Exploration of Jupiter reach Europa after a 6.5 year cruise. The spacecraft would fly by the moon 32 times to minimize radiation damage. Because of the possibility of subsurface liquid oceans on Jupiter's moons Europa, Ganymede and Callisto, there has been great interest in studying the icy moons in detail. Funding difficulties have delayed progress. The "Europa Orbiter" was a planned NASA mission to Europa, which was canceled in 2002. Its main objectives included determining the presence or absence of a subsurface ocean and identifying candidate sites for future lander missions. NASA's "JIMO" ("Jupiter Icy Moons Orbiter"), which was canceled in 2005, and
Origin of the Moon might have been a large atmosphere around the primitive Earth, which would slow the movement of the Moon by natural aerobraking before it could escape. That hypothesis may also explain the irregular satellite orbits of Jupiter and Saturn. This hypothesis also has difficulty explaining the essentially identical oxygen isotope ratios of the two bodies. This is the now discredited hypothesis that an ancient, rapidly spinning Earth expelled a piece of its mass. This was proposed by George Darwin (son of the famous biologist Charles Darwin) in the 1800s and retained some popularity until Apollo. The Austrian geologist Otto Ampherer in