Faster than Light?

Far out at the edges of the solar system, the motions of two spacecraft, Pioneer 10 and Pioneer 11, appear to be breaking all known laws of physics.

Some mysterious source of energy is causing them to speed up. They are millions of miles apart, but both of them are experiencing the same tiny amount of acceleration. It is no more than a billionth of a metre per second every second, but its cumulative effect has put them 300,000 miles further from Earth than Newton’s laws say they should be after 35 years of cruising.

The article, of course, overly dramatizes the finding, and I immediately spotted one potential flaw in the article.

...its cumulative effect has put them 300,000 miles further from Earth than Newton’s laws say they should be after 35 years of cruising.

Well, of course the craft are further away than Newton’s laws say they should be. At the speeds they are traveling relativistic effects could account for the discrepancy. A quick Google search turned up several articles explaining the crafts’ acceleration using conventional physics, including this one from 2003, which explains the gravitational attraction of the dust in the Kuiper Belt accounts for the acceleration.

If I found this in a few minutes you’d think a writer for Astronomy Now could do the same. I’m seriously thinking about going back to college even at my age and getting a degree in Astronomy so I can land a job as a science writer.

IIRC there was a debate about this issue at the Natural History Museum in NYC last year or the year before. I was out of town or would have attended.

A true mystery of the Space Age

I had a look at the quoted article and yes, as popular science writing goes, it’s pretty bad. The Pioneer Anomaly intro is just a teaser; the article is really about faster-than-light travel and teleportation—really speculative, iffy stuff.

Still, that doesn’t excuse the author getting his facts wrong; Pioneer 10 and 11 are slowing down relative to their direction of travel, not speeding up. This has been known since 1997, as this article from PhysicsWeb explains.

Relativistic effects do not explain the anomaly.

The Pioneer Anomaly is a true mystery of the Space Age, one that may lead us to a new understanding of cosmology or a reappraisal of the laws of physics. Many theoretical explanations have been suggested for it, but no-one’s been able to formulate a good hypothesis and test it due to lack of data. The ESA experiment mentioned in the linked article above is one attempt to get more. This project, which seeks to recover additional Pioneer data from old files, is another.

We live in exciting times.

Thanks for the links to the articles. Interesting reading. Whether the Pioneer Anomaly is simply a quirk of the spacecraft or some new physics, the Astronomy Now Online article is inexcusable.

From Physics World.com:

Dispassionately, the most likely cause of the anomalous acceleration of the Pioneer spacecraft is on-board systematics, but the smoking gun has not yet been found. The only other possibility is the existence of new physics. This dichotomy represents a healthy win-win situation because either one of these two explanations for the Pioneer anomaly would constitute an extremely important discovery.

The interesting part is that the unexplained acceleration reported for the two Pioneer probes is the same in both cases (see first link in earlier post). That suggests (although it does not prove) that the cause of the acceleration is also the same. And it seems to me to point towards an external cause rather than some random thermal or propellant leakage effect from the spacecraft themselves.

Again, if the effect were due to gravitational pull from proximate bodies, wouldn’t you expect to find some difference in the value in each case? The two probes are in different parts of the Solar System, and the net gravitational force vector for each should be different in both magnitude and direction.

Incidentally, the mass of the two probes is given by NASA as 258kg. for Pioneer 10 and 259kg. for Pioneer 11: nearly indentical.

I agree it is interesting that the unexplained acceleration is the same in both cases, but it is far too early to draw any conclusions about what is happening. The researchers have tons of data to crunch. One way or the other we’ll learn something from this. Best case is we get a hint about why outlying galactic stars orbit faster than Newton’s inverse square law predicts.