In the best case scenario for a gravity boost, a spacecraft approaches a planet very
closely and does a U-turn around it, heading back exactly in the direction it came
(see the image below).
If the spacecraft was initially going to the right with a speed of 1 km/s, and the
planet was going to the left at a speed of 12 km/s, what would be the fastest the
spacecraft could be going after the U-turn, in km/s?