The guided entry
"The Mars Science Laboratory was the first spacecraft to attempt a so-called guided entry on another planet.
To control its lift, which allowed Curiosity's flight computer to make a pinpoint landing, two 165-pound tungsten weights were ejected just before entry to change the spacecraft's center of mass. During hypersonic flight, thruster firings controlled the orientation of the vehicle's "lift vector" to compensate for actual atmospheric conditions as it precisely controlled its path toward Gale Crater.
About one minute and 15 seconds after entry, the spacecraft's heat shield experienced peak temperatures of up to 3,800 degrees Fahrenheit as atmospheric friction provided 90 percent of the spacecraft's deceleration. Ten seconds after peak heating, that deceleration was expected to peak at 10 to 15 times the force of Earth's gravity at sea level.
Plummeting toward Mars, the rover's flight computer continued steering the spacecraft, firing thrusters to make subtle changes in the flight path as required by atmospheric density and other variables.
The guided entry phase of flight was programmed to an end about four minutes after entry began. Six 55-pound weights then had to be ejected to move the center of mass back to the central axis of the spacecraft to help ensure stability when its braking parachute deployed.
Seconds later, at an altitude of about seven miles and a velocity of some 900 miles per hour, the huge chute unfurled and inflated to a diameter of 70 feet, delivering a 65,000-pound jolt to the still-supersonic spacecraft.
At touchdown, cables connecting the rover to the sky crane descent stage were severed, setting the stage for initial tests and checkout.
The heat shield was expected to be jettisoned about 24 seconds later, at an altitude of about five miles and a descent rate of 280 mph, exposing the rover's undercarriage to view.
A sophisticated radar altimeter then began measuring altitude and velocity, feeding those data to the rover's flight computer while a high-definition camera began recording video of the remaining few minutes of the descent.
Six minutes after entry, now one mile up and falling toward the surface at roughly 180 mph, the rover and its rocket pack were cut away from the parachute and backshell, falling like a rock through the thin martian atmosphere.
An instant later, eight hydrazine-burning rocket engines, two at each corner of the descent stage, ignited to stabilize and quickly slow the craft's vertical velocity to less than 2 mph.
About 16 seconds before touchdown, at an altitude of just under 70 feet, Curiosity was lowered on the end of a 25-foot-long bridle made up of three cables. As the support and data cables unreeled, the rover's six motorized wheels presumably snapped into position for touchdown.
Finally, seven minutes after the entry began and descending at a gentle 1.7 mph, Curiosity's wheels touched the surface of Mars. Radio confirmation of landing came in at 10:32 p.m., about 3 p.m. local time on Mars.
Curiosity's flight computer, sensing "weight on wheels," then sent commands to fire small explosive devices that severed the cables connecting the rover to the still-firing propulsion system. Its work complete, the descent stage flew away to a crash landing a safe distance away.
"We have three different signals we would use to confirm touchdown and we need all three of those things to look right before we say so," Steltzner said earlier Sunday. "One of those is a message from the spacecraft that says 'I touched down, and this is the velocity I touched down at and where I think I am.'
"The rover has an inertial measurement unit, a gyro and an accelerometer set, and we look at that stream to say the rover's not moving at all, that signal says 'I think I'm on the ground and I'm not moving.' And the third is, we wait a safe period of time and confirm we're getting continuous UHF (radio) transmission. And frankly, that's there to make sure the descent stage hasn't fallen back down on top of the rover. When all three of those signals are positive, we declare touchdown confirmation."
And that's exactly what Chen reported at 10:32 p.m."
CNet - NASA rover successfully lowered to surface of Mars
