| Themes > Science > Astronomy > Equipment and Devices > Telescope > History of Telescope > Telescopes above the Atmosphere |
Once the technical problems with telescopes were solved and instruments were invented to help detect energy beyond what we could physically see, only one big hindrance remained for astronomers - the Earth's atmosphere. Even on the tops of mountains, the atmosphere prevents the hugest telescopes from getting a really sharp picture of distant objects. But how do we get telescopes into space, and how would they operate? Rockets were one way found to get such devices above the atmosphere, but rockets could only stay up for a few minutes before falling back down. They also couldn't point a telescope in a specific direction, so only simple detectors could be used that would just scan the general area. Airplanes and big weather balloons were tried, but neither one escaped the atmosphere altogether, and they, too, couldn't stay up for a very long time. Balloons also couldn't point the equipment in a specific, steady direction. What was needed was advanced equipment with complex controls that could make a flying telescope point steadily at a chosen target. This would require an expensive satellite or spacecraft, but such projects only get financial support if scientists can show that a lot can be learned with such a telescope. To get satellites and spacecraft observatories funded, smaller projects using those rockets, planes, and balloons had to come first, to justify spending the money on putting remote-control telescopes in space. The first rocket carrying an instrument to investigate ultraviolet radiation was launched in 1949. Ultraviolet was a good choice to begin such studies, since it was already known to have a big influence on human health. A small dose of ultraviolet builds up vitamin D, but large amounts cause sunburn and skin cancer. Earth's atmosphere keeps most of the harmful ultraviolet radiation from affecting us, and that's why a detector in space could be useful, to measure the full amount that the sun and stars radiate. [V-2 rocket. image 34, p. 359 Hoskin] The ultraviolet detector was carried in the nose cone of a German V-2 rocket being fired to test its propulsion system. It was really just along for the ride, installed in the place where bombs had once been. When scientists were given this opportunity to put an experiment in the rocket, an ultraviolet photometer was chosen specifically because the photometer could read the sun's spectrum without being carefully pointed. Once launched, the rocket was above the cloud layers only for a few minutes, but the photometer was able to detect not only the sun's ultraviolet radiation, but solar X rays as well. As experiments continued, some X rays were detected from sources other than the sun. Scientists became eager to find out more about these mysterious cosmic X rays. To determine if there were enough X rays coming from outside our solar system to study in the first place, a detector which would specifically target the X-ray wavelength was built into a rocket. The first two rockets launched with this instrument ran into technical difficulties. The first rocket engine failed, and while the second rocket launched successfully, the door to the instrument area got stuck, and so the only thing detected was the inside of the rocket chamber. The third try on June 18, 1962, was finally a success. Once above 80 kilometers, the rocket doors opened and the instruments found very strong X rays coming from the southern sky. The five-minute flight had discovered a cosmic X-ray source hundreds of times brighter than anyone thought existed. After a few more rocket surveys, an X-ray detector was sent up on one of the Orbiting Solar Observatories. The more experiments done with X rays, the more promising were the discoveries. Invisible X-ray stars were found. Some were also strong radio sources. Scientists now pushed to get an X-ray telescope in space. The X-ray Explorer, nicknamed "Uhuru," was launched from Kenya in 1970. Its success led to the launch of the Einstein X-ray Observatory in 1978, which operated for two and a half years. Observations were made of 5,000 objects ranging from comets in our solar system to quasars billions of light-years away. One key achievement was the discovery of a uniform glow of X rays throughout the sky, probably coming from far outside our galaxy. If you compare it to how our eyes see the night sky as black with bright points of light scattered here and there, when viewed in the X-ray wavelength, there is no black sky at all. This bright X-ray background could mean that very hot gas exists between galaxies, or perhaps it is produced by millions of distant X-ray sources, like quasars, which are star-like radio sources. More study and observation will not only clear up such mysteries, but will likely reveal more amazing things about X-ray energy sources. [Einstein X-ray observatory] Gamma rays, which are produced by the decay of radioactive material, were first found in space by sending detectors up in balloons 20 miles above the Earth's surface. Enough interesting waves in that spectrum were discovered to argue for a small gamma-ray telescope to be included on board the second Small Astronomy Satellite (SAS-2) in 1972. SAS-2 made a gamma-ray map of the entire sky. [SAS-2 image] Gamma rays are associated with neutron stars, which are stars once bigger than our sun that have exploded and collapsed into very dense material, so dense that a piece of a neutron star the size of a grape would weigh about a billion tons. Then in 1979 the first spacecraft flown to detect gamma rays from outer space was the third High Energy Astronomical Observatory. HEAO-3 had lots of difficulties at first, because many false gamma ray readings hit the detector. Eventually scientists sorted out the false readings and learned that gamma rays and their radioactive sources are probably coming from novae, which are partial explosions of stars, a process which happens 1,000 times more frequent than supernovae, the complete explosion of a star. [High Energy Astronomical Observatory] Infrared is another wavelength in which scientists wanted to map the sky. Infrared energy is like heat, and every living thing, and even nonliving things which retain heat, emit an infrared glow, though humans can't see it. Certain snakes have an infrared detector so they can catch mice and small animals at night by sensing the prey's body heat. At one point, some scientists were anxious to view Mars in the infrared, thinking that we could sooner determine if life existed there if infrared pictures showed large concentrations of infrared energy. The first airborne infrared survey was done in a plane. The success of this and other experiments with infrared detectors eventually led to an internationally sponsored Infrared Astronomical Satellite carrying an infrared telescope, launched in 1983. This device ran mainly by computer and made almost four complete surveys of infrared energy in outer space. Since there is a lot of gas, dust, and general space debris like burned-out rocket stages orbiting Earth, many of the readings had to be thrown out as just interference. But these thrown-out readings were also kept, since a mistaken infrared source could be an unknown distant 10th planet or a dark star companion to our sun. Scientists will be able to refer to this infrared map in the future if some interesting object is discovered at a later time. [IRAS] While these specialized projects uncovered many interesting things about the universe, other astronomers insisted that we also had to have a large all-purpose telescope in orbit, one with a range of sensitivity from the infrared through the visual spectrum and into the ultraviolet. Many years of planning, development, and battles over funds finally produced the Hubble Space Telescope, named after Edwin Hubble (1889-1953), the astronomer who discovered the redshift in numerous galaxies, proving the universe was expanding. Launched in 1990, the Hubble was flown to outer space in the Space Shuttle, which limited the size of its mirror and overall structure, since it had to fit into the shuttle's cargo bay. But a 2.4-meter or 96-inch mirror in outer space still promised to get sharp images of distant star systems and clouds of gases 10 times better than possible from Earth. [Hubble] The Hubble telescope is arranged so that all instruments are installed behind the main mirror, and a hole in that mirror faces a smaller mirror which reflects images back into the instrument area for recording and analysis. A wide-field camera and a faint-object camera are on board, as well as devices for analyzing the color spectrum of very distant objects. The Hubble is not studying the sun or moon, because the light from these bodies is too bright and would damage the telescope. In fact, the Hubble is not usually pointed at any object which lies within 50 degrees of the sun or 15 degrees of the sunlit moon or Earth. The telescope completes an orbit every 95 minutes and holds steady by locking onto guide stars. Flying about 600 kilometers above the Earth, it is expected to operate for 15 years, with the Space Shuttle visiting it every three years to service it and install any updated equipment it might need. Soon after the Hubble Space Telescope was launched, a problem was discovered with its main mirror. The mirror had a very slight flattening at its edge, so slight that it was hardly detectable. But this tiny flaw produced images which just weren't the sharp quality which was expected of a space telescope. Since replacing the mirror in space would have been extremely difficult and expensive, Hubble engineers decided to trick the mirror into working properly. They built duplicates of some of the equipment that worked with the mirror and made those devices with an opposing flaw, to "correct" the defect of the slightly warped mirror. In this way, the images the Hubble produced would come out right. It was like the color problem of early telescopes all over again. Just as flint and crown glass lenses made images bend in complementary ways to produce one perfect image, the Hubble's mirror and altered equipment together create a correct image. Sometimes two wrongs can make a right! Plans are already underway for a bigger and better orbiting telescope, presently called the Next Generation Space Telescope. Early ideas for this next space telescope included possibly installing it on the moon, which would allow it a stable foundation instead of needing complex control systems to point it steadily in space. But although the moon has no atmosphere to interfere with such a telescope, there are limitations to any telescope which stands anywhere on firm ground. The telescope would be restricted to pointing only to the half of the sky it is facing, and the sun and sunlit Earth would have to be constantly avoided. The current plans are instead to put an 8-meter or 314-inch reflecting infrared telescope in deep space around the year 2007. A far-Earth orbit is planned to help keep the equipment at a colder temperature and to eliminate the problems of having to avoid a close sunlit Earth and moon so much of the time. [NGST planning drawing] While the Hubble can detect the near infrared, which is closest to the visual wavelengths, the Next Generation Space Telescope will cover longer wavelengths as well so it can study the first stars and galaxies that formed after the universe cooled. This is possible to see when looking with an infrared telescope, since the process of star formation is thought to be very violent, releasing energies hundreds of billions of times more than our sun. Even though these events happened so long ago, they still exist visibly for us, since the light we see from these distant stars was radiated billions of years ago. Understandably, once telescopes got very powerful and could see to the visible edges of the universe, the planets in our own solar system sometimes got neglected in favor of the farthest stars, nebulae, and mysterious quasars. But during the last 30 years, we no longer had to peer at planets like Jupiter and Saturn using just a mere 200-inch mirror. With robotic spacecraft, we can now travel to the planets and take our pictures close up! |
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