Astronomers discover an Earth-like explained orbiting a star 20 light years away. (a) How fast should a rocket ship go if the round trip is to take no more than 40 years in time for the astronauts aboard? (Suppose a constant speed for the rocket whenever it’s in motion, i.e. we are ignoring deceleration and acceleration phases--which may be a pretty bad approximation) (b) How much time will the trip take as gauged on Earth?