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, cosmic neutrino background, cosmic dust, and cosmic rays. The baseline temperature, determined by cosmic background radiation since the birth of the universe, is 2.7 kelvins (K) (−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 equivalence in the observable universe is a poorly understood vacuum energy of space, which astronomers label dark energy. Intergalactic space takes up most of the volume of the Universe, but even galaxies and star systems consist almost entirely of empty space.
There is no definite altitude above the Earth's surface where outer space begins. However, the Kármán line, at 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 dual hazards of vacuum and radiation. Microgravity also has a negative effect on human physiology that causes both muscle atrophy and spaceflight osteopenia. In addition to these health and environmental issues, the economic cost of putting objects, including humans, into space is very high.
- ↑ Chuss, David T. (June 26, 2008), Cosmic Background Explorer, NASA Goddard Space Flight Center, archived from the original on May 9, 2013, https://web.archive.org/web/20130509235348/http://lambda.gsfc.nasa.gov/product/cobe/, retrieved 2013-04-27.
- ↑ Freedman & Kaufmann 2005, pp. 573, 599–601.
- ↑ Trimble, V. (1987), "Existence and nature of dark matter in the universe", Annual Review of Astronomy and Astrophysics 25: 425–472, doi:10.1146/annurev.aa.25.090187.002233, Bibcode: 1987ARA&A..25..425T.
- ↑ "Dark Energy, Dark Matter", NASA Science, archived from the original on June 2, 2013, https://web.archive.org/web/20130602083852/http://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy/, retrieved May 31, 2013, ""It turns out that roughly 68% of the Universe is dark energy. Dark matter makes up about 27%.""
- ↑ Freedman & Kaufmann 2005, pp. 650–653.
- ↑ O'Leary 2009, p. 84.
- ↑ "Where does space begin? – Aerospace Engineering, Aviation News, Salary, Jobs and Museums" (in en-US). Archived from the original on 2015-11-17. https://web.archive.org/web/20151117034012/http://aerospaceengineering.aero/where-does-space-begin/. Retrieved 2015-11-10.
- Ellery, Alex (2000), An introduction to space robotics, Springer-Praxis books in astronomy and space sciences, Springer, ISBN 1-85233-164-X, https://books.google.com/books?id=Sl8LggLa5R0C&pg=PA68
- Freedman, Roger A.; Kaufmann, William J. (2005), Universe (seventh ed.), New York: W. H. Freeman and Company, ISBN 0-7167-8694-X
- O'Leary, Beth Laura (2009), Darrin, Ann Garrison, ed., Handbook of space engineering, archaeology, and heritage, Advances in engineering, CRC Press, ISBN 1-4200-8431-3, https://books.google.com/books?id=dTwIDun4MroC&pg=PA84