For most of the last century Venus was considered Earth's ''sister planet.'' It was thought there might, in fact, be quite Earthlike life on its surface. Many early science-fiction films and books depicted our nearest planetary neighbor as having lush, tropical jungles in which dinosaurs and various forms of cave people lived. However, the picture that has emerged from more than two decades of studying Venus with spacecraft and sophisticated Earth-based observations is that of a planet considerably more hostile than our tropical fantasies.
The most recent and most comprehensive of these studies has been made with the National Aeronautics and Space Administration (NASA) Pioneer Venus Mission. The spacecraft arrived at Venus in December 1978. The Pioneer Venus project consisted of two separate missions. The first was a ''Multi-Probe Mission,'' in which a total of four probes landed on the planet's surface. The scientific instruments aboard these probes, plus those on the spacecraft which had carried them, and which later burned up in Venus's atmosphere, collected detailed samples of the planet's atmosphere and cloud structure as they descended.
Among the Probe Mission's results was confirmation of a surface temperature on Venus hot enough to melt lead, and an atmosphere more than 100 times as dense as Earth's. The probes also measured the composition of the lower atmosphere. Among other things, the concentration of rare gases --argon, neon, krypton--which were found to help scientists understand the formation of that atmosphere.
The other part of the Pioneer Venus Project is the Orbiter Mission. It involves a second spacecraft, the Pioneer Venus Orbiter, which was placed in orbit around the planet. It is still returning data today and could continue functioning for many years unless plans to shut it down prematurely are carried out.
The Orbiter is a remarkable spacecraft. The instruments it carries have provided a rather startling view of our ''torrid sister''--a view that makes Venus a fascinating exhibit from which we can learn more about our own planet.
One of the most pertinent of orbiter findings has been the discovery that the high temperatures on Venus's surface are caused by a planetary ''greenhouse'' effect. Carbon dioxide gas and probably sulfuric acid combined to trap sunlight. This results in an extremely high surface temperature. Atmospheric scientists, concerned about the increasing concentration of carbon dioxide in Earth's atmosphere as fossil fuels are burned, find Venus a useful example to learn more about such ''greenhouse'' warming.
Other major discoveries include what appear to be huge volcanoes on the planet's surface, observation and apparent correlation of lightning with these volcanoes, and the discovery that Venus at one time had oceans. These oceans had subsequently boiled away as the temperature on the surface increased. Some scientists now wonder if Venus once had life in those ancient seas.
The Orbiter has also found that, unlike the Earth and most other planets, Venus has virtually no magnetic field of its own. This makes it unique among the planets we have explored. Such a magnetic field protects the planet's atmosphere from being scavenged by the solar wind--the hot gas of electrically charged particles which flows radially outward from the sun.
In the case of a strongly magnetized planet, such as Earth, the planet's magnetic field acts as kind of a ''cushion,'' preventing most of the solar wind from actually reaching the planet's atmosphere. But several times in geological history our planet's magnetic field has, for some unknown reason, undergone a sudden change and reversed its direction. During these periods, Earth's magnetic field can become very weak. Our planet's atmosphere may be directly exposed to the solar wind. What actually happens during these geological anomalies is uncertain. However, over the last three years, the Orbiter has provided much new information on what may happen to a planet's atmosphere with no magnetic field to protect it.
It has been found, for example, that the Venus ionosphere ''breathes,'' compressing and expanding as the pressure in the solar wind changes. Inside the ionosphere, regions of strong magnetic fields have been discovered despite the fact that Venus has no magnetic field of its own.
Smaller-scale magnetic structures, known as ''magnetic flux ropes,'' have also been found throughout Venus's ionosphere, the electricfied upper atmosphere. These are twisted tubes of magnetic field lines roughly 10 kilometers (61/4 miles) thick, which may stretch for thousands of kilometers. Meanwhile, ''bubbles'' or ''streamers'' of ionospheric plasma (which is a gas of electrically charged particles) observed outside Venus's atmosphere indicate that portions of the upper ionosphere of Venus are being swept away by the solar wind. Finally, on the night side of Venus, there are ''holes'' in the ionosphere where plasma seems to have been removed by some unknown process.
Our ''sister'' planet is a strange place indeed. Many important questions remain to be answered. Among them: How is the ionosphere of Venus heated by the solar wind? How much of that heat is transmitted to Venus's electrically neutral lower atmosphere? Can energy deposited by the solar wind account for any global climatic effects. How much of Venus's atmosphere is being stripped off by the solar wind? Where do the magnetic fields inside Venus's ionosphere come from? How are magnetic flux ropes created? What causes the holes in the ionosphere? These are all significant scientific questions. But what may be most interesting to Earth people is the insight Venus can provide into what happens to our own planet's atmosphere when the Earth's magnetic field is switched off.
The Pioneer Venus Orbiter is just beginning to answer these questions. Data returned each day add new pieces to the puzzle. The spacecraft is healthy. Nearly all the instruments aboard are in operating order. However, NASA has decided to turn Pioneer Venus off. The reason for this decision is not clear. However, the $6 million it takes to fund the mission each year is less than 1/10 of a cent of the NASA dollar. If this is NASA's, or the administration's, way of saving money, it just doesn't make sense.
Pioneer Venus is valuable. It is cheap to operate. And it is a national resource. The United States has no more missions planned for Venus. If the Pioneer Orbiter is switched off, the only hope of answering many of the questions scientists now are investigating lies with the Soviet Union. The USSR has recently landed two spacecraft on the surface of Venus and has another mission planned for a few years from now.