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This Viking 1 orbiter image shows the thin atmosphere of Mars. The 2001
Mars Odyssey spacecraft will repeatedly brush the top of the atmosphere to
lower and circularize its orbit around Mars. |
Following carefully calculated parameters set by navigators and
ground controllers, Odyssey's targeting will be fine-tuned with a
"trajectory correction maneuver" involving a final whisper
of hydrazine gas meted through onboard jets the size of
cake-decorating nozzles.
To enter orbit, Odyssey's propellant tanks, the size of big
beachballs, must first be pressurized, plumbing lines heated, and the
system primed before all 262.8 kilograms of propellant (579.4 pounds)
burns in exactly the right direction for 19.7 minutes.
This maneuver, called the Mars orbit insertion, will brake the
spacecraft's speed, slowing and curving its trajectory into an
egg-shaped elliptical orbit around the planet. In the weeks and
months ahead, in a process called aerobraking, the spacecraft will
repeatedly brush against the top of the atmosphere to reduce the
long, 19-hour elliptical orbit into a shorter, 2-hour circular orbit of
approximately 400 kilometers altitude (about 250-miles) desired for
the mission's science data collection.
"All of this has to be done by remote control," says
Whetsel. During the main engine firing, for instance, "there's no
time for the ground (operations team) to interact with the
spacecraft." The one-way light time to the spacecraft -- the
time it takes a radio signal to travel from the communications dishes
of the Deep Space Network here on Earth to Odyssey at Mars -- will be
about 8.5 minutes during the engine firing. "By the time you see
whether or not things are going well, in reality, it's essentially already
done. So you have to put everything on board the spacecraft ahead
of time."
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