The alien service module is approaching
Check out "jet propulsion laboratory" via google. Asteroid is in an orbit closely matching earth, at a slight angle to the eliptic.
Think this over: a alien race with tech no mo than earth has uses fission-poweredsteam rockets to accelerate a "generation ship" to escape cataclism in their home system. Route given to Betty and Barney Hill was not for trade, but for exodus.
Every 100K years or so, the ship approaches a inhabited system. Pobulation is directed toward warfare and excessive consumption in order to force fission bombs and power systems. At the appointed time, a service ship arrives to pick up uraniun and plutoniun (remember the reports of godawfuf accounting of spent fuel rods?) and taki it to the mainship.
Now, for simple atmospheric ships, take a fresnel-styled microwave collector disc, power wirh a maser on a sattilite, add a few aerodynamic modifications and inclose the receiver horn in a dome. Instant flying saucer, burn circles, spontaneous human combustion...
Using "cast iron tech" and exhausting invaded worlds, it's possible. Hairyb