QotW: How Hubble's Telescope mirror stays clean?

The Hubble telescope has been orbiting 550 kilometres above the earth for almost 31 years now. It's a wonder how it would be maintained and cleaned at that altitude. Fortunately, Otis Kingsman asks Sarah Kendrew from the European Space Agency to shed some light on this question.

Sarah - The short answer is, we don't have to clean it. Despite what we hear about dust, debris and space, space is actually extremely empty and clean so there are no particles sticking to the surface of the mirror. Outside of the Earth's atmosphere, there's also no moisture and no molecules that can cause corrosion or degradation of the mirror. A risk Hubble does face is impact from micrometeorites. These are tiny specs, smaller than sand grains, that have typically been chipped off or eroded from asteroids in the solar system, or sometimes also materials from launched rockets and other broken satellites. Those types of impacts do happen quite regularly and they can cause some weathering and damage over time.

Otis - While these grains are small, the lack of air resistance in space causes these micrometeors to travel very fast before colliding with the telescope. Fortunately, there are a few integrated features to help avoid impacts with the mirror.

Sarah - The design of the Hubble telescope includes a tube-like structure to protect the mirror and other optics. So, these micrometeorites typically collide with the tube or the outer material. The damage is mainly seen on solar panels and on the outer covering materials.

Otis - This space debris has caused five separate missions between 1993 and 2009 to replace parts of the telescope. Thankfully, these broken pieces haven't been simply discarded.

Sarah - From the parts that were returned to earth after service emissions of Hubble, scientists were actually able to study how many of these impacts had occurred and what kind of materials the meteorites were made of. That's helped us to understand the low earth orbit environment, helped us to understand better what kinds of particles are out there, how much risk they pose to satellites, and, in turn, how to design our spacecraft and our satellites to be better protected against those impacts.

Otis - So, Daniel, the Hubble telescope is prevented from getting dirty by the natural lack of particles in space to stick to the surface, alongside the careful construction of the outer shell to protect it. Thanks to this, we can still get fantastic pictures of our galaxy 31 years after its first launch without needing to clean the mirror. Next week, we'll be answering this question from listener Krzysztof -

Krzysztof - Why don't plants freeze to death during Winter?