The Mars Advanced Radar for Subsurface and Ionosphere Sounding (MARSIS) will seek evidence of underground water, either frozen or liquid, up to five kilometres beneath the surface of the Red Planet.

The antenna consists of two 20-metre long hollow booms that are folded up like a concertina on board Mars Express. When a pyrotechnic mechanism is fired, the booms will spring out like a jack-in-a-box.

The antenna was due to be deployed from the spacecraft on 20 April. This was delayed due to concerns that the antenna might swing back with a greater range of motion than expected after opening, possibly hitting the spacecraft.

Data from the most recent mathematical models carried out by the antenna's manufacturer, Astro Aerospace, USA, suggested the instrument's deployment might have more movement than previously thought.

MARSIS scientists have since been reviewing the Astro Aerospace data and making their own measurements to re-assess the likely behaviour of the antenna in space. At the end of this review period, they will make their recommendations to the ESA, after which the final decision on whether to deploy will be taken.