I am given the classical orbital elements of the orbit of a spacecraft around a planet which is not the Earth, say Venus. I assume those are referred to a reference frame whose fundamental plane is the equator of the planet, but which is its primary direction?
I am hesitating between two options: either it is the intersection of the plane of the orbit with the ecliptic, or the intersection of the plane of the orbit with the J2000 Earth equator. While the ecliptic options seems more reasonable to me, I've been reading lots of documentation on the Internet but either they are all Earth-minded (i.e. Wikipedia) or there is some implicit convention I am missing.
The thing is that this is not important if for example I am just analyzing the motion of my spacecraft around my main attractor (Venus), but when I want to look at it from an interplanetary point of view, I think I need this information.