The spacecrafts Pioneer 10 and 11 are the first spaceships from earth to reach escape velocity from the solar system. While we no longer can communicate with them we have over 20 years of data from them. Scientists have known the exact position and velocities from these ships and we know that as they leave the solar system the gravity from the solar system will cause both spacecraft to slightly decelerate as they leave the solar system. However, both spacecraft, traveling in different directions are slowing down faster than expected. Scientists still do not know why but we know that there is something we do not understand about the gravity of the solar system that is causing this phenomena.

(The Great Courses – The Inexplicable Universe: Unsolved Mysteries)