The first of three radar booms that will search for underground water on Mars has apparently deployed successfully aboard Europe's Mars Express spacecraft, despite fears that the boom would whip back and strike the craft. But the radar will not be functional until its twin deploys, an event currently scheduled for Sunday.

On Wednesday, mission officials in Darmstadt, Germany, commanded the first of two 20-metre-long antennae on the MARSIS (Mars Advanced Radar for Subsurface and Ionosphere Sounding) experiment to pop out of its storage box. It had been folded there since before the mission's launch in June 2003.

The antenna will form a "T" consisting of the two long booms and a third, 7-metre-long antenna, and was originally scheduled to be deployed in April 2004. But European Space Agency (ESA) officials postponed the date over concerns the antennae could endanger the mission by hitting or getting snagged on the spacecraft during deployment.

Now mission members confirm the first deployment appears to have gone without a hitch, with initial data suggesting the boom popped out as planned.

Team members contacted by New Scientist say they are pleased with the apparent success but remain cautious. More tests of the spacecraft's behaviour will continue on Friday to confirm the boom did indeed straighten out as required. The radar will not be able to function until the second 20-metre boom is deployed.

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