NASA Mars Exploration Program Team are now achieving such a level of accuracy in landing craft on the Martian surface that the targeted landing area for Curiosity was like an arrow hitting the bull’s-eye from hundreds of kilometres away.
Proportionally this bull’s-eye equates to a landing zone on Mars measuring 20km by 7km.Although this may sound like a huge area to you and I, bearing in mind that this spot on Mars is a minimum of 56 000 000km away, and that the projectile has been travelling full speed across this distance for the last eight months, the facts quickly regain our awe.Moreover we can easily discern real technological progress when we consider that on previous missions to the Red Planet the bar for landing robotic spacecraft “accurately” was set at 150km by 20km.
The Curiosity rover also known as the Mars Science Laboratory (or MSL) is in itself a record weight to send hurtling through the precariously thin Martian atmosphere at approximately 21 120kmph. The atmosphere of Mars merits this description because its gases provide a notoriously weak atmospheric frictional ‘brake’ for spacecraft desperately trying to slow their descent. Curiosity, the $2.5 billion exploratory 6-wheeled robot that bears no less than ten delicate investigative instruments is entirely reliant on the landing module’s braking techniques to stop its 980kg bulk from thumping into the Martian surface and leaving a crater of its own. So once again a Mars lander will step up to the mark to perform a carefully choreographed sequence of actions and protect its precious cargo. For Curiosity’s Martian exploration mission to stand any chance of success each of the lander’s descent phases must trigger the next like clockwork.
So what trick have NASA selected from their engineering repertoire to pull off the extra-terrestrial landing feat this time round?The answer is ‘Skycrane’.Although this Mini-sized rover is the heaviest of its ancestors and bears the greatest weight of public expectation to fulfil its fact-finding potential on Mars, the MSL will daringly be lowered alone, completely unprotected, onto the red terrain.After a history of cushioned landings on Mars that frequently employed no less than a cocoon of airbags, we may wonder how this latest proposal is possible.Although it might seem that this final stage of the descent is placing the MSL rover in a vulnerable position, the opposite is in fact the case.Skycrane, this ingenious new NASA invention did all the work.After the lander craft entered the Martian atmosphere and deployed the largest supersonic parachute ever to be made, the heat shield or bottom aeroshell of the lander was jettisoned approximately 8km above the surface of Mars.
Now falling at less than 448kmph, any keenly observing Martians would have got their first glimpse of Curiosity, cradled up inside the lander module’s hull. Aimed at the ground, MARDI, a radar ‘camera’ mounted in the rover took colour 1600×1200 pixel images at a rate of 5 per second for the remaining few minutes of the descent. These images provided essential terrain-mapping data for the MSL’s landing, initial surface movements and operations. Descending at around 288kmph the rover , for a heart-stopping moment, appeared to fall out of the module and plummet unhindered once again 1.8km from the ground. The ‘drop’ however was only for a second or two and was permitted solely to ensure the now redundant back shell of the lander, still drifting above, was a safe distance away when the skycrane platform came to life. This would have been a dramatic spectacle as eight variable thrust mono propellant hydrazine rocket thrusters mounted on arms beneath the platform suddenly attempted to blast Curiosity away from Mars. However due to the speed of Curiosity’s descent, this objective will not be achieved instantly. The result instead was a reduction in the MSL’s surface-bound velocity to 2.7kmph. Having at last reached a tantalisingly close 20 metres from the ground, any Martians in the vicinity were able to witness this Boeing/Lockheed Martin-built platform almost in ‘hover mode’
Although it plays a huge role in bringing Curiosity so close to the surface of Mars, somewhat surprisingly, the Skycrane platform was not to complete MSL’s journey to the Martian surface. While Skycrane could technically reduce retro rocket thrust until it finally touched the surface of Mars, this action would destroy the rover’s chances of success. The blast from the platform’s rockets on landing would have churned up the Martian surface creating huge dust clouds that would have effectively blinded the rover and hide the terrain we have been so anxious to see. For this reason alone Curiosity was gently be winched on tethers from beneath the Skycrane until its wheels make contact with Mars. Simultaneously the rover opened out its wheels and adopted its landing configuration. Curiosity will then severed its tethers and having done all it can to aid in the landing phase of the mission, the Skycrane platform blasted back up into the pink sky before executing a crash-landing a safe distance from the MSL.
(Article by Nick Parke, Education Support Officer)