In a lucky shot of the Martian sky, the rover Spirit caught a UFO on its camera. Astronomers say it could be an asteroid, or a redundant orbiting spacecraft, left in orbit from 30 year old missions to Mars.
"We may never know, but we are still looking for clues," said Dr Mark Lemmon, from Texas A&M University. Normally the rover is looking down at Martian soil with its cameras, but every now and then it looks up to probe the Mars atmosphere.
There are seven out-of-commission spacecraft still in orbit around Mars. The Russian probes Mars 2, Mars 3, Mars 5, and Phobos 2; and the American probes Mariner 9, Viking 1 and Viking 2.