The Grebulons are a race that are observing the Earth, but do not know why. During the centuries-long spaceflight the Grebulons were all in suspended animation with their memories saved to the ship's computer (which was struck by an asteroid influenced by Guide Mark II). With the loss of the backup, after the robots carrying it also fell out of the hole made by the asteroid, the Grebulons awoke with no idea where they were going or who they were. What little instructions they could extract from the wrecked computer told them to "land" somewhere and "monitor" something, so they landed on Rupert and monitored the television transmissions from Earth. Trillian later reveals that the Grebulons are a missing reconnaissance fleet from the war that she was meant to cover (which never happened because the Grebulons never arrived with their respective army).