Astronomers combine special telescopes to see deep into space. The network also provides means for studying Earth's crust
Capturing radio waves from space, which has brought some of the universe's most distant and violent objects into view, is soon to enter a fresh phase. New networks of radiotelescopes on the ground and radiotelescopes orbiting Earth will allow astronomers to see stars, galaxies, and more exotic objects such as pulsars and quasars at greater distances and in finer detail than ever before.
In addition to enriching mankind's understanding of the universe, such studies have direct applications on earth: Radiotelescopes trained on cosmic radio sources have become useful tools for studying Earth's crust, from the movement of plates to shifts along individual earthquake faults. The telescopes have also recorded minute changes in Earth's rotation rate and the amount of wobble it shows as it turns on its axis. Such information is of prime interest to geophysicists grappling with how the planet works as a whole.
Scientists from the United States, the Soviet Union, Europe, Japan, China, and Australia outlined progress on a number of radiotelescope projects and proposals at a recent International Astronomical Union symposium here. The meeting was celebrating 20 years of a radioastronomy technique known as Very Long Base line Interferometry. VLBI involves combining images from two or more distant radiotelescopes to produce an image with greater detail than can be obtained from the individual telescopes.
Of the Earth-bound projects, perhaps the most ambitious is under way in the United States. This November, scientists from the National Radio Astronomy Observatory hope to flip the switch on the first of 10 radiotelescopes that will be part of a network known as the Very Long Base line Array. By the time it's finished in 1991, the array will stretch from Hawaii to the US Virgin Islands and New England. When images obtained from the most distant 25-meter (82-foot) dishes are combined, it will be as if the images were produced from a radiotelescope 8,000 kilometers (5,000 miles) in diameter.
If the human eye could duplicate the full array's resolution of a few tenths of a milliarcsecond at its highest frequency, a person would be able to spot a football on the moon, says Ken Kellermann, a senior scientist at the observatory. ``The best ground-based optical telescopes could barely resolve the football field,'' he adds.
Initially, he says, each radiotelescope will be used for VLBI work on an ad hoc basis until six or seven are built. Once the system reaches that point, it will begin operation as a ``stand alone'' network dedicated full time to VLBI observations. If work proceeds on schedule, that should happen by mid-1990.
That would be just in time to serve as a powerful ground-based component of a VLBI system that would include Radioastron, a Soviet satellite sporting a radiotelescope with a 10-meter antenna. At the satellite's farthest point from Earth the satellite-ground station combination would span some 77,000 kilometers and resolve objects as small as 30 microarcseconds (millionths of an arcsecond) wide. If all goes according to schedule, say scientists with the Soviet Institute for Space Research, the satellite should be ready for launch in the summer of 1991 and run for about two years.
Western European scientists, who are building one of the receivers for the Radioastron project, are eying a orbiting radiotelescope themselves. The European Space Agency has given the go-ahead for feasibility studies for QUASAT, a 10- to 20-meter radiotelescope that, if given final approval in 1988, would be placed on orbit in 1996.
In addition, the Japanese are looking at the possibility of launching a five-meter radiotelescope in 1993 or '94.
The notion of blending images from orbiting radiotelescopes with those from groundbased dishes has received a boost from joint US-Japanese-Australian experiments using a US tracking and data relay satellite system.
Twice last year, and again in January, the TDRSS satellite was successfully used as the space-based component of a VLBI system that would be the equivalent of a radiotelescope with a diameter of 1.4 times that of Earth. The resolution wasn't great, researchers say, because the TDRSS satellite was never designed to serve as a radiotelescope. But the experiments, they say, have shown that the concept is valid.