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ESA Science News
http://sci.esa.int

30 Jan 2003

Landers feel the heat on space missions
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Space is certainly a cold place, but spacecraft have to face
exteremely 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."

USEFUL LINKS FOR THIS STORY

* More about Mars Express
  http://sci.esa.int/marsexpress/
* More about Huygens
  http://sci.esa.int/huygens/

IMAGE CAPTIONS:

[Image 1:
http://sci.esa.int/content/searchimage/searchresult.cfm
          ?aid=9&cid=12&oid=31286&ooid=31287]
Beagle 2's heat resistant shield protects it from heat
generated by friction with the martian atmosphere.
Parachutes deploy to slow it down further, then gas-filled
bags inflate to soften its landing.

[Image 2:
http://sci.esa.int/content/searchimage/searchresult.cfm
          ?aid=9&cid=12&oid=31286&ooid=28279]
The Beagle 2 lander, to be carried on ESA's Mars Express,
is equipped with a suite of instruments designed to look
for evidence of life on Mars.

[Image 3:
http://sci.esa.int/content/searchimage/searchresult.cfm
          ?aid=9&cid=12&oid=31286&ooid=18420]
After entering Titan's atmosphere, Huygens' parachute system
will be deployed for the 2-1/2 hour descent, during which
most of the scientific measurements will be made. This
artist's impression shows the Probe suspended from the
stabiliser parachute passing through the clouds that are
expected at around 20 km altitude. The descent will occur
during daylight to provide the best illumination conditions
for imaging the clouds and surface.

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