 Why can't Loy Chuk use time travel to send Ned back to Earth before his death?
Glacial ages, war, decadence, disease, and a final scattering of those ultimate superhumans to newer worlds in other solar systems, had done that. Loy Chuk had flown his geological expedition out from the far lowlands to the east, out from the city of Kar-Rah. With infinite care it was crated in a metal case, and hauled into the flying machine. The mummy was taken to Loy Chuk's laboratory, a short distance below the surface. Still submerged in liquid, the corpse was submitted to a flow of restorative energy, passing between complicated electrodes. Ned Vince did not know how Loy Chuk had probed his brain, with the aid of a pair of helmets, and the black box apparatus.
 No one has figured out time travel.
 The government of Kar-Rah turned down Loy Chuk's request to use time travel.  There is no such thing as time travel.  No one has figured out time travel.  Humans took the secrets of time travel with them when they left Earth.