Observatories, Space-Based

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Observatories, Space-Based

Space-based observatories are telescopes located beyond Earth, either in orbit around the planet or in deep space. Such observatories allow astronomers to observe the universe in ways not possible from the surface of Earth, usually because of interference from our planet's atmosphere. Space-based observatories, however, are typically more complicated and expensive than Earth-based telescopes. The National Aeronautics and Space Administration (NASA) and other space agencies have been flying space observatories of one type or another since the late 1960s. While the Hubble Space Telescope is the most famous of the space observatories, it is just one of many that have provided astronomers with new insights about the solar system, the Milky Way galaxy, and the universe.

The Advantages and Disadvantages of Space-Based Telescopes

Observatories in space have a number of key advantages. Telescopes in space are able to operate twenty-four hours a day, free of both Earth's day-night cycle as well as clouds and other weather conditions that can hamper observing. Telescopes above the atmosphere can also observe portions of the electromagnetic spectrum of light, such as ultraviolet radiation , X rays , and gamma rays , which are blocked by Earth's atmosphere and never reach the surface. Telescopes in space are also free of the distortions in the atmosphere that blur images. These factors increase the probability that space telescopes will be more productive and useful than their ground-based counterparts.

Space-based observatories also have some disadvantages. Unlike most ground-based telescopes, space observatories operate completely automatically, without any humans on-site to fix faulty equipment or deal with other problems. There are also limitations on the size and mass of objects that can be launched, as well as the need to use special materials and designs that can withstand the harsh environment of space, creating limitations on the types of observatories that can be flown in space. These factors, as well as current high launch costs, make space observatories very expensive: the largest observatories, such as the Hubble Space Telescope, cost over $1 billion, whereas world-class ground-based telescopes cost less than $100 million. In many cases, though, there is no option other than to fly a space observatory, because ground-based telescopes cannot accomplish the required work.

The History of Space-Based Telescopes

The first serious study of observatories in space was conducted in 1946 by astronomer Lyman Spitzer, who proposed orbiting a small telescope. In the late 1960s and early 1970s NASA launched four small observatories under the name Orbiting Astronomical Observatories (OAO). Two of the OAO missions were successful and conducted observations, primarily in ultraviolet light, for several years. NASA followed this up with a number of other small observatories, including the International Ultraviolet Explorer in 1978 and the Infrared Astronomy Satellite in 1983.

While NASA was developing and launching these early missions, it was working on something much larger. In the 1960s it started studying a proposal to launch a much larger observatory to study the universe at visible, ultraviolet, and infrared wavelengths . This observatory was originally known simply as the Large Space Telescope, but over time evolved into what became known as the Hubble Space Telescope. Hubble was finally launched by the space shuttle Discovery in April 1990. After astronauts corrected a problem with the telescope's optics in 1993, Hubble emerged as one of the best telescopes in the world. Hubble is scheduled to operate at least through 2010.

The Great Observatories

While the Hubble Space Telescope may be the most famous space observatory, it is far from the only major one. NASA planned for Hubble to be the first of four "Great Observatories" studying the universe from space, each focusing on a different portion of the spectrum. The second of the four Great Observatories, the Compton Gamma Ray Observatory (CGRO), was launched by the space shuttle Atlantis on mission STS-37 in April 1991. The telescope was named after Arthur Holly Compton, a physicist who won the Nobel Prize in 1927 for his experimental efforts confirming that light had characteristics of both waves and particles.

The purpose of CGRO, also known as Compton, was to study the universe at the wavelengths of gamma rays, the most energetic form of light. CGRO carried four instruments that carried out these observations. Data from these instruments led to a number of scientific breakthroughs. Astronomers discovered through CGRO data that the center of our galaxy glows in gamma rays created by the annihilation of matter and antimatter . Observations by CGRO of hundreds of mysterious gamma-ray bursts showed that the bursts are spread out evenly over the entire sky and thus likely originate from far outside our own galaxy. Astronomers used CGRO to discover a new class of objects, known as blazars: quasars that generate gamma rays and jets of particles oriented in our direction.

CGRO was intended to operate for five years but continued to work for several years beyond that period. In early 2000 one of Compton's three gyroscopes, used to orient the spacecraft, failed. Because the spacecraft was so heavy—at 17 tons it weighed more than even Hubble—NASA was concerned that if the other gyroscopes failed the spacecraft could reenter Earth's atmosphere uncontrolled and crash, causing damage and injury. To prevent this, NASA deliberately reentered Compton over the South Pacific on June 4, 2000, scattering debris over an empty region of ocean and ending the spacecraft's nine-year mission.

The third spacecraft in NASA's Great Observatories program is the Chandra X-Ray Observatory. The spacecraft, originally called the Advanced X-Ray Astrophysics Facility but today known simply as Chandra, waslaunched by the space shuttle Columbia on mission STS-93 in July 1999. Chandra was the largest spacecraft ever launched by the space shuttle. The spacecraft is named after Subrahmanyan Chandrasekhar, an Indian-American astrophysicist who won the Nobel Prize for physics in 1983 for his studies of the structure and evolution of stars.

Chandra carries four instruments to study the universe at X-ray wavelengths, which are slightly less energetic than gamma rays, at up to twenty-five times better detail than previous spacecraft missions. To carry out these observations Chandra is in an unusual orbit: Rather than a circular orbit close to Earth, as used by Hubble and Compton, it is in an eccentric orbit that goes between 10,000 and 140,000 kilometers (6,200 and 86,800 miles) from Earth. This elliptical orbit allows Chandra to spend as much time as possible above the charged particles in the Van Allen radiation belts that would interfere with the observations.

Although Chandra has been in orbit only a relatively short time, it has provided astronomers with a wealth of data. Astronomers have used Chandra to learn more about the dark matter that may make up most of the mass of the universe, study black holes in great detail, witness the results of supernova explosions, and observe the birth of new stars. Chandra's mission is officially scheduled to last for five years but will likely continue so long as the spacecraft continues to operate well.

The final spacecraft of the Great Observatories program is the Space Infrared Telescope Facility (SIRTF). SIRTF will probe the universe at infrared wavelengths of light, which are longer and less energetic than visible light. SIRTF is scheduled for launch in January 2003 on an unpiloted Delta rocket. Rather than go into Earth orbit, SIRTF will be placed in an orbit around the Sun that gradually trails away from Earth; this will make it easier for the spacecraft to perform observations without interference from Earth's own infrared light. Astronomers plan to use SIRTF to study planets, comets, and asteroids in our own solar system and look for evidence of giant planets and brown dwarfs around other stars. SIRTF will also be used to study star formation and various types of galaxies during its five-year mission.

Other Space Observatories

Besides NASA's Great Observatories, there have been many smaller, space-based observatories that have focused on particular objects or sections of the electromagnetic spectrum. A number of these missions have made major contributions. NASA's Cosmic Background Explorer (COBE) spacecraft was launched in 1989 on a mission to observe cosmic microwave background radiation, light left over from shortly after the Big Bang . COBE's instruments were able to measure small variations in the background, providing key proof for the Big Bang model of the universe. NASA launched a new mission, the Microwave Anisotropy Probe (MAP), in June 2000 to measure the variations in the microwave background in even greater detail.

NASA is not the only space agency to launch space observatories. The European Space Agency (ESA) has launched a number of its own observatories to study the universe. The Infrared Space Observatory provided astronomers with unprecedented views of the universe at infrared wavelengths in the mid-and late 1990s. In 1999 ESA launched XMM-Newton, an orbiting X-ray observatory similar to NASA's Chandra spacecraft. XMM-Newton and Chandra serve complementary purposes: Whereas Chandra is designed to take detailed X-ray images of objects, XMM-Newton focuses on measuring the spectra of those objects at X-ray wavelengths.

Japan has also contributed a number of small space observatories. The Advanced Satellite for Cosmology and Astrophysics spacecraft was launched in 1993 and continues to operate in the early twenty-first century, studying the universe at X-ray wavelengths. The Yohkoh spacecraft was launched in 1991 to study the Sun in X rays. The Halca spacecraft, launched in 1997, conducts joint observations with radio telescopes on Earth. The Soviet Union also flew several space observatories, including the Gamma gammaray observatory and the Granat X-ray observatory. Since the collapse of the Soviet Union, however, Russia has been unable to afford the development of any new orbiting telescopes.

Future Space Observatories

The success of past and present space observatories has led NASA, ESA, and other space agencies to plan a new series of larger, more complex spacecraft that will be able to see deeper into the universe and in more detail than their predecessors. Leading these future observatories is the Next Generation Space Telescope (NGST), the successor to the Hubble Space Telescope. Scheduled for launch in 2009, NGST will use a telescope up to 6.5 meters (21.3 feet) in diameter (Hubble's is 2.4 meters [7.9 feet] across), which will allow it to observe dimmer and more distant objects. The telescope will be located at the Earth-Sun L-2 point, 1.5 million kilometers (930,000 miles) away, to shield it from Earth's infrared radiation. NASA is also supporting the development of other new space observatories, including GLAST, a gamma-ray observatory scheduled for launch in 2006.

ESA is developing several space observatories that will observe the universe at different wavelengths. Integral is a gamma-ray observatory scheduled for launch in 2002. Planck, scheduled for launch in 2007, will build upon the observations of the cosmic microwave background made by COBE and MAP. Herschel, also scheduled for launch in 2007, will observe the universe at far-infrared wavelengths. ESA is also collaborating with NASA on development of the NGST.

In the future, space observatories may consist of several spacecraft working together. Such orbiting arrays of telescopes could allow astronomers to get better images without the need to build extremely large and expensive single telescopes. One such mission, called Terrestrial Planet Finder (TPF), would combine images from several telescopes, each somewhat larger than Hubble, to create a single image. A system of this type would make it possible for astronomers to directly observe planets the size of Earth orbiting other stars. TPF is tentatively scheduled for launch no sooner than 2011. NASA is also studying a similar proposal, called Constellation-X, which would use several X-ray telescopes to create a virtual telescope 100 times more powerful than existing ones.

In the more distant future, astronomers have proposed developing large telescopes, and arrays of telescopes, on the surface of the Moon. The far-side of the Moon is an ideal location for a radio telescope, because it would be shielded from the growing artificial radio noise from Earth. However, there are as of yet no detailed plans for lunar observatories.

see also Astronomer (volume 2); Astronomy, History of (volume 2); Astronomy, Kinds of (volume 2); Hubble Space Telescope (volume 2); Observatories, Ground (volume 2).

Jeff Foust

Bibliography

Chaisson, Eric. The Hubble Wars. New York: Harper Collins, 1994.

Smith, Robert W. The Space Telescope. Cambridge, UK: Cambridge University Press,1993.

Tucker, Wallace H., and Karen Tucker. Revealing the Universe: The Making of the Chandra X-Ray Observatory. Cambridge, MA: Harvard University Press, 2001.