def vx(self,*args,**kwargs):
        """
        NAME:

           vx

        PURPOSE:

           return x velocity at time t

        INPUT:

           t - (optional) time at which to get the velocity (can be Quantity)

           vo= (Object-wide default) physical scale for velocities to use to convert (can be Quantity)

           use_physical= use to override Object-wide default for using a physical scale for output

        OUTPUT:

           vx(t)

        HISTORY:

           2010-11-30 - Written - Bovy (NYU)

        """
        out= self._orb.vx(*args,**kwargs)
        if len(out) == 1: return out[0]
        else: return out
NAME:

           vx

        PURPOSE:

           return x velocity at time t

        INPUT:

           t - (optional) time at which to get the velocity (can be Quantity)

           vo= (Object-wide default) physical scale for velocities to use to convert (can be Quantity)

           use_physical= use to override Object-wide default for using a physical scale for output

        OUTPUT:

           vx(t)

        HISTORY:

           2010-11-30 - Written - Bovy (NYU)