Twenty five years ago today, the Space Shuttle Discovery blasted off with the Hubble Space Telescope on board. The crew were Commander Loren Shriver, Pilot Charles Bolden (who is now the NASA administrator), Mission Specialist Bruce McCandless, Mission Specialist Kathryn Sullivan, and Mission Specialist Steven Halway. None were rookies, and the Shuttle’s mission was executed flawlessly — albeit about four years late. It had been scheduled to launch in 1986, but the Challenger accident changed that. This hourlong video is the STS-31 mission highlights reel. Check out how young Charles Bolden looks. 😉
But all was not as perfect as it seemed. Less than a month after launch, Hubble completed on-orbit checkout and saw “first light”. First light is a major milestone for any telescope — it’s the first time it is used to image something. Hubble’s first light image isn’t bad, nor particularly complicated. Here’s the first image from the Wide Field Planetary Camera (which has since been replaced twice), compared against a ground-based image taken with a large observatory. Notice that in Hubble’s image, you can actually tell the bright star at the top is a double.
It’s definitely an improvement over ground-based images, but mission scientists quickly realized it wasn’t as good as they were expecting. It took a while to hunt down the problem, but ultimately they discovered that the huge primary mirror had a refractive error — in human terms, it had astigmatism. It’s a very subtle astigmatism, but enough to cause serious disappointment. Fortunately, as Hubble had been designed to be serviced on orbit, it was also fixable; the first Servicing Mission delivered COSTAR, a device that functioned essentially as a corrective lens for Hubble. The fault was traced to a tiny paint chip on the device used to measure the lens during grinding and during qualification tests afterwards. (Since the paint chip was present both times, the mirror passed testing. But the upshot was that the same device could be used to derive a corrective lens.) It’s not like the manufacturer didn’t know what they were doing; Hubble was built by people who had been building space telescopes of that size for years. Yes, really — Hubble was not the biggest space telescope when it was built. It was just the only one of that size designed to point *away* from Earth, if you follow my meaning.
Despite the blurry vision, Hubble returned spectacular images of Pluto/Charon, the Einstein Cross, and more. But each servicing mission made it better and better. Eventually, COSTAR was made obsolete, as starting with Servicing Mission 2, they started putting the correction directly into each of the cameras, and eventually COSTAR was removed altogether. It’s in the National Air & Space Museum now. Wide Field Planetary Camera 2 (successor to the original WFPC, with the COSTAR correction built directly into it) is also there, having been replaced during the final servicing mission. And as more and more devices were added or replaced, the instrument got better and better. Hubble has shown us deeper into space (and further back in time, therefore) than any other instrument.
For your everyday astronomy needs, it is now outclassed by ground-based instruments outfitted with adaptive optics, as ground-based telescopes can be extremely large, but it still dominates in two other areas: near-infrared astronomy (which is very limited on the ground) and extremely long exposures. As a space telescope, it can take amazingly steady images over an exceptionally long period of time. It first demonstrated the implications of this in 1995 when it was pointed at a seemingly empty spot in the sky just 2.5 arcminutes across. Combining 342 images taken over 10 days into a collective exposure time of about 35 hours (the exact time was different at different wavelengths; they were distributing observations across multiple sessions to avoid cosmic ray damage to the imagers) produced an image that was full of distant and previously unobserved galaxies. It was breathtaking, and Hubble has repeated the feat several more times over its career, with ever longer exposures, most recently about a year ago, showing galaxies as they existed just 100 million years after the Big Bang.
Hubble has come a long ways over its 25 years in orbit, and has many good years left in it. Its had many components replaced, including ones not designed to be replaced. But with the Shuttle gone, it is unlikely it will ever be serviced again. Its ultimate fate will probably be to have an unmanned deorbit module capture it and then steer it to a destructive reentry. Currently, Hubble vastly exceeds NASA’s risk tolerance for an uncontrolled reentry, due to the fact that most (and perhaps even all) of its enormous mirror would likely survive the plunge and present a substantial hazard.
In the meantime, though, there it is, shining in the sunlight and returning amazing science: