Thirty years ago last spring, researchers slewed the metal skeleton of a giant dish antenna toward a spot in the sky and beamed a pulse of radar energy across tens of millions of miles of space.
Some five minutes later, a faint echo of that signal was received by a second antenna, several miles to the north in California's Mojave Desert. The experiment had captured the first radar echo bounced off another planet, Venus.
In 1991, researchers are using a similar technique to bounce signals off of Saturn's moon Titan, hundreds of millions of miles distant in the outer solar system. By snagging an elusive echo from the surface of Titan -- shrouded by a dense atmosphere boasting organic chemicals possibly like those that led to life on the Earth -- they may be able to tell if Titan is partially covered by oceans of ethane, as many scientists believe.
Those experiments -- in 1961 and 1991 -- represent the earliest attempts and most sophisticated recent efforts in the field of ground-based planetary radar.
A symposium marking the 30th anniversary of the technique is being held October 2 at the California Institute of Technology in Pasadena, California, sponsored by NASA's Jet Propulsion Laboratory.
"Radar astronomy was the godfather of the planetary space program," said Dr. Nicholas Renzetti, manager of JPL's Telecommunications and Data Acquisition Science Office and organizer of the Caltech symposium.
Although such radar experiments can provide nowhere near the detailed data on planets sent back by exploration spacecraft, the ground-based technique offers one approach for scientists when no spacecraft is available.
By counting how long the radar signal took to return from Venus in the 1961 tests at JPL's Deep Space Network at Goldstone, California, researchers were able to measure precisely the distance between Earth and Venus. That in turn offered the best-ever estimate of the astronomical unit, or AU -- the standard unit of astronomical measure based on the distance between Earth and the Sun.
Researchers also found that Venus -- cloaked by perpetual clouds -- rotates in a retrograde direction about once every eight months. That fact was confirmed the next year when JPL's Mariner 2 made the first flyby of Venus in August 1962.
In later years, researchers at other institutions used various other dish antennas around the world to conduct radar studies, among them Lincoln Laboratory in Massachusetts, Jodrell Bank at Manchester in the United Kingdom, and various sites in the Soviet Union.
The largest such dish on Earth -- the Arecibo Observatory in Puerto Rico -- was used by scientists to map the terrain of Mercury, Venus and Mars.
Planets have not been the only targets for radar studies. Using the giant Arecibo dish, researchers have bounced radar off of asteroids to help determine their orbits -- and, in some cases, even their spin rates. Radar was also used to show for the first time that comets have a solid nucleus.
Today the most sensitive radar studies are made not with a single dish antenna but with a network of such antennas arrayed together. In many current experiments, scientists beam a radar signal into space from JPL's Goldstone site in the California desert and receive the bounce hundreds of miles away at the National Radio Astronomy Observatory's Very Large Array in Socorro, New Mexico.
That technique is currently being used by researchers in an attempt to resolve some of the surface features of Saturn's moon Titan. Masked by an opaque haze and orbiting the Sun a billion miles from Earth, Titan appears featureless to telescopes on Earth. Even the instruments of JPL's Voyager spacecraft provided only limited information on the moon during their flybys in 1980 and 1981.
Contrasts in "brightness" of radar echoes received in June 1989 suggest that continents or some similar features may exist in the oceans of ethane that scientists believe may cover the surface of Titan. Although Titan's environment is not hospitable to human-type life, its atmosphere appears rich in organic chemicals like those that led to life on Earth. Scientists are particularly interested in studying Titan to understand better conditions on the early Earth.
Titan and its parent planet, Saturn, will be the destination of Cassini, a NASA mission with the European Space Agency to be launched in the mid-1990s. Clues from ground-based radar studies may help to shape the investigations that the Cassini spacecraft will carry out.
Speakers at the Pasadena symposium will include:
-- Prof. Solomon W. Golomb, University of Southern California, "The First Touch of Venus";
-- Dr. Donald B. Campbell, Cornell University, "The Exploration of Venus by Radar";
-- Dr. John K. Harmon, Arecibo Observatory, Puerto Rico, "Radar Observations of Mars and Mercury";
-- Dr. Steven J. Ostro, JPL/Caltech, "Asteroid Radar Astronomy";
-- Prof. Von R. Eshelman, Center for Radar Astronomy, Stanford University, "Early Radar Astronomy at Stanford";
-- Prof. Duane O. Muhleman, Caltech, "Goldstone-Very Large Array Observations of Titan."
JPL's ground-based radar studies are sponsored by NASA's Office of Space Science and Applications with support from the Office of Space Operations.
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