Primary direction in planet-centered equatorial reference frame 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.
 A: From Seidelmann, P.K., Abalakin, V.K., Bursa, M., Davies, M.E., Bergh, C. de, Lieske, J.H., Oberst, J., Simon, J.L., Standish, E.M., Stooke, P., and Thomas, P.C. (2002). "Report of the IAU/IAG Working Group on Cartographic Coordinates and Rotational Elements of the Planets and Satellites: 2000," Celestial Mechanics and Dynamical Astronomy, v.82, Issue 1, pp. 83-111:
(Table I)
Recommended values for the north pole of rotation and the prime meridian at standard epoch (J2000 = JD2451545.0 barycentric coordinate time).
Venus: 
$ \alpha_0 = 272.16 $
$  \delta_0 = 67.16 $
$  W = 160.20 $ (at J2000, for other values, the rotation term given is $  -1.4813688d$ where $d$ is days since J2000)
The values of $\alpha_0$ and $\delta_0$ are given in standard equatorial coordinates. The prime meridian is at the center peak of Ariadne crater.
When more missions visit Venus, we may learn more about its rotation.
Making myself clear - this is one out of infinitely many possible inertial frames. Makes sense to use it though. I urge you to explore the options of SPICE toolkit since it's a neat collection of goodies and may be used to interface your simulation with real planetary and mission data from NAIF/PDS.
