The Mars Rover, named Curiosity, weighs approximately one ton, looks like a dune buggy crossed with a Lego creation, and is the size of a small SUV. It landed on Mars this past Monday morning (1:32am EST).
It landed successfully, and thank goodness; had it crashed, the future of the Mars exploration might have been in danger. (This program is costing approximately USD $2.5 billion, so you can see why.)
It took more than eight months for the Mars Rover to travel the 352 million miles to Mars, and its mission is expected to be about two years. Curiosity is nuclear powered, and is looking for the basic elements required for life: oxygen, carbon, sulfur, nitrogen and phosphorus. It won't collect things to bring back to earth; it will collect them for analysis on-board.
There have been a surprisingly large number of landings on Mars already. From 1969 to 1973, the Soviet Union worked on 11 probes, most of which were lost at various stages in the process. Two were able to transmit information back to Earth.
The US has had better luck: in 1976, two probes sent back the first colour photographs of Mars; in 1997, Pathfinder returned numerous images and extensive data, including the fact that at one point in its past Mars was warm and wet; Spirit and Opportunity landed in 2004; and most recently, Phoenix landed in 2008, and became famous for being photographed by the Mars orbiter.
Last year, a Russia-China alliance attempted to land, but failed.
Opportunity, who arrived in 2004, is still tooling around Mars, sending data back to Earth. Its twin, Spirit, stopped roving in 2009, and stopped transmitting in 2010.