How long will it take for us to know if the landing is a success? Almost 14 minutes. That's how long it currently takes a radio signal to reach Earth from Mars.
Curiosity launched on Thanksgiving weekend 2011 and traveled 354 million miles in eight months. Its target: Gale Crater on Mars.
The landing was set to happen in an area that would be facing away from Earth as the planet turned. The spacecraft was equipped with an X-band transmitter that will let the rover communicate directly with Earth, but its first attempt to phone home would be relayed by an older U.S. probe in orbit around Mars. Controllers cautioned that Curiosity could make a perfectly good landing -- but hours or even days could pass before confirmation reached Mission Control at the Jet Propulsion Lab here in Pasadena.
How long was Curiosity designed to roam the planet? One Martian year, which is 22 months for Earthlings. Spirit and Opportunity -- NASA deliberately played down expectations -- were only expected to run for 90 Martian "sols," or days, but Spirit kept on going until 2010 and Opportunity is still puttering around on Mars.
The U.S. has had its share of failures getting to Mars, but it has done better than other countries. Russia has the most dismal record: it launched 19 missions and 19 failed. You have to give them credit for trying. Phobos Grunt, a mission to return a small soil sample from one of Mars' tiny moons, ended in spectacular fashion last fall, unable even to leave Earth orbit. India just announced it would fund its first mission to Mars to launch next year.
Why do this? Aswin Vasavada, a project scientist for Mars Curiosity, said there is an elemental drive in humans.
"We are humans, we want to explore," he said. "We see a mountain, and we want to climb it. As scientists we want to answer the big questions."