Leading anomaly: What happened to NASA's old probes at 20 AU?

IIn 1972 and 1973, NASA launched the Pioneer 10 and Pioneer 11 probes. These were the first missions to Jupiter and Saturn, before heading to the asteroid belt and becoming the first two missions five Probes to reach the edge of our solar system.

With such missions, NASA was of course expecting to reach some unknowns. Even newer probes like Voyager 1 had their own share of errors, sending back data that didn't make any sense. But what NASA got from Pioneer 10 and 11 was a strange and interesting mystery. At about 20 astronomical units (AU), with one AU being the distance between Earth and the Sun, both spacecraft began accelerating toward the Sun.

“The anomalous acceleration of the Pioneer 10 and 11 spacecraft is a discrepancy between the modeled and observed radio Doppler data from the two spacecraft.” paper On the subject explains. “This discrepancy can be eliminated by incorporating a constant acceleration toward the Sun of unknown origin.”

The probes were still heading out of the solar system, but there was a force acting on them, pushing them toward the sun and slowing their progress. While this may seem trivial, the fact that both probes experienced this at around 20 AU has led some physicists to suggest that there is something wrong with our theory of gravity, specifically Newton's inverse square law.

According to the inverse square law, the force of gravity between two objects is directly proportional to their mass, and inversely proportional to the square of the distance between them. As such, you would expect the Sun's influence to weaken the farther away you move from it, but data from these spacecraft suggests otherwise.

Some tentatively suggested that this involved new physics.

Acceleration a_s “It is not associated with any abnormal or atypical movement of spaceships.” paper Proposal. “The anomaly is a manifestation of the expansion of the universe, which causes an increase in the background potential Ψ