bye bye bennu

Asteroid Bennu close encounter could help NASA predict a future apocalypse

The craft will leave Bennu in May, but on Wednesday it had one last close encounter with Bennu.

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After nearly two-and-a-half years together, NASA’s OSIRIS-REx spacecraft has said goodbye to asteroid Bennu and begun its journey back to Earth.

The spacecraft packs precious cargo for Earth, anticipated to return in September 2023: a piece of the asteroid that could reveal the origin story of the Solar System and life on Earth itself.

On Wednesday, OSIRIS-REx conducted one last flyover of Bennu before it departed from the asteroid’s side. In that last encounter, the spacecraft imaged Bennu for 5.9 hours, a little over a full rotation period of the asteroid, at a distance of 2.1 miles (3.5 kilometers). This is the closest the spacecraft has been to Bennu since the sample collection event.

Amy Simon is a planetary scientist at NASA's Goddard Space Flight Center. She explains to Inverse that the main purpose of the final flyover was to look at the sample collection site, one last time.

“When we did the sample collection and backed away, there was so much debris and dust that we couldn't actually see the surface,” Simon tells Inverse.

The main point is to be able to see what’s changed, if we left a crater, or if there's any new composition that's been kind of uncovered.”

OSIRIS-REx gathered 4 gigabytes of data during its final flyover. But scientists on Earth have to wait until the spacecraft is able to send all these data to Earth, which won’t happen until April 13 at the earliest.

The spacecraft communicates to ground control on Earth through the Deep Space Network, an array that NASA uses to communicate with a host of space missions, including the Perseverance rover on Mars. OSIRIS-REx gets between four and six hours of downlink time per day and transmits back at 412 kilobits per second — about 186 megabytes per hour. As a result, it will take several days to get the new data from OSIRIS-REx into our Earthling hands.

Here’s the background — OSIRIS-REx has orbited asteroid Bennu since December 2018. Its mission was to scoop up a sample of this ancient body and bring it back to Earth for analysis. On October 20, 2020, the craft successfully touched down on the asteroid and snagged a sample from the “rubble pile” space rock using the Touch-and-Go Sample Acquisition Mechanism robotic arm — which operated a bit like a vacuum for the loose materials on Bennu.

An image sequence that shows OSIRIS-REx stowing the samples, and closing two internal latches to keep them secure.

NASA/Goddard/University of Arizona/Lockheed Martin

What have we learned about the asteroid Bennu?

Prior to its return with the samples in stow, OSIRIS-REx has already fed scientists with plenty of new insights on the asteroid — and any other asteroids heading for a close encounter with Earth.

“We’ve actually learned quite a lot over the course of the mission,” Simon says. “We had a couple of main goals besides obviously the sample collection.”

One of the other goals of the mission was to determine how much the orbit of the asteroid changes over time as a result of its interactions with the Sun and to learn about the asteroid’s composition.

When OSIRIS-REx got to Bennu, the asteroid had actually moved 185 kilometers from where the mission’s ground team thought it was going to be, Simon admits.

“Its orbit is changing over time, which is really interesting and that sort of information is important for understanding how likely these near-Earth objects are to impact the Earth,” Simon says.

Based on the craft’s observations, the team also determined that the spin of the asteroid is gradually accelerating — at a rate of one second every 100 years. Although this may seem rather inconsequential, it indicates that the asteroid is just a “rubber pile” made up of bits and pieces of dust and rock held together by gravity rather than a solid mass.

Images captured by OSIRIS-REx also constitute the first evidence of thermal fracturing on an object without an atmosphere. Scientists can use measurements of thermal fracturing to determine the asteroid’s age.

Thermal fracturing takes place on Earth. On our planet, rocks baked in the Sun all day fracture at night as they cool. As the rocks expand and contract, the buildup of stress causes them to crack and eventually disintegrate into smaller pieces.

Much like the rocks on Earth, the rocks on Bennu also experience differences in day and night temperatures. Daytime highs on Bennu can reach almost 127 degrees Celsius (260 degrees Fahrenheit), and nighttime lows plummet to about minus 73 degrees Celsius (minus 99 degrees Fahrenheit)

Scientists believed that thermal fracturing could take place on asteroids like Bennu — but they never had any evidence to show for it until OSIRIS-REx.

When will OSIRIS-REx leave Bennu?

OSIRIS-REx will depart from Bennu’s orbit on May 10 and set off on a two-year journey back to Earth. Bennu has an orbit that intersects with Earth’s own orbital path, so a return trip is less fuel-intensive than it might be from other asteroids within our reach.

“Leaving Bennu’s vicinity in May puts us in the ‘sweet spot,’ when the departure maneuver will consume the least amount of the spacecraft’s onboard fuel,” Michael Moreau, OSIRIS-REx deputy project manager at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, said in a statement.

The spacecraft will remain in loose orbit until it leaves Bennu’s side, awaiting its scheduled departure.

The OSIRIS-REx mission goals

OSIRIS-REx launched from Earth on September 8, 2016, carrying six instruments designed to map the asteroid, document the sample site, and make other observations of Bennu.

The mission marked NASA's first attempt to land on an asteroid and return a sample of one of these curious bodies back to Earth. Two similar missions — Genesis and Stardust — visited comets in the past, and brought back samples. Genesis, however, suffered a parachute failure, damaging some of the craft’s payload.

Asteroid Bennu contains precious pristine material on its surface, likely dating back to when the Solar System formed.


Why is OSIRIS-REx bringing back asteroid samples?

Most of what we know about asteroids comes from meteorites, which are fragments that rain down to Earth as their parent bodies pass by our planet. Meteors may be significantly altered by these trips through Earth's atmosphere.

Unaltered asteroid samples are critical to understanding our origins. Scientists believe that asteroids are made up of the same ancient material that formed the Solar System billions of years ago. Studying asteroids provides us with clues on how planets, including Earth, came to exist.

Bennu orbits the Sun every 436.6 days. It is a rare, B-type asteroid — the most primitive kind of asteroid. Bennu has barely changed since it formed 4 billion years ago, and it contains relatively high amounts of carbon, organic molecules, and amino acids — considered to be the building blocks of life.

By studying samples of the asteroid in a lab, scientists will get a better understanding of its composition and the material that makes up these space bodies.

As they observe asteroids, or any other body in space, through the eyes of a spacecraft, scientists are limited by the instruments carried aboard and aren’t able to break it down to the exact minerals that are on the surface.

As a result, studying the samples in a lab is a whole different ball game.

“Yeah, it’s gonna be a lot of fun,” Simon says. “We'll learn more about the actual materials that are there and how easily they break apart, how hard they are, the different compositions.”

“I think there's going to be a lot of interesting stuff that comes out of this,” she adds.

The mission isn’t just an attempt to trace our origins — it’s also a good way to keep Earth safe.

Bennu comes within a mere 0.002 Astronomical Units to Earth every six years, and there is a very small chance it will collide with our planet in about 150 years' time. By studying Bennu's orbit and the small forces that affect its trajectory, scientists can create better models of asteroid orbits and calculate the chances that they might collide with Earth more accurately.

When does OSIRIS-REx return to Earth?

OSIRIS-REx is scheduled to return to Earth on September 24, 2023, with a bigger-than-expected payload.

The mission’s team members suspect that OSIRIS-REx grabbed a substantial amount of material from Bennu’s surface, probably exceeding the mission’s original requirement of two ounces.

“We don’t have an absolute estimate, but we knew we had plenty,” Simon says.

After the sample collection in 2020, NASA noticed a leak of materials coming from the craft’s robotic arm. It had collected far more than necessary, leaving its collector head unable to close. Thankfully, the samples were eventually successfully stowed on the spacecraft.

The TAGSAM collector leaking samples into space. Ultimately they were successfully stowed.

NASA/Goddard/University of Arizona

OSIRIS-REx’s sample capsule will travel through Earth’s atmosphere and land with parachutes at the Utah Test and Training Range.

After recovering the capsule, team members will transport it to the curation facility at NASA’s Johnson Space Center in Houston.

The agency will then distribute the sample to laboratories worldwide so that a large group of international scientists can simultaneously study the formation of the Solar System and our home planet.

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