The Pioneer 10 and 11 probes, launched in 1972 and 73, were named after the pioneers who settled the western United States, because these probes were also explorers: Pioneer 10 was the first spacecraft to pass through the asteroid belt, and both probes explored the outer planets before continuing on their path out of the solar system.
But in the early 1980s, a NASA analysis team discovered something strange: Pioneer 10 and 11 were both slowing down and veering off course.
The deceleration became known as the Pioneer Anomaly, and for a long time, it had scientists and engineers totally stumped.
It took an intense data hunt, a computer simulation, and more than 3 decades to figure out what was going on.
The team first noticed the anomaly because of the Doppler effect, which describes how the frequency of light admitting from an object changes if the object is moving.
The engineers working on Pioneer expected the light coming from the probes to be shifted by a certain amount, based on the probes' speed.
But when they measured the actual frequency of the light, it was slightly higher than they'd predicted.
They already knew that the probes would be slowing down at least a little, because of things like the pull of the Sun's gravity.
But the frequency of the light showed that Pioneer 10 and 11 were both slowing down more than they should have been, and no one knew why.
Each year they were a few hundred kilometers from where they were expected to be.
