A month from now, the Mars Science Laboratory (Curiosity) rover is set to touch down on the surface of the Red Planet and begin its mission to learn more about the possible existence of life - past or present. Curiosity will attempt to touch down using a complex and unusual landing sequence unlike any other used for previous Mars rovers ... here's how the plan will unfold.

The challenge

In the past, NASA's preferred modus operandi for landing Mars rovers has been to wrap them into a spheric "airbag" that breaks the fall and absorbs the impact with the terrain. This time around NASA is going for a much more complicated, multi-stage approach that seems to have come out of a science fiction movie.

Among the stages are a sophisticated rocket-guided entry system, a huge supersonic parachute that will be traveling almost parallel to the Martian surface, and a skycrane that will tether the rover directly onto the Martian surface while hovering just a few feet above. The entire process will be executed completely autonomously, managed not by human intervention, but by a computer algorithm made of some 500,000 lines of code. The success of this ambitious US$2.5 billion mission lays in the balance.

"Most people look at this system - particularly the skycrane at the end - and they say, 'What are you guys thinking, are you out of your mind?,'" says Pete Theisinger, project manager of the Mars Science Laboratory. "But the vehicle is too big and heavy for airbags."

Curiosity weighs 2,000 lbs (907 kg) - making it five times as heavy as the Spirit and Opportunity rovers launched in 2003 - and carries an impressive 180 lbs (82 kg) of science payload. Theisinger says that, for its size, this is the safest, simplest landing sequence that NASA could muster.

During the landing phase, the main challenge is that Mars's atmosphere is 100 times thinner than Earth's - thick enough that engineers need to worry about a heat shield, but not quite thick enough to slow the spacecraft fast enough to prevent it from crashing to the ground at high speed. Altitude on Mars ranges from minus 4 to plus 12 miles (minus 6 to plus 20 km) and the whole of the southern hemisphere has positive altitude. Until now, no attempt has been made to explore this region because engineers need the extra space to slow down the rovers.

This is going to be a very risky landing. Only 40 percent of missions to Mars have been successful, either because of engineering problems or because of the hostile Mars environment. But at the very least, should the landing falter, the data collected on it by the three current Mars orbiters - Mars Express, Odyssey and Mars Reconnaissance Orbiter - will help scientists learn from their mistakes and increase the probability of success in future missions.

The testing phase

The technology behind the landing is an interplay of hardware and software. On the software side, the computer algorithms that guide each part of the craft can be tested from Earth, simulations can be run, and new software updates can be installed - the final stable version was uploaded in the last few days of May.

Testing the hardware was not nearly as easy, since the right conditions can't be recreated on Earth. "One of the problems you have with entry and descent landing with any Martian vehicle is, how do you test it on Earth? We have the wrong atmosphere, the wrong gravity, and we would need to start at 13,000 mph (20.921 km/h) outside the atmosphere," says Theisinger.

NASA's answer was to construct a long series of compartmentalized tests, and to then stitch them together using computer simulations. The individual tests were quite elaborate, and the scientists often had to go to great lengths to simulate the conditions they would be facing on Mars.

To test the radars that will help direct the thrusters toward the landing site, the devices were flown on a helicopter over a desert landscape (representative of the Martian terrain). To characterize the high-velocity, high altitude portion of the landing sequence, the equipment was put on a F-18 accelerating toward the ground (each dive only gathered about six seconds worth of data).

Landing on Mars, with style

In the context of Mars exploration, the landing ellipse describes the area inside of which a rover has a 99 percent chance of landing. Previous Mars landers (Spirit, Opportunity, Pathfinder and Phoenix) have operated with a ballistic landing system that meant a very elongated landing ellipse: Pathfinder, for instance had a 185 by 9 miles (300 by 15 km) ellipse, and the limited mobility of the rovers meant that scientist have had very little control over exactly which terrain the rovers would find themselves in.

By contrast, Curiosity will use guided entry, including thrusters during the supersonic phase of the mission, to achieve a much smaller landing ellipse of only 4 by 12 miles (6 by 18 km). This allows scientists to select landing sites that would have otherwise been inaccessible, with the potential of a much greater scientific payoff.

Built to operate for at least two Earth years, Curiosity will be the first mission in which the rover will be able to venture outside its own landing ellipse.

The landing sequence will start at 13,200 miles (21,243 km) above the planetary surface, and will last only seven minutes. At the date of the scheduled landing, Earth and Mars will be separated by 14 light-minutes. The process will therefore be performed completely autonomously by the spacecraft, and it will be a grueling few minutes at NASA and around the world before news on the result, whether good or bad, reaches Earth.

Ten minutes before hitting the atmosphere, the "cruise stage" of the craft will separate and the final preparations for entry begin. Hitting the atmosphere at 13,000 mph, the spacecraft will start to slow down while using thrusters, guided by radars and data from the Mars orbiters, to help steer toward the landing target.

A supersonic parachute will be deployed to slow the craft down to the speed of sound and enable the rover to descend on an angle almost parallel to the Martian ground, gaining more time to slow the craft down. Meanwhile, the heat shield will separate to clear the view for MARDI, the rover's camera system, which will hopefully provide us with a spectacular, hi-def video of the descent at eight frames per second.

At an altitude of about a mile and speeds of 200 mph (320 km/h), the craft will then fire up its six landing engines, bringing the rover down very gently to only a few yards of altitude. The rover will then deploy its wheels and - this is the fancy part - a skycrane will slowly start lowering the vehicle to the ground.

After detecting touchdown, the skycrane will remove its tethers and fly away to a controlled crash far from the landing site, leaving the rover on the surface of Mars. Or, at least, that is the plan.

The NASA video below illustrates the different phases of the landing.

Source: NASA

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