Space begins and atmosphere ends for a planet...

[A. BETTIK]

A) where the shuttle begins to use aerodynamics to steer.
B) where a balloon can float no higher
C) where ionic winds dominate over atmospheric winds
D) where 100 km is a height normal to the surface

These don't seem very objective.

How about:
1) Target Planet must have an atmosphere not already collapsed by low temperature.
2) Determine target planet's "metre" according to French Academy of Sciences methodology.
3) Determine representative mass of target planet for a one target planet meter diameter sphere. For earth use something like figure 5
4) Find a height normal above the planet surface.
5) Determine orbital velocity for height in 4) and launch meter object in a circularized orbit.
6) Repeat step 4) and 5) exclusively of prior heights until meter object makes exactly one orbit but impacts planet directly under the launch position. This is the height of the atmosphere or where it ends and space begins.

Call it the Newton Napoleon Atmospheric Height Algorithm or some better title.

 

[OldHippie]

I'm not a geophysicist or astrophysicist but why wouldn't you define the difference between the atmosphere of a planet and space by some number of atmospheric particles per some defined volume, like particles per cubic meter which in turn determines A, B and C.

 

[kgp]

How far out does the tall part of a Gaussian curve extend? How many inches make a man tall? I like the ideas above, but there are two reasons to define the atmosphere. One is academic, the other practical. Practical definitions will be dependent in utility on the interaction we have with the planet. Academic ones should be easy to incorporate into papers and to measure reliably from here. My 2 bits as a scientist from a different field.

 

[mia1994]

I think space should "begin" at the point that you are free of the Earth's atmopheric, magnetic and gravitational force. Since the influence of gravity has no limits, that means that the Earth is roughly 13.75 billion light years in diameter.

Seriously though, other than a penis measuring contest there is not purpose in defining a discrete dividing line between "Earth" and "not Earth". It serves no definable purpose, and there are literally hundreds of equally arbitrary delineations you could use.

 

[mia1994]

nevermind. I'm being a fun cruncher, pretend I said nothing.

 

[omahawk]

good, mia. i would definitely consider the distance between the earth and moon as space, despite the having gravitation pull on the lunar rock.

 

[RamjetFDO]

Space Shuttle program Entry Interface defined height is 400,000 feet (or 65.832 NM)... just FYI.

Roger

 

[mia1994]

goo, I think most would. My only point is that there is not a good objective and discrete designation for what is and is not "space". NASA says right now that you are an astronaut if you've left the mesosphere (50 miles), most scientists put space around 62 miles. While the ISS orbits at 250 miles, the highest layer of the atmosphere is the exosphere which has an upper limit of around 450 miles. I've never heard a compelling reason why a designation even need be made.

Personally, since the designation is meaningless outside of it being personal to humanity, I would put the lower limit of space just beyond the maximum altitude at which a human can survive without assistance. That's about 5 miles. If not that, then the lowest altitude that you can orbit the earth at least once without continued or intermittent thrust, around 150-200 miles. If not those two, then the outer limit of the exosphere. All of these are no more or less reasonable than the mesosphere.

 

[washparkhorn]

One of my 5 year old boys went to a Kindergarten enrichment program on space yesterday (that focuses on math - his passion) and I asked him, "How far is Earth from space?"

He said - "Dad, that's a trick question. Earth is in space."

My journey as "Dad is a dumb ***" has begun.

 

[A. BETTIK]

My proposed methodology wouldn't seem work for a gaseous planet...

Also I didn't factor in orbital rotation.

 

[MaduroUTMB]

Very different definitions depending on which angle you come from. If you're a mammal trying to breathe, space begins before the Earth ends (e.g. Everest). If you're a scramjet, maybe 100,000 feet. If you're a balloon, 150k feet. If you're trying to orbit without periodically using an engine, a few hundred miles. As such, I think that trying to throw a blanket definition on things would be counterproductive.