Astronomy - Osiris-REX is Bound for Asteroid Bennu

NASA's Osiris-REX is headed to asteroid Bennu for a dramatic sample return mission.

Osiris-REX launches from the Cape.
Hap Griffin

The dusk skies lit up over Cape Canaveral on September 8th, as NASA's ambitious Origins, Spectral Interpretation, Resource Identification, Security and Regolith Explorer (Osiris-REX) mission started its long round trip journey. Its mission: meet up and explore Earth-crossing Apollo asteroid 101955 Bennu and bring samples of it back to the Earth.

“Tonight is a night for celebration, we are on the way to an asteroid,” says chief scientist Ellen Stofan in a recentNASA press release. “We're going to be answering some of the most fundamental questions that NASA works on.”

Launched at 7:05 p.m. (EDT) atop a United Launch Alliance Atlas 5 liquid-fueled rocket configured with one single external solid rocket engine, Osiris-REX is the first U.S. interplanetary launch since the MAVEN mission to Mars in 2013. NASA's next mission to another world is Mars InSight in 2018. Engineers report that Osiris-REX is currently in good health post-launch.

Osiris-REX is the third mission in the New Frontiers program, after Juno and New Horizons. The objective is to return a sample of at least 60 grams of Bennu to Earth for further study, though researchers hope to collect as much as 2 kilograms. This sample will represent the largest sample return since the Apollo missions to the Moon. NASA plans to share the bounty on return, with 4% of the material going to the Canadian Space Agency (a partner on the mission) and 0.5% going to the Japanese Space Agency (in return for sharing material returned by Hayabusa). 75% of the sample return is allocated for future study.

“We have multiple lines of evidence to show that Bennu contains remnants of the formation of the solar system,” says project scientist Jason Dworkin. “The minerals, compounds, and isotopes in Bennu recorded the environment where the planets formed and should give us information about materials available for the origin of life on Earth. These will be studied with instruments in the best labs around the planet.”

Osiris-REX will also study the Yarkovsky effect, or how the absorption and emission of solar photons change the orbit and spin of an asteroid such as Bennu over time.

Osiris-REX on Earth. The sample return capsule is the white object on top.
NASA

Circling the Sun every 1.2 years, Bennu is a carbonaceous asteroid about 246 meters across (that's its mean radius; it's not perfectly circular). Light curve studies and other observations carried out by ground-based telescopes suggest it has a bulging shape and rotates every 4.3 hours. Bennu is also known as a potentially hazardous asteroid, due to its (very low: 0.037%) probability of impacting Earth late next century.

To reach Bennu, Osiris-REX will need to change its velocity by a relatively small 5 kilometers per second, one of the prime reasons the asteroid was selected for study. Bennu was named for a bird in Egyptian mythology after an online student competition in 2013. Osiris-REX also carries a microchip with 440,000 names submitted online.

Our best look at Bennu thus far, via radio data from Arecibo.
NASA/Arecibo

To Bennu and Back

The journey to Bennu and Osiris-REX's return to Earth will be a long one. Next up is an Earth flyby next year on September 22nd, when closest approach will bring the probe to 20,000 kilometers from Earth. We should see some great images of Earth and the Moon as the team uses the pass to calibrate instruments for the Bennu encounter.

Next year's pass and several orbital correction burns will set Osiris-REX up for an approach to Bennu starting in 2018. Researchers will then map and study the asteroid up close for two years before selecting a sample site and approaching Bennu. The sample attempt is currently scheduled for July 2020. With almost negligible gravity, touchdown on an asteroid is more like docking than landing. Osiris-REX will then depart around March 2021 with its precious cargo, and the sample return capsule is set to reenter over the Utah Test and Training Range on September 24, 2023.

Osiris-REX is equipped with a battery of instruments to complete the mission, including:

OCAMS: The Osiris-REX Camera Suite developed by the University of Arizona, consisting of three cameras: an 8-inch aperture telescope known as PolyCam, which will acquire Bennu from 2 million kilometers out and image it at high resolution, MapCam, which will search for out-gassing and aid with the selection of a sample site, and SamCam, which will witness and record the sampling maneuver.

OLA: The Osiris-REX Laser Altimeter built by the Canadian Space Agency will use Light Detection and Ranging (LIDAR) to generate a three-dimensional topographic map of the asteroid's surface.

OTES: The Osiris-REX Thermal Emission Spectrometer developed by Arizona State University. This instrument will examine surface mineral composition in the 5 to 50 micron infrared range.

OVIRS: The Osiris-REX Visible and Infrared Spectrometer provided by NASA's Goddard Spaceflight Center, this instrument will characterize mineral and organic material in the blue through near-infrared 0.4 to 4.3 micron range.

REXIS: The Regolith X-ray Imaging Spectrometer, this is an innovative student collaboration experiment led by MIT and Harvard University. This will complement OTES and OVIRIS, and will characterize elements via X-ray fluorescence.

TAGSAM: The Touch and Go Acquisition Mechanism, this sampler is on the articulated arm that will reach out and touch Bennu. TAGSAM will use a burst of nitrogen gas to force surface regolith into the collection chamber. TAGSAM has three nitrogen cylinders, for three separate sampling attempts. Once successful, TAGSAM will place the sample return in the Sample Return Capsule (SRC), the only part of the mission that will return to the Earth.

An artist's conception of OSIRIS-REx performing its touch and go sampling maneuver.
NASA

“We draw heavily on Stardust's sample return capsule and successful return to Utah for Osiris-REX,” Dworkin says. NASA's Stardust mission delivered samples from Comet Wild 2 in 2006.

Getting pristine samples back to Earth is tricky. Meteorites recovered on Earth are already contaminated once they strike our planet's atmosphere. And prior to Stardust's successful mission, the Genesis solar wind sampling mission failed to deploy its parachute and slammed into the Utah desert 12 years ago to the day prior to the launch of Osiris-REX.

Since Genesis, though, sample return missions have seen success not only with Stardust but also with JAXA's Hayabusa mission, which successfully returned 1,500 micron-sized grains of asteroid 25143 Itokawa. Speaking of which, JAXA's Hayabusa 2 mission will keep Osiris-REX company in the sample return game, as it seeks to bring back material collected from 162173 Ryugu in December 2020.

A timeline of asteroid exploration.
NASA

Congrats to the Osiris-REX team, as they begin the long and exciting journey to Bennu and back.