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Outer space, or simplyspace, is the expanse that exists beyondEarth's atmosphere and betweencelestial bodies. It contains ultra-lowlevels of particle densities, constituting anear-perfect vacuum of predominantlyhydrogen andheliumplasma, permeated byelectromagnetic radiation,cosmic rays,neutrinos,magnetic fields anddust. The baselinetemperature of outer space, as set by thebackground radiation from theBig Bang, is 2.7 kelvins (−270 °C; −455 °F).
Theplasma between galaxies is thought to account for about half of thebaryonic (ordinary) matter in the universe, having anumber density of less than onehydrogen atom per cubic metre and akinetic temperature of millions ofkelvins. Local concentrations of matter have condensed intostars andgalaxies. Intergalactic space takes up most of the volume of theuniverse, but even galaxies andstar systems consist almost entirely of empty space. Most of the remainingmass-energy in theobservable universe is made up of an unknown form, dubbeddark matter anddark energy.
Outer space does not begin at a definite altitude above Earth's surface. TheKármán line, an altitude of 100 km (62 mi) abovesea level, is conventionally used as the start of outer space in space treaties and for aerospace records keeping. Certain portions of the upperstratosphere and themesosphere are sometimes referred to as "near space". The framework for internationalspace law was established by theOuter Space Treaty, which entered into force on 10 October 1967. This treaty precludes any claims ofnational sovereignty and permits all states to freelyexplore outer space. Despite the drafting ofUN resolutions for the peaceful uses of outer space,anti-satellite weapons have been tested inEarth orbit.
The concept that the space between the Earth and the Moon must be a vacuum was first proposed in the 17th century after scientists discovered thatair pressure decreased with altitude. The immense scale of outer space was grasped in the 20th century when the distance to theAndromeda Galaxy was first measured. Humans began the physical exploration of space later in the same century with the advent of high-altitudeballoon flights. This was followed by crewedrocket flights and, then, crewed Earth orbit, first achieved byYuri Gagarin of theSoviet Union in 1961. The economic cost of putting objects, including humans, into space is very high, limiting humanspaceflight tolow Earth orbit and theMoon. On the other hand,uncrewed spacecraft have reached all of the knownplanets in theSolar System. Outer space represents a challenging environment forhuman exploration because of the hazards ofvacuum andradiation.Microgravity has a negative effect on humanphysiology that causes bothmuscle atrophy andbone loss. (Full article...)
Planetary habitability is the measure of an astronomical body's potential for developing and sustaining life. It may be applied both toplanets and to thenatural satellites of planets. The only absolute requirement for life is anenergy source (usually but not necessarilysolar energy), but the notion of planetary habitability implies that many othergeophysical,geochemical, andastrophysical criteria must be met before an astronomical body is able to support life. The idea that planets beyond Earth might host life is an ancient one, though historically it was framed byphilosophy as much asphysical science. The late 20th century saw two breakthroughs in the field. To begin with, the observation and roboticexploration of other planets and moons within the solar system has provided critical information on defining habitability criteria and allowed for substantial geophysical comparisons between the Earth and other bodies. The discovery ofextrasolar planets—beginning in 1995 and accelerating thereafter—was the second milestone. It confirmed that the Sun is not unique in hosting planets and expanded the habitability research horizon beyond our own solar system.




For a full schedule of launches and deep-space rendezvous, see2025 in spaceflight. |
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