7 minutes of terror: the most nerve-racking parts of a trip to Mars are the final moments before landing.The entry, descent, and landing of a spacecraft are the most dangerous stages of any mission to the Red Planet. A spacecraft must make thousands of maneuvers in a few short minutes. A lot can go wrong. That's why engineers call this nail-biting time the "seven minutes of terror."
In November 2011, NASA launched a spacecraft carrying the rover Curiosity to Mars. Curiosity is the largest of the six rovers sent to Mars by any space agency. Because it's so big--it's about the size and weight of a Mini Cooper car--engineers had to design a revolutionary technique to gently lower it onto the planet.
THEN AND NOW
When Curiosity approached Mars in August, this new maneuver resulted in the most terrifying "seven minutes of terror" ever.
Previous Mars landers used parachutes, airbags, or finely tuned rockets to touch down. To land Curiosity, engineers took a cue from helicopters that lower their payloads using cables (see diagram, right). When the spacecraft descends to an altitude of 20 meters (66 feet), it slowly unwinds three cables to lower Curiosity to the Martian surface.
Engineers have practiced every step of the landing thousands of times in test runs. "Even though we all know in the back of our heads that it's a simulation, we still get that tight chest, rapid heartbeat, and sweaty palms during the tests of the entry, descent, and landing," says Curiosity engineer Steven Sell.
The odds of a successful Mars landing aren't good. More than half of the Mare missions from all countries combined have failed. "It's very difficult to reach Mars. We try our best to predict all of the scenarios we can possibly encounter," says Sell. "And we build the spacecraft and the rover to be able to handle as much as we can think of--and more."
Curiosity is the most advanced rover ever sent to Mars. It will collect and beam data to scientists on Earth. This will allow scientists to study the atmosphere and rocks of Mars in unprecedented detail to search for evidence of water and past life.
After dropping off Curiosity, this delivery platform crash-lands about 400 meters (1,300 feet) away to avoid disturbing the landing site.
Eight rockets create thrust to slow Curiosity's descent.
Six 50 cm (20 in.) wheels are built to climb rocks.
Engineers designed Curiosity to begin its 7-minute landing sequence when it reaches the atmosphere of Mars. They preprogrammed the spacecraft with commands that initiate each step of the landing--and deal with problems that might arise.
 TIME: 0:00 (min. and sec.)
ALTITUDE: 125 km (78 mi)
SPEED: 20,900 kph (13,000 mph)
Curiosity enters the atmosphere, and friction slows it down. A heat shield protects it from the 1,650[degrees]C (3,000[degrees]F) temperatures the friction creates.
 TIME: 4:00
ALTITUDE: 11 km (7 mi)
SPEED: 1,440 kph (895 mph)
A parachute deploys, slowing the spacecraft even more. Seconds later, the heat shield drops, lightening the load.
 TIME: 6:00
ALTITUDE: 1.6 km (1 mi)
SPEED: 360 kph (224 mph)
The delivery platform drops. Rockets fire to slow the craft even more.
 TIME: 6:45
ALTITUDE: 20 m (66 ft)
SPEED: 2.7 kph (1.7 mph)
Cables lower the rover. Curiosity's wheels deploy to prepare for landing.
 TIME: 7:00
ALTITUDE: 0 SPEED: 0
Only 1.5 seconds after touching ground, the cables are cut, and Curiosity sends photos of its landing site to scientists.