Hubble Space Telescope: History, Design, and Discoveries

Introduction: Vision for a Space Telescope

As the world's first Earth-orbiting reflecting telescope, the Hubble Space Telescope (HST) represents the culmination of the dreams of many astronomers and the fulfillment of a hope that began in the 1950s, when the United States launched instruments into space to study the Earth's atmosphere. Our planetary speck in space — namely, the Earth — is only one of hundreds of billions of planets that surely orbit other star systems; our Milky Way galaxy, composed of billions of stars and other astronomical bodies, is but one of billions of other galaxies. In order to increase our knowledge of the universe, the Hubble Space Telescope serves a vital function: since its launch in 1990, despite several crucial problems, it has revealed a universe full of mysterious bodies, nebulae, star systems, and galaxies, and has expanded the possibilities that humankind is not alone in the universe.

The first scientific idea for a telescope such as the HST came about in 1946 when Lyman Spitzer issued a proposal for a space telescope with a primary mirror between sixteen and fifty feet in diameter. Throughout the 1960s and 1970s, with the ultraviolet observatories OAO-2, OAO-3, and the IUE in the planning stages, the space telescope remained only a dream due to the lack of technology. Yet Spitzer did not give up; by the 1970s he had offered another proposal for a space telescope that would be able to provide images of very faint objects, in contrast to ground-based telescopes, which suffer from distortion caused by the Earth's atmosphere. According to Spitzer, such a telescope would not "supplement our present ideas of the universe we live in, but rather would uncover new phenomena not yet imagined and perhaps would modify profoundly our basic concepts of space and time" (42).

As the years passed, many astronomers came to understand that such an instrument would provide new opportunities for the present and for the future. In 1965, the National Academy of Sciences issued a report focusing on the construction of a space-based telescope. In 1970, the NAS convened an Astronomy Survey Committee and placed responsibility on the California Institute of Technology to set priorities for the telescope's construction. Between 1973 and 1977, the Space Telescope Working Group was formed to discuss the project in detail and to ensure that the space telescope contained the best instruments possible within certain budgetary and design restrictions.

Engineering and Construction Challenges

By the early 1980s, plans were well underway for the construction of the Hubble Space Telescope. However, several engineering problems had to be overcome — namely, the effects of gravity and moisture on the telescope's primary mirror and its internal structure. Gravity greatly affected the lightweight trusses forming the internal skeleton of the HST, which were in danger of sagging just enough to disrupt the fine alignment of all the instrument's integral parts. On the Earth's surface, there is no practical way to support every part of an object equally well, but the engineers managed to design a method to support the primary mirror at various points, reducing the overall stress upon it. As long as the mirror remained on the ground it was subject to bending from gravity, but once in space, gravity would be virtually eliminated.

The mirror itself was designed by the Perkin-Elmer Corporation, which worked for three years to solve the problem of creating such a mirror on the ground. Made of special ultra-low expansion glass, the mirror consisted of honeycomb cores with most of the glass behind the reflecting surface removed in order to decrease its total weight. Another essential property of the mirror, made by the Corning Glass Company, was its weldability, which allows a single slab of glass to be fused from individual honeycombs. As a result, the mirror — ninety-four inches in diameter — weighs under one ton. By comparison, the 200-inch mirror for the Mount Palomar telescope weighs more than fourteen tons and has only a little more than twice the light-gathering power of the HST mirror. The telescope's mirror system is based on the Ritchey-Chrétien design, which "affords wider and flatter fields of view than traditional Cassegrain systems" ("Hubble Space Telescope," Encyclopedia.com), and even though "the Hubble Space Telescope has a mirror 15 times smaller than an Earthbound telescope, it can resolve detail almost 100 times finer" (Nemiroff, Internet).

By 1985, the HST was complete and represented new frontiers in optics, detectors, and pointing accuracy. Yet as it stood ready for launch in early 1990, the scientists responsible for approving the launch faced a dilemma: the approaching solar maximum — a period of heightened solar activity that includes sunspots, active regions, and solar flares. In March of 1989, one of the largest groups of sunspots ever observed became apparent, and a solar flare emitted huge numbers of energetic particles that caused brilliant displays of the northern lights. For the telescope, this meant higher densities of gas in the atmosphere, which could affect the amount of drag on the instrument's launch rocket.

An even bigger setback had occurred in 1986 with the Challenger explosion disaster, which claimed the lives of all the astronauts aboard the space shuttle. As a result, the initial launch of the Hubble Space Telescope was delayed. During this time, however, engineers at NASA used the delay to their advantage, running numerous tests on the HST — checking all alignments related to the primary mirror and double-checking supporting devices such as internal computers, the skeletal structure, and other important systems. After all of this delay, the Hubble Space Telescope was finally launched aboard the STS-31 mission of the space shuttle Discovery on April 24, 1990.

At the time of its launch, the HST stood as an engineering marvel, defined by its scientific instruments, the ninety-four-inch primary mirror, and the detectors designed to analyze all incoming images. For the HST engineers, the CCD detectors were the most crucial aspect of the instrument, as they were designed to sense the light of an image and then convert it into electrical impulses. These detectors would work at their optimum best with a long focal length — which creates a sharper and larger image — and with the smallest possible pixels in order to produce the clearest image achievable.

The scientists at NASA were restricted by the fact that, in order to fit inside the space shuttle, the HST could be no longer than forty-two feet. The engineers therefore chose an optical system similar in design to a Newtonian reflector. In the case of the Hubble, a large primary mirror reflects incoming light to a secondary mirror supported sixteen feet ahead of the primary by a truss rod arrangement, which blocks a small portion of the incoming light. The light reflected from the primary travels around the secondary mirror and is not critically diffused. After being reflected from the secondary mirror, the light passes through a small hole in the center of the primary, where it is detected and analyzed by the onboard instruments. The actual image is formed a few feet behind the primary, and the total effective focal length equals one hundred and ninety feet. This arrangement is also referred to as a Cassegrain system and is common in Earth-bound telescopes.

Launch, Spherical Aberration, and Repair

The HST also contains four additional mirrors placed behind the small hole in the primary mirror. Three of these smaller mirrors direct part of the incoming light to fine guidance sensors, which help point the telescope in the proper direction. A fourth mirror directs a portion of the light to the wide-field planetary camera, which captures wide-field images. In addition, the HST contains instruments used to analyze the color and total intensity of light. These include beams directed to the high-resolution photometer, high-resolution spectrograph, faint object camera, and faint object spectrograph.

After all of the hard work, design, and engineering that went into creating the Hubble Space Telescope, scientists at NASA soon discovered that something was terribly wrong with the instrument. On May 20, 1990, the HST sent its first images to the Goddard Flight Center in Greenbelt, Maryland; the Marshall Space Flight Center in Huntsville, Alabama; and the Space Telescope Science Institute in Baltimore, Maryland. The subject was a star cluster over 1,300 light years away.

No one expected these first images to be spectacular — they were intended as an evaluation of the telescope's performance. Star images normally appear as bright spots surrounded by a small amount of scattered light, with most of the star's light focused by the telescope's mirrors into a small core. This first image, however, showed a sharp spike of light surrounded by a large halo and strange tendrils extending out from the central core. After extensive discussion among scientists and engineers, NASA "discovered a major flaw in the giant mirror — it was too flat on one edge by 1/50th of the width of a single human hair" (Stathopoulos, Internet). In technical terms, the primary mirror was suffering from spherical aberration: the mirror had been ground incorrectly. The curve at the center of the mirror did not match the curve at its edge, meaning the mirror was incapable of reflecting light into a tight focal point. Instead, it spread the light over a large area; while the center of the mirror focused well, the edge was out of focus, creating a large halo of light.

On June 17, 1990, Hubble's Faint Object Camera (FOC) took its first image, and almost immediately the same problems appeared. This ruled out other possible causes and necessitated a comprehensive range of tests on the primary mirror. After extensive investigation, all parties linked to the HST project concluded that a serious mistake had been made. The announcement of the spherical aberration drew considerable public criticism; some suggested that NASA place its other expensive projects on hold or eliminate them altogether. This situation was compounded by the earlier Challenger explosion, and the two failures together became a major embarrassment for NASA. It was clear that communication within the mirror-making team had broken down, and that unless the flaw was corrected, the HST would become one of the most expensive mistakes in American space history.

Incredibly, despite the spherical aberration, the HST was not a total loss. It could still make observations, but only 15% of the light could be focused into a central core instead of the intended 80%. The HST had been designed to obtain images that land-based telescopes could never hope to achieve, and without a fix it would remain a compromised scientific instrument. The engineers at NASA and those responsible for grinding the primary mirror therefore devised a bold solution: a team of astronauts would be sent into space to repair the telescope. In 1993, "NASA astronauts installed a special instrument inside the Hubble with tiny mirrors that corrected the distorted light from the primary mirror. With these corrective mirrors, perfectly-focused light reached the scientific instruments" ("Eye on the Universe," Internet).