For nearly 70 years, astronomers have made some of their most significant discoveries by tuning in to the universe with telescopes that gather radio waves instead of light.
Through these sensitive receivers, scientists have detected the big bang's afterglow, improved their ability to measure the universe's expansion, and rigorously tested Albert Einstein's theory of general relativity. Three of the five Nobel Prizes given to astrophysicists have been awarded for cosmic discoveries made at radio frequencies.
Now, a new generation of radio telescopes is moving from design sketches to construction sites, with the first scheduled to be dedicated this month.
The devices are engineering marvels - some using thousands of tiny pistons to keep football-field-size dishes from warping in heat and wind, and others using complex computer programs to link hundreds of smaller dishes. And they are expected to open a fresh round of discoveries as scientists probe the farthest reaches of space and search for extraterrestrial life.
*In Green Bank, W.Va., the National Radio Astronomy Observatory (NRAO) is slated to dedicate a new, 100-yard-wide dish this month. It is the largest fully steerable radio telescope in the world, observatory officials say.
*High in Chile's Atacama Desert, the NRAO and the European Southern Observatory are building an array of as many as 64 dishes, each 12 yards wide. They can be spread over an area more than six miles across.
*In California, the University of California at Berkeley and the Search for Extraterrestrial Intelligence (SETI) Institute are building an array of 500 to 1,000 small radio telescopes, scheduled to begin operating in 2005. The primary goal is to search for signals from alien civilizations.
*Astronomers from five countries and a consortium of European nations last week signed a pact to begin planning the mother of all radio telescopes - a connected array of dishes that, taken together, would yield a combined area of 1 square kilometer.
"It's a new age for radio astronomy," says Alyssa Goodman, who studies the cosmos in radio wavelengths at the Harvard- Smithsonian Center For Astrophysics in Cambridge, Mass.
Much of the groundwork for these new instruments has been laid during the past two decades. During that time, astronomers have solved a number of technical puzzles that prevented them from observing the full spectrum of electromagnetic waves that penetrate Earth's atmosphere.
"Now what we're doing is building the facilities that allow us to go beyond the preliminary observations we've been able to make," says Lincoln Greenhill of the Center for Astrophysics.
Frontiers of exploration
Those preliminary observations have given astronomers much to explore.
For example, astronomers had once thought that molecules couldn't exist in interstellar space, because the radiation from stars would break them apart. But researchers using radio telescopes have found dozens of types of molecules in interstellar space, including methane and water. Most recently, scientists even found a form of sugar that serves as a foundation for DNA and RNA, genetic material vital to biological activity.
Other observations have led to the discovery of pulsars - dense, rapidly spinning cinders of stars that explode as supernovae.
Now, radio astronomers want to look back to the universe's earliest epochs. By searching the very fringes of the universe - where it is still expanding - researchers want to observe objects in their earliest stages. In other words, they want to study the time when the hydrogen gas that permeates the cosmos began to coalesce into stars and galaxies.
Moreover, they want to study objects much closer to home, including our solar system, at a level of detail that rivals today's optical telescopes.
To accomplish that feat, radio telescopes must be sensitive enough to pick up the faint whispers from those early periods. They have to be able to separate individual energy sources, which appear very close together when viewed at such great distances. And that, notes Cornell University astronomer Yervant Terzian, means big scopes.
The new $74.5 million telescope at Green Bank typifies one approach to size - building a single, monstrous dish. Nearly as tall as the Washington Monument, the 7,500-ton behemoth is pushing the state of the art in dish design, according to Mark McKinnon, an NRAO scientist and deputy site director.
Aided by frequent laser measurements of the antenna's shape, thousands of computer-controlled pistons battle the effects of gravity, wind, and temperature on the curvature of the aluminum plates that form the dish's surface.
Number, not size
Yet some astronomers suggest such telescopes are nearing the limits of cost and engineering skill.
The other way to create a big surface is to build many smaller dishes and link them together using a technique called interferometry. Then, the signals from individual receivers can be synchronized and combined to give results that approach those of a single dish the size of the all the smaller dishes put together.
Here, the CFA's Dr. Goodman says, the challenge focuses more on communications links and computer hardware and software than on structural engineering. Researchers planning the proposed Square Kilometer Array in the Atacama Desert say they hope to use facility and the SETI-based project in northern California as test beds for the processing and communications techniques that the kilometer array could adopt.
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