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Outer space, or simply space, is the void that exists between celestial bodies, including the Earth. It is not completely empty, but consists of a hard vacuum containing a low density of particles: predominantly a plasma of hydrogen and helium, as well as electromagnetic radiation, magnetic fields, and neutrinos. Observations and theory suggest that it also contains dark matter and dark energy. The baseline temperature, as set by the background radiation left over from the Big Bang, is only 2.7 Kelvin (K); in contrast, temperatures in the coronae of stars can reach over a million Kelvin. Plasma with an extremely low density (less than one hydrogen atom per cubic meter) and high temperature (millions of Kelvin) in the space between galaxies accounts for most of the baryonic (ordinary) matter in outer space; local concentrations have condensed into stars and galaxies. Intergalactic space takes up most of the volume of the Universe, but even galaxies and star systems consist almost entirely of empty space.
Humans began the physical exploration of space during the 20th century with the advent of high-altitude balloon flights, followed by the development of single and multi-stage rocket launchers. Earth orbit was achieved by Yuri Gagarin in 1961 and unmanned spacecraft have since reached all of the known planets in the Solar System. Achieving orbit requires a minimum velocity of 28,400 km/h (17,600 mph); much faster than any conventional aircraft.
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun. Of the many objects that orbit the Sun, most of the mass is contained within eight relatively solitary planets whose orbits are almost circular and lie within a nearly flat disc called the ecliptic plane. The four smaller inner planets, Mercury, Venus, Earth and Mars, also called the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the gas giants, are substantially more massive than the terrestrials. The two largest, Jupiter and Saturn, are composed mainly of hydrogen and helium; the two outermost planets, Uranus and Neptune, are composed largely of ices, such as water, ammonia and methane, and are often referred to separately as "ice giants".
The Solar System is also home to a number of regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is similar to the terrestrial planets as it is composed mainly of rock and metal. Beyond Neptune's orbit lie the Kuiper belt and scattered disc; linked populations of trans-Neptunian objects composed mostly of ices such as water, ammonia and methane. Within these populations, five individual objects, Ceres, Pluto, Haumea, Makemake and Eris, are recognized to be large enough to have been rounded by their own gravity, and are thus termed dwarf planets.[e] In addition to thousands of small bodies[e] in those two regions, several dozen of which are considered dwarf-planet candidates, various other small body populations including comets, centaurs and interplanetary dust freely travel between regions. Six of the planets and three of the dwarf planets are orbited by natural satellites,[b] usually termed "moons" after Earth's Moon. Each of the outer planets is encircled by planetary rings of dust and other particles.
The solar wind, a flow of plasma from the Sun, creates a bubble in the interstellar medium known as the heliosphere, which extends out to the edge of the scattered disc. The Solar System is located within one of the outer arms of Milky Way galaxy, which contains about 200 billion stars.
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