OSIRIS-REx Earth flyby: What to Expect

OSIRIS-REx launched on September 8, 2016. Now, a year later, it's returning to its home to get a second boost on to its destination, the asteroid Bennu. The main purpose of the flyby is to change the inclination of OSIRIS-REx's orbit to match Bennu's. To tilt the orbit, the spacecraft has to pass by one of Earth's poles. So the spacecraft will approach Earth most closely over Antarctica, south of Cape Horn. Close approach altitude is 17,237 kilometers (targeting 23,595.5 kilometers above Earth's center). Due to the far southern location, OSIRIS-REx will be out of contact with all Deep Space Network stations for about an hour around closest approach.

NASA / GSFC / UA

Ground track for OSIRIS-REx Earth Gravity Assist (EGA)

On board OSIRIS-REx as it flies by Earth are 440,000 names of people who wanted to symbolically join the mission. In partnership with the mission, The Planetary Society collected the names as part of its Messages from Earth program. The collection includes the names of everyone who was a Planetary Society member at the time. One copy of those names is stored aboard the spacecraft's sample return capsule, and will plunge back to Earth in 2023. Another copy, aboard the main spacecraft, will remain in space after the sample return capsule is dropped off. The Planetary Society also helped give the target asteroid for OSIRIS-REx, Bennu, its name by running a contest won by Michael Puzio, who was 9 years old at the time.

No rocket firings are planned for the flyby; the spacecraft will coast through it. Out of an abundance of caution, engineers turned off all of its instruments about a month ago. Assuming the flyby goes according to plan, the instruments will power up about 2 hours after the flyby. Beginning about 4 hours after closest approach, OSIRIS-REx will begin testing cameras and spectrometers on Earth and the Moon. All the cameras as well as two spectrometers will see use in four science observation windows spaced across the 10 days after the flyby; only the laser altimeter and X-ray spectrometer will not get used. In total, OSIRIS-REx will acquire nearly 1000 images.

Data transmission will not happen instantaneously. The data will arrive on the ground 24 to 48 hours after the close of each of the four science observation windows. There is not yet an automated pipeline for sending OSIRIS-REx data to the Web, so it's not yet clear when or how many of the images will be posted. (I have asked and will update this post when I get a response. I probably would've had the information in time to post, had I not been so buried in the end of the Cassini mission last week!)

Following is a predicted timeline of the coming OSIRIS-REx events. Because the spacecraft is close to Earth, the difference between spacecraft event time and Earth received time is never more than a few seconds throughout the flyby period, so I am only reporting Earth received time here.

OSIRIS-REx instruments being used during the encounter:

OSIRIS-REx Camera Suite (OCAMS): Three cameras, PolyCam, MapCam, and SamCam. PolyCam is for high-resolution imaging, both long-range (providing the first resolved views of Bennu) and close-up (for sample site reconnaissance). MapCam is a color (four-filter) medium-resolution imager for reconnaissance of the surface of the whole asteroid and searching for plumes and satellites. The wide-angle SamCam is for recording the sample collection process. Learn more (PDF)
OSIRIS-REx Visible and Infrared Spectrometer (OVIRS): an imaging spectrometer sensitive to light with wavelengths from 0.4 to 4.3 microns, for compositional mapping. OVIRS is based on New Horizons LEISA. Learn more (PDF)
OSIRIS-REx Thermal Emission Spectrometer (OTES): will map the surface in wavelengths from 5 to 50 microns to map surface mineralogy and study asteroid surface temperatures. It is based on heritage from TES instruments built for Mars Global Surveyor and the Mars Exploration Rovers. Learn more (PDF) and see a photo album.

NASA / GSFC / UA

OSIRIS-REx instruments

To keep track of the events of the flyby on September 22, follow OSIRIS-REx and Jason Davis on Twitter. (I will be traveling and may not be able to live tweet.) I'll be back next week with images from the flyby!