Conceptually and technologically, the entry, descent and landing (EDL) phase for the Mars Science Laboratory (MSL) mission was the most complex one that NASA had ever attempted in any of its planetary missions. It included a combination of technologies derived from past Mars missions and some new technologies. The most challenging among these was the guided entry and a sky-crane touchdown system to land the massive (899 kilogram) rover, Curiosity, softly on the Martian surface instead of the air-bag landing used in earlier Mars missions. This ensured that the rover and its parts, in particular the 0 instruments onboard, did not suffer any damage as it decelerated from about 21,000 kilometres/hour at the top of the Martian atmosphere to zero at the surface within just seven minutes. Also, it could precisely place the rover on its wheels at its chosen landing site.
These new technologies essentially defined the four components of the EDL architecture:
Guided Entry: Precision landing technologies not only allowed Curiosity to have a safe landing but greatly improved its landing accuracy. The spacecraft was controlled by small rockets during the descent through the Martian atmosphere. As compared to the 150 km x 20 km landing ellipse (which defines the landing area with high probability) of the earlier Mars Exploration Rovers (MERs), Spirit and Opportunity, the landing ellipse for the MSL was 20 km x 7 km. This capability removed the uncertainties of landing hazards, such as steep slopes or rocky terrain that might be present in larger landing ellipses. Indeed, the spacecraft landed within 1 km of the targeted site.
Powered Descent: To ensure a robust and efficient touchdown, unlike in any of NASAs previous rover missions, the MSL used a powered descent, instead of being delivered by way of airbags. Rockets continued to control the spacecrafts descent until the rover separated from its final delivery system, the sky crane.
Bigger parachute: Like Viking, Pathfinder and the MERs, the MSL, too, was slowed down by a large parachute belonging to NASAs Mars mission heritage.The MSLs parachute is part of a long-term Mars parachute technology development effort. Parachute designs are based on the forces that the parachute will be subjected to during descent. Such load calculations are dependent on the atmospheric density, the spacecrafts velocity and the parachutes drag area and mass. While the basic design remained the same, for the MSL mission the parachute was about 10 per cent larger than the one used in the MER mission, which itself was 40 per cent larger than the Pathfinder mission. It is the largest supersonic parachute ever built.
Sky Crane: The MSL is the first mission to use the soft landing technique. Unlike earlier missions, the MSL is more capable and carries more instruments, and hence is much bigger in size. To accomplish a soft landing of the huge rover, the sky crane method was designed.
After the parachute significantly slowed down the vehicle, and the heat shield was discarded, the descent stage separated from the back shell and eased the spacecraft towards the surface. Four steerable engines then slowed down the descent stage even further to eliminate the effects of any horizontal winds. When the vehicle slowed to nearly zero velocity, the rover was released from the descent stage. The vehicle was first stabilised with the help of retro-rockets and then lowered using tether just as was done with Spirit and Opportunity. The difference, however, was that in the case of the MSL a trio of tethers and an electrical umbilical cord were used to maintain communication with the stage.
Unlike its predecessors, the MSL is designed to be ready to rove upon landing. In order to enable this, it shed its shell on the way to the surface itself. Its front mobility system the wheels and suspension was also deployed while the rover was headed to the Martian surface. The MERs, on the other hand, had to wait for their lander petals to open after touchdown. The bridle on the descent stage that connected it to the rover with tether was cut once the on-board computer sensed that touchdown was successful. The descent stage then flew away at full throttle from the rover to a crash landing far from the MSL.
In addition to these four components, the EDL phase also included the technology of descent imaging using advanced terrain-sensing technologies. Descent imaging on the MSL enabled early determination about the precise place the rover would land on the basis of the images of the Martian surface taken on the way down. This measurement helped decide which retro-rockets should be fired to keep the spacecraft within the targeted landing area. These images could allow scientists observe the geological processes at a variety of scales, sample the horizontal wind profile, and make detailed geologic, geomorphic and traverse planning and relief maps of the landing site. The technique could also be used in future missions for monitoring and avoidance of surface hazards during the descent through the atmosphere.R. Ramachandran
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