  We use planar coordinates as well as hyperbolic coordinates to separate the
de Sitter spacetime into two parts. These two ways of cutting the de Sitter
give rise to two different spatial infinities. For spacetimes which are
asymptotic to either half of the de Sitter spacetime, we are able to provide
definitions of the total energy, the total linear momentum, the total angular
momentum, respectively. And we prove two positive mass theorems, corresponding
to these two sorts of spatial infinities, for spacelike hypersurfaces whose
mean curvatures are bounded by certain constant from above.
