CAPE CANAVERAL, Florida — NASA’s Mars Curiosity Rover will undergo “7 minutes of terror” overnight tonight during its scheduled landing at 1:31 a.m. EDT on August 6, 2012.
The intense period called the entry, descent and landing (EDL) phase of the mission begins when the spacecraft reaches the top of the Martian atmosphere, traveling at about 13,200 miles per hour (5,900 meters per second).
EDL ends about seven minutes later with the rover stationary on the surface. From just before jettison of the cruise stage, 10 minutes before entry, to the cutting of the sky crane bridle, the spacecraft goes through six different vehicle configurations and fires 76 pyrotechnic devices, such as releases for parts to be separated or deployed.
The top of Mars’ atmosphere is a gradual transition to interplanetary space, not a sharp boundary. The atmospheric entry interface point — the navigators’ aim point during 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.
While descending from that altitude to the surface, the spacecraft will also be traveling eastward relative to the Mars surface, covering a ground-track distance of about 390 miles (about 630 kilometers) between the atmospheric entry point and the touchdown target.
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.
A minute after cruise stage separation, nine minutes before entry, small thrusters on the back shell halt the two-rotation-per-minute spin that the spacecraft maintained during cruise and approach phases. Then, the same thrusters on the back shell 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 solidtungsten 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 from interaction with the atmosphere gives the Mars Science Laboratory the ability to generate lift, essentially allowing it to fly through the atmosphere.
The ability to generate lift during entry increases this mission’s capability to land a heavier robot, compared to previous Mars surface missions. The spacecraft also manipulates that lift, using a technique called “guided entry,” to steer out unpredictable variations in the density of the Mars atmosphere, improving the precision of landing on target.
During guided entry, small thrusters on the back shell can adjust the angle and direction of lift, enabling the spacecraft to control how far downrange it is flying. The spacecraft also performs “S” turns, called bank reversals, to control how far to the left or right of the target it is flying. These maneuvers allow the spacecraft to correct position errors that may be caused by atmosphere effects, such as wind, or by spacecraft modeling errors. These guided entry maneuvers are performed autonomously, controlled by the spacecraft’s computer in response to information that a gyroscope-containing inertial measurement unit provides about deceleration and direction, indirect indicators of atmospheric density and winds.
During EDL, more than nine-tenths of the deceleration before landing results from friction with the Mars atmosphere before the parachute opens. Peak heating occurs about 75 seconds after atmospheric entry, when the temperature at the external surface of the heat shield will reach about 3,800 degrees Fahrenheit (about 2,100 degrees Celsius). Peak deceleration occurs about 10 seconds later. Deceleration could reach 15 g, but a peak in the range of 10 g to 11 g is more likely.
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 the descent.
The parachute, which is 51 feet (almost 16 meters) in diameter, deploys about 254 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). About 24 more seconds after parachute deployment, the heat shield separates and drops away when the spacecraft is at an altitude of about 5 miles (about 8 kilometers) and traveling at a velocity of about 280 miles per hour (125 meters per second).
As the heat shield separates, the Mars Descent Imager begins recording video, looking in the direction the spacecraft is flying. The imager records continuously from then through the landing. 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 85 seconds after heat shield separation. At this point, the spacecraft is about 1 mile (1.6 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 and a data “umbilical cord,” 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. The bridle is fully spooled out as the spacecraft continues to descend, so touchdown occurs at the descent speed of about 1.7 miles per hour (0.75 meters per second). 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.
Soon after landing, the rover’s computer switches from entry, descent and landing mode to surface mode. This initiates autonomous activities for the first Martian day on the surface of Mars, Sol 0. The time of day at the landing site is mid afternoon — about 3 p.m. local mean solar time at Gale Crater.
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 respond and adapt to the atmospheric conditions it encounters more effectively 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 380 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.
The largest variable during EDL is the length of time the spacecraft spends on the opened parachute. Curiosity could be hanging below a fully inflated chute as briefly as about 55 seconds or as long as about 170 seconds. Times given in the above description and on the accompanying graphic are for a typical case, with touchdown 416 seconds after entry.
VIDEO CREDIT: NASA
GRAPHIC CREDIT: NASA