Telescopes in space

Why would anyone put a telescope in space, where nobody can reach it to mend it? Because of the air. The air that keeps you alive is a poor window. It stops most kinds of light from ever reaching the ground: gamma rays, X-rays, most ultraviolet and most infrared. What does get through is shaken about on the way down, which is why stars twinkle. A telescope above the air sees every kind of light, and sees it steady.

The best known is the Hubble Space Telescope. It was carried up by a space shuttle on 24 April 1990 and is still working. It is the size of a bus, 13 metres long, with a mirror 2.4 metres across. It circles the Earth a little under 500 km up, at about 27,000 km an hour, and goes round once every 95 minutes.

Hubble began with a disaster. Its first pictures were blurred. The mirror had been ground to the wrong shape: too flat at the edge by 2.2 thousandths of a millimetre, about a fiftieth of the width of a hair. Hubble had been built to be visited, and in December 1993 seven astronauts flew up and fitted it with what were, in effect, glasses. The pictures have been sharp ever since.

Hubble’s successor is the James Webb Space Telescope, launched on 25 December 2021. Its mirror is 6.5 metres across, too wide for any rocket, so it is made of 18 six-sided pieces that were folded for launch and opened out in space. Each piece is coated in a film of real gold, because gold is very good at reflecting infrared light, the kind Webb is built to see.

Infrared matters for two reasons. It passes through dust, so Webb can see into the clouds where stars are being born. And the light of the earliest galaxies has been stretched on its long journey until it is infrared and nothing else. But everything warm glows in infrared, the telescope included. So Webb sits 1.5 million km from the Earth, behind a sunshield the size of a tennis court, and stays colder than 223 degrees below freezing.

Webb’s first deep picture was shown on 11 July 2022. It covers a patch of sky the size of a grain of sand held at arm’s length, and it holds thousands of galaxies. Some of their light set out more than 13 billion years ago. There are dozens of other telescopes in space, for X-rays, gamma rays and more, and every one of them is there for the same reason: to get above the air.