[SciShow Intro plays]

Hank: 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 emitting 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.

Two other probes seemed to have some trajectory issues too, though there was less data on them: Galileo, which launched in 1989 to study Jupiter and its moons; and the Ulysses spacecraft, a Sun probe that launched in 1990. If you’re going to launch something into space, you really do want to know where it’s going. So in 1994, a team of scientists decided to try to identify the source of the Pioneer anomaly.

The most likely culprit seemed to be thermal radiation. If the different components of the spacecraft were emitting heat unevenly, the extra radiation coming from one side could be enough to slow them down a tiny bit. And when I say tiny, I mean tiny — each Pioneer probe was being slowed by a force that was about 10 billion times weaker than Earth’s gravitational pull.

But over time, and in the microgravity environment of outer space, this tiny force was enough to knock the probes off-course. The thing is, the researchers didn’t know for sure that thermal radiation was the problem. That amount of force also happened to work out really well for some cosmological theories that were being developed at the time.

The team announced the Pioneer anomaly to the public in a paper published in 1998. In response, theorists published hundreds of papers about what could be causing it. They thought it might have come from things like an undiscovered planet, dark matter, or even the expansion of spacetime itself.

If some of the papers turned out to be correct, they’d prove that a lot of what we knew about our universe was just plain wrong. Around 2005, another team of researchers — including one of the authors of the 1998 paper — decided to try and simulate the missions, to see what kind of thermal radiation they were emitting and whether that could account for the anomaly. All they needed were 3D models of the spacecraft, and as much data they could get their hands on.

Except, they ran into two problems: First, the Pioneer spacecraft were designed on paper. Computer-aided design just didn’t exist at the time. So, they had to create their 3D models from scratch. Second, NASA didn’t require mission records to be archived, so there was no telling where the data was.

They did eventually get the data they needed, though it took some digging: They found the Doppler data on a navigator’s computer hard drives, and at the National Space Science Data Center. They managed to get the other science and housekeeping data from one of the other engineers who worked on the Pioneer missions — he’d decided to save a copy, even though NASA destroyed the original files.

Then there were the 400 magnetic tapes of navigational data inside some moldy cardboard boxes under a staircase at the Jet Propulsion Laboratory in California. By combining all this data with old blueprints, and consulting some of the engineers that built the Pioneers, the team created a 3D model and used it to analyze how heat radiated from each probe.

In 2012, they published their results: they finally had enough evidence to conclude that the Pioneer anomaly was caused by a force coming from the probes’ thermal radiation. Over thirty years, this tiny force pushed both probes off-course a combined distance of 400,000 kilometers — farther than the Moon is from Earth. The main heat sources were the electrical heat from spacecraft components and waste heat from their nuclear power sources – the Radioisotope Thermoelectric Generators.

The Pioneer probes were especially affected by this problem because they rotated to stabilize themselves. But that technique didn’t fix the push from thermal radiation. Ulysses and Galileo were also spin-stabilized, so it made sense that they would be affected, too. Other NASA designs are stabilized by control thrusters that do compensate for small disturbances like thermal recoil.

The Pioneer probes were retired in the 1990s, and we’ve lost communications with both of them. But it turns out that they had more in common with pioneers in the Wild West than we thought: They were packing heat!

Thanks for watching this episode of SciShow Space, and thanks especially to our patrons on Patreon who help make this show possible. And for the last four month of 2016, we’re going to be collecting all the money we raise and putting in toward a brand new project. It’s gonna be a new channel, here on YouTube, and it will either be SciShow Health, SciShow Life, or SciShow Psychology. And who is going to be picking which of these great ideas we will be executing on – it’s our patrons on Patreon. So if you wanna have a voice in that process, and also help us keep making cool stuff, You can go to patreon.com/scishow.