Janus (or S10 Janus) and Epimetheus are co-orbital satellites of
    Saturn, they exchange orbits every four years. The following
    properties are from K.R. Lang, 'Astrophysical Data: Planets and
    Stars', p.71:
 
                                      Janus       Epimetheus
   Distance from planet center:     151.4         151.5     km
                                      2.510         2.511   planet radii
                Orbital period:       0.694         0.695   days
                  Eccentricity:       0.009         0.007   degrees
                   Inclination:       0.3           0.1     degrees