One of the reasons that planetary astronomy is such a delight these days is that it is possible to find out what's really right. In the old days, you could make any guess you liked, however improbable, about a planetary environment, and there was little chance that anyone could ever prove you wrong. Today, spacecraft hang like swords of Damocles over each hypothesis spun by planetary theoreticians, and the theoreticians can be observed in a curious amalgam of hope and fear as each new burst of spacecraft planetary information comes winging in.
Back when astronomers had telescopes, eyeballs, and very little else to assist their observations, Venus beckoned as a sister world. By the late nineteenth century, it was known that Venus had about the same mass and radius as Earth. Venus is the closest planet to Earth, and it was natural to assume that it was, in other respects, Earth-like.
Immanuel Kant imagined a race of amorous quasi-humans on Venus. Emanuel Swedenborg and Annie Besant, a founder of theosophy, found - by methods described as spirit travel and astral projection - creatures very like humans on Venus. In more recent years, some of the more spectacularly audacious flying-saucer accounts - for example, those of George Adamski - populated Venus with a race of benign and powerful beings, many of whom seem to have been garbed in long hair and long white robes - clear symbolism, in pre-1963 America, of deep spiritual intent. There is a long history of wishful thinking, bemused speculation, and conscious and unconscious fraud, which produced a popular expectation that our nearest planetary neighbor is habitable by humans, and is possibly even already inhabited by creatures rather like us.
It was, therefore, with a sense of considerable surprise, and even annoyance, that the results of the first radio observations of Venus were greeted. These measurements, performed in 1956 by C. H. Mayer and colleagues at the U. S. Naval Research Laboratory, found Venus to be a much more intense source of radio emission than had been expected. From Venus' distance to the Sun and the amount of sunlight it reflects back to space, the planet should be cool. Because Venus reflects so much sunlight back to space, its temperature ought to be even less than the Earth's, despite its closeness to the Sun. Mayer's group found that Venus, at a radio wavelength of 3 centimeters, was giving off as much radiation as it would if it were a hot body at a temperature of about 600 degrees Fahrenheit. Later observations with many different radio telescopes at many different radio frequencies confirmed the general conclusion that Venus had a "brightness temperature" of about 600 degrees to 800 degrees.
Nevertheless, there was great reluctance in the scientific community to believe that the radio emission came from the Venus surface. A hot object emits radiation at many wavelengths. Why did Venus seem hot only at radio wavelengths? How could the surface of Venus be kept so hot? And finally - since psychological factors may be unconsciously compelling, even in science - a Venus hotter than the hottest household oven is simply less pleasant a prospect than the Venus populated, in the long tradition from Kant to Adamski, by gracious humans of amorous or spiritual inclinations.
This problem of the origin of the Venus radio emission was a major part of my doctoral dissertation. I wrote some twenty scientific papers concerning it between 1961 and 1968, when the problem was finally considered settled. I look back on this period with pleasure. The Venus radio story is very much like a detective story where there are clues littering the pages. Some are vital to the solution; others are false clues, leading in the wrong direction. Sometimes the right answer can be deduced by bearing in mind all the relevant facts and requiring reasonable logical consistency and plausibility.
There were several things we knew about Venus. We knew how the brightness temperature varied with radio frequency. We knew how Venus reflected back to Earth radio waves sent out by large radar telescopes. Man's first successful planetary probe - the United States' Mariner 2 - found in 1962 that Venus was brighter at radio wavelengths at its center than at its edge.
To be matched against such observations were a variety of theories. They fell into two general categories: The hot-surface model, in which the radio emission came from the solid surface of the planet; and the cold-surface model, in which the radio emission came from somewhere else - from an ionized layer in the atmosphere of Venus, or from electrical discharges between droplets in the clouds of Venus, or from a hypothesized great belt of rapidly moving electrically charged particles surrounding Venus (like those that, in fact, surround the Earth and Jupiter). These latter models permitted the surface to be cold by placing the intense radio emission above the surface. If you wanted sailing ships on Venus, you were a cold-surface model advocate.
We systematically compared the cold-surface models with the observations and found that they all ran into serious troubles. The model in which the radio emission came from the ionosphere, for example, predicted that Venus should not reflect radio waves at all. But radar telescopes had found radio waves reflected from Venus with an efficiency of 10 or 20 percent. To circumvent such difficulties, advocates of the ionospheric model constructed very elaborate hypotheses in which there were many ionized layers with especially constructed holes in them to let radar through the ionosphere, hit the surface of Venus, and return to Earth. At the same time there could not be too many holes; otherwise, the radio emission would not be as intense as observed. These models seemed to me to be far too detailed and arbitrary in their requirements.
Just before the remarkable spacecraft observations of Venus of 1968, I submitted a paper to Nature, the British scientific journal, in which I summarized these conclusions and deduced that only the hot-surface model was consistent with all the evidence. I had earlier proposed a specific theory, in terms of the greenhouse effect, to explain how the surface of Venus could be at such high temperatures. But my conclusions against cold-surface models in 1968 did not depend upon the validity of the greenhouse explanation: It was just that a hot surface explained the data and a cold surface did not. Because of my interest in exobiology, I would have preferred a habitable Venus, but the facts led elsewhere. In a paper published in 1962, I had concluded from indirect evidence that the average surface temperature on Venus was about 800 degrees F and the average surface atmospheric pressure about fifty times larger than at the surface of Earth.
In 1968, an American spacecraft, Mariner 5, flew by Venus, and a Soviet spacecraft, Venera 4, entered its atmosphere. By the year 1974 there had been five Soviet instrumented capsules that entered the Venus atmosphere. The last three touched down and returned data from the planetary surface. They were the first craft of mankind to land on the surface of another planet. The average temperature on Venus turns out to be about 900 degrees F; the average pressure at the surface appears to be about ninety atmospheres. My early conclusions were approximately correct, just slightly too conservative.
It is interesting, now that we know by direct measurements the actual conditions on Venus, to read some of the criticism of the hot-surface model published in the 1960s. The year after receiving my Ph.D., I was offered, by a well-known planetary astronomer, ten-to-one odds that the surface pressure on Venus was no more than ten times that on Earth. I gladly offered my ten dollars against his hundred; to his credit, he paid off - after the Soviet landing observations were in hand.
Theory and spacecraft interact in other ways. For example, Venera 4 radioed its last temperature/pressure point at 450 degrees F and twenty atmospheres. The Soviet scientists concluded that these were the surface conditions on Venus. But ground-based radio data had already shown that the surface temperature must be much higher. Combining radar with Mariner 5 data, we knew that the surface of Venus was far below where the Soviet scientists concluded Venera 4 had landed. It now appears that the designers of the first Venera spacecraft, believing the models of cold-surface theoreticians, built a relatively fragile spacecraft, which was crushed by the weight of the Venus atmosphere far above the surface - much as a submarine, not designed for great depths, will be crushed at the ocean bottoms.
At the 1968 Tokyo meeting of COSPAR, the Committee on Space Research of the International Council of Scientific Unions, I proposed that the Venera 4 spacecraft had ceased operating some fifteen miles above the surface. My colleague, Professor A. D. Kuzmin, of the Lebedev Physical Institute, in Moscow, argued that it had landed on the surface. When I noted that the radio and radar data did not put the surface at the altitude deduced for the Venera 4 touchdown, Dr. Kuzmin proposed that Venera 4 had landed atop a high mountain. I argued that ground-based radar studies of Venus had shown mountains a mile high, at most, and that it was exceptionally unlikely Venera 4 would land on the only fifteen-mile-high mountain on Venus, even if such a mountain were possible. Professor Kuzmin replied by asking me what I thought was the probability that the first German bomb to fall on Leningrad in World War II would kill the only elephant in the Leningrad zoo. I admitted that the chance was very small, indeed. He responded, triumphantly, with the information that such was indeed the fate of the Leningrad elephant.
The designers of subsequent Soviet entry probes were, despite the Leningrad zoo, cautious enough to increase the structural strength of the spacecraft in each successive mission. Venera 7 was able to withstand pressures of 180 times that at the surface of the Earth, a quite adequate margin for the actual Venus surface conditions. It transmitted twenty minutes' worth of data from the Venus surface before being fried. Venera 8, in 1972, transmitted more than twice as long. The surface pressure is not at twenty atmospheres, and the spectacular Mount Kuzmin does not exist.
The principal conclusion about the scientific method that I draw from this history is this: While theory is useful in the design of experiments, only direct experiments will convince everyone. Based only on my indirect conclusions, there would today still be many people who did not believe in a hot Venus. As a result of the Venera observations, everyone acknowledges a Venus of crushing pressures, stifling heat, dim illumination, and strange optical effects.
That our sister planet should be so different from Earth is a major scientific problem, and studies of Venus are of the greatest interest in understanding the earliest history of Earth. In addition, it helps to calibrate the reliability of astral projection and spirit travel of the sorts popularized by Emanuel Swedenborg, Annie Besant, and innumerable present-day imitators, none of whom caught a glimmering of the true nature of Venus.
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