Outer space

• 1. Introduction

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  Outer space holds a baseline temperature of 2.7255 K, the chill left behind by the Big Bang. That number describes the void between celestial bodies, a near-perfect vacuum of mostly hydrogen and helium plasma. It is laced with electromagnetic radiation, cosmic rays, neutrinos, magnetic fields and dust. The plasma between galaxies may account for about half of the ordinary matter in the universe, yet it holds less than one hydrogen atom per cubic metre. How did thinkers come to accept that anything could be so empty? Where does the sky end and space begin? What happens to a human body suddenly exposed to it? And who decided that no nation may claim it? The answers run from a 17th-century mercury barometer to a cosmic ray carrying 10 to the 20th electron volts.

• 2. Naming The Void

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  John Milton reached for the heavens before the word existed in its modern form. The short term space, meaning the region beyond Earth's sky, first appears in his epic poem Paradise Lost, published in 1667. That use predates the full phrase outer space by nearly two centuries. The English poet Lady Emmeline Stuart-Wortley wrote outward space in her 1842 poem The Maiden of Moscow. In astronomy, the term outer space found its first application in 1845, through Alexander von Humboldt. It spread more widely after 1901 through the writings of H. G. Wells. Theodore von Kármán gave the field its legal vocabulary. He used the term free space for the altitudes where a spacecraft escapes atmospheric drag, distinct from the airspace under a nation's sovereign control. The words spaceborne and space-based later followed, describing whatever exists or is carried out in the void above us.

• 3. How Empty Is Empty

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  A breath of air holds about 10 to the 25th molecules per cubic metre. Intergalactic space, by contrast, holds only a few hydrogen atoms in that same volume. This is the closest known approximation to a perfect vacuum, with effectively no friction, so stars, planets and moons glide freely along their orbits. The average energy density of the present-day universe sits near 5.9 protons per cubic metre, counting dark energy, dark matter and ordinary matter together. Atoms make up only 4.6% of that total, a density of one proton per four cubic metres. The thinness lets light travel almost untouched. The mean free path of a photon in intergalactic space runs about 10 to the 23rd kilometres, or 10 billion light years. Yet the universe is far from uniform. Density climbs to extremes inside galaxies, within planets, stars and black holes, then collapses to near nothing in the vast voids between them. The remaining mass-energy belongs to two unknowns, dubbed dark matter and dark energy.

• 4. The Body In Vacuum

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  At an altitude of around 19.14 km, the pressure outside matches the vapor pressure of water at body temperature. This is the Armstrong line, named after American physician Harry G. Armstrong, and above it exposed saliva, tears and the liquids in the lungs boil away. Survival there demands a pressure suit or a sealed capsule. Sudden exposure during a rapid decompression can rupture the lungs, the eardrums and the sinuses. Once deoxygenated blood reaches the brain, a person loses consciousness within seconds and dies of hypoxia within minutes. When pressure drops below 6.3 kPa, blood and body fluids boil, a condition called ebullism, and steam may swell the body to twice its size. Tissues are elastic and porous enough to resist bursting. A fitted elastic garment designed in the 1960s, the Crew Altitude Protection Suit, prevents ebullism at pressures as low as 2 kPa. Most space suits run on roughly 30 to 39 kPa of pure oxygen, close to the partial pressure of oxygen at the Earth's surface.

• 5. Living Bodies Off Earth

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  More than 50% of astronauts feel space motion sickness in their first days off Earth, with nausea, vertigo, headaches and malaise that typically last 1 to 3 days. Weightlessness then begins its slower harm. Muscles atrophy, the skeleton thins into spaceflight osteopenia, the cardiovascular system slows, red blood cell production falls and the immune system weakens. Exercise can blunt these effects but not erase them. Radiation adds a second danger across long journeys. High-energy cosmic rays carry energies that peak around 10 to the 9th electron volts and rise to an extreme 10 to the 20th, with the flux running roughly 87% protons, 12% helium nuclei and 1% heavier nuclei. On a three-year round trip to Mars, a large fraction of an astronaut's cells would be pierced by high-energy nuclei. Shielding from a spacecraft's walls and from water containers dims the particles, but the impacts produce new radiation that can reach the crew. Some hardy life endures it all. A strain of Bacillus subtilis survived 559 days exposed to low Earth orbit or a simulated Martian environment, and seeds of Arabidopsis thaliana and Nicotiana tabacum germinated after 1.5 years in space.

• 6. Where The Sky Ends

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  No physical wall marks the edge of the atmosphere, so engineers and lawyers drew their own. Proposed boundaries have ranged from 30 km out to 1,600,000 km. In 2009, a sounding rocket measured the flow of ions and found the midpoint of the transition at 118 km, where gentle atmospheric winds give way to flows reaching well over 268 m/s. The Kármán line settles the convention at 100 km. Set by the Fédération Aéronautique Internationale and used by the United Nations, it marks where a vehicle can no longer generate enough aerodynamic lift to hold itself up, and where astrodynamics must take over from aerodynamics. Below it lies the contested zone of near space, often defined from 20 to 100 km, through which commercial aircraft are typically capped near 12 km. Until 2021, the United States named anyone traveling above 50 mi an astronaut. The line still resists a single legal value, since setting it too high could choke space activity over fears of airspace violation, while spacecraft like the Space Shuttle have flown over foreign countries as low as 30 km.

• 7. A Province Of All Mankind

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  The Outer Space Treaty calls outer space the province of all mankind, and bars any nation from claiming it. It frames international space law, covers the Moon and other celestial bodies, and prohibits deploying nuclear weapons in space. The United Nations General Assembly passed it in 1963, the USSR, the USA and the UK signed it in 1967, and by 2017 some 105 state parties had ratified or acceded, with another 25 signing without ratifying. Loopholes remain. No legal ban stops conventional weapons in space, and anti-satellite weapons have been tested by the USA, the USSR, China and, in 2019, India. The 1979 Moon Treaty handed jurisdiction over all heavenly bodies to the international community, but no nation that flies humans has ratified it. In 1976, eight equatorial states met in Bogota and claimed the slices of geosynchronous orbit above their territory, a claim the world never accepted. Since 2020 the Artemis Accords have sought to align lunar exploration around the United States led Artemis program.

• 8. The Long Road To The Vacuum

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  Aristotle declared in 350 BCE that nature abhors a vacuum, a principle called horror vacui that built on Parmenides denying any void could exist. The West held for centuries that space could not be empty, and as late as the 17th century Rene Descartes argued all space must be filled. The case cracked open through experiment. Galileo Galilei knew air had mass, and in 1640 he showed a force resisting a vacuum. His pupil Evangelista Torricelli built the first mercury barometer in 1643, producing a partial vacuum and a scientific sensation. In 1648, Florin Perier carried the experiment up the Puy de Dome in central France and found the mercury column shorter by three inches. Two years later Otto von Guericke built the first vacuum pump and concluded a vacuum must lie between the Earth and the Moon. The scale came later still. In 1838 Friedrich Bessel measured 61 Cygni at a parallax of 0.31 arcseconds, over 10 light years away. In 1923 Edwin Hubble measured the distance to the Andromeda Galaxy using cepheid variables and a technique discovered by Henrietta Leavitt, proving all galaxies lay beyond the Milky Way. Georges Lemaitre proposed Big Bang cosmology in 1931. The temperature guesses converged toward truth, from Charles E. Guillaume's 5 to 6 K in 1896 to Erich Regener's 2.8 K estimate in 1933, closing in on the cold the cosmic microwave background would confirm.