For the Earth civilization, as shown in "Astra" by the time of asteroid impact - FTL-capable spacecrafts, wormholes - the asteroid threat seems rather... illogical. I mean, they have wormholes, capable of transporting the bulk of human population & terraforming machinery 5000 light years away. How could they be unable to stop the asteroid?
They could easily use the wormholes to put fusion charges of any required size (Teller-Ulam scheme allowed to boost the charge basically endlessly - 5-gigaton SUNDIAL bomb was actually under consideration in 1950s) inside the asteroid and blast it apart. Or place the mass-drivers on it, and correct the trajectory to avoid Earth impact. Or just use the wormholes to transport part of asteroid mass away, thus changing the parameters of its orbit. After all, they have years to prepare.
With all respect to author, but threat, that may force the human population of Earth to relocate must be something far more threatening. Something like galaxy gamma-ray burst; the stream of gamma radiation from supernova, coming toward Solar System. SUCH threat could really be avoided only by relocating the population to somewhere else.