Planet Mercury

Mariner 10 used Venus as a gravity assist to enable it to reach Mercury in 1974. During the fly-by, the spacecraft returned the first close-up images of the Venusian atmosphere in ultraviolet, revealing previously unseen details in the cloud cover.

Mariner 10 eventually made three fly-bys of Mercury from 1974 to 1975 before running out of attitude control gas. The probe revealed Mercury as a heavily cratered world with a mass much greater than previously thought. This would seem to indicate that Mercury has an iron core, which makes up 75 percent of the entire planet.

Biological data was returned for approximately a week (the first data of its kind). The data showed scientists how Laika was adapting to space - information important to the manned missions already being planned.

Laika was considered a hero in the Soviet Union. The first human to pilot a spacecraft, Yuri Gagarin, followed in 1961, on board Vostok I.