The Russians and the Americans are shooting at a common target, yet neither can see it and at least one shooter hopes to miss it completely. The Soviet projectile is Venus 4, a space probe that beat the U.S. Mariner 5 into space by two days in the middle of last month. Both spacecraft are bound for earth's mysterious sister planet Venus, both should arrive some time in October, and both represent attempts to settle one of the biggest planetary controversies ever waged. The Russian probe is so much larger than Mariner-it weighs more than 2,400 pounds compared to Mariner's 540-that it may even be two spacecraft in one, with a secondary capsule to land on the planet's surface while the parent vehicle flies by. Mariner 5 carries instruments to measure Venus' magnetic field, to pierce its atmosphere with radio waves, to hunt for hydrogen and oxygen in its upper atmosphere and to examine the space around it for radiation, charged particles and the solar wind. Yet all this information will be but a drop in the virtually empty bucket of information on earth's nearest neighbor outside the moon. Cloaked in a seemingly impenetrable blanket of clouds-and recently doubt has been cast even on thatVenus has intrigued and defied astronomers for centuries. The first photographs of the planet that showed even any shading were taken in 1911, and Venus has yet to be revealed as anything more distinct than a smudge. The noted British astronomer Patrick Moore spent 30 years studying vague visible shadings on Venus, looking for a clue to the hidden planet's period of rotation, but in vain. Fairly accurate estimates, however, have been made by other methods. Until 1964, the best guess at the rotation period was about 266 earth days. but that estimate, made by scientists at California's Jet Propulsion Laboratory, was accurate only to within four weeks either way. Then the huge radio telescope at Arecibo, Puerto Rico, was used to bounce radio waves off the planet's surface; Cornell researchers measured the Doppler shift of the returning waves to pin down the period at 253 days, plus or minus five days. The direction of the shift also confirmed the suspicion that Venus rotates in the opposite direction from the earth and all the other planets in the solar system except Uranus. A year later, radar measurements refined the number to 247 days, give or take five. In 1966 it was suggested that earth itself played a hand in Venus' rotation. Drs. Peter Goldreich and Stanton J. Peale of the University of California said that earth's gravity was enough to cause the same side of Venus to be facing the earth every time the two planets were lined up with the sun. If that were true, the rotation period of the planet would be 243.16 days. Only two months later, measurements at JPL confirmed the time, trimming off a half-day to compensate for an electronic error in the equipment, leaving a final estimate of 242 days 14 hours and 24 minutes. Few if any other Venusian details have been settled upon with such accuracy, however. One summary of Venus research, prepared for the National Aeronautics and Space Administration, observes with considerable understatement that there is uncertainty about the abundance of carbon dioxide in the planet's atmosphere: recent estimates range from four to 90 percent. The atmosphere of the planet, in fact. is its most discussed feature, since it has much to do with most of Venus' other characteristics-temperatures. winds, surface features, even the possiblility of life. Venus is indeed a near twin to earth, and the similarities of the planets themselves are largely responsible for the similarities, however disputed, of their atmospheres: Venus' density is about .934 times that of earth, its diameter .956 times that of earth, and the speed required to escape from its gravity .913 times that of earth. As a result, Venus is much more able than, say, Mars, which is smaller and has about a third of earth's gravity, to retain an atmosphere with gases as light as those on earth. This does not mean that our sister planet's atmosphere is the same as earth's, however. The air we breathe is about four-fifths nitrogen, with oxygen comprising almost all of the remainder; hydrogen, carbon dioxide and miscellaneous other gases make up the rest. An estimate of Venus' atmosphere is difficult with so many conflicting studies, but the NASA summary makes a reasonable attempt: the major constituent is again nitrogen. perhaps 63 percent; the next 31.5 percent is neon. based on the relative abundances of gases in the sun, minus the helium and hydrogen which would have escaped from the planet's weaker gravity; for carbon dioxide the report allows about 5 percent; with water, oxygen and all the other gases and vapors making up the remaining half percent. Water is a sore point with Venus researchers. As a key factor in the life-or-not controversy, it has scientists lined up in opposing camps. Typical of one side is Dr. John Strong, director of Johns Hopkins University's Astrophysics and Physical Meteorology Laboratory in Baltimore, Md., who in 1964 sent a spectrometer up 17 miles in a balloon to study the light reflected from Venus' clouds. His finding, Lowell Observatory
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