7 minutes of terror

Print edition : September 07, 2012

A Profile of entry, descent and landing events for a typical case. The actual event timings depended on the Martian atmospheric conditions on August 6.-NASA/JPL-CALTECH

Scientists and engineers of the National Aeronautics and Space Administration (NASA) have termed the intense phase of entry, descent and landing (EDL) of the Mars Science Laboratory (MSL) mission, with the rover Curiosity stationary on the surface, as seven minutes of terror. For, this phase was estimated to last 416 seconds. During the actual landing, this phase lasted about 436 seconds. It was so named because of the great challenges its accomplishment posed and the enormous uncertainty of achieving it given the many unknown factors about the Martian atmosphere and surface. It is perhaps the most complex landing ever attempted in the history of space exploration.

The EDL phase began when the spacecraft reached the top of the Martian atmosphere. From 10 minutes before entry, when the cruise stage is jettisoned, to the cutting of sky-crane bridle, the landing sequence involves six spacecraft configurations, the firing of 76 pyrotechnic devices for release of parts to be separated or deployed, the deploying of the largest supersonic parachute ever built, and more than 500,000 lines of code to be read by the on-board systems for operation.

The atmospheric entry interface point was set at launch to be 131.1 kilometres above the ground elevation of the landing site at Gale Crater. The entry point was, however, not directly above the landing site. While descending from that altitude, the MSL also travelled eastward relative to the red planet, covering a ground track of about 630 km between the entry point and the target.

Ten minutes before the atmospheric entry, the spacecraft shed the cruise stage. The on-board MSL Entry, Descent and Landing Instrument (MEDLI) suite on board began to take measurements. A minute after the cruise-stage separation, small rockets on the back shell stopped the 2 rpm (revolutions per minute) spin that the spacecraft had during the cruise and approach phases. Then a manoeuvre called turn to entry was performed when the same back-shell thrusters oriented the spacecraft so that the heat shield faced forward.

After this, the back shell discarded two solid tungsten weights, called the Cruise Balance Mass Devices (CBMDs), each weighing about 75 kg. This was to shift the centre of mass of the spacecraft from its earlier position during the cruise and approach on the spacecrafts spin axis. This off-setting of the centre of mass when the spacecraft experienced pressure from interaction with the atmosphere enabled the MSL to generate lift and fly through the atmosphere. Use of atmospheric lift during the entry phase increased the missions ability to deliver a heavier robotic payload as compared to the earlier landing missions.

The technique of guided entry during this phase enabled the spacecraft to use that lift to manoeuvre through unpredictable variations in the density of the atmosphere and thus improve the accuracy of landing. During the guided-entry stage, back-shell thrusters adjusted the angle and direction of the lift to control its descent and enable it to perform S turns, called bank reversals, to control its left-right departures from the target site due to atmospheric effects and spacecraft modelling errors. These actions were carried out autonomously by the spacecraft using data from the on-board gyroscope-based inertial navigation unit.

Over nine-tenths of the deceleration during the EDL phase occurs owing to friction with the Martian atmosphere before the parachute opens. Peak heating should have occurred at 75 seconds after atmospheric entry, with the temperature at the external surface of the heat shield being 2100oC. Peak deceleration, which would have been in the range 10-11 g, would have occurred 10 seconds later. A few seconds before the parachute was deployed, another set of tungsten weights, each weighing about 25 kg and called the Entry Balance Mass Devices (EBMDs), was jettisoned to shift the centre of mass back to the axis of symmetry of the spacecraft required for proper parachute-aided descent.

The parachute opened at about 260 seconds after entry, at an altitude of about 11 km, when the substantially reduced velocity was still about 1,500 km/hr. After about 20 seconds of parachute deployment, the heat shield separated and dropped away. At this point, the spacecraft was still about 8 km high and travelled at a velocity of about 440 km/hr. As the heat shield separated, the Mars Descent Imager (MDI) began to take video in the direction of flight through the landing. The rover and the descent stage rocket backpack remained attached to the back shell carrying the parachute.

The back shell and the parachute separated from the rover about 85 seconds after the heat-shield separation, at which instant the spacecraft was 1.6 km above the ground with a velocity of about 290 km/hr. There are eight retro-rockets with throttle capability on the descent stage, all of which fire for a powered descent of the spacecraft. These landing engines decelerated the spacecraft to about 2.75 km/hr and this velocity was maintained until touchdown.

Four of the eight engines were shut off at this instant. Soon afterwards, nylon cords began to spool out from the bridle on the descent stage and lower the rover with sky crane manoeuvre. The rover then detached from the descent stage (about 120 seconds after heat-shield separation) but still was attached to it by the tether and the data umbilical cord. The height then (about 12 sec before touchdown) was about 20 metres. Just before touchdown, the rovers wheels and suspension system, which also act as the landing gear, got deployed. The cords were fully spooled out as the spacecraft continued to descend. So at touchdown, too, the velocity remained at 2.75 km/hr. As soon as touchdown was sensed, the connecting cords severed and the descent stage flew out of the way, bringing the rovers velocity to zero. The stage landed 650 m away from the rovers position.

R. Ramachandran

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