The Hubble Space Telescope has touched every area of astronomy, from the Solar System to the most distant galaxies. In the public eye, it’s so well-known that many people think it’s the only world class astronomy facility. In fact, it operates in a highly competitive landscape with other space facilities and much larger telescopes on the ground. Although it doesn’t own any field of astronomy, it has made major contributions to all of them. It has contributed to Solar System astronomy and the characterization of exoplanets, it has viewed star birth and death in unprecedented detail, it has paid homage to its namesake with spectacular images of galaxies near and far, and it has cemented important quantities in cosmology, including the size, age, and expansion rate of the universe.
Ranked by size of the mirror, Hubble wouldn’t make it into the top fifty largest optical telescopes. Its pre-eminence is based on three factors associated with its location in Earth orbit. The first is liberation from the blurring and obscuring effects of the Earth’s atmosphere. Ground-based telescopes typically make images far larger than their optics would allow because turbulent motion in the upper atmosphere jumbles the light and smears out the images. Hubble gains in the sharpness of its vision by a factor of ten relative to a similar-sized telescope on the ground. Earth orbit also provides a much darker sky, which affects the contrast and depth of an image. The difference you might see in going from a city center to a rural or mountain setting is only part of the story; natural airglow and light pollution affect even the darkest terrestrial skies. The last feature of a telescope in Earth orbit is its ability to gather wavelengths of radiation that would be partially absorbed or even quenched by the Earth’s atmosphere. Hubble has taken advantage of this by working at invisibly long infrared wavelengths and invisibly short ultraviolet wavelengths.
Launched in 1990, the Hubble Space Telescope (HST) is well into its third decade of operations, and it’s easy to take for granted the beautiful images that are released almost weekly. But it was not an effortless journey for NASA’s flagship mission. In 1946, Yale astronomy professor Lyman Spitzer wrote a paper detailing the advantages of an Earth-orbiting telescope for deep observations of the universe. The concept had been floated even earlier, in 1923, by Hermann Oberth, one of the pioneers of modern rocketry. In 1962, Spitzer was appointed chair of a committee to flesh out the scientific motivation for a space observatory. The young space agency NASA was to provide the launch vehicle and support for the mission. NASA had demonstrated the great potential of space astronomy, but also the risks — two of their four missions failed. After the National Academy of Sciences reiterated its support of a telescope in space in 1969, NASA started design studies. But the estimated costs were $400 to $500 million and Congress balked, denying funding in 1975. Astronomers regrouped, NASA enlisted the European Space Agency as a partner, and the telescope shrunk to 2.4 meters. With these changes, and a price tag of $200 million, Congress approved funding in 1977 and the launch was set for 1983. More delays followed. Making the primary mirror was very challenging and the entire optical assembly wasn’t put together until 1984, by which time launch had been pushed back to 1986. The whole project was then thrown into limbo by the tragic loss of the Challenger Space Shuttle in January, 1986. When the shuttle flights finally resumed, there was a logjam of missions so more years slipped by.
Hubble was finally launched on April 24, 1990 by the shuttle Discovery. A few weeks after the systems went live and were checked out, euphoria turned to dismay as scientists examined the first images and saw they were slightly blurred. The telescope could still do science but the original goals were compromised. Instead of being focused into a sharp point, some of the light was smeared into a large and ugly halo. The primary mirror had an incorrect shape. It was too flat near the edges by a tiny amount, about 1/50 of the width of a human hair. Hubble’s mirror was still the most precise mirror ever made, but it was precisely wrong. The spherical aberration problem may be ancient history now, but at the time it was a public relations nightmare for NASA. Its flagship mission could only take blurry images. Commentators and talk show hosts lampooned the telescope and David Letterman presented a Top Ten list of “excuses” for the problem on the Late Night Show.
What went wrong? When the primary mirror was being ground and polished in the lab by Perkin-Elmer, they used a small optical device to test the shape of the mirror. Because two of the elements in this device were mis-positioned by 1.3 millimeters, the mirror was made with the wrong shape. This mistake was then compounded. Two additional tests carried out by Perkin-Elmer gave an indication of the problem, but those results were discounted as being flawed! No completely independent test of the primary mirror was required by NASA, and the entire assembled telescope was not tested before launch, because the project was under budget pressure. NASA was embarrassed by the failure. Their official investigation put it succinctly: "Reliance on a single test method was a process which was clearly vulnerable to simple error." As the old English idiom says: penny wise, pound foolish. The propagation of a small problem into a huge one recalls another aphorism from England, where a lost horseshoe stops the transmission of a message and the result affects a critical battle: for the want of a nail, the war was lost.
The problem was fixed in 1993. It helped that the mirror flaw was profound but relatively simple; the challenge was reduced to designing components with exactly the same mistake but in the opposite sense, essentially giving the telescope prescription eyeglasses. Installing the corrective optics was probably the most challenging mission for astronauts since the Apollo Moon landings. Seven astronauts spent thousands of hours training for the mission, learning to use nearly a hundred tools that had never been used in space before. They did a record five back-to-back space walks, each one grueling and dangerous, during which they replaced two instruments, installed new solar arrays and replaced four gyros. The first servicing mission was a stunning success. It also played significantly into the vigorous debate in the astronomy and space science community over the role of humans in space. NASA had always bet that the public would be engaged by the idea of space as a place for us to work and eventually live. But after the success of Apollo, public interest and enthusiasm waned. Most scientists think that it’s cheaper and less dangerous to create automated or robotic missions than to service them with astronauts. Hubble was of course designed to be serviced by astronauts, but the problems they were able to solve in orbit, coupled with the positive public response (and high TV ratings during the space walks) persuaded many that the human presence was essential and inspirational.
More servicing missions followed. Each one rejuvenated the facility and kept it at the cutting edge of astronomyresearch. All of the original instruments have been replaced, along with computers, power system, and gyroscopes. The Hubble Space Telescope is reminiscent of the ship of Theseus, a story from antiquity where every plank and piece of wood of a ship is replaced as it plies the seas.
Hubble’s continual rejuvenation is a major part of its scientific impact. The instruments built for the telescope are state-of-the-art, and competition for time on the telescope has consistently been so intense that only one in eight proposals get approved. All this comes with a hefty price tag. Estimating the cost of Hubble is difficult because of how much to assign to the Shuttle launches and astronaut activities, but a 25-year price tag of $8 billion is probably not far from the mark. For reference, the budget was $400 million when construction started and the cost at launch was $2.5 billion. Compared to slightly larger 4-meter telescopes on the ground, Hubble generates 15 times as many scientific citations (one crude measure of impact on a field) but costs 100 times as much to operate and maintain. Regardless of its cost, the facility sets a very high bar on any subsequent space telescope. As Malcolm Longair, Emeritus Professor at the University of Cambridge and former Chairman of the Space Telescope Science Institute Council has observed: "The Hubble Space Telescope has undoubtedly had a greater public impact than any other space astronomy mission ever."