The item in Science Alert's A Harvard Astrophysicist Says Outer Space Is Actually Closer Than We Think (see also Science; Outer space may have just gotten a bit closer) talks about the recent Acta Astronautica article by Jonathan McDowell The edge of space: Revisiting the Karman Line, and ends with the quip:

That doesn't mean we're going to see any commonly used definitions change soon, though: McDowell first proposed his 80-kilometre boundary line in 1994 - over 25 years ago. Perhaps we need a new term for it instead: the McDowell line.

I think the idea is that once something that's been in Earth orbit drops to the Karman line in circular orbit, it's "toast" in that it's fate is fairly well sealed and it has hours to perhaps a day at most before burning up.

Earlier, the article says:

So back to McDowell. He chose for his proposed boundary the 80-kilometre mark, just below the mesopause - the boundary between the lower mesosphere and the upper thermosphere, and the coldest point in the Earth's atmosphere.

And this is because of the satellites. McDowell analysed over 90 million points of orbital data from 43,000 satellites dating back to 1957, using archives maintained by the North American Air Defence Command.

Most of the satellites fly pretty high, but he identified 50 that flew below the 100-kilometre mark, down as low as the 80-kilometre mark, over two or more complete revolutions of Earth.

"Are you going to say [these satellites are] in space and then not in space every 2 hours?" he told Science. "That doesn't seem very helpful."

While orbits in natural decay will circularize first before burning up, I'm trying to understand if able-bodied spacecraft with some remaining propulsion in elliptical orbits can dip below the Karman line once or twice, and then at apoapsis boost back to an orbit with a higher periapsis so that they can "Die Another Day."

Question: Have spacecraft ever dipped below the Karman line and then safely returned to spaceflight?

edit: Responding to comments, yes, aerobraking would count, as long as it involved dropping below the Karman line in the Earth's atmosphere temporarily. Also, 'dipped' means going below for a short time, such as a fraction of an orbit as discussed in the links above. Not landing, then being launched again at a later date.

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    $\begingroup$ Somewhat relevant in-development technology. Not suggested to fly below the Karman line, but designed to operate long-term in a region where atmospheric drag would otherwise lead rapidly to deorbit. $\endgroup$ – Jack Jul 26 '18 at 11:41
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    $\begingroup$ Does aerobraking count? $\endgroup$ – gerrit Jul 26 '18 at 13:38
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    $\begingroup$ You might want to rephrase as "returned to stable orbit" rather than "lived to tell"; almost every crewed craft ever has done the latter, as well as many camera packages, ICBM reentry vehicles, etc. $\endgroup$ – Russell Borogove Jul 26 '18 at 15:26
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    $\begingroup$ SpaceX Dragon does it, but between the dip and return it makes a stop for refurbishing and being put on top of a new booster. $\endgroup$ – SF. Jul 26 '18 at 19:46
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    $\begingroup$ Works for me... $\endgroup$ – Russell Borogove Jul 27 '18 at 0:11

The idea behind the Karmán line is that the air is so thin that a plane would need to move at orbital velocity to maintain a steady horizontal flight. This means the dynamic pressure should be about the same as sticking your hand out an airplane window. It seems possible given an eccentric enough orbit and a large enough cross sectional density.


I hope this helps.


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