The Pioneer 10 spacecraft sailed out of the solar system this morning, becoming the first man-made object to enter interstellar space and to flash back reports on solar winds and cosmic rays from beyond the planets.
At 8 a.m., the indefatigable 570-pound spacecraft sent out a series of faint beeps and whines on an eight-watt transmitter. The sounds, from beyond Neptune's orbit, took four hours and 20 minutes to hurtle across 2.8 billion miles to tracking stations on Earth.
As they have for the past 11 years, researchers at NASA's Ames Research Center in Mountain View, Calif., quickly scanned the readings to seek out unusual data. By measuring the time it took the signals to reach tracking stations, scientists were able to ascertain that Pioneer had crossed Neptune's orbit and achieved the historic milestone of becoming the first man-made object to pass beyond the solar system.
Normally, Pioneer would have had to pass the planet Pluto. But Pluto follows such an erratic orbit that it is now inside the circular path of the dependable Neptune, and will be for the next 17 years.
Pioneer sent a second series of radio transmissions about 6 this evening. Pete Waller, a spokesman for Ames, said the latest readings from Pioneer's compact instrument panel indicate that the solar wind--a steady stream of subatomic particles sent hurtling from the sun across the solar system--is becoming increasingly less dense as the ship moves farther away.
Pioneer also is monitoring the magnetic field in the region, an indication that the residual strength from the solar system's large outer planets is being felt beyond the system's boundaries.
The latest instrument readings also indicate a three-degree change in the direction of the solar wind. "Considering the distance, it's amazing it could do that," Waller said. The change presumably only means that the solar wind is no longer being taken in any one direction by the gravitational pull of the planets.
Pioneer also is reporting that the solar wind in the area is moving at a speed greater than 1 million mph and that its temperature is 15,000 degrees Centigrade. Little of that is surprising to scientists who likely will take months to analyze the data completely and debate the significance of the readings.
The intrepid space traveler, which was designed for a 21-month mission, seems assured of a long and productive life as it continues through an endless void of space for billions of years.
Herbert A. Lassen, Pioneer's designer, said the probe should continue to exist for at least one revolution of the galaxy, which takes about 250 million years. Some expect Pioneer 10 to survive through 10 or 20 galactic cycles.
"The chance of someone finding it in a million years is almost nil," Lassen said. "But if you start talking about these kinds of time periods, God only knows who might find it."
As Pioneer 10 moves farther from the Earth, communication is becoming more difficult. Signals should be picked up for only about another decade. But since Pioneer is passing through a relatively benign region of space, there is little to disturb it. "We don't see anything inhibiting its future life except the normal statistical use of its hardware," said William J. Dixon, a systems engineer for the Pioneer program. Most of the critical devices have backup systems and these have not been used, he added.
Pioneer 10 will continue to measure the speed, density and direction of the solar wind. The spacecraft will spend most of its time defining the sun's extended atmosphere, or heliosphere, beyond the solar system. The heliosphere is a magnetic bubble, believed to be tear-shaped, created by the solar wind. Pioneer 10 could prove the true shape of the heliosphere. The spacecraft may also discover its boundary, called the heliopause, which some scientists believe is 5 billion miles from Earth.
"That's the one milestone left," Lassen said.
Lassen said Pioneer 10 already has accomplished its three primary objectives: studying and passing through the asteroid belt between Mars and Jupiter, measuring Jupiter's radiation belts and magnetosphere and discovering that the solar wind extends beyond the known planets. Lassen said that what Pioneer discovers on its journey beyond the solar system may not be appreciated for decades.
"This is now a 19-year-old child to me," he said. "It's just about reached its maturity and it's ready to go out on its own. When I think about the time it will go on, it's mind-boggling."