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The International Aviation Federation (FAI) considers the space border at 100 km (330,000 ft) above sea level. So if an orbiting body goes below that altitude and continues its orbit without propulsion, do they count as two (and more) spaceflights (or a 'double spaceflight') for the FAI? Consequently they should even though it's weird.

An example would be the two GoldenEye satellites from the James Bond movie reaching an altitude of exactly 100 km and maybe lower (the FAI defines space as above, not at, 100 km).

Giovanni
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    Is there any reason the FAI would care if a satellite made "multiple spaceflights" as a result of its orbit crossing the 100km line? – Russell Borogove Sep 11 '20 at 16:45
  • I recommend expanding the acronym "FAI" and adding a short description and/or link explaining what it is. – Chris Sep 11 '20 at 16:54
  • @RussellBorogove Because they defined the space border at 100 km. If a satellite goes below that line and continues orbiting without propulsion, well... – Giovanni Sep 11 '20 at 17:05
  • they defined the space border at 100 km but orbits slightly above 100 km do decay very fast. There is no sharp border – Uwe Sep 11 '20 at 17:32
  • @Uwe A sharp border might be set where sats definitely will be decelerated enough by air drag to fall to Earth, under any circumstance. A sat in a highly elliptical orbit can have a perigee below what the FAI calls the Karman line and still complete multiple orbits. The orbit will change due to air drag but the sat will keep orbiting. If a sat can orbit without propulsion you can't possibly call that airborne flight. But if they set the atmosphere border at 100 km, they must consequently consider those sats as having "left space". – Giovanni Sep 11 '20 at 17:53
  • This is why some people suggest changing the definition of space to 80 km. An object can orbit below 100 km for a time (Especially elliptical), but below 80 km and you will be reentering within a few orbits. – PearsonArtPhoto Sep 11 '20 at 19:37
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    @giovanni - Is this question largely hypothetical? Do you know of any real-life satellites that have gone below 100 km multiple times? Also, I'm not sure that the FAI really tracks what is "in space". Finally, the only sharp border, as you describe in your comment is the ground. Anything else, you can theoretically have a rocket to get you through the atmosphere and back out. – Carlos N Sep 11 '20 at 20:09
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    This sounds like a quibble over scorekeeping for a game I'm not sure anybody is actually playing. – Tristan Sep 11 '20 at 21:01
  • At 160 km an orbit decays in about a day, at 185 km about a week, at 300 km about 6 months, at 400 km several years, above 600 km over a decade. These times depend on solar activity. Not easy to define a border. – Uwe Sep 11 '20 at 22:19
  • Tristan’s got it. Yes, by FAI rules such a satellite “enters space” multiple times. No, neither the FAI nor anyone else cares. – Russell Borogove Sep 12 '20 at 02:20
  • @CarlosN I didn't describe the ground in my comment, but a thick enough atmosphere that decelerates the sat to prevent it to continue its orbit, regardless of circumstance. See my answer to "How far do you have to be from Earth to be in space?" – Giovanni Sep 12 '20 at 05:32
  • @Tristan No, you're wrong. – Giovanni Sep 12 '20 at 05:33
  • @PearsonArtPhoto Unfortunately I can't upvote comments but I'd upvote yours if I could. – Giovanni Sep 12 '20 at 05:34
  • @Uwe This counts for circular orbits but in highly elliptical orbits you can go below 100 km and continue orbiting multiple times. The Molniya orbits are extremely elliptical with a very low perigee. Jonathan McDowell talks of sats going below 100 km (The edge of space: Revisiting the Kármán line). – Giovanni Sep 12 '20 at 05:38

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First of all, this can actually happen. There is an effort going on to change the boundary of space to 80 km, not so much for this definition, but because technically a satellite operating at 90 km would be invading a countries air space. Below 80 km, the reentry time is no more than 2 orbits for any known mission, but a satellite can be stable for weeks below 100, particularly in an elliptical orbit.

As for what happens now, well, you won't win any records by dipping below 100 km and then back up, unless it is maintaining a low stable orbit. That doesn't seem likely to be changing anytime soon.

PearsonArtPhoto
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