  We investigated the impact of three different modifications of Newtonian
gravity on motions of Keplerian objects within the Solar System. These objects
are located at distances of the order of the distance to the Oort cloud. With
these three modifications we took into account a heliocentric Dark-Matter halo
as was indicated by Diemand et al, Modified Newtonian Dynamics (MOND) and a
vacuum-induced force due to a locally negative cosmological constant
$\Lambda_-$ derived by Fahr & Siewert. In gravitationally bound systems it
turns out that all three modifications deliver the same qualitative results:
Initially circular orbits for the pure Newtonian case are forced to convert
into ellipses with perihelion migrations. The quantitative consideration,
however, of the orbital parameters showed strong differences between MOND on
the one side, and Dark-Matter and $\Lambda_-$ effects on the other side.
