Updated 12/30/03



 

Damaged detectors or no,
Chandra snapped this image
in the X-ray spectrum.
Note how it differs from
the visible and radio
images of the same location.

Courtesy Chandra X-Ray Observatory.





 




Here's what's seen in the radio section of the spectrum.

 

And here's what it looks like in visible light.

Space: The junkman cometh

Perilous protons
Ever since the first satellites were launched, scientists have known that charged particles in space could monkeywrench instruments. Now The Why Files has learned that Chandra, the new, expensive and ultra-accurate X-ray telescope launched last summer, has burned its retina on energetic protons. A glowing ball.

The sun blows a huge amount of protons out into space, creating the solar wind. Some of the protons are trapped by Earth's magnetic field in the Van Allen radiation belts. It turns out that Chandra's orbit passes through the outermost belt, where a high number of low-energy protons cooked one of the sensitive detectors, causing "physical, permanent damage," says Dan McCammon, professor of physics at the University of Wisconsin-Madison.

The silicon-based detectors in question, called charge-coupled devices (CCDs), produce electrons when struck by X-rays. Accuracy is critical: Losing just a few electrons can degrade its performance.

And that's just what happened during the first weeks of Chandra's operation, says McCammon. Protons came down the nested mirrors and struck CCDs on the Advanced Imaging CCD Spectrometer, knocking silicon atoms out of place and creating traps for electrons. "It did degrade one set of detectors," says McCammon. "That's a surprise, we've not had CCDs up that high before."

In fact, the protons caused as much damage in a couple of weeks as the designers expected to happen in 10 years. Now that the problem is understood, the solution is simple: the detector is moved out of the focal region when the protons are intense -- during the small fraction of Chandra's orbit that passes through the dangerous radiation belt.

McCammon points out that the damage doesn't affect the razor-sharpness of the images that is Chandra's speciality. Special software can compensate for much of the loss in accuracy of measuring the energy of the X-rays, and astronomers who need the best possible energy measurements can use one of the two "upside down" CCD chips that weren't damaged by the protons.

Getting weaker
The Van Allen belts cause a different problem closer to Earth, in an area east of Brazil that's sometimes called the Sargasso Sea of satellites. In this so-called South Atlantic Anomaly, where sensitive instruments run amok and hardened scientists cringe, Earth's magnetic field is weak, and radiation from the Van Allen belts dips closer to the ground.

Normally, the radiation is more than 1,000 kilometers up, but in the Anomaly, it sinks within a few hundred kilometers of the Earth's surface -- prime satellite territory.

Ever since a 1968 mission, scientists have known about the Anomaly, and they've learned to cope by changing schedules and installing shields. Enough about this radiation stuff. What are the dangers of meteoroids and space junk?


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The Why Files
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