Oberon (moon) at the core–mantle boundary. The eutectic temperature of this mixture is 176 K. If the temperature dropped below this value the ocean would have frozen by now. Freezing of the water would have led to expansion of the interior, which may have also contributed to the formation of canyon-like graben. Still, present knowledge of the evolution of Oberon is very limited. So far the only close-up images of Oberon have been from the "Voyager 2" probe, which photographed the moon during its flyby of Uranus in January 1986. Since the closest approach of "Voyager 2" to Oberon was 470,600 km,
Discovery and exploration of the Solar System human settlement in space began with the Soviet space station Mir, which was continuously occupied for close to ten years, from 1989 to 1999. Its successor, the International Space Station, has maintained a continuous human presence in space since 2001. In 2004, U.S. President George W. Bush announced the Vision for Space Exploration, which called for a replacement for the aging Shuttle, a return to the Moon and, ultimately, a crewed mission to Mars. Bodies imaged up close: See also: Meteorites and Cosmic dust Discovery and exploration of the Solar System Discovery and exploration of the Solar System is observation,