Moons of Mars attention when Houston's claim was repeated in earnest by a Soviet scientist, Iosif Shklovsky, who, based on a later-disproven density estimate, suggested Phobos was a hollow metal shell. Searches have been conducted for additional satellites. In 2003, Scott S. Sheppard and David C. Jewitt surveyed nearly the entire Hill sphere of Mars for irregular satellites. However scattered light from Mars prevented them from searching the inner few arcminutes where the satellites Phobos and Deimos reside. No new satellites were found to an apparent limiting red magnitude of 23.5, which corresponds to radii of about 0.09 km using an albedo of
Phobos (moon) presence of voids and indicate that it is not a solid chunk of rock but a porous body. The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third being empty. Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. The first was "Mariner 7" in 1969, followed by "Viking 1" in 1977, "Mars Global Surveyor" in 1998 and 2003, "Mars Express" in 2004, 2008, and 2010, and "Mars Reconnaissance Orbiter" in 2007 and 2008. On August 25, 2005, the "Spirit" rover, with an excess of