On Monday August 6th, at 1:31 am EDT (0531 GMT), following a 254 day, 352 million mile (567 million kilometer) journey through the Solar system, NASA’s Mars Science Laboratory (MSL) will land its newest rover, “Curiosity”, on the surface of the red planet.
In what is NASA’s most ambitious and risky Mars mission ever, the entry, descent and landing phase (EDL) is particularly hazardous. So much so, that the time from MSL’s entry into the Martian atmosphere to its landing of Curiosity on the Martian surface has even been dubbed by NASA, the ‘Seven Minutes of Terror’.
With a size comparable to a Volkswagen Beetle, and a weight of approximately 1 ton (907kg), the Curiosity rover is about twice as long and five times as heavy as its rover predecessors, Spirit and Opportunity. While the smaller rovers were light enough to have their landings cushioned by an airbag system, Curiosity’s size precludes this option. The Viking landers which visited Mars in the 1970’s as well as the Phoenix lander in 2008 were set down on Mars using “legs”. Again, the size of Curiosity would make this approach too risky – an extremely flat surface would be needed to avoid instability, and a legged landing could potentially kick up enough Martian dust to damage Curiosity’s scientific instruments.
Instead, in what NASA scientist Adam Steltzner of NASA’s Jet Propulsion Laboratory (JPL) describes as the “least crazy” of all possible methods that could be used to land the rover, NASA will employ a novel and elaborate sequence that will slow the spacecraft from a staggering 13,000 mph (21,000 kph) to a final descent speed of about 2 mph (3.2 kph).
The first step in slowing the spacecraft will use friction provided by the Martian atmosphere – similar to the way the Earth’s atmosphere is used to reduce the speed of a returning space shuttle or capsule. However, with an atmosphere 100 times thinner than that of Earth, the friction can only slow the spacecraft to about 1000 mph. A heat shield (the largest ever flown in space) will need to withstand the scorching 3,800 Fahrenheit (2,100 degree Celsius) heating of atmospheric entry.
Next, the spacecraft will deploy the largest and strongest supersonic parachute ever built. Designed to withstand 65,000 pounds of force, the chute measures a massive 51 feet (16 meters) in diameter. The heat shield will then separate and fall away giving the Curiosity rover its first “look” at the red planet. In fact a camera on the bottom of the rover (the “Mars Descent Imager”) will begin videoing the action from this point until landing (about 2 minutes).
With the spacecraft now only 1 mile above the ground and barreling towards it at approximately 180 mph (290 kph), the parachute separates, leaving the rover and a key piece of new equipment called the “Sky-Crane”. This novel apparatus will actually lower the rover with a tether to a soft landing on the surface of Mars. Attached to the Sky Crane are eight “retrorockets”. Firing these rockets enables the descent speed to be reduced to about 1.7 mph (0.8 meters per second).
At an altitude of only 66 feet (20 meters), and a mere 12 seconds before scheduled touchdown, nylon cords begin to lower the rover itself from the sky crane. The sky crane continues to lower, with Curiosity dangling 20 feet (6 m) below.
As the Curiosity rover comes to rest on the Martian surface, it will wait 2 seconds to confirm that it is indeed on solid ground before firing the last of a total of 76 pyrotechnic explosions used throughout its descent – this time to cut the cords attaching it to the sky crane above. With its job done, the sky crane will fly away – to crash land at a safe distance of at least 500 feet (150 meters) from Curiosity. The sky crane maneuver provides a safer alternative to a purely rocket powered descent, which could see the jets kicking up dust and debris, potentially damaging sensitive instrumentation.
Assuming the Curiosity rover is intact after this dramatic and intricately choreographed landing sequence, scientists will need to wait the 14 minutes it takes for signals to travel the distance from Mars to Earth before knowing the rover’s fate. When Earth receives the signal that MSL is entering the Martian atmosphere, Curiosity will already be on the surface of Mars.
With a total cost of $2.5 billion dollars, there is a lot to lose if the mission fails. Looking at history gives little comfort. Curiosity is the latest in a string of Martian landing missions that stretches back more than 40 years. Of 16 prior spacecraft destined to land on Mars, only 6 were complete successes.
There is also much to gain if the mission succeeds. More so than in previous missions, NASA has chosen a site of particular scientific interest in hopes of determining whether conditions have been favorable for microbial life.
In the case of Curiosity, a risky landing may be the price for great scientific reward.
Next up: the science behind Curiosity’s mission.