14
$\begingroup$

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.

$\endgroup$
  • 1
    $\begingroup$ 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
  • 1
    $\begingroup$ Does aerobraking count? $\endgroup$ – gerrit Jul 26 '18 at 13:38
  • 5
    $\begingroup$ 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
  • 5
    $\begingroup$ 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
  • 1
    $\begingroup$ 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...) $\endgroup$ – SF. Jul 26 '18 at 19:49
2
+50
$\begingroup$

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.

$\endgroup$
  • $\begingroup$ 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! $\endgroup$ – uhoh Mar 27 at 12:47
  • 1
    $\begingroup$ @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. $\endgroup$ – PearsonArtPhoto Mar 27 at 12:53
  • 1
    $\begingroup$ 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? $\endgroup$ – uhoh Mar 27 at 13:01
  • $\begingroup$ 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. $\endgroup$ – uhoh Mar 27 at 21:03
  • $\begingroup$ 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. $\endgroup$ – PearsonArtPhoto Mar 27 at 21:21
-2
$\begingroup$

I believe this could be done with the Karman Line of Mars much easier than Earths. A spacecraft could make multiple passes through the atmosphere dipping below the karman line and back out to decelerate from interplanetary speeds. Yes I know the lines would not look like that but maybe a bulk of the speed could be shed off on a deep skip of the atmosphere of Mars out the first pass to create a Mars skip reentry.

enter image description here

$\endgroup$
  • $\begingroup$ I approached this in a physical way not so much a legal way. The spacecraft would have to speed up and shed just enough speed off or push through the atmosphere to maintain the original orbit. This would only be practical for space exploration if it could be used to shed speed. $\endgroup$ – Muze Mar 27 at 21:20

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.