Emily LakdawallaSep 19, 2017

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.

Ground track for OSIRIS-REx Earth Gravity Assist (EGA)
Ground track for OSIRIS-REx Earth Gravity Assist (EGA) Image: NASA / GSFC / UA

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. Assuming everything goes well with the spacecraft through and after the flyby, you can expect to see the first photos on Tuesday, September 26; watch for NASA press releases and also check asteroidmission.org.

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 travels within geosynchronus radius
Geosynchronous satellites orbit at Earth's equator at an altitude of 42,164 kilometers. OSIRIS-REx will be far to the south of the equator when it passes this distance, so there is no impact hazard.
22 Sep 08:3222 Sep 11:3222 Sep 15:3222 Sep 17:32
Loss of contact
Because its closest approach is so far south, it will sink below the horizon as seen from the Canberra deep space communications station. It will be out of contact for about an hour.
22 Sep 09:4522 Sep 12:4522 Sep 16:4522 Sep 18:45
Closest approach
Closest approach happens at an altitude of 17,237 kilometers and relative speed of 8.5195 kilometers per second over 74.73°S, 88.06°W, in Antarctica just south of Cape Horn.
22 Sep 09:5122 Sep 12:5122 Sep 16:5122 Sep 18:51
Regain contact
OSIRIS-REx rises above the horizon at the Goldstone station; communications should be regained shortly after.
22 Sep 10:4022 Sep 13:4022 Sep 17:4022 Sep 19:40
OSIRIS-REx travels above geosynchronous orbit
Again, to the south of the equator, so there is no impact hazard.
22 Sep 11:1122 Sep 14:1122 Sep 18:1122 Sep 20:11
Instruments turn on
With the flyby over, the spacecraft can test its instruments on the receding Earth and Moon.
22 Sep 11:5222 Sep 14:5222 Sep 18:5222 Sep 20:52
Observation window 1 begins (EGA+1)
During this period, OSIRIS-REx will travel from 100,000 to 220,000 kilometers from Earth. In 5 hours of science, PolyCam will look at the Pacific Ocean northwest of Hawai'I; SamCam will take panchromatic images of Earth; MapCam will take color images of Earth; and NavCam1 will take mosaics and images of Earth and the Moon.
22 Sep 13:5222 Sep 16:5222 Sep 20:5222 Sep 22:52
Observation window 1 ends22 Sep 18:5222 Sep 21:5223 Sep 01:5223 Sep 03:52
Lunar closest approach
Altitude 265,457 km (about two-thirds the average Earth-moon distance) (time approximate, about 10 hours after Earth closest approach)
22 Sep 19:5122 Sep 22:5123 Sep 02:5123 Sep 04:51
Observation window 2 begins (EGA+3)
Earth is about 1.2 million kilometers away. Both NavCams will picture Earth and the Moon. SamCam, MapCam, and PolyCam will all take mosaics on the Moon. OVIRS and OTES then point at Earth.
24 Sep 17:0024 Sep 20:0025 Sep 00:0025 Sep 02:00
Observation window 2 ends24 Sep 22:0025 Sep 01:0025 Sep 05:0025 Sep 07:00
Observation window 3 begins (EGA+6)
Earth is about 2.8 million kilometers away. MapCam and PolyCam will shoot mosaics of Earth. Both NavCams will shoot mosaics of the Moon and Earth. No SamCam.
27 Sep 17:0027 Sep 20:0028 Sep 00:0028 Sep 02:00
Observation window 3 ends27 Sep 22:0028 Sep 01:0028 Sep 05:0028 Sep 07:00
Observation window 4 begins (EGA+10)
Earth is about 5 million kilometers away. These are primarily spectroscopy observations with OTES and OVIRS. Cameras will take context images, and dark frames for calibration purposes.
1 Oct 17:001 Oct 20:002 Oct 00:002 Oct 02:00
Observation window 4 ends1 Oct 20:001 Oct 23:002 Oct 03:002 Oct 05:00

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.
OSIRIS-REx instruments
OSIRIS-REx instruments Image: NASA / GSFC / UA

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!

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