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Hubble is in space and the mirror is exposed, so how has the mirror stayed clean? Is it possible that space debris and micro particles could hit it?

Brandon Doyle, Albion, New York
Hubble
The Hubble Space Telescope floats against the background of Earth.
NASA
After 20 years of providing some of mankind’s most spectacular pictures of the universe, the Hubble Space Telescope’s 7.9-foot-diameter (2.4 meters) primary mirror remains remarkably robust and resilient. Astronomers have not measured any degradation in the amount of light the telescope collects and sends to its onboard instruments. This is true even at short-ultraviolet wavelengths, where any contamination would be the most pronounced.

That said, the mirror has probably undergone micrometeorite erosion, but there is no direct way to measure the amount of “pitting.” To be safe, engineers turn Hubble’s aft shroud — which has a micrometeorite bumper — into the directions of meteoroids during the peak of the Leonid and Perseid meteor showers. Likewise, scientists think Hubble’s collision with oxygen atoms at 350 miles (560 kilometers) altitude also pits the mirror.

When astronauts installed new instruments during the past space shuttle servicing missions, the process introduced additional contaminants. Water and hydrocarbons outgas from the new hardware when exposed to the vacuum of space. However, we have no evidence that this has affected the mirror.
 
Even after 20 years in space, the graphite epoxy truss that holds the entire optical telescope assembly together still outgasses. As it does, its dimensions change slightly. The effect is that the telescope’s focus changes as the distance between the primary and secondary mirror shrinks slightly. Once a year, engineers tweak the telescope’s focus by a few microns.

There is no practical way to repair the mirror, short of bringing Hubble back to Earth. But NASA abandoned that scheme in the 1980s because, ironically, the space shuttle landing would have introduced serious contamination issues for the telescope. What’s more, the costs, not to mention the risks of transporting the telescope to and from space, would have been excessive. — Ray Villard, Space Telescope Science Institute, Baltimore, Maryland
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