In 2004, NASA sent two rovers to Mars. Each rover had scheduled a three-month mission to explore the surface, after safely bouncing onto the surface of Mars in an cushion of airbag-like balloons. In a marvel of engineering and dedication Spirit lasted six years, and Opportunity is still advancing our scientific knowledge about Mars to this day.
In just over three weeks a new rover, Curiosity, will land on Mars. At twice the size and five times the weight of the older rovers, the airbag manoevre won't work for the landing this time. Intead, it make a high-plunge into the Martian atmosphere in a head-shielded probe, which will release a jet-powered "skycrane" which will, hopefully, safely lower the rover to the surface:
From atmosphere entry to touchdown, the entire process will take just seven minutes. But right now it takes radio signals fourteen minutes to travel from Mars to Earth, so the entire landing sequence has to be automated ... and flawless. Once the probe has landed, NASA scientists will have what they call "seven minutes of terror" to wait until they learn whether the landing was a success.
I'll be keeping my fingers crossed on August 6. Spirit and Opportunity have been unqualified successes, and Curiosity has the potential for much, much more.
NASA: Mars Science Laboratory