OSIRIS-REx Gets the Gravity Assist and Scores!

How exactly (and why) does a spacecraft get a "gravitational assist" from a planet en route to another planet? Where does that energy come from? The Astroquarks take a look at gravity assists and the OSIRIS-REx mission to grab some bits of a Near-Earth Asteroid and bring them back to Earth. Plus, the field of gravitational wave astronomy expanded now with the detection of a new black hole collision by two separate gravitational wave observatories. It's a weighty episode.