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Fact Sheet: June 25, 1998Advanced X-ray Astrophysics Facility (AXAF)Observing the High Energy universe NASA's Advanced X-ray Astrophysics Facility, (AXAF), scheduled to be launched and deployed by the Space Shuttle in late January of 1999, will be the largest and most powerful X-ray observatory ever built. AXAF images will show fifty times more detail than any previous X-ray telescope. It is a revolutionary telescope that combines the ability to make sharp images while measuring precisely the energies of X-rays coming from cosmic sources. The impact AXAF will have on X-ray astronomy can be compared to the difference between a fuzzy black and white picture and a sharp color picture. X-rays are a high-energy, invisible form of light. They are produced in the cosmos by gas that has been heated to millions of degrees by violent and extreme conditions. Much of the matter in the universe is so hot that it can be observed only with X-ray telescopes. Exploding stars, black holes and vast clouds of hot gas in galaxy clusters are among the fascinating objects that AXAF is designed to study. Spacecraft and Orbit X-rays from space are absorbed by the Earth's atmosphere, so X-ray observations must be in orbit above the Earth. After deployment from the space shuttle, a built-in propulsion system will boost AXAF to an elliptical, high Earth orbit. This orbit that will take the spacecraft more than a third of the way to the moon before returning to its closest approach to the Earth of 10,000 kilometers (6200 miles). The time to complete an orbit will be 64 hours and 18 minutes. The spacecraft will spend 85% of its orbit above the belts of charged particles that surround the Earth. Uninterrupted observations as long as 55 hours will be possible and the overall percentage of useful observing time will be much greater than for the low Earth orbit used by most satellites. The Observatory The Observatory has three major parts: the X-ray telescope, whose mirrors will focus X-rays from celestial objects; the science instruments which record the X-rays so that X-ray images can be produced and analyzed; and the spacecraft, which provides the environment necessary for the telescope and the instruments to work. Because of their high-energy, X-rays cannot be focused in the same way as visible light. They reflect only if they strike a mirror at grazing angles, like a pebble skipping across the surface of a pond. X-ray telescope mirrors are shaped like barrels, rather than the familiar dish shape of optical telescope mirrors. In the AXAF mirror assembly, incoming X-rays graze off two sets of four precisely polished cylindrical mirrors coated with the highly reflective metal iridium. The AXAF mirrors are the largest, most precisely shaped and aligned, and smoothest X-ray mirrors ever constructed. If the surface of the Earth was as smooth as the AXAF mirrors, the highest mountain would be less than six feet tall! Calibration tests at Marshall Space Flight Center show that the mirrorswill produce images fifty times sharper than the best previous X-ray telescope. This focusing power is equivalent to the ability to read a newspaper at a distance of half a mile. The telescope mirrors will reflect cosmic X-rays onto a small area called the focus. The science instruments that will be used at the focus are the High Resolution Camera (HRC) and the AXAF CCD Imaging Spectrometer (ACIS). These instruments, which are housed in the Science Instrument Module, record the number, position and energy of the cosmic X-rays. This information can be used to make an X-ray image and to study other properties of the source. Besides the focal plane instruments, AXAF will have two sets of gold gratings, which can be used with either of the science instruments for accurate measurement of the energies of the X-rays. The spacecraft module contains computers, communication antennas and data recorders to transmit and receive information between the observatory and ground stations. The onboard computers and sensors, with ground-based control center assistance, command and control the vehicle and monitor the condition of the observatory. The spacecraft module also provides rocket propulsion to move and aim the entire observatory, an aspect camera that tells the observatory its position relative to the stars, and a Sun sensor that protects it from excessive light. Electrical power is provided by solar arrays that also charge three nickel-hydrogen batteries that provide backup power. Science AXAF images will allow scientists to study how the elements necessary for life are created and spread throughout the galaxy by exploding stars. The cloud that collapsed to form the Sun, Earth, and other planets was composed mostly of hydrogen and helium, with a small amount of heavier elements such as carbon, nitrogen, oxygen and iron. These and other heavy elements necessary for life can only be made deep in the interior of massive stars where they remain until a catastrophic explosion spreads them throughout space. About every fifty years in our galaxy, one of these massive stars explodes as a supernova. The shell of matter thrown off by the supernova creates a bubble of multi-million degree gas called a supernova remnant. This hot gas will expand and produce X-radiation for thousands of years. When heavy elements are heated to high temperatures, they produce X-rays of specific energies. AXAF s images and precise measurements of these X-rays will tell how much of each element is present. Detailed measurements of the X-radiation from super-hot gas swirling near the event horizon of a black hole will provide new information about the warping of space and other effects of extreme gravity. Black holes produce some of the most intense X-ray sources in the universe. The X-rays detected by X-ray telescopes do not come from within a black hole, but from super-hot gas that is swirling toward a black hole. As gas and dust particles swirl toward a black hole, they speed up and form a flattened disk. Friction caused by collisions between the particles heats them to extreme temperatures. Just before they pass beyond the event horizon, they produce X-rays as their temperatures rise to many millions of degrees. AXAF will measure accurately the motion of particles near black holes. This information will provide new understanding of the extreme gravity around black holes. Astronomers have proposed that supermassive black holes may explain the mysterious objects called quasars. These objects radiate as much energy per second as a thousand normal galaxies from a region having a diameter less than a millionth of the size of one galaxy. Because the matter closest to the event horizon radiates most of its energy as X-rays and gamma rays, AXAF will present an unequaled view into the inner workings of these violent cosmic whirlpools. AXAF will precisely measure the temperature and pressure of hot gas in distant clusters of galaxies formed when the universe was very young. This data will then be compared with hot gas in nearby clusters to determine how clusters of galaxies evolve and to test theories for the origin and evolution of the universe. More than half of all galaxies are members of groups or clusters of galaxies. The Milky Way is part of a group of galaxies called the Local Group. X-ray observations have shown that most clusters of galaxies are filled with vast clouds of multi-million degree gas. The mass of this gas is greater than all the stars in all the galaxies in a cluster of a thousand galaxies. The Xray producing hot gas found in a typical cluster of galaxies also presents astronomers with a grand puzzle. Over time this extremely hot gas should escape the cluster since the galaxies and gas do not provide enough gravity to hold it in. Yet in clusters of all ages the gas remains. Scientists have concluded that some unobserved form of matter, called dark matter, is providing the extra gravity needed to hold the gas in the cluster. An enormous amount of dark matter is needed. About three to ten times as much matter as that observed in the gas and galaxies. This means that most of the matter in the universe may be dark matter! The dark matter could be collapsed stars, planets or black holes. Or strange subatomic particles that produce no light, and can only be detected through their gravity. Detailed measurements of the size and temperature of the hot gas clouds in galaxy clusters by AXAF could help solve the dark matter mystery. Operations The Smithsonian Astrophysical Observatory controls science and flight operations of the advanced X-ray observatory for NASA from Cambridge, Mass. The Smithsonian manages two electronically linked facilities the Operations Control Center and the Science Center.The Operations Control Center is responsible for directing the observatory s mission as it orbits Earth. A control center team will interact with the observatory three times a day to acquire science and housekeeping information from its recorders. The control center team also will send new instructions to the observatory as needed, and receive scientific information from the X-ray observatory to the Science Center. The Science Center is an important resource for scientists and the public. The Science Center will provide user support to researchers, including science data processing and a science data archive. The Science Center will work with NASA and the scientific community to inform the public of discoveries made by scientists using the observatory. NASA and Partners The Advanced X-ray Astrophysics Facility program is managed by the Marshall Space Flight Center for the Office of Space Science, NASA Headquarters, Washington, D.C. TRW Space and Electronics Group (now NGST) of Redondo Beach, Calif., is the prime contractor and has assembled and tested the observatory for NASA. Using glass purchased from Schott Glaswerke, Mainz, Germany, the telescope s mirrors were built by Raytheon Optical Systems Inc., Danbury, Conn. The mirrors were coated by Optical Coating Laboratory, Inc., Santa Rosa, Calif., and assembled by Eastman Kodak Co., Rochester, N.Y. ACIS was developed by Pennsylvania State University, University Park, Pa., and the Massachusetts Institute of Technology (MIT), Cambridge. The HRC was built by the Smithsonian Astrophysical Observatory. One diffraction grating was developed by MIT, the other by the Space Research Organization Netherlands, Utrecht, Netherlands, in collaboration with the Max Planck Institute, Garching, Germany. The Ball Aerospace & Technologies Corporation of Boulder, Colo., developed the aspect camera and the Science Instrument Module. Advanced X-ray Astrophysics Facility Technical Details
For more information, contact Wallace Tucker 617.496.7998 [ Press Index ] [ Fact Sheets ] |
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Revised: February 20, 2008
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