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Pioneer Non-Anomaly

April 21, 2012 by

Artist depiction of Pioneer spacecraft in deep space.

Artist depiction of Pioneer spacecraft in deep space. Image Credit: NASA

Pioneer 10 and 11 launched in 1972 and 1973, respectively, and were Earthkind’s  first explorers of the outer planets and emissaries to deep space.  Pioneer 10 became the first spacecraft to pass through the asteroid belt and observe Jupiter up-close, providing us with details of the gas giant’s interior, atmosphere, magnetic fields, and some of the most breath-taking images of Jupiter we had ever seen. Pioneer 11 wasn’t far behind, and after making its own observations of Jupiter, it went on to Saturn to open our eyes to the mighty ringed planet in the same way Pioneer 10 had done for Jupiter. (But this isn’t a story about the accomplishments of the Pioneer program; I’ll save that for another day.)

In addition to all of the data and images sent back, however, those two Pioneers also sent back a mystery. As early as 1980, it was noticed that the spacecrafts were experiencing an acceleration force toward the sun of .000000000874 m/s2 (meters per second, per second)1. To be clear, this does not mean the Pioneers are heading back towards the Sun. Pioneer 10 and 11 are cruising away from the Sun at a speed of around 132,000 kilometers per hour (82,000 miles per hour) and 175,000 kilometers (110,000 miles per hour), respectively, and this force is 10 billion times smaller than the acceleration we feel from the Earth’s gravitational pull. Nonetheless, the force is real and our instruments and techniques are precise enough to notice.

Many plausible causes were considered to explain the anomaly, including:

perturbations from the gravitational attraction of planets and smaller bodies in the solar system; radiation pressure, the tiny transfer of momentum when photons impact the spacecraft; general relativity; interactions between the solar wind and the spacecraft; possible corruption to the radio Doppler data; wobbles and other changes in Earth’s rotation; outgassing or thermal radiation from the spacecraft; and the possible influence of non-ordinary or dark matter.

In 1994, a thorough, long-term, collaborative study was undertaken to try and solve the anomaly. Initial results from that study were released in 1998, with a detailed analysis following in 2002. All known systematics were tested and calculated, yet that 8.74±1.33×10−10 m/s2 deceleration force2 remained. The origin of the anomaly was still unaccounted for, though the leading theory was that it was the result of anisotropic thermal radiation (don’t let the big words intimidate you, this just means heat was being radiated from the Pioneers in a certain direction). In 2004, another paper was published, proposing a deep space mission to solve the anomaly once-and-for-all.

But now, that expensive deep-space mission won’t be necessary, according to a paper just submitted by astrophysicist Slava Turyshev and his team of scientists and engineers, with thanks, in no small part, to The Planetary Society and its members.3

With funds provided by The Planetary Society, Turyshev and his team were able to collect and compile great volumes of data from the two Pioneer missions. The data had to come from a variety of different sources and came in any number of formats, media, and condition. According to Bruce Betts, Director of Projects at The Planetary Society:

“This was not an easy (or quick) task. These missions lasted for more than 30 years. Imagine all the people, computing formats, and hardcopy and electronic storage devices involved over that period, and you’ll start to get an idea of the problem.”

Boxes of Pioneer data tapes.

Boxes of data tapes from the Pioneer missions. Image Credit: The Planetary Society

Think of what you would have to go through if I handed you a 5.25″ floppy disk that contained… well, it couldn’t contain much compared to the amount of data we exchange today, but whatever it was, it was something you needed. Imagine trying to find the hardware to read the disk, and then the intermediary hardware and software that would be required to get the data from the disk onto one of today’s modern machines so you could even utilize it. If you consider how much technology has changed between now and floppy disks, you can only begin to imagine how much it has changed since the 1970s and how cumbersome compiling all of this data, let alone securing it, must have been. I digress.

Once Turyshev and his team were able to assemble the more-complete data picture, they were able to isolate the source of acceleration: that anisotropic thermal radiation. Again, Bruce Betts:

Why was the thermal emission from the spacecraft anisotropic and slowing the spacecraft down? First of all, because the Pioneer spacecraft were spin-stabilized and almost always pointed their big dishes towards Earth. Second of all, because two sources of thermal radiation (heat) were then on the leading side of the spacecraft. The nuclear power sources, more formally Radioisotope Thermoelectric Generators (RTG), emitted heat towards the back side of the dishes. When the dishes reflected or re-radiated this heat, it went in the direction of travel of the spacecraft. Also, the warm electronics box for the spacecraft was on the leading side of the spacecraft, causing more heat to spill that direction. Photon pressure, the same type of thing used in solar sailing, then preferentially pushed against the direction of travel, causing a tiny, but measurable, deceleration of the spacecraft – the Pioneer Anomaly.

At the end of the day, there are a few take-home lessons to be learned. First, Occam’s Razor proved itself once again (some of the suggestions to account for the Pioneer Anomaly were the need to invoke a new type of exotic physics). The second is that you can’t just apply Occam’s Razor and say that anisotropic thermal radiation is the simplest theory and therefore correct, you have to painstakingly collect all of the data needed to prove it — and more importantly, you have to have the experts that are willing to put forth the years decades of research to solve the mystery. Finally, you take in the account that this was made possible with the help of citizen scientists and those of us that contribute to furthering our understanding of the Universe, through means such as The Planetary Society4.

This new paper will undoubtedly generate more discussion about the Pioneer Anomaly and others will work to verify or disprove its results, but at this point it seems pretty safe to say that one of space physic’s mysteries is no more.


  1. 8.74±1.33×10−10 m/s2
  2. In physics, acceleration is a change in velocity over time. It does not only apply to an increase in speed. Traveling in your hot-rod sports car, as you step on the gas and race up to 100 miles per hour, you’re experiencing acceleration. When you notice a brick wall rapidly closing in from ahead of you and you stand on the brakes, you’re experiencing a negative acceleration. Acceleration is just a change from a constant velocity.
  3. Are you a member of The Planetary Society? I am. You should be, too! Sign up!
  4. Again, join!

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