Imagine a spacecraft entered an orbit around the Earth whose perigee is low enough into the atmosphere so that it reenters and lands after a revolution, without having to perform a reentry burn. While this surely can't be called a suborbital spaceflight, I wonder whether this would count as a full orbital spaceflight respectively as a full revolution around the Earth, even if it may have entered the atmosphere just before it completed a full revolution relative to the stars (but did that during its further atmospheric glide flight or free fall).

The Space Shuttle lowered its perigee to a 165,000 ft (50 km) for reentry. If a spacecraft achieved an orbit with that perigee and entered the atmosphere after about a revolution without a reentry burn and would land back on Earth, would that count as a full orbital flight or something inbetween an orbital and a suborbital flight or would it depend on other factors?

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    $\begingroup$ @OrganicMarble The FAI for instance? $\endgroup$ – Greenhorn Dec 19 '20 at 14:33
  • $\begingroup$ Welcome to Stack Exchange! I think this is a good first question here, and what does or does not "count" as orbital flight may have been addressed in previous questions and answers, and those answers may be that there is some flexibility in the term. I think that for example "sub-orbital" means that the trajectory would intersect the surface of an airless Earth, and so "orbital flight", if actually used regularly, would likely mean won't intersect an airless Earth and everybody just leaves the atmosphere out of it because that trajectory then depends on the shape, mass and other specifics. $\endgroup$ – uhoh Dec 20 '20 at 0:45
  • $\begingroup$ @uhoh But would it be a completed orbit if you reentered the atmosphere before completing the revolution (relative to the stars) but completing it when you further glide in the atmosphere without propulsion though by the help of airflow below your Space shuttle's wings? $\endgroup$ – Greenhorn Dec 20 '20 at 7:10
  • $\begingroup$ @Greenhorn: The problem with bringing the atmosphere into it, is where to draw the line. I.e., we all would probably agree that the ISS is in orbit, and yet, it needs to constantly be reboosted because of aerodynamic drag. It is actually very hard to come up with a definition that does not accidentally exclude the ISS or include sailplanes. Kind of like it is impossible to find a definition that makes Pluto a planet, but does not mean that all of a sudden hundreds of rocks become planets, too. $\endgroup$ – Jörg W Mittag Dec 20 '20 at 20:37

There is a gray area here, but probably yes.

If it were called a "suborbital flight" the assumption would be that the orbit's path would intersect the surface of the Earth.

Since the perigee of your orbit is in the denser part of the atmosphere but does not intersect the surface, if it weren't for the drag force the spacecraft would continue to orbit the earth without hitting it.

Parsing the words is a little difficult because any spacecraft in LEO is literally orbiting in the atmosphere; the ISS for example has to constantly reboost it's altitude to stay around 400 km for example.

There have been proposals to move the Karman line from 100 km down to 80 km because (to make a long story short) at 100 km it's really still orbital flight, at least for a while, and several spacecraft make regular dips down to 180 km.

  • $\begingroup$ But the atmosphere belongs to Earth too. An orbit must be free fall, and if a part of it is due to areodynamic lift, even without propulsion I wonder whether it's not something inbetween orbital and sub-orbital ("semi-orbital" e.g.). A glider (sailplane) could also go around the Earth without propulsion but that's not an orbit because it's no free fall. $\endgroup$ – Greenhorn Dec 20 '20 at 7:13
  • $\begingroup$ The atmosphere of Earth has nothing to do with the definition of orbit or sub-orbit. It does have to do with the initial velocity and altitude of the object, not aerodynamic drag or solar pressure. $\endgroup$ – Scottie H Dec 23 '20 at 0:38
  • $\begingroup$ I like your answer, so rather than write a second one that says the same thing I've added some details to yours. Please check it out, feel free to edit further or roll back the changes if you like. Thanks! $\endgroup$ – uhoh Feb 6 at 5:24

I would like to answer this correctly this time.

So, orbits are about velocity.

That means if during that revolution the artificial satellite was at a velocity faster than the rotation of the planet (by that I mean horizontal velocity, not vertical) that is technically an orbit. The atmosphere has nothing to do with the orbit.

(you can definitely tell I understand orbital physics more than I did before lol)

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    $\begingroup$ Plenty of things go "faster than the rotation of the planet" but aren't in orbit. It's only 1000 mph at the equator. SR-71s were not in orbit when they were flying 2000+ mph $\endgroup$ – Organic Marble Feb 8 at 18:25
  • $\begingroup$ Even on Concord you could watch the Sun slowly setting in the East $\endgroup$ – Slarty Feb 9 at 15:35

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