A 鈥渄ivining rod鈥� to search for underground water on Mars will be deployed on Europe鈥檚 Mars Express spacecraft in the first week of May 2005, after a year of delays.
Mars Advanced Radar for Subsurface and Ionosphere Sounding 鈥� or MARSIS 鈥� consists of wires strung inside three long fibreglass tubes. These will seek water 鈥� which might provide oases for life 鈥� as deep as several kilometres below the Martian surface.
The tubes, currently folded and stored onboard Mars Express, were originally scheduled for deployment in April 2004. But mission managers postponed the date when computer simulations showed that a similar antenna planned for launch later in 2005 could swing back and damage the spacecraft upon deployment. So the MARSIS team spent several months last autumn running vacuum-chamber tests of the antenna material and modelling the deployment on computers.
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The research now reveals that there is indeed a 鈥渉igh likelihood鈥� that one of the three tubes 鈥渨ill whip back and strike the spacecraft鈥�, says MARSIS manager William Johnson of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California, US. 鈥淏ut the antenna is quite light and flexible 鈥� about the diameter and strength of a toilet paper tube,鈥� he adds. 鈥淪o the impact would most likely not cause any damage.鈥�
Centre of gravity
More worrisome than a direct hit is the small chance that a tube could get caught on the spacecraft or tangled up with itself. In that case, MARSIS would not work and could leave the spacecraft with a new and unanticipated centre of gravity, forcing mission managers to tweak how they control the probe. 鈥淭here is some risk attached to this,鈥� Johnson told New 杏吧原创.
But officials at the European Space Agency (ESA), which manages Mars Express, have apparently decided it is worth the risk and will soon announce a deployment date 鈥� anticipated for April 2005. They had earlier resolved to wait until at least March 2005 for deployment, when the spacecraft will spend less time swinging through the planet鈥檚 shadow during each orbit. Such an 鈥渆clipse鈥� requires Mars Express to rely on its battery power, which has the potential to fail.
The MARSIS experiment鈥檚 own requirements also affect its deployment. The radar rods must be close to the planet to function 鈥� limiting its use to just 28 minutes in each 6-hour elliptical orbit. And it works best over the night side of Mars. That half-hour operational window is maximised in April, May and June, when the spacecraft鈥檚 close passes occur over the planet鈥檚 darkened northern latitudes.
Priority data
The three sections of the antenna 鈥� two of which are a remarkable 20 metres long, while the third measures 7 m 鈥� will each be deployed separately over the course of about two weeks. MARSIS team members will then have a further two weeks to organise their experiment in a 鈥渃ommissioning鈥� phase, taking priority over the spacecraft鈥檚 other six instruments in sending data back to Earth.
But the experiment鈥檚 success is anything but certain. MARSIS works by sending out pulses of radio waves and analysing the time delay and strength of the waves that return. The theory is that some of the longer wavelength waves may penetrate Mars鈥檚 porous rocky soil and bounce back when they encounter a transition between two materials with different electrical properties, such as an underground pool of water.
Similar radar experiments have been done on Earth, but over ice, where 鈥渦sually you sort of know what to expect鈥�, says Johnson. That is not the case with Mars. 鈥淲e don鈥檛 even know if we鈥檙e going to get an echo from the subsurface,鈥� he says. Johnson adds that any signal the team does get will be difficult to interpret, as waves that bounce back from the surface at large angles can mimic those that come from underground.
鈥淚t may be a dud. There may be nothing there,鈥� Johnson says. 鈥淏ut MARSIS has fantastic potential. It may tell us more than any other combination of instruments we鈥檝e ever sent to Mars.鈥�
