The aliens are machine based artificial intelligence. The mother ship will land on Mars rather than Earth. From there natural Martian resources will be mined to expand their machine colony. Machines, would prefer artic Mars over tropical Earth. Additionally, it is much easier to defend Mars from attack by Earthlings as any approaching Earthling missiles can easily be detected and destroyed.

The aliens ships can travel at superluminal speeds, but cannot arrive in a system at these speeds as the amount of energy released as gravitational waves would cause significant destruction to anything within 100 AU and cause LIGO detection within 1000 AU. Therefore, the alien ships prefer to hide behind interstellar comets (actually stellar comets ejected from the Oort cloud by their superluminal arrival.)

The scout ship arrived via Oumuamua in 2017. Oumuamua actually accelerated out of the system, which was unexpected. This was caused by the scout ship accelerating to superluminal speeds.

Unlike man-made spaceships which travel via orbital trajectories, superluminal ships can travel in straight lines to rendezvous with other ships or systems.