From: Arizona State University
Posted: Tuesday, October 16, 2001
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.
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.
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
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