Shortly after landing on the planet, all communications from the crew ceased, even when the ship departed. Concerned, the British Space Programme sent Charles Van Lyden in the Recovery 7 to rendezvous with Mars Probe 7. Van Lyden, like Lefee and Michaels, was captured by aliens, who sent three of their ambassadors to Earth in the ship. Shortly after arrival, UNIT was transporting Mars Probe 7 to the space control centre when it was stolen by General Carrington's men. The ship was later relocated by the Third Doctor, but when it was opened it was found to be empty, except for a tape recorder — which was not part of the ship's standard equipment — containing a looped recording of Van Lyden's voice requesting clearance for re-entry, which played every time communication with the ship was attempted. (TV: The Ambassadors of Death)
Behind the scenes
The Mars Probe missions are ignored by the Doctor Who television story The Waters of Mars, which referred to the human colony founded in 2058 as being made up of the "very first humans on Mars". Further complicating matters, an on-screen obituary in The Waters of Mars mentions that the captain of the Bowie Base One colony, Adelaide Brooke, along with two other unidentified astronauts, had already landed on Mars when Adelaide was 42 in the year 2041.
There is further ignoring of this milestone in The Christmas Invasion which features the first British space probe to land on Mars, with no mention of Mars Probe 7.