Pioneer 10 to Test Portion of Einstein’s Relativity Theory

The Pioneer 10 spacecraft, now farther away from Earth than any other device built by humans, will be asked to prove part of Einstein’s theory of relativity later this year. If it does so, it will have succeeded where scores of scientists have failed.

The venerable spacecraft, now in its 16th year and celebrating the fifth anniversary of its departure from the solar system, will be asked in December to measure infinitesimally small changes in its distance from the Earth, changes that could only be caused by gravity waves hitting the Earth and the tiny spacecraft, John Anderson of the Jet Propulsion Laboratory said here Monday.

Einstein predicted that the universe is awash with gravity waves, but so far, no one has been able to prove it. Several gravity-wave detectors are operating and others are being built around the world in an effort to detect the waves, which should cause very slight movement when they pass through an object, just as a ripple in a pond causes a twig to move. However, the movement is so small that no one has succeeded yet in detecting a single wave.

Anderson believes that gravity waves passing through the solar system should make both the Earth and the spacecraft “jiggle” a little, and that jiggle should cause a tiny change in the distance between the two.

Doppler Shift

That change, in turn, should show up in the telecommunications link between Earth and Pioneer as a slight doppler shift--a change in wavelength caused by two objects moving either toward or away from each other, Anderson said.

For 10 days this December, NASA will use its tracking facilities in California, Spain and Australia to monitor Pioneer 10 on a continuous basis in search of a tiny “jiggle.”

“If we have sufficient sensitivity, we should be able to detect it,” Anderson said.

That would be a major achievement in view of the fact that the spacecraft is now more than 4 billion miles away. It takes 12 hours and 26 minutes for signals traveling at the speed of light to go from the control room at NASA’s Ames Research Center to Pioneer and back.

Pioneer 10 was launched on March 3, 1972, and after flying past Jupiter and Neptune, it was sent off to become the first spacecraft to leave the solar system.

Some scientists argue that it is still in the solar system, because it is still influenced by the sun, but project manager Richard O. Fimmel said the departure from the region of the planets qualifies as leaving the solar system.

Major Challenge

Pioneer 10, one of a series of spacecraft built by TRW of Redondo Beach, is now so far away that just keeping track of it is a major challenge.

NASA tracks the spacecraft with its huge radio antenna in the California desert. But the amount of energy hitting the 70-meter dish in the form of Pioneer transmissions is so slight that, according to Fimmel, it would take 11.4 billion years to collect enough energy to light a 7-watt light bulb for 1/1000 of a second, he said.

As it speeds on its journey, Pioneer 10 is giving scientists their first look at the outer fringes of the solar system. Several scientists reported that the data from Pioneer has forced them to rethink some of their ideas--the sun’s influence extends farther than some had thought, for example--but by and large Pioneer has simply confirmed theories already widely believed.

“There has been nothing really spectacular, but lots of good, solid work,” said James Van Allen of the University of Iowa, principal investigator on Pioneer who is best known for his discovery of the Earth’s radiation belts.

With any luck, the radioactive isotopes that supply Pioneer’s electricity will keep it working for another “seven, eight, nine or maybe even 10 years,” Fimmel said.

At some point, millions of years down the line, some other intelligent life might spot the spacecraft plunging through a distant neighborhood and snare it as it passes through.

If so, there will be a message aboard. The craft carries a plaque designed to tell any others out there that we are here.