By Paul Rincon
BBC News science reporter
A radar on the Mars Express spacecraft which will look for water and ice under the Red Planet is due to be deployed after a delay of over a year.
Controllers are worried the radar's long antenna booms will swing back and hit delicate components on the probe.
Deployment will take place in a window from 2 to 12 May, with the Marsis instrument's three booms set to open out of the spacecraft separately.
The radar altimeter will search for water up to 5km (3 miles) beneath Mars.
Results from the instrument are eagerly awaited; it is thought the greatest reservoir of retained water on the Red Planet could be found beneath the planet's surface.
Some think such underground reservoirs could provide a habitat for microbial life.
The Mars Advanced Radar for Subsurface and Ionosphere Sounding (Marsis) is a collaboration between scientists in Europe and the US.
Two 20m-long (65ft) hollow fibreglass "dipole" booms comprise the instrument's primary antenna, while a 7m-long (23ft) "monopole" boom acts as a receive-only antenna.
They are folded up onboard the European Space Agency's (Esa) Mars Express orbiter and spring out when pyrotechnic mechanisms are fired.
Early in 2004, the antenna manufacturer, California-based Astro Aerospace, told Marsis team members about the results of new mathematical models they had carried out on the antenna deployment.
These suggested the booms might swing back with a greater range of motion than predicted by simulations conducted four years previously.
Fears the booms might hit delicate components on the spacecraft prompted mission controllers to call a last-minute halt to the deployment scheduled for 27 April 2004.
Marsis' main antenna is folded up like a concertina in a box
Meanwhile, engineers based at the US space agency's (Nasa) Jet Propulsion Laboratory (JPL) in California carried out a comprehensive investigation of the instrument's dynamics during deployment.
The investigation concluded there was a possibility that one or all of Marsis' antenna components could whip back and strike the spacecraft.
An Esa review board concluded that if the lightweight booms did hit the spacecraft, the impact energy would be low. Even so, say researchers, they could still do damage.
"Spacecraft are generally very fragile things. What it might do if it whizzed back would be to damage some of the instruments such as the camera," Professor Iwan Williams of Queen Mary College, London, and a co-investigator on Marsis told the BBC News website.
Needless to say, mission controllers will be playing it safe. Prior to each deployment, the spacecraft will be placed in a "robust" attitude control mode, which allows it to tumble freely while the boom extends before regaining standard pointing to the Sun and Earth.
The dipoles will be unfurled first followed by the monopole. This will allow controllers to evaluate success and decide whether to proceed with the next phase.
Opening of the first dipole boom is understood to be scheduled for 4 May, with each deployment separated by three to four days. But all this could change according to the circumstances.
Esa admits it has been trying to keep the deployment low-profile to take the pressure off mission controllers.
"We don't want to go blowing our trumpets until we've deployed at least the first two booms," said a spokesman for the agency.
Successful unfurling of the two dipole booms is the minimum needed to get Marsis working. Deployment of the monopole "is a bonus", said the spokesman.
Despite the risks, mission scientists like Wlodek Kofman of the University of Grenoble, France, say they are optimistic about the outcome.
"We need these data. There is a competition here. In about a year, we will have a new radar on Mars," he told the BBC News website.
That instrument is Sharad (SHAllow RADar), which will be carried to the Red Planet aboard Nasa's Mars Reconaissance Orbiter (MRO).
If Marsis deploys successfully, scientists could begin collecting data during the instrument's commissioning phase, due to begin on 17 May.
Marsis scientists have a certain amount of priority to use the radar during the Martian night. This is because the planet's atmosphere is ionised during the day, which makes it harder for radio waves to penetrate.
Although Mars Express' primary mission is due to end in November, scientists are confident Esa will grant an extension, allowing the instrument team to gather the data they need.
Deployment operations will be carried out from Esa's European Space Operations Centre (Esoc) in Darmstadt, Germany.