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NASA's Space X-Ray Telescope Going Partially Blind



 
 
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Old November 6th 03, 07:38 PM
JimO
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Default NASA's Space X-Ray Telescope Going Partially Blind

NASA's Space X-Ray Telescope Going Partially Blind


Contact: Claire Bowles

44-207-331-2751
New Scientist

Heavens are dimmed for Chandra space telescope
NOTE: This release has been updated on 6 November.
ONE of NASA's highest profile space telescopes is losing its sight. The $2
billion Chandra X-Ray Observatory is suffering from a mysterious build-up of
grease on an optical filter in front of one of its cameras, blocking almost
half the light at some frequencies.

Since being placed in orbit by the space shuttle in 1999, Chandra has been
studying X-rays emitted by astronomical objects such as quasars and black
holes. It is expected to carry on working for up to 15 years.

Jane Turner, an astrophysicist at NASA's Goddard Space Flight Center in
Greenbelt, Maryland, was one of the first scientists to spot something
strange in her data. She compared data from an instrument on Chandra called
the Advanced CCD Imaging Spectrometer (ACIS) with similar data from a
European spacecraft called the X-ray Multi-Mirror (XMM) telescope, and found
some discrepancies in the low-energy region of the X-ray spectrum.

This low-energy data is useful for determining how much gas there is between
the instrument and an X-ray source because it can show how much light is
absorbed. If you didn't know about the contamination, she says, it could
look as if there was a gas cloud in front of the object of study.

Scientists soon identified a problem with a filter in front of the
instrument, which allowed them to add a correction factor to their data. "It
slowed everyone down at first, but these things happen," she says.
Astronomers had expected to see a certain amount of contamination on the
filter.

Some materials used on spacecraft evaporate in a vacuum and tend to settle
on the coldest surfaces nearby. ACIS is at about -100 C. But the level of
contamination is much higher than anyone anticipated.

"There is ten times as much contamination as we expected at launch," says
Herman Marshall, an astrophysicist with the Center for Space Research at
MIT. In the three years since the launch, a layer of grease 0.37 micrometres
thick has built up on the filter. That's thicker than the filter itself, he
says.

Scientists are not yet sure what is causing the build-up. Analysis of the
contamination shows that it contains carbon and fluorine, which points to a
problem with a fluorocarbon lubricant called Braycote used on the spacecraft
or with other sources of fluorine such as Teflon-coated screws.

Braycote was chosen because it does not normally evaporate at low
temperatures. But Marshall thinks the contamination may have occurred when
molecules in the lubricant were broken down by mechanical stresses and then
bombarded with radiation.

These breakdown products could then have evaporated and settled on the
filter. The contamination only affects a small percentage of the data
Chandra is collecting. "The issue is mainly with sources at lower energies
and lower temperatures," says Dan Schwartz, a physicist at the Smithsonian
Astrophysical Observatory in Cambridge, Massachusetts.

But in this region of the spectrum the contamination is absorbing about half
of the light, he says. "I would guess about 20 to 25 per cent of people have
added uncertainty in their data, but it probably doesn't affect scientific
conclusions." Now engineers are working out how to remove the grease. Next
month, they may heat up the instrument in the hope of boiling away the
contaminants.

A "bakeout" has its own risks, because too much heat could damage the camera
or the filter. Or the contamination could settle somewhere worse, says
Chandra programme manager Keith Hefner. So far, the other instruments on
board are unaffected. "The vehicle is still performing well," says Hefner.


###
Written by Emily Singer


 




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