Thirty years ago, NASA scientists noticed that two of their spacecraft, Pioneer 10 and Pioneer 11, were veering off course slightly, as if subject to a mysterious, unknown force. In 1998, the wider scientific community got wind of that veering—termed the Pioneer anomaly—and took aim at it with incessant, mind-blowingly detailed scrutiny that has since raised it to the physics equivalent of cult status. Now, though, after spawning close to 1000 academic papers, numerous international conferences, and many entire scientific careers, this beloved cosmic mystery may be on its way out.
Slava Turyshev, a scientist at the Jet Propulsion Laboratory (JPL) in Pasadena, Calif., and Viktor Toth, a Canada-based software developer, plan to publish the results of their strikingly comprehensive new analysis of the Pioneer anomaly in the next few months. Their work is likely to bring a conclusion to one of the longest and most tumultuous detective stories of modern astrophysics.
NASA launched Pioneer 10 in the spring of 1972 and Pioneer 11 one year later. The spacecraft's joint mission was to gather information about the asteroid belt, Jupiter, Saturn (in the case of Pioneer 11), and their moons. As they hurtled past those various celestial objects, the probes measured previously unknown properties of their atmospheres and surfaces; they also photographed Jupiter's Red Spot and Saturn's rings up close for the first time. Then, after completing their "flyby" missions in the mid-1970s, the Pioneers kept going. Carrying identical plaques depicting a man and a woman, the atomic transition of hydrogen, and the location of our planet within the galaxy—a message to aliens—the probes became the first manmade objects ever to plunge beyond the solar system into the inconceivable cold and dark of interstellar space.
JPL scientists continued Doppler tracking the Pioneers far into deep space. They sent and received a continuous stream of radio transmissions to and from both Pioneers, logging the velocity of each everywhere along its trajectory. An astronomer named John Anderson led the analysis of the Doppler ranging data. He and his team intended to use the data to study subtle gravitational effects in the outer solar system, far from the overwhelming influences of the sun and larger planets. It was thought, for example, that the Pioneers might oscillate in tune with low-frequency gravity waves.
Of course, in order to detect such curiosities in the motion of the spacecraft, the scientists needed to know exactly what to expect in the first place; this required the construction of an algorithm of truly staggering complexity. Contributing factors to the predicted Doppler shift included: the deceleration experienced by the Pioneers as they struggled against the gravitational pull of the sun, planets, moons, asteroid belts, and comet clouds, the positions and thus gravitational fields of which move constantly; the tiny push on the spacecraft by the sun's radiation, which weakened with time as the spacecraft moved progressively farther away, and also changed as the angle of the spacecraft changed; the increase in the delay time between the bounce of a radio wave and its reception back at Earth as the spacecraft grew more distant; the gravitational drag on the radio waves from the sun; the additional frequency shift in the radio transmissions caused by the rotation of the Earth… and the list goes on. Anderson synthesized that headache's worth of cosmic influences into a single algorithm. But unfortunately it didn't seem to work.
In 1980, he noticed a small discrepancy between the Doppler shifts he expected to receive based on his algorithm and the actual, measured shifts of the radio signals coming from the spacecraft. Their expected and actual motions weren't quite matching up. As they moved outward against the gravitational pull of the sun and planets, the spacecraft were, of course, slowing down. But the problem was they were slowing down too much. Each year, both of the spacecraft were a few hundred miles farther behind where they should have been on their respective paths, according to the algorithm. That isn't much in the context of space travel, to be sure, but it isn't trivial either. The constant, extra acceleration amounted to 8.74 x 10-10 m/s2 directed toward the sun– a factor ten billion times smaller than the acceleration due to gravity, but still, undeniably, there.
Anderson's first reaction was to think his algorithm must have been missing something. Some tiny influence on the motion of the spacecraft must not have made it into the mathematical mix. A few years of thinking and discussing led him and his immediate team to the conclusion that the anomalous acceleration must have been caused by "outgasing" - fuel dripping from the thrusters, exerting a recoil force against the spacecraft as it dripped. Since by that point the craft were cruising through interstellar space without propulsion, the scientists thought the fuel drops would soon finish dripping and the effect would go away. But perplexingly, it didn't: Over the next decade, the spacecraft racked up billions of frequent flier miles--but thousands less than they should have.
In 1994, Anderson received an email out of the blue from Michael Martin Nieto, a cosmologist at Los Alamos National Laboratory near Santa Fe, NM. Nieto had lately become interested in alternatives to Newton's inverse square law for gravity, including a new theory called MOND (modified Newtonian dynamics), and so he contacted Anderson to find out how sure NASA was about the strength of gravity based on their observations of the motions of spacecraft. Anderson replied that, as a matter of fact, gravity didn't seem to be working right for the Pioneers.
When Nieto read the exact value of the small, anomalous acceleration experienced by Pioneers 10 and 11, he almost fell off his chair. (In typical physics-speak: "My office had a hard floor and my computer chair had wheels, so when I arched my back in a "wha?" reaction the chair started rolling.") There was a profound cosmic coincidence afoot: As Nieto immediately noticed, the value of the Pioneer anomaly almost exactly equaled the so-called "cosmic acceleration"—the speed of light 'c' multiplied by the Hubble constant 'H'—suggesting the anomaly's cause lay within the foundation of physics.
Right then and there, Nieto signed on to work with Anderson at JPL, got a major investigation of the Pioneer anomaly off the ground, and has spent most of his energy studying it ever since. Why? "The Pioneer anomaly could be the first evidence that gravity deviates from an inverse-square dependence," he said recently. "It could be huge."
Are you this guy?
You're this guy, aren't you?
I never knew this was happening but now that I know of it, is this thing happening to the Voyager Probes as well?
No, at least not quantifiably so. See my comment above.
Nice article by Natalie Wolchover, geared for the public rather than written in geekspeak. Since spacecraft navigation, which is intimately tied to this problem, is an extremely arcane subject, the few inaccuracies and gloss-overs committed in the article are easily forgivable.
I led the Orbit Determinastion task for Voyager II after Saturn, and stayed with it through the Uranus and Neptune encounters, the last of which was in 1989. Voyager used hydrazine thrusters to maintain attitude, and one of the hardest parts of the orbit determination task was accounting for the non-gravitational accelerations due to the thrusting, which was complicated by the fact that the thruster plumes impinged on the spacecraft structure in complicated and time-varying ways. If my memory doesn't betray me, the thrusters would pulse typically 20-30 times per day, and it was impossible to accurately account for their effects. Thus, the Voyager spacecraft were just not good candidates for analyzing the sub 1e-12 km/s^2 accelerations talked about in the article, since the uncertainties in the thrusting were about the same magnitude.
We did see acceleration from the RTG (Radioisotope Thermal Generator) that Voyager used, which became prominent in the navigation problem after the Saturn flyby, when solar radiation pressure became less of a factor, but it was correlated with the thrusting, and so it was hard to separate the effects from one another
I must say that I was never much of a believer in the cosmological origin of the acceleration. In fact, I distinctly remember having a conversation with John Anderson around 1989 when we discussed the Pioneer anomaly, and I suggested that the spacecraft RTGs would be a likely source. A pity that this was not quantitatively investigated in more detail a lot earlier.
Well I have read the article and enjoyed it immensely. I do have a theory of my own which may mor accurately explain the probes slow down. Einsteins theories which assume that space itself has substance much like a sponge whereby the sun and each planet having mass, will dent the sponge much like laying a bowling ball on a bed. Now the sun being the densest object in our solar system will obviously cause the largest and deepest dent. That dents shape is the entire reason we call such things "gravity wells" the sun has a gravity well, each mass in the solar system has a gravity well, those effects regardless of what you may have been taught are logically additive once you leave the region of a local gravity well you enter the system gravity well. It's as if the probes are heading up hill on there way out and I believe you will see in another few years...you mayfind that the speed of these probes will pickup considerably and be noted as another unexplainable event. Physics is wonderful.
I hate be a buttinsky, but must say that anyone reading the Old Testament as Christopher Columbus did, would quickly realize that anyone suggesting that the earth was flat was a complete ignoramus. Every real sailor has seen the earth's curvature from the crow's nest.
Now, I must ask a very ignorant question regarding the Pioneer anomaly.
If the Sun gives off a solar wind, is it even remotely possible that our Milky Way galaxy also exudes some sort of a galactic wind? If so, how would the galactic wind effect change the Solar based calculated constants?
Also, would Einstein's calculation of E = MC squared be affected by having the effect of altering C as a constant 300,000 KM/sec?
I believe it is fundamental errors caused by using Eintein's two relativities: constant speed of light, relativistic redshift, relativist mass. removing all these non-sense, we will have a better read of the anomaly.
Based on pure Newton's law, the anomaly should be 0.69e-10 m/s^2.
I've written one thesis about it but not ready to publish. please reply to this post, if you want to preview it.
Solar system is a combination of gravity wells, coused by the sun and planets, but it should also have some density and weather, coused by radiation from bodies in it. Until probes were inside that bublle, they had some weight and thermal equilibrium with the rest, but outside the system they should be lighteer and wormer then sorrounding mass, causing sort of solar buoyancy efect.
Perhaps that's also connected with reasons, why solar bodies are round in shape, that principle is also a force. Our artificial probes are breaking that laws, we couldn't observe in any other objects in space because those paths are resault of natural motion only.
Nice article and well-written. One of the very few long articles on popsci that I've read fully.
Article was interesting and was well presented. From reading all of the comments, it seems we will have to wait for the results to be published. It seems like a waist of energy to predict before the results are in but apparently it is a lot of fun. (Vegas odds on Niburu appears to be a non-bet.)
..for me , article don't say news , only hopes ...much more interesting i find in wikipedia , at word 'pioneer anomaly ' , in the part of discussion , the last one ... at ' Level of normal solutions in Pioneer ' cosmonautics ' ... i should like other opinions in that last one ..
Eleventh planet anyone ? I mean you know, thats the first thing I would look for...
Hey, the third word in the captcha was captcha... better check the math. There goes Webster!
It is impossible to calculate all of the variables that might cause a drag on the actual spacecrafts or the radio energy itself for that matter. RF is subject to gravitational distortion, same as light waves. Every object in the universe has a potential of gravitational effect, including minute particles and single molecules that might cause cumulative friction or static charge. There are undoubtedly forces that science can't even measure or haven't even been discovered yet. This is really a great subject to ponder and scientific curiosity is the mother of physics. Whatever knowledge is gained from this study will fine tune future calculations and may open up a whole new understanding of the laws of physics.
What a wonderful article.
I was completely sucked in to the story. It captures, among other things, splendidly how much grunt work data analysis can be. This article truly stands out over the superficial articles one normally encounters in print.
I look forward to reading many more pieces by Natalie Wolchover.
This article was interesting and informative. However, I'd like to offer a correction to one of the illustrations. The one entitled "Mac Quadro Computer Processes Pioneer Data" is wrong. The diplay *is* from a Mac but it is a virtual control panel modelled after telemetry hardware that's been long taken out of service. Probably done with LabView or similar software.
But I still enjoyed the article.
Pioneer passed close to the Earth's equator on its way out so it could have been influenced by Earth's rotation. A very small effect but there is no good theory for spinning gravitational fields so it could be what we are looking for.
In 1980, he noticed a small discrepancy between the Doppler shifts he expected to receive based on his algorithm and the actual, measured shifts of the radio signals coming from the spacecraft. Their expected and actual motions weren't quite matching up.www.thaicartrick.com