On 02 March 1973, the NASA probe Pioneer 10 was launched into space on the top of an Atlas/Centaur/TE364-4 launch vehicle that pushed it away from Earth at 32,193 mph en route to a date with Jupiter. The massive planet’s gravity helped accelerate the probe to 82,021 mph.

It was 05 April 1973 that Pioneer 11 was launched at the apex of a similar vehicle. It too took the quick road to Jupiter, but didn’t stay long before headed off to Saturn.

As both departed their mission they each were moving in a direction askew the solar ecliptic. The trajectories afforded the rocket scientists at NASA to do their stuff. With presumably reliable estimates of how many particulates the probes would encounter, the bean counters should have been able to predict the speed of each probe through at least the end of the solar system.

But it seems their numbers weren’t crunched quite right. Both the Pioneer craft are slowing. Or, more directly, the craft are not where they were expected to be because an unexplained acceleration back towards the sun. Scientists proffered multiple possible explanations:


  • Observation error – Dust or other particulates between Earth and the probes causing a change in the Doppler Effect of the signals.
  • Bad math – Someone forgot to carry when originally calculating the trajectories … or someone rounded wrong.


  • Gas leak – Both crafts are suited with a radioisotope thermoelectric generator which has a helium exhaust; if that exhaust were leaking it would cause some slowing, and they are identical so a design flaw in one would be reflected in both.
  • Drag – from more dust than expected, solar wind, cosmic rays, or tiny UFOs slamming into it. A good possibility, since the craft are nearing the Kuiper Belt, and no one has really seen it.
  • Radiation pressure – Maybe the built in radio shooting out all those signals is creating some thrust.
  • Static cling – just like the way a skirt clings to a girl’s legs when she’s in a hurry, maybe the probes have built up an electric charge that is trying to stick it to the sun.

Scientist Nightmares

  • Dark energy – interaction with a force we haven’t observed or proved yet. Great.
  • New physics – something Einstein hadn’t gotten to yet. Maybe something waiting in the offing that we’ll see when we marry Quantum Mechanics to General Relativity.
  • Einsteinian Foible – maybe he was just wrong.

Finally, in 2012, NASA scientists concluded that the anomaly is due to thermal recoil force. “The effect is something like when you are driving a car and the photons from your headlights are pushing you backward,” the paper’s lead author explains. “It is very subtle.”