus, very
active. Mission scientists plan to capture detailed pictures
of the comet's nucleus and gather data on the violent jets of
gas and dust shooting out of it. Deep Space 1 could become just
the second spacecraft to study a comet from a distance less than
2000 km; the first was the European Space Agency's Giotto mission
that flew by Halley's comet in 1986.
DS1's historic trip to comet Borrelly almost didn't happen. In
November 1999, after successfully testing a
dozen new space flight technologies, Deep Space 1 seemed
as good as dead when its primary guidance system (called the
"Star Tracker," ironically not one of the 12 experimental
devices) unexpectedly stopped working.
"By the time the Star Tracker failed, DS1 had already
achieved more than it set out to do, so it could have been retired
to rest on its laurels," says Marc Rayman, the Deep Space
1 Project Manager at the Jet Propulsion Laboratory. Instead,
JPL engineers conducted a thrilling long-distance rescue of the
disabled craft just in time to set sail for comet Borrelly.
A 300 million kilometer service call...
Much as a mariner might judge directions at sea from the appearance
of constellations, DS1's Star Tracker measured the craft's orientation
in space based on patterns of stars it could see within a 9-degree
field of view (about the size of the bowl of the Big Dipper).
Without the Tracker, DS1 didn't know which way to thrust when
ground controllers commanded it to move. The spacecraft carries
another camera that can also see stars (the Miniature
Integrated Camera and Imaging Spectrometer, or MICAS for
short), but that experimental device wasn't designed to act as
a guidance system. Nevertheless, engineers figured that with
a bit of ingenuity they might be able to use it as a replacement.
 There
were formidable obstacles. For one, the camera's field of view
is 100 times smaller than the Star Tracker's. Also, while the
Star Tracker could estimate the probe's orientation in space
4 times every second, MICAS produces a computer file that takes
more than 20 seconds to transfer to the computer for analysis.
Updates from the camera would be at least 80 times slower than
the original guidance system.
Left: This figure illustrates approximately the difference
between the field of view of the original Star Tracker (similar
in size to the bowl of the Big Dipper) and the field of view
of the MICA camera (about the size of the full Moon). Compared
to the Star Tracker, MICA has a severe case of tunnel vision!
Undaunted, the DS1 team set to work. The situation was reminiscent
of a scene in the Tom Hanks film "Apollo 13" where
one of the characters dumps a sack full of items like duct tape
and old socks onto a table and challenges the mission's engineers
to save the astronauts. (Needless to say, they succeeded, and
a NASA legend was born.)
|
"Since DS1 has successfully
completed its primary goals, NASA decided that it could afford
to go ahead with a risky extended mission. The talented DS1 folks
at JPL are working very hard to squeeze a bonus science mission,
a comet encounter, out of the spacecraft. It is risky, and there
might be problems in the future which prevent us from completing
the mission to Comet Borrelly, but the decision to try for the
comet is much more preferable than just turning the spacecraft
off." --Paul Hertz, Program Executive for Deep Space
1 at NASA Headquarters. |
Deep Space 1 was hundreds of millions of kilometers away,
so like the fabled engineers of Apollo 13, DS1 specialists couldn't
tinker with the spacecraft's hardware. The only option was to
reprogram the onboard computer and teach it to use MICAS as a
substitute Star Tracker. It was a heady race as they labored
to complete the complex task in time to begin thrusting toward
comet Borrelly before July 2000. A later start would cause DS1
to miss the comet.
"We could have taken longer to complete the software and
been more certain that everything would work," says Rayman,
"but we opted to take the greater risk [by finishing early]
in order to have a chance at the greater prize, an encounter
with a comet."
On May 30, 2000, Deep Space Network antennas began transmitting
the newly-written software to DS1, 300 million km away. The programs
had worked well in a Deep Space 1 simulator at JPL, but no Earth-bound
device can perfectly replicate conditions in space. Team members
were anxious to discover whether the programs and procedures
would actually work on the spacecraft.
"The software was broken into 90 separate files, each of
which was radioed from one of the 34-meter diameter antennas
of the Deep Space Network," recounted Rayman. "Receiving
these faint signals was the spacecraft's main antenna, which
is only 30 centimeters (about one foot) in diameter. Without
the Star Tracker, that antenna could only be pointed to Earth
using an
innovative method the DS1 team had developed earlier this
year."
Below: The Deep Space Network facility in Canberra,
Australia. [more
information]
 On June 3rd, after 81 of the 90 files were
received, the spacecraft experienced a brief problem that triggered
a safety reset procedure. Deep Space 1 pointed its main antenna
back at the Sun, rebooted the computer and deleted all the new
files! Working rapidly, the operations team returned the spacecraft
to normal and began re-sending the software. Their skillful response
saved the day, but the episode added new urgency to an already
tight schedule.
Thanks to an allocation of extra transmission time by the Deep
Space Network, the software was in place and ready for testing
on Deep Space 1 by June 8, 2000. Ground controllers put the new
guidance system through its paces in a three-week series
of nail-biting trials. It passed with flying colors days
before the deadline.
"We brought the ion engine to maximum throttle on June 28
and began the journey toward comet Borrelly. This was an extremely
difficult task, with an enormous number of engineering problems
to solve," says Rayman. "Given the complex work that
had to be completed in such a short time, it is remarkable that
everything went so smoothly and that the team could get ahead
of schedule."
"It reminds me of an episode of Star Trek,"
continued Rayman. "I was always amused that Federation engineers
could take completely unknown technology, understand it and make
it work right away. In The Enterprise Incident, Capt.
Kirk stole a cloaking device from a Romulan ship. He beams on
board the Enterprise with this gadget under his arm and rushes
over to Scotty, telling him that he has to install it and make
it work. Scotty begins to muse about how he'll have to connect
it to the deflector shield control. 'You've got 15 minutes,'
Kirk barks. Fifteen minutes to take a completely alien technology,
install it in the complex Enterprise, and make it work? But of
course he does. When the threatening Romulan ships are bearing
down on the Enterprise and Kirk needs the cloaking device to
be activated, he orders Scotty to throw the switch. 'It'll likely
overload,' Scotty warns. But at just the last possible instant,
Kirk orders him to do it. Scotty does and it works! Sometimes
I feel that our situation was the same, but it was MICAS under
Kirk's arm! We took a system that was designed to perform a totally
different job (validate the design of a novel science camera)
and from hundreds of millions of kilometers away turned it into
a completely different instrument in a remarkably short time.
Truly, this rescue is one for the history books."
More about Comet Borrelly....
Comet Borrelly moves around the Sun once every 6.9 years in
an elliptical path that will place it near perihelion (closest
approach to the Sun) when Deep Space 1 is expected to arrive
in Sept. 2001. During the encounter, the pair will be 1.34 AU
from the center of the solar system, roughly midway between the
orbits of Earth and Mars. (1 AU is the distance from the Earth
to the Sun, or 149.6 million kilometers).
 Right:
This false-color image of comet 19P/Borrelly was obtained on
1995 Feb. 5.947UT with a 36-cm, f/11 S-C telescope, R filter
and a CCD. The exposure time was 5 minutes. Copyright ©
1995 by H.
Mikuz.
"Borrelly is very well studied from the ground, which makes
an encounter that much more interesting because it allows a calibration,
or a 'ground truth,' on comet observations," explains Marc
Rayman. "It is also one of the most active short period
comets."
Discovered as a 9th magnitude object in 1904, comet Borrelly
has never been a dazzling naked-eye comet like its more famous
cousins, Halley, Hale-Bopp and Hyakutake. One reason is that
Borrelly never comes closer to the Sun than 1.34 AU. The brightness
of a comet typically increases as the inverse 4th power of its
minimum distance from the Sun. For example, the brilliant "Great
Comet of 1996," Hyakutake, passed just 0.23 AU from the
surface of our star. (There are comets called sungrazers
that pass even closer than that.) Comet
LINEAR S4, which is expected to become visible to the unaided
eye later this month, makes its closest approach on July 26,
2000, at a distance of 0.75 AU.
Although Borrelly is not known by stargazers for spectacular
displays, it is expected to be active when DS1 arrives in its
vicinity next year. Tiny bits of dust and rocky debris boiling
off the comet's core will shoot by DS1 at speeds of 17 km/s or
more. Unlike NASA's Stardust spacecraft, en route to comet Wild-2,
or the ESA Giotto probe that passed within 600 km of comet Halley
in 1986, Deep Space 1 does not carry a shield to protect it from
hazardous debris. The lack of protection could cause problems
if Deep Space 1 comes too close to Borrelly. As Giotto approached
Halley it was struck by a piece of dust that knocked the craft
slightly off course. Although the craft was well shielded, several
of Giotto's instruments, including the camera, were damaged by
flying meteoroids.
 Left:
This image of Comet Halley's nucleus was taken by the European
Space Agency Giotto
spacecraft during a flyby on March 13, 1986. Scientists estimate
that about 10% of the surface was boiling off into space due
to solar heating 0.89 AU from the Sun. The stuff that boiled
off Halley in 1986 may one day be seen again during an eta
Aquarid meteor shower.
Scientists aren't yet sure how close to Borrelly Deep Space 1
can safely fly. That decision will probably be made next year
when astronomers have a better understanding of the comet's nucleus
and its coma. Even from a "safe" distance of 1000 km
or more, DS1 is likely to return dazzling pictures of the nucleus.
(The camera that DS1 engineers have pressed into service as a
star tracker will have to do double duty during the critical
encounter.) The spacecraft also carries an ion
mass spectrometer and a near-infrared
spectrometer. Together, these instruments should provide
unique information on the mineral content of Borrelly's nucleus
and the chemical composition of gases in the coma.
"We temporarily shut down the ion mass spectrometer after
it experienced an internal discharge last year," notes Rayman.
"We have a means of operating it that should allow the instrument
to function normally, but we haven't yet completed the work on
that. The infrared (IR) spectrometer works extremely well (and
has collected the best IR spectra of Mars available in the 1.3
- 1.9 micron range).
"There is also a suite of sensors on Deep Space 1 designed
to assess the effect of the experimental ion propulsion system
on the spacecraft and the space environment. They consist of
a retarding potential analyzer, 2 Langmuir probes, 2 pairs of
quartz crystal microbalances and calorimeters, and plasma wave
and magnetic field sensors. We reprogrammed these sensors to
take data during the
asteroid Braille flyby in 1999 and they will probably take
data during the encounter with Borrelly, too." It seems
that on Deep Space 1, nothing goes to waste!
"There is still a lot more work ahead to reach the comet,"
concluded Rayman. "Remember,
this spacecraft was not built to encounter a comet. It was designed
as a technology testbed -- a flying laboratory. The objective
[of reaching a comet] seemed impossible to meet once the star
tracker failed, but now we've given DS1 another chance!"
Stay Tuned to Science@NASA for updates and news about Deep
Space 1's exciting journey to comet Borrelly. Deep Space
1 is managed for NASA's Office of Space Science by the Jet Propulsion
Laboratory, a division of the California Institute of Technology. |
Join our growing
list of subscribers - sign
up for our express news delivery and you will receive
a mail message every time we post a new story!!!
Headlines