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Tuesday, 23 October, 2001, 08:06 GMT 09:06 UK
Countdown to Red Planet arrival
Odyssey Nasa
Scientists are hoping for good luck this time
By BBC News Online science editor Dr David Whitehouse

After 200 days of interplanetary travel, covering more than 460 million kilometres (285 million miles), the American space agency's 2001 Mars Odyssey spacecraft is preparing to go into orbit around the Red Planet.

It will be a nervous time for Nasa. The last time it attempted to send spacecraft to Mars, in 1999, both the Mars Climate Surveyor and the Mars Polar Lander failed.

Odyssey was launched on 7 April and should arrive at Mars in good condition. "The spacecraft, ground system and flight team are ready for Mars orbit insertion," said Matthew Landano, Odyssey project manager at the agency's Jet Propulsion Laboratory in California.

When it gets there it will carry out a survey of the composition of the planets's surface and look for warm and wet regions on which future landing missions could set down.

Insertion burn

Scientists will be hoping for a change of luck, but they know that of the 30 missions sent to Mars by three countries over 40 years, less than one-third have been successful.

Odyssey Nasa
Odyssey will carry out a surface survey
Matthew Landano said: "We up-linked the sequence of commands that control the orbit insertion on 15 October. Now, we will closely monitor the spacecraft's progress as it approaches Mars and executes the orbit insertion burn."

To enter orbit, Odyssey's fuel tanks must be pressurized, plumbing lines heated, and the system primed before 262.8 kilograms (579.4 pounds) of propellant are ignited for 20 minutes to push the spacecraft in exactly the right direction.

The burn is scheduled for 24 October at 0426 GMT. Shortly after the start of the burn, the spacecraft will pass behind Mars and will be out of contact for about 20 minutes.

Atmosphere brushing

The spacecraft's speed should decrease, allowing it to enter a highly elliptical Martian orbit.

Over the following weeks and months, Odyssey will repeatedly brush against the top of the atmosphere in a process called aerobraking, to reduce the long, 19-hour elliptical orbit into a shorter, 2-hour circular orbit of approximately 400 km (250 miles) altitude desired for the mission's science data collection.

Odyssey carries several scientific instruments to map the chemical and mineralogical makeup of Mars.

It has a gamma-ray spectrometer that includes a neutron spectrometer and a high-energy neutron detector, a thermal-emission imaging system, and a Martian radiation environment experiment.

See also:

12 Oct 01 | Sci/Tech
Giant storm shrouds Mars
27 Jun 01 | Sci/Tech
Martian water hunt leads to poles
23 Jun 00 | Sci/Tech
Water may flow on Mars
23 Jun 00 | Sci/Tech
What now for Mars?
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