 Why can't Loy Chuk use time travel to send Ned back to Earth before his death?
His car hit the white-painted wooden rail sideways, crashed through, tumbled down a steep slope, struck a huge boulder, bounced up a little, and arced outward, falling as gracefully as a swan-diver toward the inky waters of the Pit, fifty feet beneath.... Time, the Brother of Death, and the Father of Change, seemed to wait.... 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. The final instrument he used to test the mummy, looked like a miniature stereoscope, with complicated details.
 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.