At closest approach, OSIRIS-REx will be only 10,711 miles above our planet’s surface, well inside the orbit of geosynchronous satellites. This map shows the circumstances of the encounter:

During the brief hours before and after closest approach, the spacecraft could shine with as much reflected sunlight as a 9th to 11th magnitude star–well within reach of backyard telescopes under the ground track. Observing tips are available from NASA.

NASA is visiting Bennu … before it visits us.  Bennu crosses Earth’s orbit every six years. In 2135, the asteroid may enter a “keyhole” between the Earth and the Moon where the gravitational pull of our own planet will tweak Bennu’s orbit, potentially putting it on course for Earth later that century. Currently, the odds of a collision 150+ years from now are no more than about 1 in 2700–small, but enough to prompt an $800 million space mission.

Arriving in late 2018, OSIRIS-REx will spend more than a year flying in close proximity to Bennu using five instruments to survey the asteroid. The resulting maps will pinpoint a safe sampling site, where the probe can each out with a mechanical arm and gather material from the asteroid’s surface. If all goes as planned, samples will be returned to Earth in the Fall of 2023, when a capsule containing bits of Bennu will land at the Utah Test and Training Range.

Source: Spaceweather.com