Osiris-REX's First Views of Asteroid Bennu

NASA's Osiris-REX asteroid sample return mission spies target Bennu for the first time. Now the spacecraft is setting up for its close approach in December.

It may look like a dot moving against a starry background right now, but 101955 Bennu will swell into a brave new world begging for exploration in the coming months.

Osiris-REX (the Origins, Spectral Interpretation, Resource Identification, Security, Regolith Explorer) officially entered its close approach phase on August 24th, reaching just 1.2 million miles (2 million kilometers, just under five times the Earth-Moon distance) from asteroid 101955 Bennu. From Osiris-REX's vantage point, Bennu is now an unresolved, +11th magnitude "star," moving across the constellation Serpens in the background.

“Now that Osiris-REX is close enough to observe Bennu, the mission team will spend the next few weeks learning as much as possible about Bennu's size, shape, surface features, and surroundings before the spacecraft arrives at the asteroid,” says Dante Lauretta (University of Arizona) in a recent press release. “After spending so long planning for this moment, I can't wait to see what Bennu reveals to us.”

Launched on September 8, 2016, from Cape Canaveral Air Force Station in Florida atop an Atlas V rocket, OSIRIS-REx will spend 505 days at the asteroid, studying its composition and mapping its surface. The highlight of the mission plan is a 60-gram to 2-kilogram (0.1- to 4.4-pound) sample return to Earth, which would mark a first for American missions. The sample return, coupled with the in situ study of Bennu, is expected to shed light on the formation and evolution of the early solar system, as well as the possible mechanism that delivered the building blocks of life to early Earth.

But getting to Bennu wasn't easy. The asteroid orbits the Sun once every 1.2 years with an orbit inclined six degrees relative to the ecliptic. This required Osiris-REX to fly by Earth on September 22, 2017, to slingshot the spacecraft into a path that would intercept the asteroid. Osiris-REX made an additional scheduled course correction maneuver this past July.

Now that Bennu is visible via the spacecraft's PolyCam camera, science operations can begin.  Osiris-REX will start scanning the region around Bennu looking for dust or possible moonlets. On October 1st, Osiris-REX will begin a series of four maneuvers to fine-tune its approach, as it begins to station keep with the asteroid.

Welcome to Bennu

About half a kilometer in diameter, Bennu was discovered by the LINEAR survey project on the night of September 11, 1999.  Although the rock is on the list of potentially hazardous Earth-crossing asteroids, though its closest approach for this century is about 465,000 miles (748,000 kilometers) on September 23, 2060.

Third-grader Michael Puzio picked the name Bennu, after the Egyptian mythological bird, that won The Planetary Society's “Name That Asteroid!” competition.

Modeling Bennu's structure and composition may help predict how it will interact with solar wind pressure via the Yarkovsky effect. Modeling will also help understand how the asteroid's orbit will change over the coming centuries. Osiris-REX is armed with a battery of science instruments, including the REXIS X-ray spectrometer, the OLA laser altimeter, the OVIRIS visible and infrared spectrometer, and the OTES thermal spectrometer. The OCAMS camera suite contains PolyCam, an 8-inch aperture telescope for capturing high-resolution images, MapCam looking for out-gassing and possible satellites, and SamCam for imaging during sample acquisition. Sample collection from the asteroid's surface is expected to occur in July 2020 after a mapping and site selection phase. The Sample Return Capsule is projected to return to Earth over the Utah desert in September 2023.

“Bennu's low gravity provides a unique challenge for the mission,” says Rich Burns (NASA Goddard Space Flight Center) in a recent press release. Bennu will be the smallest object explored by a planetary mission to date.

A similar drama is now unfolding as the Hayabusa 2 team explores asteroid 162173 Ryugu.

The tale of two asteroids is far from over, as exploration of Bennu and Ryugu gets underway.