This Sunday will see one of the most dramatic events in the history of space exploration. On August 5, 2012 at 10:31 p.m. U.S. PDT (August 6, 05:31 GMT), the nuclear-powered science rover Curiosity will reach Mars and begin one of the most complex, most daring landings ever attempted. Ironically, it may end in one of two ways – a triumph for the men and women of NASA as the Curiosity lander sends back its first signals from the Red Planet or utter silence. And the key to this is a heat shield that is so new to the field of planetary exploration that it is an experiment in itself.

Mars is the Bermuda Triangle of the Solar System. Over half of all missions end in failure with probes failing to reach orbit and landers vanishing. One of the reasons for this is the atmosphere of Mars, which is one of the most hostile for explorers when it comes to landing. The Moon’s vacuum makes landing on rockets a relatively simple maneuver. And though Earth's atmosphere can burn up entering vehicles, it also helps to slow them down enough to effect a safe landing on wings or parachutes.

Entering the Martian atmosphere

The thin atmosphere of Mars is like neither of these. With a pressure of only 1/100th that of Earth’s, there’s too much of it to avoid burning up and not enough of it to slow down properly. When
Curiosity's entry vehicle hits the Martian atmosphere at 13,200 mph (21,250 km/h) and suffers over 10 g’s in deceleration forces, it will be protected by the largest aeroshell ever built.

This remarkable vehicle will literally fly through the Martian sky. Tilted at an angle and protected by its heat shield, the aeroshell will use the aerodynamic characteristics of the shield and tiny thrusters to turn itself into a lifting body that can turn and even bank in lazy “S” curves to help slow it down. This principle was used on the Apollo Command Module, but this is the first time it’s been used on a planetary mission.

The key to surviving this fiery ordeal is the 14.8 ft (4.5 m) diameter heat shield. It’s of an unprecedented size for such a mission, but the Curiosity lander is itself unprecedented. At 1,982 lbs (899 kg), Curiosity is the size of a 4X4 – a robotic, nuclear-powered SUV with a laser capable of vaporizing rock. This requires a very large entry vehicle to get it to the ground with the whole thing coming in at 5,359 lbs (2,431 kg). This, in turn, has to be sealed in the largest ever aeroshell, a protective pod that looks like an upside-down bowl with a plate under it. That plate is the largest ever planetary mission heat shield.

This heat shield isn’t just big, it’s also of a new design previously used on the Stardust mission to collect samples from a comet’s tail and return them to Earth. The heat and stress on the shield will be greater than any Mars probe has ever encountered, with temperatures reaching 3,800° F (2,100° C). To protect against this, the shield is made of tiles of phenolic impregnated carbon ablator (PICA) invented by the NASA Ames Research Center. Though NASA did its best to design the shield using the best models of the Martian atmosphere, models are still models, so the shield was built with a wide margin of error.

NASA hopes to learn a great deal from the shield’s performance and, in order to profit from the experience, the shield has been fitted with the Mars Science Laboratory Entry, Descent, and Landing Instrument (MEDLI) Suite. This is a pack of 14 sensors consisting of seven pressure sensors to collect atmospheric data and seven plugs fitted with multiple temperature sensors. Since the heat shield works by ablation, that is, it protects the vehicle by burning away, the temperature sensors are fitted to the shield at different depths, so material loss can be plotted. All fourteen plugs are hooked into an electronics box that feeds the data to the lander where it is combined with information about changes in velocity.

From this data, scientists and engineers hope not only to build up a profile of the Martian atmosphere, but to also evaluate the effectiveness of the shield. During the descent, measurements are taken eight times per second and this is fed back to the lander.

As to the shield, its reward is that 24 seconds after the parachute deploys at an altitude of seven miles (11 km), the heat shield separates and what’s left of it plummets to the ground. It’s job is done, though the spacecraft continues its descent with the aeroshell jettisoning and Curiosity lowered to the surface by a rocket-powered sky crane. The data collected from the MEDLI Suite will be held by Curiosity and transmitted to Earth during the first month after landing.

Assuming it’s there to do the transmitting. We'll have our answers soon enough.

In the video below, NASA team members detail some of the challenges faced in getting the Curiosity Mar's rover to the Martian surface.

Source: NASA

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