Today, September 22nd, 2017, the OSIRIS-REx mission passed by the planet Earth, just a little over a year since its launch, for a gravity assisted trajectory change to send it on its way to its target, the asteroid Bennu.
Gravity assists are used often in missions as a method of fuel conservation. Traveling around the solar system bounding from planet to planet certainly takes longer, but can allow for more massive, more capable payloads than a more direct route could have launched. The Voyagers used gravity assists to get to their next mission goals, Cassini used assists to make it to Saturn, as have other previous and as will many future missions.
In the case of OSIRIS-REx, this assist is being used to make an orbital plane adjustment – quite a fuel-demanding process – to get the probe into the proper orbital path to meet up with Bennu in 2018.
It’s an interesting enough little process worth noting. If you want to read some details, check out the link below.
http://www.asteroidmission.org/?latest-news=nasas-asteroid-bound-spacecraft-slingshot-past-earth