Some practically ancient technology has recently been used to rule out some exotic theories of physics. The Pioneer 10 and 11 probes, which were the first successful space missions to Jupiter and Saturn, respectively, have still provided data since their launch in the 1970s. After the Pioneers observed their target planets, they still transmitted radio signals back to Earth, and NASA would also try to bounce radio waves off the probes. Similar to how you can tell if an ambulance or train is rushing towards you or moving away by the change in pitch, NASA could use the Doppler effect to measure the velocity of the probes along their paths.
Astronomers wanted to use this data to study the influence of gravity farther out in the solar system, where the mass of the Sun and outer planets wouldn’t overwhelm smaller effects. Of course, studying “small effects” in deep space required accounting for many variables. John Anderson, the lead analyst of the Doppler data, started to notice small differences between his model’s predictions and the Pioneers’ actual velocity and position data beyond Neptune’s orbit. The probes were experiencing a small acceleration towards the Sun, slowing them down. By small, they mean really small: the acceleration was calculated as being 8.74 x 10-10 meters per second per second. (To help put that in context, it would take over 400 years of constant acceleration of that amount to get up to the average person’s walking speed)
For the probes to be experiencing that acceleration means they need to be experiencing some sort of force. Anderson’s team originally wrote it off as unexpected gas leaks from the Pioneers’ thrusters. But the anomalous acceleration persisted even after the thrusters should have run out of fuel. And that started to concern astronomers. Anderson’s model accounted for all known gravitational sources in the outer solar system, and even more complicated effects such as radiation pressure from the Sun. So something astronomers didn’t know about must have been the cause of the anomaly.
Astronomers and physicists have come up with dozens of interesting explanations for the Pioneer anomaly that would change our understanding of the universe. Some have proposed previously unknown clusters of dark matter in the solar system, but this was ruled out when the Voyager probes never showed this acceleration and our models of planetary orbits never showed such problems. Others proposed rewriting gravity. Ironically, an alternative theory to dark matter, known as Modified Newtonian dynamics (or MOND) was also invoked to explain the anomaly, as it proposes that gravity behaves differently at extremely low accelerations (intriguingly, also on the order of 10 to -10 meters per second per second). Others proposed that time passed at different rates for objects depending on their acceleration through the gravitational field of space, and Anderson’s group actually considered these theories for a while (in some way I truly don’t understand).
And others proposed less exciting explanations. Tiny amounts of gas could be produced and leak from the Pioneer’s nuclear power sources. And some thought even the tiny amounts of heat radiating off the probes could cause push the probes off course. No one seems to have taken the gas theory very seriously. And Anderson’s team (now working with Slava Turyshev) wrote off the heat theory in the early 2000s, saying any force based on the heat should have decreased with distance from the sun.
Viktor Toth, a programmer who seems to do theoretical physics in his free time (seriously), helped change their minds. Toth argued that Anderson’s team couldn’t rule out thermal effects unless they did a detailed analysis. In an interesting story showing the merits of preserving old research data, Toth helped Turyshev save or find copies of nearly all the Pioneer mission’s Doppler and temperature telemetry. Somewhat amazingly, the team (now with Toth) managed to recreate a full CAD model of Pioneer 10 based on the original engineering drawings and using the temperature data, ran a computer simulation measuring the thermal emissions in all directions. Their model showed that the radiation could be responsible for all but about 20% of the acceleration.
The model seems pretty convincing. But their figures look like they show the “thermal recoil” effect decreasing further from the sun, while the anomaly still seems constant. So maybe there’s still room for some anomalous physics.