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oof of the Naval Research Laboratory in Washington； D。C。； at Venus and measured the flux of radio waves arriving at Earth。 This was not radar： No radio waves were bounced off Venus。 This was listening to radio waves that Venus on its own emits to space。 Venus turned out to be much brighter than the background of distant stars and galaxies。 This in itself was not very surprising。 Every object warmer than absolute zero （…273°C） gives off radiation throughout the electromagnetic spectrum； including the radio region。 You； for example； emit radio waves at an effective or 〃brightness〃 temperature of about 35°C； and if you were in surroundings colder than you are； a sensitive radio telescope could detect the faint radio waves you are transmitting in all directions。 Each of us is a source of cold static。
What was surprising about Mayer's discovery was that the brightness temperature of Venus is more than 300°C； far higher than the surface temperature of the Earth or the measured infrared temperature of the clouds of Venus。 Some places on Venus seemed at least 200° hotter than the normal boiling point of water。 What could this mean？
Soon there was a deluge of explanations。 I argued that the high radio brightness temperature was a direct indication of a hot surface； and that the high temperatures were due to a massive carbon dioxide/water vapor greenhouse effect—in which some sunlight is transmitted through the clouds and heats the Surface； but the surface experiences enormous difficulty in radiating back to space because of the high infrared opacity of carbon dioxide and water vapor。 Carbon dioxide absorbs at a range of wavelengths through the infrared； but there seemed to be 〃windows〃 between the CO2 absorption bands through which the surface could readily cool off to space。 Water vapor； though； absorbs at infrared frequencies that correspond in part to the windows in the carbon dioxide opacity。 The two gases together； it seemed to me； could pretty well absorb almost all the infrared emission； even if there was very little water vapor—something like two picket fences； the slats of one being fortuitously positioned to cover the gaps of the other。
There was another very different category of explanation； in which the high brightness temperature of Venus had nothing to do with the ground。 The surface could still be temperate； clement； congenial。 It was proposed that some region in the atmosphere of Venus or in its surrounding magnetosphere emitted these radio waves to space。 Electrical discharges between water droplets in the Venus clouds were suggested。 A glow discharge in which ions and electrons rebined at twilight and dawn in the upper atmosphere was offered。 A very dense ionosphere had its advocates； in which the mutual acceleration of unbound electrons （〃free…free emission〃） gave off radio waves。 （One proponent of this idea even suggested that the high ionization required was due to an average of 10；000 times greater radioactivity on Venus than on Earth—perhaps from a recent nuclear war there。） And； in the light of the discovery of radiation from Jupiter's magnetosphere； it was natural to suggest that the radio emission came from an immense cloud of charged particles trapped by some hypothetical very intense Venusian magnetic field。
In a series of papers I published in the middle 1960s； many in collaboration with Jim Pollack；* these conflicting models of a high hot emitting region and a cold surface were subjected to a critical analysis。 By then we had two important new clues： the radio spectrum of Venus； and the Mariner 2 evidence that the radio emission was more intense at the center of the disk of Venus than toward its edge。 By 1967 we were able to exclude the alternative models with some confidence； and conclude that the surface of Venus was at a scorching and un…Earthlike temperature； in excess of 400°C。 But the argument was inferential； and there were many intermediate steps。 We longed for a more direct measurement。
* James B。 Pollack made important contributions to every area of planetary science。 He was my first graduate student and a colleague ever since。 He converted NASA's Ames Research Center into a world leader in planetary research and the post…doctoral training of planetary scientists。 His gentleness was as extraordinary as his scientific abilities。 He died in 1994 at the height of his powers。
In October 1967—memorating the tenth anniversary of Sputnik 1—the Soviet Venera 4 spacecraft dropped an entry capsule into the clouds of Venus。 It returned data from the hot lover atmosphere； but did not survive to the surface。 One day later； the United States spacecraft Mariner 5 flew by Venus； its radio transmission to Earth skimming the atmosphere at progressively greater depths。 The rate of fading of the signal gave information about atmospheric temperatures。 Although there seemed to be some discrepancies （later resolved） between the two sets of spacecraft data； both clearly indicated that the surface of Venus is very hot。
Since then a progression of Soviet Venera spacecraft and one cluster of American spacecraft from the Pioneer 12 mission have entered the deep atmosphere or landed on the surface and measured directly—essentially by sticking out a thermometer—the surface and near…surface temperatures。 They turn out to be about 470°C； almost 900°F。 When such factors as calibration errors of terrestrial radio telescopes and surface emissivity are taken into account； the old radio observations and the new direct spacecraft measurements turn out to be in good accord。
Early Soviet landers were designed for an atmosphere somewhat like our own。 They were crushed by the high pressures like a tin can in the grasp of a champion arm wrestler； or a World War II submarine in the Tonga Trench。 Thereafter； Soviet Venus entry vehicles were heavily reinforced； like modern submarines； and successfully landed on the searing surface。 When it became clear how deep the atmosphere is and how thick the clouds； Soviet designers became concerned that the surface might be pitch…black。 Veneras 9 and 10 were equipped with floodlights。 They proved unnecessary。 A few percent of the sunlight that falls on the top of the clouds makes it through to the surface； and Venus is about as bright as on a cloudy day on Earth。
The resistance to the idea of a hot surface on Venus can； I suppose； be attributed to our reluctance to abandon the notion that the nearest planet is hospitable for life； for future exploration； and perhaps even； in the longer term； for human settlement。 As it turns out there are no Carboniferous swamps no global oil or seltzer oceans。 Instead； Venus is a stifling； brooding inferno。 There are some deserts； but it's mainly a world of frozen lava seas。 Our hopes are unfulfilled。 The call of this world is now more muted than in the early days of spacecraft exploration； when almost anything was possible and our most romantic notions about Venus might； for all we then knew； be realized。
MANY SPACECRAFT CONTRIBUTED to our present understanding of Venus。 But the pioneering mission was Mariner 2。 Mariner 1 failed at launch and—as they say of a racehorse with a broken leg—had to be destroyed。 Mariner 2 wor