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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? Please use the 100 km line, not the proposed 80 km discussed at the beginning.

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.

uhoh
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    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. – Jack Jul 26 '18 at 11:41
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    Does aerobraking count? – gerrit Jul 26 '18 at 13:38
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    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. – Russell Borogove Jul 26 '18 at 15:26
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    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. – SF. Jul 26 '18 at 19:46
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    BTW, there was a spaceplane concept, that would dip into the atmosphere to change inclination... I could look up what it was if it's something satisfying the requirements. (never went beyond design phase...) – SF. Jul 26 '18 at 19:49
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    Works for me... – Russell Borogove Jul 27 '18 at 00:11
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    Some rockets loose altitude during the 1st/2nd stage transition/coasting/2nd stage early burn: Ex Ariane 5 https://www.narom.no/wp-content/uploads/2016/10/ariane5_1.jpg but it seems to happen well over the karman line for all the rockets I searched for. – Antzi Jul 27 '18 at 01:37
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    Chinese Chang’e-5-T1 technology demonstrator mission used a “skip re-entry” during its descent https://spaceflightnow.com/2014/11/01/chinese-pathfinder-probe-returns-from-flight-around-the-moon/. I can't find trajectory parameters, but seemingly it crossed Karman line twice. – Heopps Jul 27 '18 at 10:47
  • One of the main problems for "continued spaceflight" after Karman line dip is energy source. Could solar cells survive it? Looks like could not. So what will be the energy source after the dip? – Heopps Jul 27 '18 at 10:55
  • @gerrit is that artist representation accurate? Did they really aerobrake on Mars with solar panels fully extended? If so, wow, I really underestimate how puny the atmosphere on Mars is. – Magic Octopus Urn Jul 27 '18 at 16:34
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    @MagicOctopusUrn I don't know, but it'd make a good question for this site! – gerrit Jul 27 '18 at 22:33
  • I know of some spacecraft deorbits I've been involved with in which control of the spacecraft is maintained below 100 km during reentry. So in theory, it would be possible to have a 90ish or higher perigee, and the very next apogee raise perigee. I don't know of any spacecraft that has done this, however. – Carlos N Dec 05 '18 at 21:58
  • @Heopps I've just added a bounty. – uhoh Mar 27 '19 at 07:09
  • Ha this was asked 9 months before my question and I didn't see it until now: https://space.stackexchange.com/questions/35322/is-there-any-kind-of-research-on-sub-orbital-rendezvous-ing-with-a-space-tug-o

    the idea being that it might be better at some point with infrastructure more evolved having in-orbit-refueling it might be smart to pick up or refuel vehicles on the way up as (s)low as possible

     – Prototypist Jul 21 '23 at 15:04

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I've been able to find a few rocket bodies and payloads that have done so, at least for some period of time prior to decay. These objects might have been in a state of decay prior to entering in these low objects. The two satellites I could find were MOLNIYA 2-12 and MOLNIYA 1-30. I don't know if they were actually able when they did so, but they certainly could have survived after a single dip in the upper atmosphere by a small burn at apogee after such a pass. I can't tell you for exactly how long, the orbital data is limited for such old satellites, but they both survived multiple orbits when dipping below the Karman line.

Any satellite that can do this will be in a highly eccentric orbit. These are almost always some kind of military satellite, either spy satellites using the low passes to get in really close for high resolution imaging, or some kind of communication satellites, particularly in a Molniya orbit. As a result, it can be difficult to get a lot of reliable information on these. There likely has been spy satellites that have survived brief dives when active below the Karman line, but that information would not be public. Still, per the very paper you cited, only 50 objects with highly elliptical orbits have survived any amount of time with a periapsis below 100 km, 4 days was the highest. None of these appeared to be functioning, being in the atmosphere so low would cause a satellite to tumble severely.

What I think the argument is that the issue becomes a legal one. Any object that is below 100 km now is considered an aircraft, and technically needs the permission of the country over which it is flying to enter that air space, while it does not need the permission if it is above 100 km.

PearsonArtPhoto
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  • would you consider the two MOLNIYA satellites you mentioned to be "...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.'" when they did this? If so, can you add a timeframe and verification? Dead spacecraft won't count otherwise the answer will probably be all of them! – uhoh Mar 27 '19 at 12:47
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    @uhoh It's basically impossible to know if they were able-bodied, coming from cold-war Soviet Russia. They did not raise their perigee after they started dipping below the Karman line, however, if the spacecraft was capable they certainly could have. Edited my answer to clarify a few points. – PearsonArtPhoto Mar 27 '19 at 12:53
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    Last paragraph (re legal) is fascinating; either reentering spacecraft are constantly violating international law or there's an exception for spacecraft that are burning up? So I've just asked Do owners of reentering spacecraft notify the countries' whose airspace they are likely to violate and seek permission? – uhoh Mar 27 '19 at 13:01
  • So you haven't actually named any spacecraft that match the criteria I've laid out in the question. The "legal criteria" that it wouldn't happen seems vague and unsupported. I don't think this is really an answer to the question as asked. – uhoh Mar 27 '19 at 21:03
  • I looked at the paper you linked, and combined with what I've seen, all of the spacecraft were end of life, but survived in orbit for at least 2 days with perigees that low. I don't think a functioning spacecraft would do that, too risky. – PearsonArtPhoto Mar 27 '19 at 21:21
  • Yes, but "Do you think that..." is not how the question is written. I worded the question carefully, then more recently added the bounty, in order to get a fact-based answer naming "...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.'" The "it's legally an aircraft and so nobody would do it" is unsupported legal speculation. – uhoh Mar 27 '19 at 21:29
  • Okay, let me put it very simply. One of the absolute world experts on the subject wrote a paper on it that you referenced. He specifically called out that one of the 50 objects he identified by looking through millions of data points survived for 4 whole days. No one would intentionally do that with such a short lifetime for anything that was designed to be operational, unless it is some top secret stuff that the public couldn't possibly know about. – PearsonArtPhoto Mar 27 '19 at 21:57
  • But that paper is about unpowered vehicles reentering the atmosphere in circular orbits. I've specified powered vehicles in elliptical orbits. The reason I cited that paper is to distinguish the two cases. "While orbits in natural decay will circularize first before burning up, I'm trying to understand if able-bodied spacecraft with some remaining propulsion..." orbits circularize first before decaying see also this answer. – uhoh Mar 27 '19 at 22:01
  • Craft dipping below 100 km don't count as aircraft. During its reentry the Space Shuttle flew below 100 and also below 80 km above foreign countries (Latin American or Bahamas obviously), there is not really a legal definition. In the USA the FAA's legislation ends above 60,000 ft (18.3 km) (you're above class A airspace). – Giovanni Sep 02 '20 at 14:24