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Outer space

The interface between the Earth's surface and outer space. The Kármán line at an altitude of 100 km (62 mi) is shown. The layers of the atmosphere are drawn to scale, whereas objects within them, such as the International Space Station, are not.

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 this plasma 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.

Outer space does not begin at a definite altitude above the Earth's surface. However, the Kármán line, an altitude of 100 km (62 mi) above sea level, is conventionally used as the start of outer space in space treaties and for aerospace records keeping. The framework for international space law was established by the Outer Space Treaty, which entered into force on 10 October 1967. This treaty precludes any claims of national sovereignty and permits all states to freely explore outer space. Despite the drafting of UN resolutions for the peaceful uses of outer space, anti-satellite weapons have been tested in Earth orbit.

Humans began the physical exploration of space during the 20th century with the advent of high-altitude balloon flights, followed by manned rocket launches. Earth orbit was first achieved by Yuri Gagarin of the Soviet Union in 1961, and unmanned spacecraft have since reached all of the known planets in the Solar System. Due to the high cost of getting into space, manned spaceflight has been limited to low Earth orbit and the Moon.

Outer space represents a challenging environment for human exploration because of the hazards of vacuum and radiation. Microgravity also has a negative effect on human physiology that causes both muscle atrophy and bone loss. In addition to these health and environmental issues, the economic cost of putting objects, including humans, into space is very high.

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Major features of the Solar System (not to scale)

The Solar System consists of the Sun and its planetary system of eight planets, their moons, and other non-stellar objects. It formed 4.6 billion years ago from the gravitational collapse of a giant molecular cloud. The vast majority of the system's mass is in the Sun, with most of the remaining mass contained in Jupiter. The four smaller inner planets, the terrestrial planets, are primarily composed of rock and metal. The four outer planets, the giant planets, are substantially more massive than the terrestrials. The Solar System also contains a number of regions populated by smaller objects. The asteroid belt, which lies between Mars and Jupiter, is composed of material similar to that of the terrestrial planets. Beyond Neptune's orbit lie the Kuiper belt and scattered disc, linked populations of trans-Neptunian objects composed mostly of ices. These populations include objects large enough to have been rounded by their own gravity, termed dwarf planets. Identified dwarf planets include the asteroid Ceres and the trans-Neptunian objects Pluto, Eris, Haumea, and Makemake. Six of the planets, at least three of the dwarf planets, and many of the smaller bodies are orbited by natural satellites, usually termed "moons" after the Moon, Earth's natural satellite.

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Phases of the Moon
Credit: Tom Ruen

An animation of the phases of the Moon. As the Moon revolves around the Earth, the Sun lights the Moon from a different side, creating the different phases. In the image, the Moon appears to get bigger as well as "wobble" slightly. Tidal locking synchronizes the Moon's rotation period on its axis to match its orbital period around the earth. These two periods nearly cancel each other out, except that the Moon's orbit is elliptical. This causes its orbital motion to speed up when closer to the Earth, and slow down when farther away, causing the Moon's apparent diameter to change, as well as the wobbling motion observed.

Astronomical events

2 July, 19:24 New moon and total solar eclipse
4 July, 05:39 Moon occults Mars
4 July, 22:11 Earth at aphelion
5 July, 04:52 Moon at perigee
9 July, 17:00 Saturn at opposition
14 July Pluto at opposition
16 July, 07:15 Moon occults Saturn
16 July, 17:17 Moon occults Pluto
16 July, 21:31 Full moon and partial lunar eclipse
20 July, 23:51 Moon at apogee
21 July, 12:36 Mercury at inferior conjunction
29 July, 06:00 Southern Delta Aquariids peak
31 July, 20:36 Moon occults Venus


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