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, and was expected to work for 15 years through to 2014.

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.

According to New Scientist, Turner 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.

This press release by New Scientist was issued in relation to a story by Emily Singer that will appear in the November 8 edition of New Scientist

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