When NASA’s 2001 Mars Odyssey spacecraft reaches Mars on October 23,
Arizona State University geologist Philip Christensen will be as nervous
as a scientist can be, watching a critical experiment enter a key phase.
He has an important instrument aboard the spacecraft which is entering
a difficult stage in its journey and all he can do is sit and watch
television, waiting for word of success or failure.

Make no mistake about it, science can be risky business, particularly
when your experiment is millions of miles away — so far away it takes
a radio transmission traveling at the speed of light eight and a half
minutes just to reach it.

Christensen, Korrick Professor of Geology, whose life’s work as a
planetary geologist has been intimately involved in the triumphs and the
tragedies of exploring Mars. Though recent failures of the Mars Climate
Orbiter and Mars Polar Lander spacecraft have received a lot of media
attention, Christensen’s experience with the difficulties of research
in space has an even deeper history.

Christensen has been doing planetary research since he was a student
(working with the Mariner 9 and subsequent Viking missions), but his
first big research project was directing the thermal emission
spectrometer experiment on the Mars Observer mission — years of
careful design and planning by Christensen and his team that came
to a sudden end on August 21, 1993 as the spacecraft disappeared
approaching Mars.

Almost half a decade later, Christensen got another shot with a similar
experiment on the Mars Global Surveyor (MGS), which was launched in
November of 1996. Though the MGS and Christensen’s Thermal Emission
Spectrometer have since been spectacularly successful, the mission was
not without worry and difficulty. A problem with a solar panel put
everything in doubt as the spacecraft entered orbit, and the planned
"aerobraking" maneuver had to be extended for a year before an orbit
appropriate for the science experiments could be achieved.

"Putting a spacecraft into orbit around Mars is like throwing a football
from here to the moon and putting it through a basketball hoop," said
Greg Mehall, Systems Engineer for the Thermal Emission Imaging System
(THEMIS), a new instrument under Christensen’s direction on the 2001
Mars Odyssey spacecraft.

Odyssey will arrive at Mars at 7:30 p.m. Pacific Daylight Time on October
23. As it nears its closest point to the planet over the northern
hemisphere, the spacecraft will fire its main engine for approximately
20 minutes to allow itself to be captured into an elliptical, or
egg-shaped, orbit. This is the only time the main engine is used during
the entire mission.

About ten minutes after the engines are fired, the spacecraft will go
behind Mars and radio contact will be lost for about 30 minutes, as the
engines continue burning. At this point, the engineers and scientists
can only wait until the spacecraft comes out from behind the planet.
Christensen and his team will simply sit and watch the proceedings on
NASA TV, nervously anticipating a new transmission from the orbiting
spacecraft — a signal that they can begin relax.

If all goes exactly as planned. "You can’t plan it completely," said
Christensen. "Going into orbit is definitely risky — you are halfway
across the solar system with a spacecraft traveling three miles per
second meeting a planet traveling three miles per second and everything
has to work precisely right including equipment and an engine that have
been out in interplanetary space for six months.

"NASA is very good at this though, and has put forth an incredible
effort to insure it works right this time. I have a great deal of
confidence in JPL and Lockheed Martin’s ability to pull it off."

Following the "orbit insertion," aerobraking will begin, as the
spacecraft skims the atmosphere during each orbit’s close passes to
the planet. Over a three-month period, friction with the atmosphere
should slowly bring the into a planned circular orbit 400 km above
Mars and allow the science instruments to be turned on. The 2001 Mars
Odyssey Mission is managed by the Jet Propulsion Laboratory, Pasadena,
Calif., for NASA’s Office of Space Science, Washington, DC.

Members of the media are welcome to join the THEMIS team as they
observe the Mars Orbit Insertion. For more information, contact Jim
Hathaway at 480-965-6375.

Orbit timeline:

Monday, 10/22

9:30 p.m. Twenty-two hours prior to the orbit insertion burn, engineers
will disable the system level fault protection to prevent the
vehicle from entering "safe mode" and shutting down.

Tuesday, 10/23

5:00 p.m. Two and a half hours before the burn, a final adjustment
of the spacecraft’s stabilization system will be made (a
procedure called "reaction wheel desaturation").

7:20 p.m. Nine and one half minutes prior to Mars orbit insertion,
the propulsion system is pressurized. At that time, the
telecommunications system is switched from the high gain
antenna to the low and medium gain antennas for receive
and transmit capability, and a downlink carrier-only (no
telemetry) signal is established.

7:30 p.m. Orbit insertion begins as the main engine is fired. The
burn starts 12 minutes, 51 seconds prior to periapsis, the
spacecraft’s closest point to Mars. The total burn duration
is just under 20 minutes.

7:40 p.m. Roughly 10 minutes into the burn, the spacecraft will pass
behind Mars as seen from Earth, and the carrier signal will
be lost.

8:00 p.m. Odyssey will emerge from behind Mars and the carrier signal
will be reacquired roughly 30 minutes after the start of
the burn. Shortly thereafter, downlink telemetry will be
reestablished, and the flight team will evaluate the
spacecraft’s health and status.

It is expected that Odyssey will be captured into an orbit that is between
15 and 23 hours, with an average period expected to be about 19 hours. If
the post-Mars orbit insertion orbit period is greater than 19 hours, a
period reduction maneuver using the spacecraft’s small thrusters will be
performed three orbit revolutions after the burn. The period reduction
maneuver will be sized to reduce the orbit period to 19 hours.

For more on THEMIS, and other NASA/ASU programs relating to Mars, see:
http://clasdean.la.asu.edu/news/planetimage.htm