Topics · Space Codex
What is deep space?
Deep space usually means beyond the Earth–Moon system: first interplanetary space, then interstellar space past the heliopause, about 120 AU out.
Beyond Earth's neighbourhood
At a glance
Key points
It is not empty: dust, cosmic rays, magnetic fields and thin gas fill the void between stars.
- Voyager 1 entered interstellar space in 2012
- The cosmic microwave background is light left over from the young universe
- Most of the universe's volume is near-vacuum between galaxies
Where it begins
There is no single border. Radio engineers follow the International Telecommunication Union, which treats everything 2 million km or more from Earth, a little over five times the Moon's distance, as deep space. Mission planners often use the term for anything beyond the Earth–Moon system. Astronomers add a second line: the heliopause, where the Sun's wind gives way to interstellar gas and interstellar space begins.
A sense of scale
Sunlight takes 8.3 minutes to reach Earth and about four hours to reach Neptune. Voyager 1, the most distant spacecraft, is projected to reach one light-day from Earth on 18 November 2026, after almost 50 years of travel. Proxima Centauri, the nearest star, is 4.24 light-years away, more than 1,500 times farther than that, and the Andromeda galaxy lies about 2.5 million light-years off.
Not quite empty
Air at sea level packs about 25 billion billion molecules into every cubic centimetre. Just beyond the heliopause Voyager 1 measured about 0.08 electrons per cubic centimetre, and the gas between galaxies is thinner still. Even so, the near-vacuum carries magnetic fields, cosmic rays and dust, and every part of it is bathed in the cosmic microwave background, the cooled afterglow of the young universe.
Common misconceptions
Space is not 'cold' the way a winter day is: with so little matter around, heat leaves a spacecraft mainly as radiation, so probes must shed heat in sunlight as well as stay warm in shadow. And crossing the heliopause did not take the Voyagers out of the Solar System. The Sun's gravity still rules the icy Oort cloud, whose inner edge lies 2,000 to 5,000 AU away, many times farther than either probe has gone.
Related entries
- The heliosphereThe bubble blown by the solar wind
- Voyager 1First spacecraft in interstellar space
- Oort cloudThe Solar System's distant icy shell
- Cosmic microwave backgroundThe oldest light in the universe
Sources
- NASA Science: Voyager interstellar mission (opens in a new tab)
- NASA Exoplanet Archive (IPAC/Caltech) (opens in a new tab)
- NASA Science: Voyager 1: what is a light-day? (opens in a new tab)
- NASA NTRS: Deep Space Communication (opens in a new tab)
- Gurnett et al. 2013, Science 341:1489 (opens in a new tab)
Last reviewed against these sources. Built from public NASA, ESA and JPL data; not affiliated with or endorsed by any agency.