Satellite astronomy

(astronomy) The study of astronomical objects by using detectors mounted on earth-orbiting satellites or deep-space probes; allows observations that are not obstructed by the earth's atmosphere.

The study of astronomical objects by using detectors mounted on Earth-orbiting satellites or deep-space probes so that observations unobstructed by the Earth's atmosphere can be made. The Earth's atmosphere allows only a narrow slice of the visible and near-infrared spectrum through, in addition to much of the radio band. Even at visible and near-infrared wavelengths, atmospheric turbulence distorts the light and smears the images of ground-based telescopes, limiting their resolution to about 1 arc-second.

This article discusses orbiting telescopes that observe electromagnetic radiation that is blocked by the Earth's atmosphere in the microwave, infrared, ultraviolet, x-ray, and gamma-ray regions of the electromagnetic spectrum. For a discussion of the Hubble Space Telescope, which was launched in 1990 to make visible, ultraviolet, and near-infrared observations at a higher resolution than previously possible, See also Space Telescope, Hubble.

Cosmic background radiation

The shortest radio wavelengths are blocked so that studies of the high-frequency end of the spectrum of the cosmic microwave background radiation, which is the thermal blackbody radiation at approximately 3-K temperature and is the signature of the big bang, must be made from high-altitude balloons, rockets, or satellites. The Cosmic Background Explorer (COBE) satellite carried out such studies from 1989 to 1991 and provided discoveries of historic importance, as did the Wilkinson Microwave Anisotropy Probe (WMAP), launched in 2001. See also Cosmic background radiation.

Infrared astronomy

At infrared wavelengths longer than about 2 micrometers (that is, some four times that of visible light), the atmosphere is largely opaque, and again astronomy must be done from balloons, rockets, and satellites. In 1983 the United States launched the Infrared Astronomical Satellite (IRAS) into low Earth orbit, where it carried out the first all-sky survey in four different infrared-wavelength bands over the next 2 years.

The European Space Agency's Infrared Space Observatory (ISO; 1995–1998) was designed to do detailed studies of selected regions of the sky with better angular resolution, wider wavelength coverage, enhanced imaging and spectroscopic capabilities, and higher sensitivity than IRAS. In 2003, the National Aeronautics and Space Administration (NASA) launched the Spitzer Telescope, the last of its four great observatories, with infrared arrays that were substantially larger and more sensitive than those on ISO. See also Infrared astronomy.

Ultraviolet astronomy

Since the Earth's atmosphere is opaque to ultraviolet (UV) light with wavelengths shorter than about 310 nanometers, ultraviolet astronomy had to await the space age. Astrophysical problems were explored with the several ultraviolet detector systems flown on the OSO (Orbiting Solar Observatory) and OAO (Orbiting Astronomical Observatory) satellite series in the 1960s and early 1970s, as well as with the subsequent IUE (International Ultraviolet Explorer) satellite. A much more powerful tool became available in 1990 with the launch of the Hubble Space Telescope. In June 1990 the ROSAT satellite was launched by NASA for the Germans, and carried out the first far-ultraviolet sky survey with the Wide Field Camera supplied by United Kingdom investigators. The first dedicated ultraviolet astronomy satellite, the Extreme Ultraviolet Explorer (EUVE), was launched in 1992 and operated until January 2001.

The Wide Field Camera ultraviolet telescope on ROSAT and the ultraviolet telescopes on EUVE detected some 400 far-ultraviolet objects in several bands in the wavelength range 6–80 nm. EUVE also enabled ultraviolet spectra on a great variety of objects to be measured, from coronae of relatively nearby stars (such as alpha Centauri) to measurements of an unexpectedly bright ultraviolet halo around the active galaxy M87 in the Virgo cluster of galaxies.

The major leap forward in ultraviolet observations, both imaging and spectra, of galactic objects came with the Hubble Space Telescope. Even with its initially blurred vision, this instrument made countless discoveries by obtaining much higher spatial resolution in the ultraviolet than ever before. Its images of SN1987A were able to resolve the expanding ring of ejecta from the bright supernova and the presupernova shell of gas expelled by the giant. With the enhanced ultraviolet sensitivity of the repaired Hubble Space Telescope, significant studies of the nature of quasars and distant galaxies became possible. See also Galaxy, external; Supernova.

The Far Ultraviolet Spectroscopic Explorer (FUSE), launched in 1999, studies a wide range of astronomical problems in the 90.5-118.7-nanometer wavelength region through the use of high-resolution spectroscopy. The FUSE bandpass complements the spectral coverage provided by the Hubble Space Telescope, which extends down to about 117 nm.

X-ray astronomy

X-ray astronomy can be done only from above the Earth's atmosphere. The first Small Astronomy Satellite (SAS 1; 1970), designated Uhuru, carried two proportional-counter x-ray detectors. The OSO satellites also carried cosmic x-ray detectors (proportional counters similar to Uhuru) and contributed much to the detailed understanding of individual sources. The Astronomical Netherlands Satellite (ANS; 1974) was the first x-ray observatory. Both it and the SAS 3 satellite (1975) contained a variety of x-ray detectors. A major increase in sensitivity for x-ray astronomy was achieved with the High Energy Astronomical Observatory (HEAO) satellite (1977).

The HEAO 2 satellite, or Einstein Observatory (1978), provided the first x-ray images of celestial objects and detected thousands of new sources. The European satellite EXOSAT (1982) provided follow up x-ray imaging and spectroscopy to Einstein. It was particularly sensitive to very low energy x-ray sources, although its total collecting collecting area was much less than that of the Einstein Observatory.

ROSAT (1990–1999) was a relatively large x-ray telescope with approximately twice the sensitivity of the Einstein Observatory. ROSAT carried out the first all-sky imaging x-ray survey, discovering more than 60,000 sources, and thus was as revealing as the Palomar survey for optical astronomy. The all-sky survey was followed by pointed observations.

In 1993 the Japanese x-ray program launched ASCA, the first x-ray imaging telescope with energy response extending up to 8 keV. This was a major breakthrough since it allowed imaging and spectroscopy of objects in the x-ray light of their iron emission lines.

NASA's Rossi X-ray Timing Explorer (RXTE), launched in 1995, carries three telescopic instruments that cover a wide energy range, 2–200 keV, provide accurate timing and measurement of x-ray sources, and can detect emissions as brief as 10–100 microseconds.

The Italian-Dutch x-ray satellite BeppoSAX (1996) is characterized by a very wide spectral coverage (0.1–300 keV) with balanced performances from its low- and high-energy instrumentation. It discovered the x-ray afterglow of gamma-ray bursts, enabling them to be more precisely located and allowing follow-up radio and optical observations that confirmed that the bursts are of cosmological origin.

The Chandra X-ray Observatory (CXO; 1999), the third of NASA's great observatories, has the highest spatial and energy resolution of any x-ray observatory in orbit. At the focus of this 50-foot-long (15-m) telescope are two detectors, the Advanced CCD Imaging Spectrometer (ACIS) and the High-Resolution Camera (HRC). The ACIS uses 10 charge-coupled devices (CCDs) to measure the energy, location, and arrival time of each photon, while the HRC uses a microchannel plate. See also Charge-coupled devices.

The European Space Agency's X-ray Multimirror Observatory, renamed XMM-Newton (2000), carries three advanced telescopes, each containing 58 concentric mirrors, nested so as to offer the largest possible collecting area. Its highly eccentric orbit enables long, uninterrupted observations. See also X-ray astronomy.

Gamma-ray astronomy

Gamma rays, more energetic than x-rays, still do not penetrate the Earth's atmosphere. SAS 2, launched in 1972, first established the existence of gamma-ray point sources (such as the Crab and Vela pulsars) at energies of about 100 MeV. A more sensitive follow-up mission, COS-B, was launched by the European Space Agency in 1975.

The Compton Gamma-Ray Observatory (CGRO; 1991–2000), the second of NASA's great observatories, carried four high-sensitivity gamma-ray detectors: EGRET (Energetic Gamma-Ray Experiment Telescope, a much larger and more sensitive spark chamber than on COS-B), for the energy range 30 MeV–10 GeV; COMPTEL (Compton Telescope), a Compton-scattering telescope sensitive in the poorly explored 1–30-MeV band; OSSE (Oriented Scintillation Spectrometer Experiment), a scintillator detector sensitive in the 100-keV–10-MeV band, and thus able to detect hard x-ray sources; and BATSE (Burst and Transient Source Experiment), a detector system specifically designed to study gamma-ray bursts and determine their arrival directions. Each of these instruments obtained numerous significant discoveries. See also Gamma-ray astronomy.