← Did You Know?
🔭Space8 min read

The Hubble Telescope

It launched with a flawed mirror, was rescued by astronauts wearing its spectacles, and has now outlived the spacecraft that could reach it. It is running on its last gyroscope.

In plain English

The point of putting a telescope in space is not to get closer to the stars. The distance gained is nothing. The point is to get above the air.

Stars twinkle because their light arrives through hundreds of kilometres of moving atmosphere, smeared by every gust and thermal on the way down. From orbit there is no air, so a modest mirror sees razor-sharp, and sees colours of light the atmosphere blocks entirely. Hubble's mirror is 2.4 metres across, which no longer even makes the top forty on the ground. Above the sky, it rewrote the textbooks anyway.

It has been doing so since April 1990. Five crews of astronauts flew up to repair and upgrade it while it orbited at 27,000 kilometres per hour. No spacecraft exists that can reach it any more, and since 2024 it has been steering with one gyroscope. It is still working.

Five things to file under "wait, what?"

  • It launched broken, with the most precisely wrong mirror ever made. The main mirror was ground to the wrong shape by about two microns, a fiftieth of a hair's width, because a testing instrument had been assembled a millimetre out of place. The grinding itself was superb; the target was wrong. For three years the world's most expensive telescope could not focus.

  • The fix was a pair of glasses, fitted by hand. Because the mirror was precisely wrong, its error could be precisely cancelled. In 1993 astronauts installed corrective optics that intercepted the light on its way to the instruments: spectacles for a telescope, fitted on spacewalks 550 kilometres up. The before-and-after images made the front pages worldwide.

  • Its most famous discovery came from photographing nothing. In late 1995, Hubble was pointed at a patch of apparently empty sky the size of a grain of sand held at arm's length, for ten days, over 340 orbits. The gamble was ridiculed as a waste of scarce time. The patch turned out to contain roughly three thousand galaxies. There is no empty sky; there is only sky you have not stared at long enough.

  • It aims like a marksman standing on a merry-go-round. Hubble circles Earth every 95 minutes, yet holds its aim to about seven thousandths of an arcsecond, which is like keeping a laser fixed on a five-pence piece from hundreds of miles away, for hours. That steadiness, more than the mirror, is the real engineering miracle.

  • It is running out of ways to point. Of the six gyroscopes astronauts fitted in 2009, most have failed. Since June 2024 Hubble has operated on a single gyro, with another held in reserve, working more slowly but still producing science. With the Shuttle retired there is no way to service it, and no rescue mission has been approved. Depending on how strongly the Sun inflates the upper atmosphere, drag brings it down some time between roughly 2030 and 2040.

The full story

The long road up

An orbiting telescope was proposed seriously in 1946, decades before anything could launch one. It took until 1990, and a budget that overran into billions, for Hubble to fly. Weeks later came the spherical aberration diagnosis, and "Hubble trouble" became a late-night punchline. The recovery, mounted in 1993 by seven astronauts over five spacewalks, remains among the most delicate repair jobs ever attempted, and it turned the punchline into the most productive scientific instrument in history.

Four more servicing missions followed, the last in 2009. Each swapped instruments, gyroscopes and computers, so the Hubble working today is a ship of Theseus: almost every replaceable part has been replaced. What cannot be replaced any more is anything at all, because nothing flying can reach it.

What it actually found

Its namesake, Edwin Hubble, discovered in the 1920s that the universe is expanding. The telescope's founding job was to measure how fast, a number astronomers had argued over by a factor of two for decades. Hubble the machine settled it, which pinned down the age of the universe at about 13.8 billion years.

Then it helped find something nobody ordered. In 1998, measurements of distant supernovae, with Hubble doing crucial work, showed the expansion is speeding up. Something, labelled dark energy and still unexplained, makes up most of the universe. Two of the site's other stories, on the Rubin Observatory and on Vera Rubin herself, pick up that thread, because the astronomer the new observatory is named for found the first hard evidence for the universe's other missing ingredient.

Add the deep fields, proof that galaxies grew and collided over cosmic time, the first chemical sniffs of atmospheres on planets around other stars, and one more thing easy to overlook: Hubble made the universe public. Its images are the ones on classroom walls.

The slow goodbye

Hubble orbits low enough that the thinnest wisp of atmosphere still drags on it, and every year it sinks a little. When the Sun is active, the upper atmosphere swells and the sinking quickens, which is why its re-entry date is a range, not a year. A privately funded plan to boost it higher was studied and not taken up. The likeliest end is a robotic vehicle guiding it to a safe re-entry over ocean, some time in the 2030s.

Until then it works, one gyroscope at a time, alongside the successor that now out-sees it in the infrared. The James Webb Space Telescope gets the headlines, and this site tells its story too. But Webb sits four times further away than the Moon, untouchable by design, and was built on an assumption Hubble proved: that a telescope above the sky is worth almost any trouble.

Go deeper

For the curious:

On YouTube:

Keep going

More things worth knowing

All topics →