Outer space, or simply space, is the expanse that exists beyond the Earth and between celestial bodies. Outer space is not completely empty—it is a hard vacuum containing a low density of particles, predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, neutrinos, dust, and cosmic rays. The baseline temperature of outer space, as set by the background radiation from the Big Bang, is 2. 7 kelvins (−270. 45 °C; −454. 81 °F). The plasma between galaxies accounts for about half of the baryonic (ordinary) matter in the universe; it has a number density of less than one hydrogen atom per cubic metre and a temperature of millions of kelvins. Local concentrations of matter have condensed into stars and galaxies. Studies indicate that 90% of the mass in most galaxies is in an unknown form, called dark matter, which interacts with other matter through gravitational but not electromagnetic forces. Observations suggest that the majority of the mass-energy in the observable universe is dark energy, a type of vacuum energy that is poorly understood. Intergalactic space takes up most of the volume of the universe, but even galaxies and star systems consist almost entirely of empty space.
Ganymede: Ganymede is both the largest moon of Jupiter, our Solar System's planetary behemoth, as well as the largest moon in our entire Solar system. Observations of Ganymede by the HST in 2015 suggested the existence of a subsurface saline ocean. This is because patterns in auroral belts and rocking of the magnetic field hinted at the presence of an ocean. It is estimated to be approximately 100 kilometers deep with a surface situated below a crust of 150 kilometers. Dr. Soderblom further explained to the press that the gravity signatures of the larger craters especially may shed new light into the number of impacts Earth's Moon, and other bodies in our Solar System, suffered during the asteroid-rampage that characterized the Late Heavy Bombardment. Until 2004, no spacecraft had visited Saturn for more than twenty years. Pioneer 11 took the very first close-up images of Saturn when it flew past in 1979. After that flyby, Voyager 1 had its rendezvous about a year later, and in August 1981 Voyager 2 had its brief, but glorious, encounter. Nearly a quarter of a century then passed before new high-resolution images of this beautiful, ringed planet were beamed back to Earth.