In June 1990, two months after the most expensive scientific instrument in history reached orbit, NASA convened a press conference to announce that it could not focus. The Hubble Space Telescope — named for Edwin Hubble, the astronomer who showed that the universe extends beyond our galaxy and is expanding — had ridden the shuttle Discovery to orbit on 24 April 1990, at a cost of roughly 1.5 billion dollars, carrying hopes astronomers had nursed since Lyman Spitzer first proposed an observatory above the atmosphere in 1946. Its 2.4-metre primary mirror was described as the most precisely polished large mirror ever made. It was also, engineers now confirmed, precisely the wrong shape. Stars that should have been pinpoints smeared into fuzzy haloes. The great eye in the sky was myopic.
The flaw was spherical aberration: the mirror's outer edge had been ground too flat by roughly two microns, about one-fiftieth the width of a human hair — invisible by any everyday standard, catastrophic in precision optics. Investigators traced it to the contractor Perkin-Elmer, where the device used to check the mirror's shape, a null corrector, had been assembled with one lens out of position by about 1.3 millimetres. The mirror had been polished exquisitely to match a faulty template, and two cruder tests that flagged a problem were dismissed as unreliable. The ridicule was instant. Senator Barbara Mikulski branded the telescope a "techno-turkey", late-night comedians feasted, and the film Naked Gun 2½ flashed up Hubble alongside the Titanic and the Hindenburg in a gallery of famous disasters.
Spectacles fitted in orbit
The rescue rested on a strange consolation: because the mirror was so perfectly wrong, its error could be characterised exactly — and cancelled. Corrective optics could do for Hubble what spectacles do for a short-sighted eye. The engineer Jim Crocker liked to say the packaging of the idea came to him in a German hotel bathroom, watching a shower head slide on its adjustable rail: small mirrors on movable arms could be swung precisely into the telescope's light path. The result was COSTAR, a phone-box-sized unit that deployed coin-sized corrective mirrors in front of three of Hubble's instruments, while a replacement camera, WFPC2, carried its own correction built in.
In December 1993 the shuttle Endeavour caught up with Hubble, and over eleven days its crew performed a then-record five back-to-back spacewalks — Story Musgrave and Jeffrey Hoffman alternating with Kathryn Thornton and Tom Akers — to install COSTAR and WFPC2 along with new solar arrays and gyroscopes. It was the most ambitious servicing job ever attempted in orbit, and it worked completely. When the first corrected images were unveiled in January 1994, jubilant officials declared the trouble with Hubble over, and the pictures made front pages worldwide.
The ten-day stare
What followed turned embarrassment into legend. In December 1995 Hubble's director, Robert Williams, spent his personal discretionary observing time on a gamble some colleagues called a waste: pointing the telescope for ten consecutive days at a patch of apparently empty sky in Ursa Major no bigger than a grain of sand held at arm's length. The 342 stacked exposures became the Hubble Deep Field, an image swarming with roughly 3,000 galaxies, some of whose light had been travelling for over ten billion years. Successors went deeper still — the 2004 Ultra Deep Field caught around 10,000 galaxies — and rewrote our sense of how crowded the cosmos is: every blank scrap of sky, it turns out, is wallpapered with galaxies.
The repaired telescope went on to do exactly what its namesake would have wished. It watched the fragments of comet Shoemaker-Levy 9 slam into Jupiter in 1994, probed the atmospheres of planets around other stars, and helped pin down the rate of cosmic expansion — the Hubble constant — and with it the age of the universe, about 13.8 billion years; its observations of distant supernovae also supported the 1998 discovery that the expansion is accelerating. Astronauts returned four more times, the last in 2009, when COSTAR, no longer needed because every newer instrument carried its own corrective optics, was removed and sent to the Smithsonian. The telescope's spectacles had become museum pieces.
Hubble became, by most measures, the most productive telescope in the history of astronomy, the source of observations behind tens of thousands of scientific papers and of images — the Pillars of Creation among them — recognised by people who could not name another telescope. Yet its origin story remains the better parable: a perfect mirror ground perfectly wrong, and the fix flown up and fitted by hand — proof that the sharpest vision can begin with a corrected mistake.
Quiz nuggets
- Hubble launched aboard the shuttle Discovery on 24 April 1990, carrying a 2.4-metre primary mirror.
- Its spherical aberration came from a mirror edge ground too flat by about two microns — roughly one-fiftieth the width of a human hair.
- Servicing Mission 1, flown by Endeavour in December 1993, installed the COSTAR corrective optics during a record five consecutive spacewalks.
- The 1995 Hubble Deep Field stared at one small patch of Ursa Major for ten days and revealed roughly 3,000 galaxies.
- Hubble was serviced five times between 1993 and 2009; the retired COSTAR unit went to the Smithsonian.