In an experiment as ambitious as it is unprecedented, astronomers will attempt to measure the winds in the atmosphere of Venus when the cloud-covered planet is 135 million miles away next month.
The test will take place Dec. 9, when four small pioneer space probes descend into the atmosphere of Venus and beam back radio signals that will not only track their descent, but also will identify even their slightest wobbles as winds in the upper atmosphere blow them off course.
"We should be able to measure which way the winds are blowing on Venus and how fast they're blowing to within one mile an hour." Dr. Charles C. Counselman of the Massachusetts Institute of Technology said yesterday at a press conference."This experiment is as precise as seeing the minute hand move on a watch that's three thousand miles away."
When the four probes fall toward Venus, where upper-atmospheric winds are likely to be 200 miles an hour, they will send back radio signals whose arrival at earth can be timed to within one-trillionth of a second.
The signals from the four probes will be picked up by four antennas, one at Goldstone in California's Mojave Desert, a second at Santiago, Chile, on the west coast of South America, a third in Australia and the fourth on the island of Guam in the South Pacific.
The antennas form a ring around the Pacific Basin which Venus will be almost directly above on Dec. 9 when the probes descend into the planet's atmosphere. Any three of the antennas - with the fourth available as backup - will form a triangle which scientists can use to measure the time it takes for the signal from each probe to reach earth.
If the winds on Venus deflect any of the probes, the change of position will alter the times that a probe's radio signals arrive at the antennas.From that time difference, scientists can calculate how much the probe has been moved and from that displacement, the velocity of the winds.
"This experiment has never been done before and it measures how the probe moves, not how the air moves," Counselman said, "but the measurements we'll be making are as precise a measure of what's happening to the air as it is to the probes."
The wind speed experiment is just one of the 30 that will be carried out by the four probes and a fifth spacecraft that will orbit the planet when as they arrive at Venus next month after traveling almost 300 million miles from earth.
The five spacecraft cost the National Aeronautics and Space Administration $175 million and represent the most ambitious attempt so far by either the United States or the Soviet Union to explore our nearest planetary neighbor.
The Soviets have landed four spacecraft on the surface of Venus, the last two of which (Venera 9 and 10 in 1975) sent back two pictures of the surface of Venus. The Soviet spacecraft also were first to find that the temperature on the surface is 1,000 degrees Fahrenheit under a pressure 90 times as great as the barometric pressure on earth. This extreme heat gives Venus the name "hell in the heavens."
Even now, two Soviet spacecraft are in flight to Venus about three weeks behind the five American spacecraft. Each of the Soviet mother ships will release a smaller craft whose mission it will be to land on the surface of Venus. American scientists say the Soviet landers are little different from the earlier ones and carry nothing as advanced as the U.S. spacecraft.
From what Soviet scientists have told their American colleagues, there have been only three changes in the current Soviet spacecraft. They have removed floodlights installed on the earlier craft because they thought the surface would be too dark to photograph even in daylight. And two instruments that measure atmospheric conditions have been replaced with more reliable ones.
On the other hand, the pioneer spacecraft now nearing Venus carry some of the newest and most advanced instruments that ever have flown into space.
"These instruments dwarf whatever has been attempted at Venus before," said Dr. Richard Goody of Harvard University. "Our spacecraft were built to answer the most crucial questions that remain about the origins and evolution of the planet Venus."
Among the new instruments is a radar-mapping device built into the Pioneer to begin orbitting Venus Dec. 4, five days before the four Pioneer probes arrive and descend into the 50-mile-thick atmosphere. Besides making its own observations, the orbiter will also act as radio relay to earth for the four probes.
The orbiter radar will map the surface of the planet the same way radar charts storms and aircarft on earth. Since no cameras can penetrate the thick clouds surrounding Venus, the radar will be the closet look yet at the surface of the planet.
So far,radar echoes of Venus have revealed what appears to be a basin the size of Hudson Bay in Canada, a large volcano that could be second in size to Mount Olympus on Mars and a canyon as wide as 60 miles and as long as 600 miles. One crater has been spotted on Venus that is as big as Alaska.