----- Original Message -----
From: Ron Baalke
Sent: Thursday, January 30, 2003 4:00 PM
To: [EMAIL PROTECTED]
Subject: Landers Feel The Heat On Space Missions (Beagle 2, Huygens)
ESA Science News
http://sci.esa.int
30 Jan 2003
Landers feel the heat on space missions
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.
