Pioneer spacecraft 10 and 11 were launched in 1972 and 1973 and are heading out of our solar system at a speed of about 30,000 miles per hour. Scientists have been monitoring them throughout their flights. They are about 92.12 AUs away from the sun. (An Astronomical Unit is the distance between the Earth and the sun, about 93 million miles).

Scientists have discovered that both Pioneer spacecraft are about 240,000 miles closer to the sun than they should be. Researchers don’t know if there is an unknown boundary on the outer edge of our solar system, an unknown force, or the Pioneers’ own emitted infrared photons from their energy sources that are slowing down the spacecraft.