Outer Space

The expanse that exists beyond Earth's atmosphere and between celestial bodies. Contains ultra-low particle densities constituting a near-perfect vacuum of predominantly hydrogen and helium plasma, permeated by electromagnetic radiation, cosmic rays, neutrinos, magnetic fields, and dust.

Physical Properties

• Baseline temperature: 2.7 K (−270°C), set by background radiation from the Big Bang
• Composition: Mostly hydrogen and helium plasma; remaining mass-energy is dark-matter and dark-energy (unknown forms)
• Intergalactic plasma: Accounts for ~half of all baryonic matter in the universe; less than one hydrogen atom per cubic metre, kinetic temperature of millions of kelvins

Intergalactic space takes up most of the volume of the universe. Even galaxies and star systems consist almost entirely of empty space.

Boundary with Earth's Atmosphere

There is no well-defined physical boundary. Air pressure decreases steadily with altitude until it merges with the solar wind. The karman-line at 100 km is the conventional working boundary used in space treaties and aerospace records.

Regions

Space near Earth is divided into overlapping regions depending on distance and governing physics. Moving outward from Earth:

• geospace — upper atmosphere and magnetosphere
• interplanetary-medium — interior of the heliosphere, out to ~160 AU
• interstellar-medium — space between stellar systems within a galaxy
• Intergalactic space — the dominant volume of the universe

Legal Status

The outer-space-treaty (in force 10 October 1967) precludes national sovereignty claims and permits all states to freely explore space.