Answer
An almost identical problem is done on page 101.
Let v represent Superhero's velocity after the throw, let M represent the mass of Superhero, and let 1000M be the mass of the asteroid.
Set the momentum before the throw, which is zero, equal to the momentum after.
$$0 = (1000M)(800 m/s) + (M) v$$
$$v = -800,000 m/s$$
We conclude that Superhero should recoil at a speed of 800,000 meters per second. His final momentum is equal and opposite to that of the asteroid, because the system's initial and final momentum were both zero.
Of course, superhero comics have never been known for their faithfulness to the laws of physics, chemistry, or biology.
