The intense period called the entry, descent and landing (EDL) phase of the mission begins when the spacecraft reaches the Mars atmosphere, traveling at about 13,200 miles per hour (5,900 meters per second). It ends about seven minutes later with the rover and descent stage stationary on the surface — one about to begin its mission and the other with its job completed.
The atmospheric entry interface point — the aim point for the flight to Mars — is set at 2,188.6 miles (3,522.2 kilometers) from the center of Mars. That altitude is 81.46 miles (131.1 kilometers) above the ground elevation of the landing site at Gale Crater, though the entry point is not directly above the landing site.
Ten minutes before the spacecraft enters the atmosphere, it sheds the cruise stage. The Mars Science Laboratory Entry Descent and Landing Instrument (MEDLI) Suite begins taking measurements. The data MEDLI provides about the atmosphere and about the heat shield’s performance will aid in design of future Mars landings.
Beginning a minute later, small thrusters on the back shell halt the two-rotation-per-minute spin that the spacecraft had maintained during cruise and approach phases. Nine minutes before entry, the back shell thrusters orient the spacecraft so the heat shield faces forward, a maneuver called “turn to entry.”
After the turn to entry, the back shell jettisons two solid-tungsten weights, called the “cruise balance mass devices”. Ejecting these devices, which weigh about 165 pounds (75 kilograms) each, shifts the center of mass of the spacecraft. During the cruise and approach phases, the center of mass is on the axis of the spacecraft’s stabilizing spin. Offsetting the center of mass for the period during which the spacecraft experiences dynamic pressure of interaction with the atmosphere gives the Mars Science Laboratory the ability to fly through the atmosphere with an angle of attack, generating lift. The mission manipulates that lift, using a technique called “guided entry,” to steer out unpredictable variations in the density of the Mars atmosphere and improve the precision of landing on target.
As the spacecraft interacts with the upper atmosphere, small thrusters on the back shell can adjust the angle and direction of tilt, enabling the spacecraft to fly a series of “S” curves. The S curves reduce the net horizontal distance that the spacecraft covers as it descends. Using fewer of them prevents undershooting the target area; using more of them prevents overshooting the target area. The guided entry maneuvers can also correct for net drift to the left or right due to winds. The spacecraft autonomously controls the guided entry movements in response to information that its gyroscope-containing inertial measurement unit provides about deceleration and direction, indirect indicators of atmospheric density and winds.
More than nine-tenths of the deceleration before landing results from friction with the Mars atmosphere before the parachute opens. Peak heating occurs about 80 seconds after atmospheric entry, when the temperature at the external surface of the heat shield will be about 3,800 degrees Fahrenheit (about 2,100 degrees Celsius). Peak deceleration occurs about 10 seconds later.
The span of time from atmospheric entry until touchdown is not predetermined. The exact timing and altitude for key events depends on unpredictable factors in atmospheric conditions on landing day. The guided entry technique enables the spacecraft to be more responsive to unpredictable atmospheric conditions than any previous Mars mission. The span between the moment the spacecraft passes the entry interface point and a successful touchdown in the target area of Gale Crater could be as short as about 369 seconds or as long as about 460 seconds. Times for the opening of the parachute could vary by 10 to 20 seconds for a successful landing.
After the spacecraft finishes its guided entry maneuvers, a few seconds before the parachute is deployed, the back shell jettisons another set of tungsten weights to shift the center of mass back to the axis of symmetry. This set of six weights, the “entry balance mass devices”, each has a mass of about 55 pounds (25 kilograms).
Shedding them re-balances the spacecraft for the parachute portion of its descent. The parachute deploys about 255 seconds after entry, at an altitude of about 7 miles (11 kilometers) and a velocity of about 900 miles per hour (about 405 meters per second). After about 24 more seconds, the heat shield separates and drops away when the spacecraft is at an altitude of about 5 miles (about 8 kilometers) and a velocity of about 280 miles per hour (125 meters per second). The Mars Descent Imager begins recording video of the ground beneath the spacecraft. The rover, with its descent-stage “rocket backpack,” is still attached to the back shell on the parachute. The terminal descent sensor, a radar system mounted on the descent stage, begins collecting data about velocity and altitude.
The back shell, with parachute attached, separates from the descent stage and rover about 80 seconds after heat shield separation. At this point, the spacecraft is about 0.9 mile (1.4 kilometers) above the ground and rushing toward it at about 180 miles per hour (about 80 meters per second). All eight throttleable retrorockets on the descent stage, called Mars landing engines, begin firing for the powered descent phase.
After the engines have decelerated the descent to about 1.7 miles per hour (0.75 meters per second), the descent stage maintains that velocity until rover touchdown. Four of the eight engines shut off just before nylon cords begin to spool out to lower the rover from the descent stage in the “sky crane” maneuver. The rover separates its hard attachment to the descent stage, though still attached by the sky crane bridle, at an altitude of about 66 feet (about 20 meters), with about 12 seconds to go before touchdown.
The rover’s wheels and suspension system, which double as the landing gear, pop into place just before touchdown. When the spacecraft senses touchdown, the connecting cords are severed and the descent stage flies out of the way, coming to the surface at least 492 feet (150 meters) from the rover’s position, probably more than double that distance. The rover’s computer switches from entry, descent and landing mode to surface mode. The time of day at the landing site is mid afternoon — about 3 p.m. local mean solar time at Gale Crater.
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