The second of three radar booms has been deployed on Europe's Mars Express spacecraft - but it is unclear whether the operation has been successful.
The antennas are part of the Marsis instrument, which will look beneath the Red Planet's surface for what are expected to be vast reserves of water.
If the deployment went smoothly, it should be possible for this search to begin within about a week.
Marsis is able to see water up to 5km (3 miles) below the planet's surface.
On Tuesday, mission controllers at the European Space Agency's operations centre in Darmstadt, Germany, released the second of two identical, 20m-long (65ft) booms which comprise Marsis' primary antenna.
They performed a series of manoeuvres designed to heat the boom evenly in sunlight before reorienting the spacecraft towards Earth so it could begin transmitting data.
Controllers should know whether the operation was a success by Thursday.
The antenna boom had been folded up inside the spacecraft since launch in June 2003. The command sent from Earth fired a pyrotechnic mechanism on Mar Express that caused the boom to spring out like a "jack-in-the-box".
Deployment of the third and final boom in the Mars Advanced Radar for Subsurface and Ionosphere Sounding (Marsis) instrument is planned for 17 June, but the experiment can operate without it.
Marsis could begin collecting data on 21 June, as part of its initial commissioning phase.
The first boom was deployed in May, but mission controllers later discovered that it had not completely straightened out. Swinging the orbiter round to warm the boom's cold parts in sunlight sorted out the problem.