High-Energy Astrophysics Satellites

A general discussion of the Highlights of High-Energy Satellites is available below. More detailed information, presented as a function of mission lifetime or mission energy range, is available. Furthermore, detailed information on individual satellites which detected non-solar X-rays and/or gamma-rays (intentionally or not) can be accessed directly.

Highlights of High-Energy Satellites

A 1949 sounding rocket experiment showed that the Sun emitted X-rays (Friedman et al 1951) and another sounding rocket experiment in 1961 showed that other celestial sources emitted X-rays. Observations of celestial X-ray sources must be carried out above most of the Earth's atmosphere, which absorbs this part of the electromagnetic spectrum. The numbers, sizes, and sophistication of X-ray detection systems have increased rapidly since their simple beginnings in the 1960s.

The Past

The first dedicated gamma-ray astronomy mission was, in fact, the first high-energy astrophysics satellite as well. Explorer-11 was launched in 1961. The instrument package weighed 30 pounds, was 20 inches high and 10 inches in diameter. They believed that they detected 22 cosmic gamma-rays. The first orbiting mission which was dedicated to celestial X-ray astronomy was Uhuru, launched in 1970. (Other important satellites preceded it, such as OSO-3 and Vela 5B, but these were not dedicated astrophysics missions). Uhuru had a simple complement of instrumentation covering the energy range 2-20 keV. It provided the first comprehensive view of the entire X-ray sky. The final catalog of detected point sources contained 339 objects, which were primarily binary stellar systems, supernova remnants, Seyfert galaxies and clusters of galaxies.

Many important missions followed Uhuru in the 1970s, including ANS, Copernicus, SAS-3, Ariel V, and OSO 8. These missions discovered X-ray bursts, numerous pulsars, and obtained the first detailed X-ray spectra. Another great leap forward occurred in 1977 with the launch of NASA's first High Energy Astrophysical Observatory (HEAO-1). HEAO-1 carried 4 major instruments, covering the energy band 0.2 keV to 10 MeV. It generated a catalog of approximately 850 point sources during its 18 month lifetime, allowed the first comprehensive measurement of the diffuse X-ray background, and performed time variability studies on millisecond time scales. The Einstein Observatory (HEAO-2) was launched in 1978, carrying the first fully imaging X-ray telescope. Following this was EXOSAT, Ginga, and many other highly successful missions.

The Present

There are currently eight on-going missions dedicated to celestial X-ray and gamma-ray astronomy: the Röntgen Satellite (ROSAT), the Kvant module attached to the space station Mir, Granat, the Array of Low Energy X-ray Imaging Sensors (ALEXIS), Compton Gamma-Ray Observatory (CGRO), Advanced Satellite for Cosmology and Astrophysics (ASCA), the Rossi X-ray Timing Explorer (RXTE), and the Italian Satellite for X-ray Astronomy (SAX). These are supplemented by a host of other missions, which have other primary goals, but which contain instruments sensitive to high-energy cosmic radiation (such as Ulysses, Wind, and the Defense Meteorological Satellite Program (DMSP) series). Both individually and collectively, these satellites have provided an unprecedented view of the high-energy universe. ROSAT has produced a catalog containing over 62,000 point sources; ASCA has measured the energy spectrum of objects with unprecedented resolution; CGRO has monitored the mysterious gamma-ray burst phenomena over five years with unrivaled source location and timing information.

The Future

The next ten years is filled with the expected launches of high-energy missions which will observe the universe with increased timing resolution, spectral resolution, broad-band energy ranges, and multi-wavelength coverages. These missions include Advanced X-ray Astrophysics Facility (AXAF), Spectrum-X-Gamma, Astro-E, and the X-ray Multi-Mirror (XMM) satellite. The missions also represent the movement toward multi-national missions that has become more and more important in recent years. Collaborations between the United States, Russia, Japan, the European Space Agency, as well as Space Agencies from Italy, Argentina, and other nations are all involved in these exciting new missions.

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