Why Did Pioneer 10 and 11 Slow Down After Passing Jupiter?

Stories from New Scientist.
Aug. 18 2012 7:03 AM

The Pioneer Anomaly Is Solved

Two long-distance spaceships slowed down mysteriously. An astrophysicist figured out why.

Launched on March 2, 1972, Pioneer 10 was the first spacecraft to travel through the asteroid belt, and the first spacecraft to make direct observations and obtain close-up images of Jupiter.
Pioneer 10, shown launching on March 2, 1972, was the first spacecraft to travel through the asteroid belt and the first spacecraft to make direct observations and obtain close-up images of Jupiter.

Photo by NASA Ames Resarch Center via Wikimedia commons.

Astrophysicist Slava G. Turyshev has explained away decades of exotic speculation over the Pioneer anomaly, the puzzling slowdown of two NASA probes. He works at NASA's Jet Propulsion Laboratory in Pasadena, Calif., and is an authority on using deep space to test the nature of gravity.

Victoria Jaggard: What drew you to try to explain the anomaly, the unexpected slowing of the Pioneer 10 and 11 spacecraft that emerged in the 1980s?
Slava Turyshev: The whole solar system is a lab which I use to test general relativity. We know the solar system obeys all laws given to us by Einstein and Newton. With the Pioneer anomaly, suddenly we saw a very unusual tiny force that fell right in between Newton's gravity and Einstein's general relativity. That prompted people to think that maybe the spacecraft was sensing the presence of a new type of physics. It was either a major discovery or a puzzle that, in the solving, would help us build better craft to study gravity. It was a win-win situation for me.

VG: What were the initial thoughts on a cause?
ST: Engineers thought it was due to expected leakage from the propellant system, just as water still drips from a hose after you've turned off the tap. But then we saw this same small pushing motion for months and years, even when we didn't use the propulsion system, so people started to think maybe something else was going on.

VG: You meticulously modeled Pioneer 10 using design and flight data to test a mundane cause of the anomaly. Tell me about that.
ST: Before our latest work, there was some expectation we would see part of the anomaly was due to the spacecraft's thermoelectric generators producing a lot of heat. This heat recoils and gives a minuscule push against the craft. We didn't know that this push would match the profile of the anomaly, but that is just what we found in our latest work.

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VG: Is this is the final word on it?
ST: We still have an uncertainty in our study of less than 18 percent. But for me, this is the answer. Some may argue it is not final, but in my mind we did a good job, and it's very clear what happened.

VG: Why has it taken so long to reach this point?
ST: Lack of proper data storage was a huge problem. In the 1970s and 1980s, mission data was recorded on magnetic tapes, and to study the Pioneer anomaly we needed the probe's navigational data. But mission tapes were normally saved for only a few months and then thrown away, so you're lucky if you can find what you need. The only data available from Pioneer 10 was from planetary flybys, which were kept to study gravity around planets. Then we had to figure out how to read it. You need a proper machine with the right software, and you need to "upconvert" the data to modern formats so it can be used in today's computer modeling systems. That took years.

VG: Any more cosmic conundrums in your sights?
ST: I'd like to see more work on the search for gravitational waves, which are ripples in the fabric of space-time predicted to exist by general relativity. They open up a completely different way of looking at the universe, because these waves would allow us to detect phenomena beyond what we can see with light. What's past the event horizons of black holes? Are there other universes? Do wormholes exist?

This article originally appeared in New Scientist.

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