Outer space is frequently defined as the area outside the atmosphere of Earth (or another planetary or stellar body).

Deep space is defined as either a synonym for outer space (Google's definition) or space beyond the limits of the solar system or space well outside the earth's atmosphere

So, my question: is there a standard definition of these terms? That is, does any official body (NASA, etc.) use these terms to mean something specific? Or are they merely synonyms with rather wide definitions?

  • There's also "inner space," but not "shallow space."
    – Sven Yargs
    Apr 1, 2014 at 18:31
  • There's no body governing the use of English terms. Popular astronomical terms? Are you kidding? Wikipedia shows the confusion: Astronomy: deep space : Empty regions of the universe in outer space // Extrasolar space // Extragalactic space // Intergalactic space [formatted] Apr 1, 2014 at 18:53
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    There is also NASA's "Deep Space Network" that explores the furthest points of our solar system.
    – ermanen
    Apr 1, 2014 at 19:06
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    They don't have strong definitions, but I wouldn't say they are synonymous. Outer space is kind of vague, as if to say "out there somewhere". Deep space gives a feeling of being very very far away. It evokes a feeling of being off the edges of the map.
    – milestyle
    Apr 1, 2014 at 21:38
  • 3
    This question appears to be off-topic because it is about astronomical definitions, not English. Please try astronomy.stackexchange.com
    – Robusto
    Apr 2, 2014 at 2:04

3 Answers 3


"Outer space," or just "space," is the more general term, and usually refers to anything outside the earth's atmosphere. It can even refer to places that are inside the atmospheres of other planets. For example, the obscure Robert Heinlein short story "Tenderfoot in Space" is set on the surface of Venus.

I think the current usage of "deep space" is roughly anything more than 1 astronomical unit from the earth. NASA had a series of probes called Deep Space 1 and Deep Space 2, which went to Mars and the asteroid belt. I think this shows that the term doesn't imply the outer solar system (gas giants).

There is a "Deep Space Network," and WP's article on it provides the following detailed definition that supports the above interpretation:

Deep space is defined in two different ways. The first is when a mission gets sufficiently far from Earth that it is always in view of one of the tracking stations. This distance, about 16,000 km or 10,000 miles, was the definition used during Apollo and early days of the DSN.[1] The more modern definition is from the International Telecommunications Union, which sets aside various frequency bands for deep space and near Earth use. According to this definition, deep space starts at a distance of 2,000,000 km from the Earth's surface.[2] In particular, this means that missions to the Moon, and the Earth–Sun Lagrangian points L1 and L2, are considered near space and cannot use the deep space frequencies.


I think the answer lies in semantics and logic.

Outer space is everywhere outside of an atmosphere. It is simply the space between celestial bodies. You can say anything is outer space.

Deep space is in relation to something - meaning far away (how far, I don't know). Our Moon is not in deep space compared to us. However our Moon would be considered as deep space to 99% of the entire universe.

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    Agreed. "Outer space" is really a term from before the sixties and commonplace space travel.
    – Drew
    Apr 2, 2014 at 2:14

In my experience, "deep space" refers to a farther off area than "outer space". It either stipulates "space beyond the terrestrial planets(Mercury, Venus, Earth, Mars)" or "space beyond the solar system" (usually the latter). Outer space simply refers to the outer atmosphere and beyond.

EDIT: It should be noted that the definition likely varies by locality.

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