[X] My Assistant

[ncc1701d]

What do NASA scientists use for the "origin of longitude" when calculating orbits for Spacecraft that orbit Venus?
I assume they use the Venus Equator for the "reference plane" but what else do they use to get the "Longitude of asscending node" ?

According to Wikopedia for a geocentric orbit around earth they use earths equator as the reference plane and the "First Point of Aries" as the "origin of longitude"

My interest is in orbits around Venus though and finding the the "Longitude of asscending node" so I need to know what they use for "origin of longitude".
thanks.

[Phil Stooke]

Hi - I don't do this kind of work myself so I can be corrected if anyone knows better... but mission planners and data users would want to know things like what part of the planet was under the orbit at any given time, so I think any references to the spacecraft's orbit inclination etc. would have to refer to the equator of the planet itself. For instance, a polar orbit designed to give global imaging coverage needs to be polar relative to that planet's pole, not Earth's.

And if you are trying to create a 'celestial sphere' for Venus or any other world, that has absolutely got to be tied to the planet's rotation, with the celestial pole directly above the rotation pole for that body.

Phil

[mcaplinger]

...mission planners and data users would want to know things like what part of the planet was under the orbit at any given time, so I think any references to the spacecraft's orbit inclination etc. would have to refer to the equator of the planet itself.

True, but one has to use orbital elements with some caution because, as Jason points out, they could be given in any defined reference frame. Most often orbital elements are stated in the "Mean Equator and IAU vector of J2000" frame (see, e.g., http://naif.jpl.nasa.gov/pub/naif/toolkit_...mes.html‎ ) but the nice thing about using NAIF/SPICE SPK files is that you can state the frame you want and the software makes any needed transformation to give it to you.

Often when orbital elements are supplied it's not stated what the frame is, and in those cases one would have to make assumptions -- one reason why I don't like using elements if I can avoid it.

"mean equator" has to be used because of precession and nutation; the spin axis of a planet is always moving, albeit slowly.