It is believed that the aliens transit through interstellar space through the use of a wormhole FTL drive system. This is largely speculative, based on interrogation of alien leaders and engineers. The vast majority of UFOs in the Sol System seem to travel using conventional gravity drives. Outside of an atmosphere, in the vacuum of space, it is possible to reach near-relativistic speeds using alien gravity drives. X-COM probes launched using micro-Elerium reactors have reached transit speeds of .6c without difficulty. It seems possible that a trip from Earth to Mars, using for example an Avenger Fighter/Transport, could take as little as 30 minutes to an hour, depending on the relative orbital positions of the two planets.