Consequently, a number of outstanding scientific questions remain concerning their origins, evolution, physical nature and composition. A recent study funded by the British National Space Centre (BNSC) aims to answer these questions by sending two small spacecraft to orbit both moons, followed by a landing on one of them.

The proposed mission would not only demonstrate some impressive new developments in British space technology, but would also be conducted for a much lower cost than has hitherto been necessary for such ambitious projects.

The mission, currently known by its working 'codename' of M-PADS - Mars Phobos and Deimos Survey - will be described on Friday, 2 April 2004 by Dr. Andrew Ball (Open University), during the RAS National Astronomy Meeting in Milton Keynes.

The mission, proposed by UK company QinetiQ in partnership with Dr. Ball and Professor John Zarnecki (Open University), involves the launch of two mini-spacecraft stacked on top of each other. One spacecraft will carry instruments for remote observations of the moons and in situ measurements of their environment in Mars orbit. The other will carry a lander to be delivered onto the surface of either Phobos or Deimos.

Each spacecraft would use solar-electric propulsion to reach Mars orbit, with lightweight solar arrays to provide electrical power for an ion engine. Since ion engines have a very high fuel-mass efficiency, the Earth-Mars transfer and Mars orbit insertion manoeuvre would be achieved with only a small amount of propellant.

The ion engine also enables the spacecraft to carry a larger scientific payload than usual, so compensating for the small size of the spacecraft. Similarly, on reaching Mars, the spacecraft will have great flexibility to select their chosen orbit, with little impact in terms of required fuel mass. The orbiter spacecraft will spiral down from high Mars orbit to rendezvous with Phobos and Deimos in turn, before the spacecraft carrying the lander spirals down to achieve the first ever touchdown on a Martian moon.

The scientific objectives of M-PADS are as follows:

The measurement requirements imply both surface and orbiting remote sensing instruments, so M-PADS is expected to accommodate a 60 kg orbital payload and a 16 kg lander payload.

"The latest developments in spacecraft technology allow the Open University to contemplate some very exciting, ambitious new missions," said Andrew Ball. "Although only a small mission, M-PADS would do what all of the previous, large missions have so far been unable to achieve at the Martian moons."

M-PADS is being offered as a possibility for the 'Mars Micro Mission' slot in ESA's proposed Aurora programme, but it could also be tailored to fit into ESA's science programme since its state of technology readiness is appropriate for a Phase A study to start in 2007.

After the recent, brief pre-Phase A study, more work needs to be funded to produce a fully detailed mission proposal, one of a number of future mission possibilities that the Planetary and Space Sciences Research Institute at the Open University is pursuing.

Historical Observations

The Red Planet has almost always been the primary objective of space missions to the Martian system, with the moons receiving only brief attention. The first spacecraft to image Phobos from a distance was Mariner 7, during its brief flyby of Mars in 1969. In the 1970s, the Mariner 9, Viking 1 and Viking 2 Mars orbiters provided the first detailed pictures, showing that both are irregular in shape. Phobos measures 13.3 x 11.1 x 9.3 km, while Deimos is 7.5 x 6.1 x 5.2 km.

Further observations were performed by Mars Pathfinder and Mars Global Surveyor, but sadly the only missions to have Phobos as their primary objective - the Soviet Union's Phobos 1 & 2 launched in 1988 - failed to fulfil their potential due to loss of the spacecraft. Some additional measurements of Phobos are planned by the European Space Agency's Mars Express spacecraft, currently in orbit around Mars, but these will largely neglect Deimos.

Both Phobos and Deimos are asteroid-like in appearance - in fact, they may be asteroids captured into Mars orbit by its gravity. Although each is pockmarked with impact craters, the surface of Deimos appears smoother and more sparsely cratered. The most notable feature on Phobos is a large crater named Stickney, which measures about 10 km in diameter. Both moons are tidally locked into "synchronous" rotation, always presenting the same face towards Mars.

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