 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 "Mariner 9" in 1971, "Viking 1" in 1977, Phobos 2 in 1989 "Mars Global Surveyor" in 1998 and 2003, "Mars Express" in 2004, 2008, 2010 and 2019, and "Mars Reconnaissance Orbiter" in 2007 and 2008.