Imagine the weight of a nagging suspicion that what held your world together, a constant and consistent presence you had come to understand and rely on, wasn't what it seemed. That's how scientists feel when they ponder gravity these days.
For more than three centuries, the basics of gravity were pretty well understood.
Newton described the force as depending on an object's mass. Though it extends infinitely, gravity weakens with distance (specifically, by the inverse square of the distance). Einstein built on these givens in developing his theory of relativity.
Then more than a decade ago a researcher noticed something funny about two Pioneer spacecraft that were streaming toward the edge of the solar system. They weren't where they should have been.
Something was holding the probes back, according to calculations of their paths, speed and how the gravity of all the objects in the solar system -- and even a tiny push provided by sunlight -- ought to act on them.
Now scientists have proposed a new mission to figure out what's up with gravity.
Pioneer 10 and 11 launched in 1972 and 1973. Today each is several billion miles away, heading in opposite directions out of the solar system.
The discrepancy caused by the anomaly amounts to about 248,500 miles (400,000 kilometers), or roughly the distance between Earth and the Moon. That's how much farther the probes should have traveled in their 34 years, if our understanding of gravity is correct. (The distance figure is an oversimplification of the actual measurements, but more on that in a moment.)
Scientists are quick to suggest the Pioneer anomaly, as they call it, is probably caused by the space probes themselves, perhaps emitting heat or gas. But the possibilities have been tested and modeled and penciled out, and so far they don't add up.
Which leaves open staggering possibilities that would force wholesale reprinting of all physics books:
- Invisible dark matter is tugging at the probes
- Other dimensions create small forces we don't understand
- Gravity works differently than we think
Slava Turyshev at NASA's Jet Propulsion Laboratory is one of a handful of scientists who wrestle mentally with the Pioneer anomaly every day. He is not paid to work specifically on the problem, so he has to juggle the disturbing thought with his regular research, which involves other aspects of gravity and, significantly, whether theories that explain the glue of the whole universe might one day match neatly with those describing the invisible, subatomic world.
"I have been working on [the Pioneer anomaly] for more than 11 years now, and was never funded to do this job," Turyshev tells SPACE.com. "I guess this says a lot about my devotion to solve this mystery."
Data from the Galileo and Ulysses spacecraft suggest the anomaly may have affected them, too. But neither has been far enough from the Sun -- the dominant source of gravity in the solar system -- to firmly distinguish any possible discrepancy from noise in the data, Turyshev says. Galileo was crashed into Jupiter last year, and Ulysses will never go farther than it has.
That leaves two data points -- one from each Pioneer craft. Turyshev pointedly considers the pair as one data point, so as not to inflate the case for strange new physics. He looked at the two Voyager spacecraft, also exiting the solar system, but says their design involved "numerous attitude-control maneuvers" that "can overwhelm the signal of a small external acceleration."
NASA engineers have made their last communications with the Pioneer probes, so the two table-sized robots are carrying the unsolved mystery silently to the stars.
New mission proposed
The Pioneer anomaly was discovered by John Anderson, also of JPL, in the 1980s. For years he didn't publish what he'd noticed. Then he discussed it with physicist Michael Martin Nieto at the Los Alamos National Laboratory. Nieto says he "almost fell off my chair."
Nieto jumped into the investigation, and the two were later joined by Turyshev. They dug deeper into the data, even tracking down retired NASA scientists for some of it.
Unraveling the enigma will require a new mission, the researchers say. NASA, however, doesn't have such a project on its agenda and has not expressed much interest in one. Europeans, for reasons both historic and having to do with a current strong desire to better grasp gravity, seem more interested in investigating the problem.
So Anderson's team recently proposed to the European Space Agency a "mission to explore the Pioneer anomaly" using the latest accelerometers and advanced navigation methods. All possible sources of onboard radiation would be eliminated in "the most precisely tracked spacecraft ever to go into deep space," the group writes in the September issue of Physics World magazine.
The idea has "very high chances" of being chosen for future study, Turyshev thinks. If funded, it could launch as early as 2015.
If the mission were to find a natural, cosmic cause to the Pioneer anomaly, the revelation would rank right up there with other apple-on-the-head moments in the history of physics.
"If the anomaly is due to some new physical mechanism, this discovery would have a truly fundamental impact," Turyshev said.
One candidate is dark matter. This unknown stuff seems to infuse the universe and, though invisible, has a collective gravitational impact greater than all known matter, including stars and planets. Dark matter is inferred to exist because, without it, galaxies would fly apart. Every galaxy must be loaded with the stuff, astronomers conclude, based on how stars are bound to orbit the centers of the galaxies.
But dark matter's effects have been presumed to operate across large expanses, both within and between galaxies. There is no evidence of it controlling anything on a scale so small as our solar system.
Another idea is that gravity tugs slightly harder at things farther away. That radical suggestion, if proved true, would force a modification of Einstein's general theory of relativity and might eliminate dark matter as a player.
Yet one more exotic possibility: Dimensions exist beyond the four we know (three directions and time). Models of string theory propose that higher dimensions could provide weak forces that act in ways we don't yet comprehend.
No fancy theory in existence, however, properly explains the Pioneer data.
The Pioneer anomaly is not actually a measure of how far the Pioneer probes did or didn't travel.
Instead, scientists bounced microwave signals off each probe and noticed an unexpected drift in the Doppler frequency as the probes got farther away. The technique is akin to noting the sound change in a siren as an ambulance races first toward you, and then away from you. The Doppler effect is a shortening or lengthening of sound waves (or microwaves, or any waves) forced by an object's movement.
The drift showed that the Pioneers were being accelerated toward the Sun (or, rather, decelerated in their movement away from the Sun) by a tiny but inexplicable amount. The level of drift is equal to a gravitational effect 10 billion times weaker than the pull of Earth.
Though tiny, the signal is clear, other scientists agree.
Despite 11 years of devotion to the mystery, Turyshev is the first to admit that the "most obvious explanation" would be an unknown onboard effect. Perhaps excessive internal heat or leaks of propulsion gas are providing a wee bit of thrust that adds up over the years.
Yet despite a lot of testing, "no unambiguous, onboard systematic problem has been discovered," he said. "This inability to explain the anomalous acceleration of the Pioneer spacecraft with conventional physics has contributed to the growing discussion about its origin."
Even if the anomaly is caused by the Pioneer probes themselves, figuring it out will be useful says Turyshev, who is the proposal leader for the U.S. group.
"Finding it would help us to build a better spacecraft for the needs of fundamental physics," he said. "These craft would much more stable, quieter and would allow us to go even deeper in our quests of studying the fabric of fundamental and gravitational physics."
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