Some of the largest telescopes were built, such as the Mount Palomar 200-inch reflector, but it was soon clear that we needed something else. All the mighty telescopes were built were on the earth's surface. It was necessary to have a telescope which did not have to peer through the murky and turbulent atmosphere of the earth―a telescope in space.
Nearly a century ago, astronomers in the United States started building giant reflecting telescopes―those were unprecedented in size and vision. One of these, the 100-inch Mount Wilson reflector, was to make one of the greatest scientific discoveries ever. It was made by astronomer Edwin Hubble. He looked at the distant galaxies with the reflector, and discovered that the universe was expanding.
In 1946, an astrophysicist named Dr. Lyman Spitzer (1914 - 1997) proposed that a telescope in space would reveal much clearer images, of even farther-off objects, than any ground-based telescope. This was an outrageous idea, considering that no one had yet launched a rocket into outer space. As the US space program developed and excelled in the 1960s and 1970s, Spitzer lobbied NASA and Congress to develop a space telescope. In 1975, Congress approved funds for the space telescope, and NASA named Lockheed Martin Aerospace Company as the prime contractor to oversee its construction.
The Hubble Space Telescope (HST) took 8 years to build, held five scientific instruments, consisted of more than 400,000 parts, and had 26,000 miles of electrical wiring. HST was reported to be 50 times more sensitive than ground-based telescopes, with 10 times better resolution. After a long delay due to the Challenger disaster, HST went into orbit in 1990. Almost immediately after it was deployed, astronomers found that they could not focus the telescope. They found that the primary mirror had been ground to a wrong dimension at the Perkin-Elmer Corporation's factory. Although the defect in the mirror was less than one-fiftieth the size of a human hair, it caused the HST to suffer spherical aberration and produce fuzzy images.
Scientists came up with a replacement contact lens called COSTAR (Corrective Optics Space Telescope Axial Replacement) to correct the defect in the HST. COSTAR consisted of several small mirrors that would intercept the beam from the flawed mirror, correct the defect, and relay the corrected beam to the scientific instruments at the focus of the mirror. COSTAR replaced one of the scientific instruments when it was installed during a 1993 servicing mission by shuttle astronauts. When the HST was tested after the servicing mission, the images were vastly improved. Now, all on the instruments placed in HST have built-in corrective optics for the mirror's defect, and COSTAR is no longer needed.
HST has spent 20+ years in space. It has observed around 20,000 celestial targets, gathered more than 5 terabytes of data, and spawned nearly 3,000 scientific papers. It has made essential contributions to our knowledge of the age and evolution of the universe. It has sharpened and extended our view of the cosmos.
HST cannot observe the sun, because the intense light and heat would fry its sensitive instruments. Therefore, it is always pointed away from the sun. In addition, it cannot observe Mercury or Venus too, because they are too close to the sun. Certain stars cannot be observed by it, as they are too bright for some of its instruments. In addition to the brightness of objects, the orbit of the HST also limits what can be seen. Sometimes, targets are obstructed by the Earth itself during the orbit. This can limit the time spent observing a given object. Also, the HST passes through a section of the Van Allen Radiation Belts, where charged particles from the solar winds are trapped by the Earth's magnetic field. These encounters cause high background radiation, which interferes with the detectors of the scientific instruments. No observations can be done during these periods. The telescope orbits the Earth in around 90 minutes, or 15 orbits in a day, at 7,500 m/s.
Despite its flawed early history, the HST has performed well, yielding much scientific data and beautiful images. However, plans are underway for a new space telescope, called the Next Generation Space Telescope (NGST). NGST will be even more sensitive than HST, and provide better images of even distant objects. The age of optical space telescopes started by HST promises to revolutionize astronomy as much or more than Galileo's first use of the telescope did long ago.