Phobos (moon) tidal forces. Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars. Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a large planetesimal. The high porosity of the interior of Phobos (based on the density of 1.88 g/cm, voids are estimated to comprise 25 to 35 percent of Phobos's volume) is inconsistent with an asteroidal origin. Observations of Phobos in the thermal infrared suggest a composition containing
Exploration of Mars a focus on Phobos. Phobos 1 lost contact on the way to Mars. Phobos 2, while successfully photographing Mars and Phobos, failed before it was set to release two landers to the surface of Phobos. Roughly two-thirds of all spacecraft destined for Mars have failed without completing their missions, and it has a reputation as a difficult space exploration target. Missions that ended prematurely after Phobos 1 and 2 (1988) include (see Probing difficulties section for more details): Following the 1993 failure of the Mars Observer orbiter, the NASA Mars Global Surveyor achieved Mars orbit in 1997. This mission was