Mission controllers are holding off deploying the second antenna boom of Mars Express's MARSIS experiment until they can explain an anomaly that ocurred during the unfolding of the first boom.

The 20m-long boom is divided into 13 sections, so that it can fold into the space available in the craft. The Mars Express team in Darmstadt, Germany, started to unfurl the boom on 4 May, but by 7 May they spotted a problem with one of the sections.

All but one of the 13 sections of the boom had correctly locked into position, but mission controllers have, so far, been unable to confirm that one piece, possibly piece 10, has positively locked into position.

The plan now is to take whatever time is needed to evaluate the impact of this on the rest of the experiment. It is possible that the last piece not being positively locked could have implications for unfolding the second boom. It is also possible that everything is fine, and that all pieces are locked in place.

The aim of the MARSIS experiment (Mars Express Sub-Surface Sounding Radar Altimeter) is to map the structure of the sub-surface of Mars, to a depth of up to five kilometres, using low frequency radio waves sent by the two 20m antennae. ®

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