First spacecraft to orbit another planet

The first spacecraft to be placed into orbit around a planet other than Earth is Mariner 9 (USA). This probe was placed into Mars orbit by a 15-min orbital insertion burn, initiated at 00:17:39 UTC on 14 November, 1971.

Both NASA and the Soviet Union had sent probes to other planets before (the first being the Soviet Venera 3 probe) but Mariner 9 was the first mission to carry out the complex manoeuvres required to enter a stable orbit. Previous missions had either flown past their targets or entered the atmosphere directly from the interplanetary-cruise stage of the mission.

Both the United States and the Soviet Union planned ambitious missions for the 1971 Mars launch window (the period where Mars and Earth are aligned in a way that makes the journey from one planet to another relatively easy). The first scheduled launch was NASA's Mariner 8, but this probe was destroyed when the rocket carrying it failed. As a result, the Soviet Mars 2 and Mars 3 missions were the first to begin their journey. They launched on 19 and 28 May 1971, respectively. These were more-or-less identical orbiters, designed to deploy a small lander on arrival in Mars orbit.

The United States didn't get another mission launched until 30 May 1971, but the greater speed of the Mariner 9 spacecraft allowed it to overtake the Soviet probes, eroding Mars 2's 11-day head-start to reach Mars 13 days sooner. The first pictures received from Mariner 9 were somewhat disappointing, as they showed Mars as a featureless clouded blob. This turned out to be the result of one of the planet-wide dust storms that are now known to periodically break out on Mars. Later transmissions provided never-before seen views of the complex geography of another planet, with deep rocky gorges and high mountains.