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Herschel passes a new milestone (Forwarded)



 
 
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Old February 21st 07, 09:28 PM posted to sci.astro
Andrew Yee
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Default Herschel passes a new milestone (Forwarded)

ESA News
http://www.esa.int

21 February 2007

Herschel passes a new milestone

The heart of ESA's infrared space telescope, Herschel, has successfully
completed a vital round of tests. The cylindrical cryostat will now be
loaded with the spacecraft's instruments before more tests and Herschel's
eventual launch in 2008.

Herschel's cryostat is a complex vacuum flask, 2.5 metres high and 2
metres wide. It is vital to the mission because Herschel's instruments
need a temperature of 1.7 Kelvin (*271.3 degrees Celsius) to operate
within their most sensitive range. The instrument surroundings must
eqaully be very cold, to enable Herschel to see to see the infrared
emission from cool matter in the Universe. If the cryostat and instruments
reach higher temperatures they will emit infrared, overwhelming that from
the celestial objects.

Herschel must sit in the full glare of the hot Sun because it uses a solar
panel to turn sunlight into electrical power. The cryostat is therefore
essential to cool the instruments. The cryostat sits behind the solar
panel, isolated from it by several layers of insulation, and contains two
tanks that will be filled with liquid helium.

The first procedure in the just completed cycle of tests was the 'bake
out'. In this process, the empty cryostat was heated to 80 degrees
centigrade. Heating drives out any volatile substances including any
residual water left inside the cryostat. Water reduces the quality of the
insulation so its removal is vital to the lifetime of the cryostat. "In
effect, the bake out cleans the cryostat," says Thomas Passvogel,
Herschel's Project Manager.

Next, the engineering team simulated the launch campaign. The cryostat was
cooled down and the tanks were filled with liquid helium just as they will
be five days before launch. The main tank holds 2250 litres of superfluid
helium. The small tank is used to extend the ground 'hold' time of the
system. The team monitored the helium temperature increase in the main
tank, while cooling the cryostat with helium evaporating from the small
tank. Every second day, they topped-up the small tank.

When they do this for real, at the launch site in Kourou, French Guiana,
the cryostat will be able to withstand a launch delay of one day before
the temperature in the main tank gets too high to launch safely. If the
delay is any longer, the rocket will be rolled back to the assembly
building and the engineers will have to recondition the main tank, refill
it and subcool it.

The final tests in the current round were to monitor the behaviour of the
cryostat with the outer vessel at ambient temperature; then to load it
into the Large Space Simulation chamber at the European Science and
Technology Centre (ESTEC) and monitor it again, this time under the
conditions that Herschel will find in space. "We need these two extremes
so that we can correlate computer models to predict what will happen
inside the cryostat between launch and when it cools to its final
operating temperature," says Passvogel.

Now that Herschel's cryostat has passed all these tests and supplied
excellent data for the team, the task of placing the instruments inside
must begin. Then there will be more tests, in preparation for the July
2008 launch.

For more information:

Thomas Passvogel, ESA Herschel/Planck Project Manager
Email: thomas.passvogel @ esa.int

[NOTE: Images and weblinks supporting this release are available at
http://www.esa.int/esaSC/SEMDJUBE8YE_index_1.html ]


 




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