Phobos is the closer and larger of the two Martian Moons. Both moons were discovered in 1877. A small, irregularly shaped object with a mean radius of 11 km, Phobos is seven times more massive than Mars' outer moon, Deimos. Currently the exploration of Phobos has been difficult and incomplete because of its inconvenient placement inside of Mars's gravity well.
Exploration and History
The first unmanned spacecraft to image Phobos was the Mariner-7 flyby probe in 1969 which discovered only that it was irregularly shaped. In 1971 the Mariner-9 Orbiter of Mars took several pictures of Phobos while Mars was suffering from a Planet wide dust storm (obscuring its surface and preventing mapping to take place).
The Viking 1 & 2 Orbiter in 1973 also performed science studying Phobos as did Vikings 3 & 4 Orbiters and Mars Explorer Orbiter in 1975, the two Mars High Data Orbiters and the Mars Explorer Orbiters in 1977 and 1981.
It wasn't until August-October 1982 that humans finally got their first close up look at Phobos from the Mars spacecraft's highly eccentric Mars Capture orbit. Although no actual landing was attempted by the crew itself, an automated Lander containing a Sample Return device and mobile Rover was deployed returning several kg of samples and leaving behind a rover studying the planet InSitu.
The second Mars Expedition in 1983-1984, the third in 1985-1986 and the fourth in 1988-1989 all explored Phobos in this close-range yet remote Telerobotic way. A manned Landing presented too much of a drain on fuel necessary to return to Earth. The establishment of a 24 Person Mars Orbital Base in 1989 by the fourth crew didn't help much although it did expand the capabilities of crews operating telerobots.
As of 1990 no human landing on Phobos has yet occurred, but it's assumed such a Exploratory mission might take place in the early 2000s.