They might more properly be called satellites, however, because they are extremely small, only a few kilometers in diameter.

In fact, some scientists think Phobos and Deimos could be asteroids that somehow ended up orbiting Mars instead of crashing into the planet, or they could be leftovers from the time of planetary formation. Another option is that the moons are fragments of Mars, blasted off the planet's surface by a large asteroid or comet impact.

Whatever their origins, in terms of space missions the moons have been overshadowed by the planet they orbit. If a spacecraft is sent on a 6-month journey of many millions of miles, it seems anti-climatic to have it plop down on a tiny asteroid-like body when Mars itself is so close, beckoning with its many different regions to explore and a complex history to unravel.

Russian scientists, however, have been trying to send a spacecraft to Phobos, the larger of the two Mars moons, for many years.

Phobos 1 and Phobos 2 were launched within 5 days of each other in July 1988. Phobos 1 lost communications due to a software glitch that September, but Phobos 2 had better luck and made it all the way to Mars orbit. Before the spacecraft could send its two probes to Phobos, however, the spacecraft's signal was lost due to a computer malfunction.

The Russian Federal Space Agency proposed sending a new mission to Phobos in 1999. Called Phobos-Grunt ("grunt" being the Russian word for "soil"), this mission aims to gather a sample of soil from the moon's surface and send it back to Earth for analysis.

Although an unmanned sample return mission sounds complicated, the Russian space program has succeeded many times in conducting similar missions closer to home.

From Our Moon to Mars
Between 1959 and 1976, the USSR sent a fleet of robotic spacecraft to investigate Earth's Moon. Luna 16, Luna 20, and Luna 24 each dug up lunar soil and then sent a sample back to Earth via an ascent vehicle located on top of the lander.

These lunar soil samples are now housed at the Vernadsky Institute of Geochemistry and Analytical Chemistry in Moscow, Russia.

The director of the Institute, Erik Galimov, thinks samples from Phobos could help answer many mysteries about the martian moon's origin and composition.

He says that we have been able to learn some details about Phobos by gazing at the moon with telescopes, or by quick flybys of Mars orbiters such as ESA's Mars Express.

These glimpses have shown that Phobos is made of dark material that does not reflect much light. This makes scientists suspect it has a chemical composition similar to that of carbonaceous chondrite asteroids.

But other aspects of the moon can't be learned simply by staring. For one thing, Galimov says that remote instruments can't tell us the absolute age of Phobos, while a sample would allow us to measure that precisely.

A sample also would allow scientists to measure the chemical isotopes, which could then be compared to the isotopic signatures of martian meteorites that have landed on Earth to see how similar Phobos is to the planet it orbits. Similar studies were conducted on lunar samples, and they indicate that the Earth and our Moon formed from the same kind of source material.

A closer examination of Phobos might reveal if it is one large hunk of rock, or a gravitationally-bound rubble pile as many scientists now suspect. The question of whether Phobos contains water ice, a potential resource for future Mars explorers, also could be answered.

"I think it will be a very positive contribution, because we would have in our hand samples from the Mars area," says Galimov.

One attraction Phobos holds over Mars is that it costs much less money to go there. Phobos has no atmosphere to contend with, and much lower gravity. Even so, the latest estimated cost for the entire Phobos-Grunt mission is 2.4 billion rubles (about 72 million US dollars). Adding to the complications are the number of instruments that have been added the design plan over the years (the current count is 11 instruments - most NASA spacecraft have 3 or 4 instruments onboard).

One of the planned Phobos-Grunt experiments is the Living Interplanetary Flight Experiment, or LIFE, provided by the U.S.-based Planetary Society. This experiment would carry various microorganisms, as well as a soil colony from the Arctic permafrost, to see how well they endure space travel.

The experiment is controversial because of planetary protection issues - scientists hope to find evidence for life on Mars someday, and are careful to sterilize spacecraft so we don't inadvertently carry life there.

If Phobos-Grunt should accidentally crash on Phobos or Mars, the life contained in the spacecraft probably would not survive for long in the harsh environment, but most scientists don't want to risk contamination regardless.

The launch of Phobos-Grunt was scheduled to take place this October, but it now seems likely the launch date will be pushed back to 2011 at the earliest. Whenever the mission does launch, it will take 3 years for the lander to reach the small moon, touch down on the surface, and then return its sample to Earth.

More Missions
The Russians are not the only ones who dream of Phobos. Another mission headed by Optech Inc. and the Mars Institute, and funded as a concept study by the Canadian Space Agency, is called PRIME, for "Phobos Reconnaissance and International Mars Exploration."

The PRIME mission would be composed of an orbiter and lander, and each would carry 4 instruments designed to study various aspects of Phobos's geology. Yet another mission concept, the Phobos-Deimos Sample Return Mission (SRM), is being studied by NASA's Glenn Research Center.

This mission would rely on low-cost electric propulsion to land on Phobos and Deimos and bring back samples from the two moons.

These mission studies are proceeding in part on the philosophy that Phobos could be key to the future human exploration of Mars. Not only does the ease of landing and taking off from Phobos make it a good pit stop between Earth and Mars, but because Phobos is tidally-locked, always showing the same face to Mars, it could be a stable location for a Mars communications relay or some sort of monitoring station.

At present, neither PRIME nor the Phobos-Deimos SRM has a projected launch date.

In the meantime, spacecraft currently orbiting Mars can study Phobos and Deimos whenever they happen to pass by. The MER rovers Spirit and Opportunity also have caught glimpses of the moons, most notably when either Phobos or Deimos has passed in front of the sun.

Neither moon is large enough or positioned so as to block out the sun entirely like our Moon does during a solar eclipse, but perhaps they cast a brief shadow on the rovers as they flit by overhead.

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