“The time evolution of the Moon–Earth distance remained an unsolved problem for half a century,” Rufu said. “However, these new geological proxies for the history of the Earth-Moon system allow us to calculate the Moon’s axial tilt and the extent of the PSR over time.”
In 2009, NASA crashed a two-ton Atlas Centaur rocket body, part of the Lunar Crater Observation and Sensing Satellite (LCROSS), near the Moon’s south pole. It hit the floor of Cabeus crater, creating a pile of debris that was examined for the presence of water and other chemicals in lunar regolith. A Shepherd satellite was traveling four minutes behind Centaur and several Earth-orbiting satellites, including the Hubble Space Telescope, monitored the impact.