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Last Updated: Wednesday, 28 April, 2004, 11:45 GMT 12:45 UK
Mars probe radar search postponed
By Paul Rincon
BBC News Online science staff

Mars Express, Esa
There is concern the antenna might swing more than expected
Scientists have delayed the deployment of a radar on Europe's Mars Express probe, which is currently in orbit around the Red Planet.

The decision was made after new data suggested the 40m-long antenna might swing with a greater range of motion than expected after it opened up.

This led to concerns that the device could swing back and hit Mars Express.

The Marsis radar altimeter will search for water up to 5km (3 miles) beneath the surface of the Red Planet.

The new data came from mathematical models carried out by Astro Aerospace, the California-based company that built the antenna.

Safety limit

If a beam attached to a rigid object is pulled to one side and then let go it will oscillate back and forth. The number of oscillations depends on the "damping factor".

Mathematical models made four years ago suggested the damping factor would not play a significant role in the successful deployment of the antenna because it was made of such a lightweight material.

But new test results suggest the antenna's motion after it springs out is more dependent on the damping factor than previously thought.

"The end result is not in question," William T K Johnson, of Nasa's Jet Propulsion Laboratory, told BBC News Online.

"[The antenna] always goes out and deploys properly. But there was some thought that it might be able to swing back and strike a portion of the spacecraft.

"It's a very lightweight material so even if it did strike, it would be very improbable that it would cause any damage. But still, you don't want to hit the spacecraft."

Test period

Mars Advanced Radar for Subsurface and Ionosphere Sounding (Marsis) will seek evidence of underground water, either frozen or liquid. It is thought the greatest reservoir of retained water on Mars could be found beneath the planet's surface.

Marsis is designed to discern boundaries between layers of material as deep as 5km under the surface. It will also examine the structure and variability of the Martian ionosphere, the top layer of the atmosphere.

The antenna consists of two 20-m-long elements that will spring out from the spacecraft like a jack-in-the-box.

Project scientists now plan to review the findings before scheduling a new date for deployment.

It is expected that the radar altimeter will not be switched on for a further two to three weeks.

After the command is given to deploy the antenna, Mars Express will be monitored for a further 10 days to determine how the spacecraft performs with the antenna extended.


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