Slashdot Mirror
Chandra Losing Its Sight To Grease
lgreco writes "The new scientist has this article about NASA's Chandra X-Ray Observatory that is losing part of its sight due to grease built up on one of its optical filters. I guess it will take a space shuttle mission to clean the filter or install one of these mini windshield wipers ..."
Heavens dimmed for Chandra space telescope
17:15 07 November 03
Exclusive from New Scientist Print Edition. Subscribe and get 4 free issues.
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.
Chandra x-ray telescope
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 did not 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.
Evaporate and condense
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 10 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.
Bake out
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. In December, 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.
Emily Singer
Return to news story
(C) Copyright Reed Business Information Ltd.
Unfortunately Chandra's orbit is way out of reach of the shuttle -- orbiting between 133,000 km (82,646 mi) and 16,000 kilometers (9,942 mi) from the Earth.
Two wrongs don't make a right, but three lefts do.
The problem only shows up with only one filter at the lower end of the filter's spectral range. The amount of buildup and its effect on various frequencies has been measured. There is even a model for how the buildup has increased over time. A review of older data and instrument calibration may be able to further validate and refine this buildup model. In most cases old data that used this filter can be corrected to a reasonable degree. In nearly all cases, this buildup has a nil impact on the scientific conclusions that have thus far been produced.
NASA is examining a potential correction to the buildup. They are considering what is known as a "bakeout": going through a warm-up and cool-down cycle to boil off the contamination. They are going through a risk / reward analysis at the moment. The reward of a buildup reduction is being compared to the risk that the bakeout will move the contamination to other instruments. It is possible that the bakeout will happen in mid December, or it will be moved from mid-December to a later date or canceled altogether.
So while the buildup is not good / unfortunate, it appears to be neither fatal nor mission threatening at the moment.
P.S. To those who asked about the using the remaining on-board fuel to lower the bottom of the orbit to a shuttle serviceable level. The remaining on-board fuel is insufficient to do that ... a rough calculation shows that it
is 1-2 orders of magnitude too small.
chongo (was here)