Planetary Probes: the end of an era; Emphasis now may shift to low-cost flights

Before the dawn of recorded history, mankind first discovered that five bright stars did not behave like all the rest. These celestial lights traveled erratically across the night sky, so the Greeks gave them the name "planetes," or wanderers.

Efforts to solve the problem of the motion of the sun, moon, and planets visible to the naked eye (Mercury, Venus, Mars, Jupiter, and Saturn) kept early philosophers busy. Such efforts were the forge upon which astronomy was shaped into a modern science that revolutionized mankind's view of the universe and its place within it.

In the last two decades, humanity has taken a forward step comparable to the early, dimly remembered recognition that the "wanderers" were somehow different from other stars. Dispatching its robot eyes and ears across the dark void of interplanetary space, mankind has resolved the five planets known to the ancients from hazy disks seen in the eyepieces of Earth-bound telescopes into sharply defined worlds of spectacular uniqueness and diversity.

"When historians of the future look back at this period, they will recall it as the time when mankind first reached out and explored the solar system," passionately proclaims Carl Sagan, Cornell astronomer and host of the TV series "Cosmos."

This modern exploration of the planets presents today's scientists with as difficult a problem, and with as potentially revolutionary answers, as the erratic motion of the planets presented to Aristotle, Copernicus, and Newton. This problem is summarized in the single question: Why are the planets, including Earth, the way they are?

Mercury is an airless, heavily cratered ball strikingly similar to the moon. Venus, commonly called Earth's twin, has a massive atmosphere of carbon dioxide, potent sulfuric acid smog, and broiling surface temperatures of 900 degrees Fahrenheit. Mars is an Antarctic planet, veiled with a tenuous atmosphere and strangely water deficient. Its surface is pocked with giant volcanoes and slashed by a Grand Canyon that makes Earth's seem puny by comparison. Jupiter is a giant ball of gas with planet-sized moons unlike anything in the inner solar system. One of them, Io, sports geyser-like volcanic eruptions 60 miles high. Saturn is similar to Jupiter, but smaller. It's the only planet less dense than water. And, of course, Saturn presents its own puzzle: the magnificent and mystifying rings.

"I like to think that what we are doing is comparable to what Galileo did using the newly developed telescope. We are utilizing the most advanced technology of our age to learn as much as we can about our universe," says Edward C. Stone of the California Institute of Technology.

As they attempt to unravel the similarities and differences among these various worlds, scientists hope to achieve several goals:

* They hope to learn more about our home planet, Earth. "We are continually trying to relate what we encounter to Earth," explains Dr. Charles Barth of the University of Colorado, who has been involved since the early Ranger missions to the moon. By studing the atmospheres of other planets the experts hope to increase their capabilities for predicting terrestrial weather and climate variations.They also search for clues about the effects on the atmosphere of human activities such as the release of Freon gas and the operation of large numbers of supersonic transports. By studying the geology of the surfaces of other planets, they hope to gain insight into Earth's early geologic history, which has been wiped out by the erosive action of millenia of wind and rain.

* They hope to garner enough evidence to reconstruct the processes that must have taken place more than 5 billion years ago when the sun and planets condensed out of a cloud of dust and gas. How did this primordial material clump and cluster? What were the temperatures and pressures at the various points in this solar nebula where the different planets formed, and how did they effect each planet's chemical makeup? Why were the conditions on Earth favorable for the formation of life?

Ultimate answers to these questions, especially the last one, could have a profound effect on humanity's concept of itself. In fact, planetary exploration has already had a major effect. A few decades ago, the evolution of life was considered a cosmic accident, the result of a combination of events so rare that the chance of intelligent beings existing elsewhere in the galaxy was almost nonexistent.

Today, scientists believe that "we are the average products of solar evolution," says Bruce Murray, director of the Jet Propulsion Laboratory and one of the original planetary scientists. Actually, all the bizarre planets in the solar system that we have studied are surprisingly similar, he says. "Earth is the watery planet, and that makes all the difference," Dr. Murray explains. And Earth is watery because of the distance it formed from the sun. Venus was too close and hot. Mars was too far away and cold.

"There must be many, many watery planets in the galaxy. So there must also be many sentient beings as well," he argues.This feeling, shared by a number of scientists and philosophers, represents a major shift in intellectual thought.

For two decades the US has conducted a vigorous unmanned planetary exploration program, averaging more than two flybys, landings, or orbital operations per year. During that period the greatest gap in discovery was a year and a half between the 1962 Venus flyby of Mariner 2 and the photos of the moon radioed back by Ranger 7. At the same time, this has required only around 5 percent of the total space agency budget.

Today, however, the ambitious program faces a five-year hiatus because of lack of funding under the previous administration in Washington. Following the Voyager 2 flyby of Saturn last month, the next planetary event will not be until 1986 when this same space probe crosses the billion-plus miles from Saturn to Uranus. In the interim, the National Aeronautics and Space Administration is scheduled to put a telescope in orbit. Also on the books for the late 1980s: another mission to Jupiter, consisting of an orbiter and an atmospheric probe, and a Venus orbiter that will use a powerful radar to map the planet's surface through the clouds to an accuracy equivalent to that of the television pictures that other spacecraft have taken of Mars.

Many of the scientists involved fear that such scientific adventures will be sacrificed on the altar of fiscal austerity, and that the US will lose its scientific and technological leadership in planetary exploration. The most optimistic talk is about a renaissance in the 1990s, after the space shuttle is fully operational.Others are considering a "low cost" planetary program that would utilize smaller, cheaper probes with fewer instruments and much more circumscribed scientific objectives, as opposed to super-sophisticated machines like the Vikings that landed on Mars or the Voyagers that have photographed Jupiter and Saturn.

Mariner 2 and its successors have revolutionized mankind's understanding of the solar system and so of the universe. In doing this, they have presented Earth-bound scientists with an entirely new set of questions and puzzles.It took man thousands of years to answer the problem of the motion of the planets. It iw not clear how long it will take us to answer the question of why the planets are as they are. Two decades of US unmanned planetary exploration Encounter Target/type Spacecraft date 12/14/62 Venus/flyby Mariner 2 07/31/64 Moon/impact Ranger 7 02/20/65 Moon/impact Ranger 8 03/24/65 Moon/impact Ranger 9 07/14/65 Mars/flyby Mariner 4 06/02/66 Moon/impact Surveyor 1 09/14/66 Moon/orbiter Lunar Orbiter 1 11/10/66 Moon/orbiter Lunar Orbiter 2 02/08/67 Moon/orbiter Lunar Orbiter 3 04/20/67 Moon/impact Surveyor 3 06/08/67 Moon/orbiter Lunar Orbiter 4 07/22/67 Moon/orbiter IMP-5 08/05/67 Moon/orbiter Lunar Orbiter 5 09/11/67 Moon/impact Surveyor 5 10/19/67 Venus/flyby Mariner 5 11/10/67 Moon/impact Surveyor 6 02/10/68 Moon/impact Surveyor 7 07/30/69 Mars/flyby Mariner 6 09/04/69 Mars/flyby Mariner 7 11/13/71 Mars/flyby Mariner 9 06/15/73 Moon/orbiter RAE-2 12/03/73 Jupiter/flyby Pioneer 10 02/05/74 Venus/flyby Mariner 10 03/29/74 Mercury/flyby Mariner 10 09/21/74 Mercury/flyby Mariner 10 12/02/74 Jupiter/flyby Pioneer 11 03/16/75 Mercury/flyby Mariner 10 06/19/76 Mars/orbiter Viking 1 orbiter 07/20/76 Mars/lander Viking 1 lander 09/07/76 Mars/orbiter Viking 2 orbiter 09/03/76 Mars/lander Viking 2 lander 12/04/78 Venus/orbiter Pioneer Venus orbiter 12/09/78 Venus/atmos. Pioneer Venus probes Probes 03/05/79 Jupiter/flyby Voyager 1 07/09/79 Jupiter/flyby Voyager 2 09/01/79 Saturn/flyby Pioneer 11 11/12/80 Saturn/flyby Voyager 10 08/25/81 Saturn/flyby Voyager 2