A fleet of four Pioneer spacecraft fell through the superheated atmosphere of Venus today, surviving pressures that reached 90 times what they are on earth and temperatures of 900 degree Fahrenheit at the planet's surface.
Three of the four spacecraft apparently crashed on landing, but the fourth continued to send signals back to earth for more than an hour after striking the surface, even though the temperature inside the spacecraft had climbed to 260 degrees Fahrenheit.
"It's hard to believe everything went so perfectly," Pioneer-Venus project manager Charles F. Hall said today at the National Aeronautics and Space Administration's Ames Research Center, where the mission to Venus is being directed. "It has been a classic mission."
None of the four spacecraft was built to land on Venus, and only the largest of the four was equipped with a parachute to slow its descent to the surface. That one, weighing 700 pounds, landed near the equator on the sunlit side of the planet, while the three smaller probes (200 pounds apiece) fell at places scattered from the north to the south poles in both the day and night regions.
All four survived the worst planetary conditions ever encountered by U.S. spacecraft. One was buffeted with a force equal to 378 times that of gravity when it flew into the thick atmosphere of Venus at a speed of almost 26,000 miles an hour just after 1 p.m. EST.
All four spacecraft decelerated sharply as they punched though the clouds of Venus' upper atmosphere, their speeds dropping from 26,000 to 425 miles an hour in 38 seconds. In that time, the four fell 83 miles to a height of 42 miles above the surface.
The gravitational forces were so strong on the spacecraft that timing devices turned off their instruments until the worst of the entry was over. The instruments were turned back on only when their descent was slow enough to be considered safe.
Falling through clouds deeper than any of the earth's oceans, the four spacecraft encountered winds of up 200 miles an hour and rains made up of almost pure sulfuric acid. There is almost no moisture in the atmosphere of Venus to weaken acids that build up in the turbulent clouds around the planet.
In the 55 to 60 minutes it took the four spacecraft to fall to the surface, they sent a continuous stream of what project manager Hall called "beautiful and excellent data." The spacecraft measured temperatures and pressures all the way to the surface, studied the size of particles that make up the atmosphere and identified even the tiniest traces of gas making up the atmospheric components.
All the time the four spacecraft were falling toward the surface, their radio signals were being picked up by four giant antennae in four locations along the edge of the Pacific.
One antenna is in California's Mojave Desert, a second on Guam, a third near Canberra, Australia, and the fourth outside Santiago, Chile. So critical was the operation of the four antennae during the descent period that the Chilean army banned traffic on the Pan American Highway for three hours to prevent the noise from interfering with the radio reception from Venus.
"This was the most difficult tracking job we'd ever attempted with the deep space network," Hall said after all four spacecraft were on Venus. "We were locking up on the signals of five interplanetary spacecraft all at once from a distance of 34 million miles -- something we'd never done before."
Seven instruments aboard the large probe and the three instruments on each of the smaller probes all worked the way they were intended, from the tops of the clouds right down to the surface. The instrument aboard the small probe that survived the landing sent signals back to earth from the surface for 67 minutes.
None of the 16 instruments had been flown in space before, which was one of the most trying aspects of the mission.
"I can't remember one of these things working the first time we tried them on earth," said Steve Dorfman, project manager for Hughes Aircraft Corp., which built the Pioneer spacecraft. "I can't tell you how elated I am that everything worked the first time we tried it on Venus."
In addition to the four spacecraft, the antennae were communicating with an 810-pound Pioneer in orbit around Venus. Its mission will be to observe the planet for the next 243 days, the length of a full year on Venus. The antennae also were talking to the "bus" that carried the four descent probes from earth to the environs of Venus, a journey that took seven months and covered almost 300 million miles.
Even the bus was targeted at the atmosphere of Venus today. to a spot 120 miles above the surface near the south pole of the planet. Unlike the probes, the bus carried no heat shield to protect it from the searing temperatures, and it burned up in the planet's atmosphere in less than fou minutes.
In the days ahead, the 114 scientists involved in the Pioneer-Venus mission will be poring over the data that came back today from the four probes and was stored on tape recorders.
"The science we're getting back right now will answer amny of the prime questions we have about the fantastic atmosphere of this planet," said Harvard University's Dr. Richard Goody. "Its presure on the surface is equivalent to being below 3,000 feet of ocean. Its temperature is 250 degrees hotter than the melting point of lead, and it is completely screened with clouds in a way that no other of the inner planets is.
"Now, this is a peculiarly different atmosphere to our own and yet the assertion is made that Venus is a sister planet to our own. Why that difference?"