ASTRONOMY: RADIO, X-RAY, AND INFRARED

Radio, Infrared, and X-ray Astonomy

Radio astronomers of the 1960s were as interested in sending out radio waves as they were in receiving them. In 1964 the radio dish at Arecibo in Puerto Rico was used this way. It bounced radar off planets in the solar system and detected the returning waves, allowing astronomers to make more-accurate measurements of orbits around the sun, distances from Earth, tilts of the axes of the planets, and speeds of rotation on the axes than had been possible before.

Mapping Venus

Three groups of astronomers used radar during the decade to map Venus, which is covered by clouds, so its surface is not visible to ordinary telescopes. Cornell astronomers used the Arecibo observatory. The Jet Propulsion Laboratory (JPL) in Pasadena, California, and Lincoln Laboratory at the Massachusetts Institute of Technology (MIT) bounced radar off the "veiled planet." The Cornell group found mountains on Venus. Rough spots (such as mountains) scattered the radar more than smooth spots. The MIT group used the scatter technique to look at surface features over smaller areas. The JPL group calculated the rotation of Venus on its axis. It was in the opposite direction from all the other planets.

The Development of VLBI Technology

Perhaps the most important development of the decade in radio astronomy was the development of "very long baseline interferometry," or VLBI, a technique developed in Australia that came to be used by astronomers around the world. Simply put, the limiting part of radio astronomy is the size of the antenna. The bigger the antenna, the more information obtained from it. The Australians worked on a mathematical principle to develop a new "giant" receiver. The trick was to link electronically two or more radio antennas at different places. The reception was the same as it would have been for one large dish as big as the distance between the connected smaller dish antennas, The development of VLBI eventually allowed scientists to hear deep into space. The first practical VLBI system was used at Cambridge University in 1960. Linking antennas more than ten kilometers apart by cable was not practical because of the distortion in the connecting cable. American and British astronomers used microwave signals sent to a separate base station from different antennae. The Owens Valley system in the United States connected distant dishes this way in 1960. The first modern VLBI was the U.S. National Radio Astronomy Observatory, which used computers to record and transmit the data from each of the antennae, employing synchronized time at each antenna by using atomic clocks. Now the signals from each dish could be compared exactly at the base station.

The Aerobee Rocket

The first X-ray detector used in astronomy was launched on an air force Aerobee rocket in 1962. The detector was a highly sensitive X-ray telescope designed by American physicists and astronomers, including Riccardo Giacconi. The Aerobee rocket flew 150 miles up for six minutes after takeoff from New Mexico on 18-19 June. It detected a strong X-ray source in Scorpius. The X-ray source in Scorpius had no visible light source to correspond to it, suggesting that it was a neutron star, resulting from a large, dense star that had exploded. Such explosions cause massive changes in the atoms of the star: protons are crushed into electrons, destroying the atoms, and only the neutrons remain. The Scorpius star was found to be ten miles in diameter by a Naval Research Lab rocket with an X-ray detector. Its mass was found to be one billion tons per cubic inch. The explosion left the star burning so hot it gave off X-rays but no light. Another source was later found in the Crab nebula. Still other X-ray detectors were launched in the 1960s. The Orbital Solar Observatories pointed X-ray detectors toward the Sun, which, while not a strong X-ray source, does emit some X-rays.

The Development of Infrared Astronomy

Infrared astronomy was developed in the 1960s. Infrared detectors require long periods of time without motion to be useful. Water vapor in the atmosphere is the main interfering substance, so infrared astronomy is best done using high altitude balloons rather than rockets. The first major infrared astronomical expedition was a manned mission launched by the navy in November 1959. The balloon rose to eighty thousand feet. The infrared telescope was pointed at Venus, but the motion of the balloon caused by the men and the design made the results unreliable. In 1961 the air force took over the infrared experiments and switched to unmanned balloons. In 1963 Martin Schwarzschild of Princeton launched an unmanned balloon that looked at the atmosphere of Mars. He detected water vapor around the red planet. In 1964 the air force group launched a balloon from Holloman Air Force Base in New Mexico that looked at Venus again. It did not go where it was expected to, and one of its detectors worked backward. Even so, it located Venus after nine minutes of trying. The observation lasted over two hours. Water vapor was found around Venus also. As the decade continued, the air force, Princeton, and others continued infrared astronomy. The technology of the infrared telescopes and the balloons improved. While infrared astronomy was used in learning about distant stars, it was mainly a tool for studying the solar system.

Sources:

"Ultraviolet 'Stars' Found," Science News Letter, 77 (6 February 1960): 85;

"Venus Observed," Science News, 93 (24 February 1968): 183.

X-ray telescope An instrument used to focus X-rays into an image. Most X-ray telescopes are grazing-incidence telescopes, based on a technique first developed in the 1940s and 1950s. A complete unit may combine a number of individual mirrors mounted concentrically, inside one another. The most commonly used types are Wolter telescopes. Since the mid-1980s normal-incidence X-ray telescopes have been developed, exploiting the reflecting properties of multi-layer coatings on conventional mirrors. However, their efficiency is restricted to only a very narrow wavelength range, determined by the particular coating used. More recent research has led to the development of micropore optics, which consist of a large array of small holes in a silicon wafer, with one wall of each hole being an X‐ray reflecting surface. In all designs of telescope, the X‐rays are focused on to an X‐ray detector, such as a gas scintillation proportional counter, a proportional counter, a CCD spectrometer, or a microchannel plate detector.

X-ray pulsar A regularly variable X-ray binary, in which the pulsation is associated with the spin period of the compact companion, a magnetized neutron star; abbr. XP. Periods range from a few seconds to a few minutes. These pulsations are thought to be caused by the magnetic field channelling the accreting gas on to the poles of the star, producing localized ‘hot spots’ which move in and out of view as the star spins. An example of such a system is Hercules X-1.

X-ray transient A burst of X-ray emission that, after rising to a maximum brightness, fades until no longer detectable. Examples are novae, supernovae, cataclysmic variables, stellar flares, and active galactic nuclei. Many sources originally detected as transients have subsequently been found to be weak quiescent X-ray sources.

X-ray nova A nova-like outburst at X-ray wavelengths, which may also have an optical counterpart. The main difference between X-ray and optical novae may be that the compact object is a neutron star or black hole rather than a white dwarf. See also X-ray transient.

X-ray astronomy See astronomy