The planet Mars possesses two small moons named Phobos (Fear) and Deimos
(Terror). Although their existence has been known since their discovery in
1877 and a number of long-range observations have been made by Earth-based
telescopes and spacecraft that have visited Mars, the satellites remain only
partially studied, particularly Deimos.

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:

  • Distinguish between different models of the origin of Phobos and Deimos;

  • Establish (or disprove) a link between the moons and known asteroid types;

  • Study how Phobos and Deimos have been affected by their association with Mars, and how Mars and its environment have been affected by the presence of satellites;

  • Search for frozen volatiles such as water ice in their interiors;

  • Determine whether the grooves on Phobos are the result of collision with ejecta from impacts on Mars or the surface expression of internal features, e.g. cracks induced by impacts;

  • Find out how and why Phobos and Deimos differ, e.g. in surface morphology and composition;

  • Measure differences in surface and sub-surface properties.

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.

NOTES FOR EDITORS

The 2004 RAS National Astronomy Meeting is hosted by the Open University,
and sponsored by the UK Particle Physics and Astronomy Research Council
(PPARC).