Landers feel the heat on space missions
Space is certainly a cold place, but spacecraft have to face extremely high temperatures when they are exposed to the Sun's radiation. However, there are other extreme situations in which spacecraft are subject to tremendous heat. ESA's spacecraft must endure temperatures from hell...
When a lander plunges at high speed through the atmosphere of an alien planet, things can get very hot. This rise in temperature comes from the friction between the landing craft and the atmosphere. The heat can become as intense as several thousands of degrees Celsius. Con McCarthy is a senior engineer on Mars Express, due to start its journey to the Red Planet later in 2003. He explains the process is similar to putting the brakes on when driving a car. "When you apply the brakes to a fast-moving car, they convert all the energy being used in the car's forward motion into heat. This makes the brake disks burning hot. Similarly, when a lander enter into a planet's atmosphere at very high speed, a great deal of heat will be generated by friction."
Landers have to be well prepared to withstand such vicious temperatures. The heat shields of landers are high-tech products, composed of material capable of protecting and isolating the spacecraft from the heat during the atmospheric descent. Engineers use two main technologies when building heat shields. The first one is based on material called ablative. This material can absorb the heat by melting and decomposing during the descent. The second technology consists of material called radiator. This material is designed to reject the heat by radiating it into space, thereby protecting the spaceraft. Radiator material has to be a very efficient spacecraft insulator, especially at very high temperatures. Ablative material uses conventional technology and is usually cheaper and heavier. Radiator material uses more advanced technology, is lighter, and is generally more expensive. However, it is usually reusable (such as on NASA's Space Shuttle, for example).
"The heat shield of Beagle 2, ESA's Mars Express lander, is made of ablative material that is like a composite of cork," says McCarthy. "Having absorbed the heat, part of it burns off, dissipating the heat." The atmosphere on Mars is much thinner than on Earth. However, it still behaves as a kind of thick soup, slowing down the lander. When entering the Martian atmosphere at a speed of 25-30 times the speed of sound (which is about 330 metres per second), the heat shield will have to cope with temperatures of up to 1000 degrees Celsius.
On the other hand, ESA's Huygens probe, which reaches Saturn's moon Titan in 2004 on-board NASA's Cassini spacecraft, has a heat shield that behaves mainly as a 'radiator', composed of silicon fibres in resin. Huygens will be the first lander to penetrate the Titan's thick atmosphere. Its heat shield will protect it from temperatures as high as 1800 degrees Celsius as it speeds towards the surface at 25 times the speed of sound. If the temperature during the descent rises to very high levels, this material can start melting and partially behave as an ablator, to improve the heat dissipation when necessary.
How do you know which type of heat shield technology to choose? In general, engineers tend to go for the most economic suitable solutions. "However," says Kai Clausen, ESA's senior expert on the Huygens probe, "there are several parameters to take into account. The final choice is driven by a combination of elements. First of all, the different materials have to be compatible with the atmosphere's chemical composition and density. Secondly, different materials have different thermal and mechanical behaviour. It is up to the experts to choose the one that best responds to the so-called lander entry profile. This profile is the angle and the speed with which the lander enters the atmosphere, combined with the atmosphere's density and height."