Retrograde or Prograde Orbit?

[Dunbaratu]

wow lot of misinformation in here.

quite simply, prograde and retrograde are velocity vectors toward and away from the orbiting body's direction of travel relative to the body of reference.

That doesn't fit the definition of it that people are using for easterly or westerly orbits around a planet, though, because if the body of reference is the planet then the direction of travel of the planet itself is around the sun, not around its own axis.

[PringleMan]

That doesn't fit the definition of it that people are using for easterly or westerly orbits around a planet, though, because if the body of reference is the planet then the direction of travel of the planet itself is around the sun, not around its own axis.

That is not true either. Your reference frame has to be with the local acceleration, or else normal physics fails. That is to say, if you are in orbit around a planet, you MUST set your reference to the center of that planet, and not to the center of the sun. Your accelerations due to your orbit around that planet are MUCH MUCH greater than the accelerations to the star.

By standard definition, prograde refers to a counter-clockwise motion with regards to a body that is rotating counter-clockwise. The easiest case of this is the Earth. When we launch prograde, that is the simplest and easiest (energy wise) motion that is possible to achieve. In contrast, prograde for Venus would be a clockwise motion, and prograde for Uranus would be a "north-south" motion (I think, not sure which way Uranus is tilted)

Whenever talking about physics and motion, always make sure that you are setting the correct reference, that is the one that has the most bearing on your motion. We could technically work out all physics on the Earth with regards to the moon or the sun, but that would be shear insanity.

edit: And just to make sure that everyone is clear, normal physics only works in an Inertial reference frame, that is one that is not under significant acceleration. That is why you MUST select the primary body of mass in a system as your reference frame. A planet orbiting a star is not under significant acceleration in relation to a spaceship orbiting a planet. By making your reference frame the planet, you avoid the really really messy mathematics necessary to make the other case work.

Edited June 12, 2013 by PringleMan

[Temstar]

Funny that this confusion arises among the young people today, when the fact is, people who are old enough to have experienced (or at least got stories from parents/grandparents) Apollo, are in fact aware of retrograde orbits, because that's how the CSM/LM orbits the moon when arriving there. The reason being that the LM should make an approach from east to west, so that when it encounters the terminator of the moon (which as of this time is in the "middle" of the moon, i.e. first quarter moon), the LM lands in such a way that the commander and LMP can see long shadows, which are crucial for distance and depth perception, important for manual control of the LM's descent. Thus, on T.V. newscasters interviewing Apollo astronauts and mission planners would often mention "retrograde orbit" and explain it to the public too. That is also how I got the idea, from watching archived footage of such interviews and TV broadcasts about Apollo.

Actually this isn't true. The LM could enter a prograde Lunar orbit and land on the terminator fine with the CMD and LMP able to see the shadows out the windows. The reason why Apollo inserted into Lunar orbit retrograde was for safety. On the way to the moon the Apollo stack would fly in a free return trajectory so that they enter Lunar SOI retrograde. When they go behind the moon (and so all alone and out of contact with the ground) they need to fire the service module engine to insert into a retrograde lunar orbit. If something goes wrong and the SM engine does not fire then their retrograde motion means the moon slingshots them back to the safety of Earth, hence free return trajectory.

If the Apollo stack enters Lunar SOI in prograde direction and the SM engine does not successfully fire to insert them into Lunar orbit then the moon will fatally slingshot them out to a solar orbit. Hence for the reason of safety a retrograde Lunar orbit was chosen despite the fact that it means the LM would have to pack a little bit more delta-V in both descent and ascent stages.

[RoboRay]

And you don't think it's ambiguous that "retrograde direction" has two different meanings here? Yes, it's ambiguous.

It's not ambiguous, because it doesn't have two different meanings. You are misinterpreting what the word means and how it's applied.

That doesn't fit the definition of it that people are using for easterly or westerly orbits around a planet, though, because if the body of reference is the planet then the direction of travel of the planet itself is around the sun, not around its own axis.

Please refer to the material I linked earlier.

Edited June 12, 2013 by RoboRay

[Colonel_Panic]

That doesn't fit the definition of it that people are using for easterly or westerly orbits around a planet, though, because if the body of reference is the planet then the direction of travel of the planet itself is around the sun, not around its own axis.

It sure does. The surface of a planet has a prograde vector around the planet's axis. Since prograde and retrograde are just velocity vectors, literally anything that is moving in relation to anything else has a prograde vector. This comes into play because the planets are rotating, so if you want to match your direction of orbit around the direction of a planet's rotation (to ease takeoff and landing) you want to know the surface's prograde relative to the planet's center. That said, some of the ways I've seen this concept described in this thread are incorrect.

All of the bodies in the kerbol system, so far as I am aware, rotate easterly in the same direction as their orbits, so if you enter a 180 inclination orbit by capturing from the inside, you are orbiting in the surface retrograde. If you enter a 0 inclination orbit by capturing from the outside, you are moving to surface prograde.

[Mr Shifty]

Definition of prograde from Merriam Webster: "having or being a direction of rotation or revolution that is counterclockwise as viewed from the north pole of the sky or a planet."

From Oxford: "proceeding from west to east; direct. The opposite of retrograde."

From the Free Online Dictionary: "Having a rotational or orbital movement that is the same as most bodies within a celestial system. In our solar system, prograde movement for both rotating and orbiting bodies is in a counterclockwise direction when viewed from a vantage point above the Earth's north pole."

This is different than the definition used in the game wherein it typically refers to your vessel's forward velocity vector, regardless of the direction of orbit. Hence, some ambiguity.