 Why can't Loy Chuk use time travel to send Ned back to Earth before his death?
A scream of brakes, the splash into icy waters, a long descent into alkaline depths ... 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.... Movement was slower now, as he began to sink, trapped inside his wrecked car. Time, the Brother of Death, and the Father of Change, seemed to wait.... A launching catapult for the last space ships built by the gods in exodus, perhaps it was—half a million years ago. 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. A strange accident, happening in a legendary antiquity, had aided them in their quest for knowledge. The final instrument he used to test the mummy, looked like a miniature stereoscope, with complicated details. Medical and biological sciences were far advanced among Loy Chuk's kind. It might move, speak, remember its past! With infinite care it was crated in a metal case, and hauled into the flying machine. Flashing flame, the latter arose, bearing the entire hundred members of the expedition.
 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.