Space telescopes

Last updated: April 13, 2009.

Telescopes have told us most of what we know about the Universe. But because Earth's atmosphere scatters or distorts much of the radiation given off by distant objects, Earth-bound telescopes can answer only a fraction of our questions about space. The only real way to study the Universe is with a space telescope mounted on a satellite outside Earth's atmosphere.

Photo: Redeployment of the Hubble Space Telescope (HST). Picture courtesy of Great Images in NASA

How telescopes work

Photo: Earth-bound telescopes (like the historic, 26-inch refractor telescope at the U.S. Naval Observatory in Washington, D.C., shown here) can pick up only so much—hence the need for telescopes that travel into space. Photo by Seth Rossman courtesy of US Navy.

All telescopes work by collecting electromagnetic radiation (such as light, infrared, microwaves,or X-rays) given off by distant objects. Different kinds of space telescopes specialize in collecting different kinds of radiation. A satellite called the International Ultraviolet Explorer (IUE) launched in 1978 gained much information about objects such as supernovae (very bright exploding stars) by analyzing short-wavelength ultraviolet rays. These rays cannot be properly studied on Earth because they are scattered by the atmospheric ozone layer. High-energy gamma rays are given off during violent events in space. They have been studied by NASA's Compton Gamma-Ray Observatory, launched in 1991. At the other end of the electromagnetic spectrum, the Infrared Astronomical Satellite (IRAS) launched in 1983 was able to discover more than 350,000 new sources of infrared radiation, including six new comets. The $2 billion Hubble Space Telescope, launched in 1990, began a new era in space observation, sending back razor-sharp colored pictures of distant galaxies. It is so powerful that it can pick out objects 100 times fainter than the best telescopes on Earth.

Space telescopes have come a long way since the first Orbiting Solar Observatories (OSO) were launched in 1962. NASA's Next Generation Space Telescope (NGST), planned for a launch in 2007, will study infrared sources to find out more about how the first stars and galaxies in the Universe were formed.

Types of telescopes

Distant objects give off radiation in different bands of the electromagnetic spectrum. Astronomers can use a range of different telescopes to analyze the whole range of electromagnetic radiation, from long-wavelength radio waves to short-wavelength gamma rays. Here are some examples:

• Radio waves are picked up by dish-shaped radio telescopes such as the 1000 ft (305 m) Arecibo Observatory in Puerto Rico.
• Cosmic microwaves cannot penetrate the lower atmosphere. They are studied by satellites such as Cosmic Background Explorer (COBE).
• Because water in the lower atmosphere absorbs infrared, instruments such as the Infrared Astronomical Satellite (IRAS) must study it in space.
• Visible light can be picked up by conventional telescopes such as the 16.6 ft (5.08 m) Hale telescope on Mount Palomar in California, USA.
• Much ultraviolet light is absorbed by Earth's ozone layer, but satellites such as the International Ultraviolet Explorer (IUE) can study it from space.
• The Roentgen Satellite (ROSAT) observes X rays in space. The atmosphere prevents these dangerous, high-energy rays from reaching Earth telescopes.
• Most gamma rays are blocked out by Earth's atmosphere, but can be studied in space by the Compton Gamma-Ray Observatory.

Photo: Our eyes can see only the relatively narrow range of electromagnetic radiation we call visible light; we need telescopes to detect other kinds of electromagnetic radiation, on Earth or from space.