Anomalous Acceleration of Pioneer 10 and 11

Pioneer Spacecraft

I have been thinking about space travel and I started poking around on the net and I came across an abstract (summary) of a paper some rocket scientists had written. These guys put in some time studying the radio signals from the spacecraft and have concluded it is not going as fast as they think it should.

The difference between how fast Pioneer 11 is going and how fast they think it should be going is about:

8 x 10^{-8} cm/s^2

which is the same as eight hundred picometers per second squared. (A picometer is one trillionth of a meter. Pretty stinking small.) "Per second squared" means this is a change in velocity, or acceleration. This works out to be about two centimeters per second per year (2 cm/s per year), BACK TOWARD THE SUN, so it is slowing down, or decelerating. Pioneers velocity when we last heard from it (1995) was 11.6 km/s.

• 11.6 km/s is 11,600 m/s.
  
• 2 centimeters is 0.02 meters.
  
• 11,600 meters divided by .02 meters is 580,000

So it will take 580,000 years for Pioneer 11 to come to a complete stop. At it's current velocity it will be four million years before it comes to another star, so if this acceleration (deceleration) holds true, we should see Pioneer 11 back here in about one million years.

Update September 2016 replaced missing picture.