Reaching orbit with Air-raft

[Agemegos]

Originally Posted by Malenfant

Erm, I'm pretty sure that being in orbit (assuming you're not thrusting or rotating) IS effectively identical to being in freefall. That's why people in orbit (e.g. in the ISS) are in zero gravity when they're only about 100 km above the earth (where the gravitational field strength pulling you down is still pretty close to 9.8 m/s² because that's still very close to Earth)

Yeah, but I'd sympathise if someone didn't what to say that anyone who jumps a ditch or falls out of a tree was "in orbit". So I would accept a definition of "orbit" that excluded free-fall trajectories which intersect the ground.

Furthermore, I recognise the term "powered orbit" for trajectories in which the object is not in free fall, but is accelerating continuously.

[Malenfant]

Originally Posted by Brett

Yeah, but I'd sympathise if someone didn't what to say that anyone who jumps a ditch or falls out of a tree was "in orbit". So I would accept a definition of "orbit" that excluded free-fall trajectories which intersect the ground.

But "freefall" has a specific meaning that applies here. It means "the motion of a body when gravity is the dominant force on it". When you're jumping in the air, you are in freefall for the time it takes for you to fall from the peak of your motion to the ground (or even from when you leave the ground, technically).

See e.g. http://en.wikipedia.org/wiki/Free_fall

Examples of objects in free fall include:

* A spacecraft (in space) with propulsion off (e.g. in a continuous orbit, or on a suborbital trajectory going up for some minutes, and then down).
* An object dropped at the top of a drop tube.
* An object thrown upwards or a person jumping off the ground at low speed (i.e. as long as air resistance is negligible in comparison to weight). Technically, the object or person is in free fall even when moving upwards or instantaneously at rest at the top of their motion, since the acceleration is still g downwards. However in common usage "free fall" is understood to mean downwards motion.

"Orbit" has a specific meaning too, but you can get paths that intersect planets are still technically orbits. (See http://en.wikipedia.org/wiki/Orbit#Understanding_orbits )

An "orbit" that goes completely around an planet is essentially "a path that keeps missing the ground". It's not unlike when they say in the Hitchhikers books that "flying is throwing yourself at the ground and missing" ;).

I may sound like I'm being pedantic, but these things have specific meanings, and saying that they mean something else is only going to confuse people.

[Agemegos]

Originally Posted by Malenfant

But "freefall" has a specific meaning that applies here. It means "the motion of a body when gravity is the dominant force on it". When you're jumping in the air, you are in freefall for the time it takes for you to fall from the peak of your motion to the ground (or even from when you leave the ground, technically).

Yes, and that is exactly why I say that a boy who falls out of a tree or who jumps across a ditch is in free fall.

"Orbit" has a specific meaning too, but you can get paths that intersect planets are still technically orbits. (See http://en.wikipedia.org/wiki/Orbit#Understanding_orbits )

That article classifies trajectories that intersect the ground or the atmosphere as sub-orbital trajectories, distinct from orbits.

I may sound like I'm being pedantic, but these things have specific meanings, and saying that they mean something else is only going to confuse people.

Well, if you say that a little boy who falls out of a tree is in orbit until he hits the ground that is going to confuse people. But he is definitely in free fall, and therefore the two things cannot be the same.

[ak_aramis]

Actually, all orbits eventually either intersect the orbited body or result in tidal locking, given enough time. That said, an orbit that intersects the body orbited must take it around at least one full time around that body; if it's less than one full time around, it's suborbital.

Almost all satellites are in orbits which will decay and eventually intersect earth.

[Malenfant]

Originally Posted by ak_aramis

Actually, all orbits eventually either intersect the orbited body or result in tidal locking, given enough time. That said, an orbit that intersects the body orbited must take it around at least one full time around that body; if it's less than one full time around, it's suborbital.

Almost all satellites are in orbits which will decay and eventually intersect earth.

That's incorrect.

An orbit will only decay if there is drag on it, or if other perturbations change it so that it intersects the planet - otherwise it'll be stable forever. Satellites in low orbits close to the atmosphere may decay as things like heating from solar flares can change the height of the atmosphere and cause unanticipated additional drag. Satellites orbits that are higher up, beyond the range of an extended atmosphere will not decay though.

Also, orbits do not necessarily end in collision or tidelocking - given enough time, the orbiting body may be forced outwards by tidal evolution so far that it orbits the sun instead of the planet. Even though the moon is tidelocked to face the Earth, orbital evolution is continuing to push its orbit outwards, and will continue to do so until either the earth's rotational period is equal to the moon's orbital period, or until the moon is lost into a solar orbit (whichever happens first. Either won't happen before the Sun expands into a red giant, which might render the whole thing moot if it engulfs the earth/moon system).

[ak_aramis]

Originally Posted by Malenfant

That's incorrect.

An orbit will only decay if there is drag on it, or if other perturbations change it so that it intersects the planet - otherwise it'll be stable forever. Satellites in low orbits close to the atmosphere may decay as things like heating from solar flares can change the height of the atmosphere and cause unanticipated additional drag. Satellites orbits that are higher up, beyond the range of an extended atmosphere will not decay though.

Also, orbits do not necessarily end in collision or tidelocking - given enough time, the orbiting body may be forced outwards by tidal evolution so far that it orbits the sun instead of the planet. Even though the moon is tidelocked to face the Earth, orbital evolution is continuing to push its orbit outwards, and will continue to do so until either the earth's rotational period is equal to the moon's orbital period, or until the moon is lost into a solar orbit (whichever happens first. Either won't happen before the Sun expands into a red giant, which might render the whole thing moot if it engulfs the earth/moon system).

There is no non-theoretical orbit without drag, Dr. Thomas, and you know that. That drag may be minimal, but it's present, since space is not true vacuum. (If it were, Traveller's low-relativistic velocities wouldn't be an issue...)

And until tidelock, the orbiting body and body orbited both produce drag on each other due to gravity effects. In the case of tiny satellites, negligible effect on the orbited, and much more profound on the satellite.

[Malenfant]

Originally Posted by ak_aramis

There is no non-theoretical orbit without drag, Dr. Thomas, and you know that. That drag may be minimal, but it's present, since space is not true vacuum. (If it were, Traveller's low-relativistic velocities wouldn't be an issue...)

Yeah, so there's one or two atoms in every cubic centimetre in space. But it would take a ridiculously long time for the orbit of anything bigger than a dustgrain to be affected by "drag" because of that (and if we're talking about dustgrains then you need to include things like the Yarkowsky Effect, which accounts for light pressure from photons).

For something orbiting a planet, it's simply not an issue. You may as well raise "all things will eventually decay into photons" as if it were a valid point.

And until tidelock, the orbiting body and body orbited both produce drag on each other due to gravity effects. In the case of tiny satellites, negligible effect on the orbited, and much more profound on the satellite.

It's actually because angular momentum is being transferred between the two bodies, and both objects are being heated up as energy is disspiated within them. And while your statement is otherwise correct, it's not really relevant to the point I was making.

[MrBackman]

If you are in in (unpowered) orbit implies that you are in freefall, being in freefall does NOT imply that you are in orbit. If you jump off a trampoline you are in free fall but certainly not in orbit.

[malloyd]

Originally Posted by Malenfant

Yeah, so there's one or two atoms in every cubic centimetre in space. But it would take a ridiculously long time for the orbit of anything bigger than a dustgrain to be affected by "drag" because of that (and if we're talking about dustgrains then you need to include things like the Yarkowsky Effect, which accounts for light pressure from photons).

Come to think of it, shouldn't that matter to the large satellites too? I know you can get measurable position changes from photon pressures on the trajectories of interplanetary probes over the course of just a few years, and there is a constant perturbative force away from the sun on the sattelites. I suppose the perturbation could average out to zero over the course of a year, but it seems unlikely it would unless the Earth's orbit were perfectly circular and satellite periods are exactly integer divisions of a sidereal year.

I'd think the tidal and magnetic drag would be pretty signficant on anything not in geosynchronous orbit too, and signficant lunar perturbations on anything not co-orbiting with the moon. Ancient artifacts that have been sitting in close stable orbits around planets for a few million years may well be science fantasy.

[MrBackman]

Originally Posted by malloyd

<snip>I'd think the tidal and magnetic drag would be pretty signficant on anything not in geosynchronous orbit too, and signficant lunar perturbations on anything not co-orbiting with the moon. Ancient artifacts that have been sitting in close stable orbits around planets for a few million years may well be science fantasy.

Yes, and matters get worse around smaller objects such as the moon where high density points on the surface (from asteroid impacts) make the gravity field significantly differ from the point mass ideal required by mr Newton. Low orbits over the moon are more or less impossible to maintain over long periods, that's why we have no 'permanent' sattelites around it.

No, those Ancient things should be parked ON asteroids, moons etc to be credible - or better yest; inside complicated cave labyrinths guarded by monsters, just like magical items of D&D does.