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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.

Selected article

Mars

The recorded history of Mars observation dates back to the era of the ancient Egyptian astronomers in the 2nd millennium BCE. Detailed observations of the position of Mars were made by Babylonian astronomers, and ancient Greek philosophers and Hellenistic astronomers developed a geocentric model to explain the planet's motions. Indian and Muslim astronomers estimated its size and distance from Earth. The first telescopic observation of Mars was by Galileo Galilei in 1610. The first crude map of Mars was published in 1840. When astronomers mistakenly thought they had detected the spectroscopic signature of water in the Martian atmosphere, the idea of life on Mars became popular. During the 1920s, the range of Martian surface temperature was measured; it ranged from −85 °C (−121 °F) to 7 °C (45 °F). The planetary atmosphere was found to be arid with only trace amounts of oxygen and water. Since the 1960s, multiple robotic spacecraft have been sent to explore Mars. The planet has remained under observation by ground and space-based instruments and the discovery of meteorites on Earth that originated on Mars has allowed laboratory examination of the chemical conditions on the planet.

Selected image

Olympus Mons
Credit: United States Geological Survey

A composite image of Olympus Mons on Mars, the tallest known volcano and mountain in the Solar System. This image was created from black-and-white imagery from the USGS's Mars Global Digital Image Mosaic and color imagery acquired from the 1978 visit of Viking 1.

Astronomical events

1 August, 03:12 New moon
2 August, 07:04 Moon at perigee
9 August, 23:06 Mercury at greatest western elongation
12 August, 09:53 Moon occults Saturn
12 August, 22:14 Moon occults Pluto
13 August, 12:00 Perseids peak
14 August, 06:06 Venus at superior conjunction
15 August, 12:29 Full moon
17 August, 10:36 Moon at apogee
30 August, 10:37 New moon
30 August, 15:47 Moon at perigee
31 August Comet 322P/SOHO at brightest

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