  In this paper, we study the communication problem from rovers on Mars'
surface to Mars-orbiting satellites. We first justify that, to a good extent,
the rover-to-orbiter communication problem can be modelled as communication
over a $2 \times 2$ X-channel with the network topology varying over time. For
such a fading X-channel where transmitters are only aware of the time-varying
topology but not the time-varying channel state (i.e., no CSIT), we propose
coding strategies that code across topologies, and develop upper bounds on the
sum degrees-of-freedom (DoF) that is shown to be tight under certain pattern of
the topology variation. Furthermore we demonstrate that the proposed scheme
approximately achieves the ergodic sum-capacity of the network. Using the
proposed coding scheme, we numerically evaluate the ergodic rate gain over a
time-division-multiple-access (TDMA) scheme for Rayleigh and Rice fading
channels. We also numerically demonstrate that with practical orbital
parameters, a 9.6% DoF gain, as well as more than 11.6% throughput gain can be
achieved for a rover-to-orbiter communication network.
