Since arriving at the asteroid Bennu in December, NASA’s OSIRIS-REx mission has been mapping its surface to determine the best spot to collect samples in 2020. This will be the first NASA mission to return samples collected from an asteroid, and the samples are expected back on Earth in 2023.
But Bennu, which is 70 million miles from Earth, has been full of surprises. The asteroid is much more rugged than expected, so four potential sites where the spacecraft can “tag” Bennu have been chosen with care. The mission scientists have considered the safety and accessibility of each site, according to NASA.
Bennu is covered in boulders rather than the large areas of fine-grain material that scientists expected. In order for the craft to bump the asteroid and send up material for the sampler to collect it, the material must be less than an inch in diameter.
“We knew that Bennu would surprise us, so we came prepared for whatever we might find,” said mission principal investigator Dante Lauretta of the University of Arizona, Tucson. “As with any mission of exploration, dealing with the unknown requires flexibility, resources and ingenuity. The OSIRIS-REx team has demonstrated these essential traits for overcoming the unexpected throughout the Bennu encounter.”
Initially, the sample site was going to have a radius of 82 feet, but there’s no space on Bennu of that size without boulders. Now, the scientists are targeting sites between 16 and 33 feet in radius.
“Although OSIRIS-REx was designed to collect a sample from an asteroid with a beach-like area, the extraordinary in-flight performance to date demonstrates that we will be able to meet the challenge that the rugged surface of Bennu presents,” said project manager Rich Burns of NASA’s Goddard Space Flight Center. “That extraordinary performance encompasses not only the spacecraft and instruments, but also the team who continues to meet every challenge that Bennu throws at us.”
The four sites are named after birds found in Egypt: Nightingale, Kingfisher, Osprey and Sandpiper. The asteroid, its surface features and the spacecraft itself have been named after Egyptian deities and mythological birds, according to NASA.
All four sites are diverse in their location and features.
Nightingale is the most northern spot, with multiple possibilities for sampling in a small crater contained within a larger crater that contained dark, fine-grain material. Kingfisher is also in a small crater near the equator, free of large rocks and showing a signature of including hydrated minerals. Osprey is also on the equator in a small crater, showing signs of carbon-rich material and diverse rocks, and has several areas that could be sampled. Sandpiper is to the south and includes hydrated minerals on the flat wall of a larger crater.
Now, the spacecraft will further analyze the four sites using its five scientific instruments over the next four months. The scientists still need to determine how much material is at each site that could actually be collected for a sample. The final two potential sites, a primary site and a backup, will be selected in December, according to NASA.
Time to understand and deal with any challenges was built into the mission, including more than 300 days of extra time.
The mission
The OSIRIS-REx mission and the asteroid Bennu got to meet face to face on December 3. OSIRIS-REx has been orbiting the asteroid since December 31.
Bennu is the smallest body to ever be orbited by a spacecraft, just a little bit wider than the height of the Empire State Building, according to NASA. Bennu has a shape comparable to that of a spinning top, and it’s a “rubble pile” asteroid, a grouping of rocks held together by gravity rather than a single object.
The mission — which stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer — launched in September 2016 and will spend two years up close and personal with Bennu.
The sample from Bennu, a near-Earth asteroid, could help scientists understand not only more about asteroids that could impact Earth but about how planets formed and life began.
The asteroid could pass close to Earth, closer than the moon, in 2135, with even closer approaches possible in 2175 and 2195. A direct hit is unlikely, but the data gathered during this mission can help determine the best ways to deflect near-Earth asteroids.
The greatest surprise of the mission happened only a few days in. An unexpected observation occurred, signaling activity on Bennu, the researchers said.
The mission science team detected particle plumes ejecting from the surface January 6, followed by additional plumes over the past two months. That makes Bennu an active asteroid that is regularly ejecting material into space, which is rare. This is the first time scientists have had close-up observations of particle plumes erupting from an asteroid’s surface.
A suite of seven studies published this year in the journals Nature, Nature Astronomy, Nature Geoscience and Nature Communications detail the spacecraft’s observations.
The particles range from centimeters to tens of centimeters, and some of them are slow-moving while others are quickly making their way through interstellar space. Some fall back onto the surface, while others go into orbit around it, like small satellites. The researchers don’t understand why this is happening, but it poses no risk to the spacecraft.
“The discovery of plumes is one of the biggest surprises of my scientific career,” Lauretta said.
OSIRIS-REx’s instruments have confirmed that hydrated minerals, including magnetite, are abundant and widespread on the asteroid. The asteroid is full of valuable materials that may even contain clues about how life began. Bennu is essentially a leftover from the formation of our solar system billions of years ago, although some of the minerals inside it could be even older.
Asteroids could even serve as fuel stations for robotic and human missions if we can unlock the hydrogen and oxygen inside them, NASA said.
Bennu is also older than expected, between 100 million and 1 billion years old, and probably originated in the main asteroid belt. Bennu probably broke off of a larger asteroid in the belt between Mars and Jupiter a couple billion years ago. This knocked it through space until an orbit close to Earth locked it in place.
The spacecraft also detected numerous larger boulders on the surface, with more than 200 boulders that are over 32 feet in diameter. The asteroid’s surface is like a time capsule, with different regions representing different eras, from the remains of Bennu’s parent asteroid to recent activity.
