NASA's OSIRIS-REx is an ongoing mission at asteroid 101955 Bennu, a carbon-rich rock hosting traces of the early solar system. The spacecraft arrived at Bennu on Dec. 3, 2018, and successfully went into orbit around the asteroid on Dec. 31, 2018.

In 2023, OSIRIS-REx's sample return capsule is scheduled to bring a piece of Bennu back to Earth. But first, the spacecraft will conduct an in-depth study of the rocky body and help scientists learn more about how sunlight can shift an asteroid's position.

Visiting an asteroid

The $800-million OSIRIS-REx (Origins-Spectral Interpretation-Resource Identification-Security-Regolith Explorer) mission was named a finalist for NASA's New Frontiers mission class in 2009, along with missions to study Venus (Surface and Atmosphere Geochemical Explorer, or SAGE) and the moon (MoonRise). In 2011, OSIRIS-REx was chosen as the winning proposal.

The New Frontiers program consists of a series of medium-class spacecraft missions intended to help us better understand the solar system. OSIRIS-REx is the third mission chosen for that program; the previous selectees were New Horizons, which flew by dwarf planet Pluto in 2015 and by an object called 2014 MU69 in 2019, and the Juno mission, which arrived in orbit around Jupiter in 2016.

OSIRIS-REx launched on Sept. 8, 2016. It briefly returned home in September 2017 to complete a speed-boosting flyby of Earth. The probe arrived at Bennu on Dec. 3, 2018. In the month after its arrival, OSIRIS-REx took detailed measurements of Bennu's shape and mass before precisely slipping into orbit.

OSIRIS-REx achieved its orbit on Dec. 31, 2018, just hours before New Horizons flew by 2014 MU69. The asteroid probe set two records: Bennu is the smallest body a spacecraft has ever orbited (the space rock has a diameter of 1,640 feet, or 500 meters), and OSIRIS-REx performed the closest orbit ever of a small body, at just 1 mile (1.6 kilometers) from the surface.

Part of the spacecraft's next job is to look for a safe place to land; mission managers will pick two semifinalist locations by July 2020, and OSIRIS-REx will touch down at one of those spots on the surface shortly after that.

OSIRIS-REx will follow in the footsteps of Japan's Hayabusa, which, in 2010, became the first spacecraft to bring a bit of an asteroid back to Earth. A successor Japanese sample-return mission, called Hayabusa2, is on its way to asteroid Ryugu (also known as 1999 JU3), with an arrival set for 2020. OSIRIS-REx, however, is the first U.S.-led asteroid sample-return mission.

An artist’s illustration of NASA’s OSIRIS-REx spacecraft approaching the asteroid Bennu. (Image credit: NASA/Goddard/University of Arizona)

Sampling an asteroid

Asteroids are the bits and pieces left over from the formation of the planets, and as such, they serve as rudimentary blueprints of the early solar system. Studying them allows scientists to see what the early solar system was like and to get a handle on how planets formed.

NASA's Dawn mission visited the asteroid Vesta before moving on to the dwarf planet Ceres. In contrast to the Dawn mission, OSIRIS-REx will bring home a piece of Bennu for in-depth laboratory studies that aren't possible from orbit. The OSIRIS-REx team hopes to collect at least 2 ounces (60 grams) of material to achieve their scientific goals.

Understanding how asteroids form will not only provide insight about the early solar system, but also help scientists better understand the space rocks' makeup today. That could be helpful should any such object threaten to collide with Earth. The research could also assist in future efforts to mine asteroids.

"The mission will develop important technologies for space exploration that will benefit anyone interested in exploring or mining asteroids, whether it's NASA or a private company," Dante Lauretta, the principal investigator for OSIRIS-REx, said in a statement. (Lauretta was previously the mission's co-principal investigator, along with fellow planetary scientist Michael Drake, who died in 2011 at age 65.)

OSIRIS-REx will also help astronomers study the Yarkovsky effect on asteroids. This phenomenon occurs when heat from the sun provides a tiny push that alters the movement of an asteroid or other object. Although the push is minimal, it can build up over time, helping to change the path a space rock travels. But it can be challenging to study this effect, as it varies based on the shape of each asteroid.

"The knowledge from the mission will help us to develop methods to better track the orbits of asteroids," Jim Green, director of NASA's Planetary Science Division, said in a statement.

Now that OSIRIS-REx has reached Bennu, five instruments onboard the spacecraft are working to study, map and analyze the asteroid in unprecedented detail:

OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) — By measuring visible and near-infrared light, OVIRS will hunt for organics and other minerals.

OSIRIS-REx Thermal Emission Spectrometer (OTES) — Using the thermal infrared, OTES will take Bennu's temperature and map mineral and chemical abundances. Together, OVIRS and OTES will map Bennu over a wave of interesting wavelengths to help select the best site to sample the asteroid.

OSIRIS-REx Camera Suite (OCAMS) — A three-camera suite will help map Bennu. PolyCam, the largest, will acquire the first images of Bennu, taking them from 1.2 million miles (2 million kilometers) out, and capture high-resolution images of the sample site. MapCam will hunt for satellites and dust plumes around the asteroid, map it in color, and take the photographs to create topographic maps. SamCam will document the collection of the sample and its capture.

OSIRIS-REx Laser Altimeter (OLA) — By scanning the entire surface of Bennu, OLA will send back data to create highly accurate 3D models of the asteroid surface.

Regolith X-ray Imaging Spectrometer (RExIS) — Studying X-ray emissions of Bennu will help in the creation of a map showing the abundance of different elements on the surface. Unlike other imaging instruments, RExIS will examine the composition of the asteroid at the level of individual atomic elements.

Mission timeline

Since its launch, the spacecraft has made two deep-space maneuvers. The first took place on Dec. 28, 2016, and set the spacecraft up for its gravity assist with Earth. Almost a year later, OSIRIS-REx made a flyby of the planet, on Sept. 22, 2017, to pick up speed for its journey to Bennu.

At its closest approach, over Antarctica, the spacecraft was roughly 10,700 miles (17,200 km) from the surface. The flyby increased OSIRIS-REx's speed by about 8,500 mph (13,000 km/h) and was executed flawlessly. The spacecraft then continued on its journey to Bennu.

Along the way, the OSIRIS-REx snapped some stunning images of Earth and its moon from a distance of a little more than 3 million miles (5 million km).

On June 28, 2018, the spacecraft aced its second and final major deep-space maneuver before making a series of successful asteroid-approach maneuvers to put it close to Bennu.

Once the asteroid has been studied in significant detail, scientists will identify a region to sample. That's where the Touch-And-Go Sample Acquisition Mechanism (TAGSAM) will come into play. As OSIRIS-REx draws closer to the asteroid, TAGSAM will blast a sample of pure nitrogen gas at the rock's surface. Regolith (dust and broken rock) blasted from the surface will be pushed into the chamber of the sampler. To complete all of the planned science, OSIRIS-REx needs to collect at least 2 ounces (60 g) of asteroid material, but the goal is to collect 5 ounces (150 g) to account for any errors in measurement. The spacecraft will carry three bottles of nitrogen gas to allow for multiple collection attempts, and TAGSAM can carry as much as 70 ounces (2,000 g). To weigh the sample in the low-gravity environment, scientists will measure the angular acceleration of the spacecraft before and after the sample is collected.

Although OSIRIS-REx will sample Bennu, the spacecraft won't fully land on the space rock's surface. Contact during the collection process will be brief, lasting only a few seconds. "We kiss the surface," Drake told reporters in 2011.

OSIRIS-REx's brief contact with the surface of Bennu is scheduled for July 2020.

The spacecraft will depart Bennu in March 2021 for the journey back to Earth. OSIRIS-REx will arrive back home in September 2023 and will drop a return capsule, which will parachute into the Utah desert. The spacecraft will then move into a stable orbit around the sun.

The near-Earth asteroid Bennu, as seen by NASA's OSIRIS-REx probe on Dec. 2, 2018. (Image credit: NASA/Goddard/University of Arizona)

Meet Bennu

Originally known as 1999 RQ36, the rock OSIRIS-Rex will visit was renamed Bennu by 9-year-old Mike Puzio in a 2013 contest. Bennu is an Egyptian god usually depicted as a gray heron. Puzio chose the name because he thought TAGSAM and the solar panels resembled the bird god's neck and wings.

Bennu is one of several thousand near-Earth objects orbiting within 120 million miles (190 million km) of Earth. Among those, Bennu was one of less than 200 objects whose orbit was well-known and sufficiently similar to Earth's orbit. This asteroid orbits the sun every 436 days and comes very close to Earth every six years.

Bennu's also relatively large, at about 1,650 feet (500 m) wide. Rocks smaller than 650 feet (200 m) spin too rapidly for a spacecraft to safely land on them. Only a handful of the asteroids in appropriate orbits were large enough to allow a visit.

Among those deemed appropriate, Bennu has a more-attractive composition. The asteroid is relatively rich in carbon-based material; the asteroids that may have brought water and organic material to Earth and helped kick off life would have looked like Bennu.

"We're going for something rich in organics, which might have had something to do with life getting started," Drake said.

Bennu's close orbit also makes it a potentially hazardous object for Earth, but contrary to what some media reports have said, this space rock isn't a definite threat. NASA scientists calculated that there is a 0.037 percent (or 1-in-2,700) chance that it will strike Earth in the last quarter of the 22nd century. For that to happen, the asteroid would need to have its orbit altered in its 2,135th flyby of Earth.

If the asteroid does collide with Earth, the impact won't destroy the planet, though it will wreak havoc wherever it touches down. Experts say that an asteroid of this size would likely devastate the local area around the impact, but it wouldn't cause mass extinctions.

Still, the advanced warning provided by OSIRIS-REx could help scientists take preventative measures long before an impact.

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This article was updated on Jan. 25, 2019 by Space.com contributor Elizabeth Howell.