During these missions , the Viking 1 orbiter flew within 80 km of the surface of Phobos .
The team was able to determine its mass and density , while the Viking 2 orbiter flew by Deimos within 26 km on October 17 , 1977 .
During these missions , the Viking 1 orbiter flew within 80 km of the surface of Phobos and the team was able to determine its mass and density , while the Viking 2 orbiter flew by Deimos within 26 km on October 17 , 1977 .