By Paul Rincon
BBC News Online science staff
The deployment of a radar on Europe's Mars Express probe which is to look for water and ice under the Red Planet has been delayed until later this year.
There have been concerns the antenna could swing back and hit Mars Express
The 40m-long Marsis antenna was due to open out of the spacecraft on 20 April.
The delay is linked to concerns that the antenna might swing back with a greater range of motion than expected after opening, hitting the spacecraft.
The Marsis radar altimeter will search for water up to 5km (3 miles) beneath the surface of the Red Planet.
The antenna consists of two 20m-long hollow booms that are folded up like a concertina aboard Mars Express. When a pyrotechnic mechanism is fired, the booms spring out like a jack-in-the-box.
Data from the most recent mathematical models carried out by the antenna's manufacturer, Californian-based Astro Aerospace, suggested the instrument's deployment might be more "dynamic" than previously thought.
Concerns that the antenna might swing back and hit delicate components on the spacecraft led the radar's science team to call for a halt to deployment.
Marsis scientists have since been reviewing the Astro Aerospace data and making their own measurements to re-assess the likely properties and movement of the antenna in space.
At the end of this review period, they will make their recommendations to the European Space Agency (Esa), which will take the final decision on whether to deploy.
Marsis looks set to miss a deployment window over the summer as a result of the review. It will then have to wait until the end of a Mars solar conjunction, which lasts from 31 August to 1 October.
This conjunction will put Earth and the Red Planet on opposite sides of the Sun, making communication with Mars Express more difficult.
The antenna will be 40m long when extended
Rudolf Schmidt, Mars Express project manager told BBC News Online: "Sometime after summer, no precise date is agreed, we will reach the level of confidence to make a decision about deployment.
"Should the new results still indicate a risk that the boom may hit the spacecraft, we will analyse the associated impact energy, potential area to be hit, sensitivity for damaging and so on."
The only variable scientists can change about the deployment of the instrument itself is the temperature of the box in which the antenna is stored. But as one scientist points out: "It's either cold or very cold."
The deployment will cause a two-week interruption to the work of the other instruments but Dr Schmidt added that he was confident a compromise would be reached with the instruments' science teams.
"Once we know that we are ready to deploy, we will talk to the scientists to see how to phase the deployment into the ongoing science observations," he said.
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.