Asteroid Bennu: Will the best-characterized asteroid in the solar system hit the Earth or not?

• Asteroid Bennu was likely formed in the Main Asteroid Belt between Mars and Jupiter.
• Bennu is as tall as the Empire State Building.
• Bennu could impact the Earth on September 24, 2182.

Asteroid Bennu was in the news recently for an astonishing discovery. NASA scientists revealed that the asteroid has a surface that appears similar to plastic balls. The discovery dates back to October 2020, when NASA successfully collected a sample from the asteroid.

During the sampling event, the sampling head of the OSIRIS-REx (Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) spacecraft had sunk by 1.6 feet (0.5 meters) into the surface of the asteroid. The space agency found that Bennu's exterior is made of loosely packed particles that are haphazardly packed together. The spacecraft would have sunk right into the asteroid if it hadn't fired its thruster to back away after collecting dust and rocks.

This souvenir from Bennu is the largest sample collected by a NASA mission since Apollo astronauts brought back lunar rocks. According to University of Arizona planetary scientist Dante Lauretta, Bennu is considered the best-characterized asteroid in the solar system. But, of all asteroids, why was Bennu chosen for the sample return mission?

And, what will happen if Bennu hits the Earth?

Made of rubble

Asteroid Bennu is ancient. In fact, it has seen more than 4.5 billion years of history. Bennu is thought to have broken off from a much larger, carbon-rich asteroid some 700 million to two billion years ago, according to NASA, and is made from lots of pieces of rocky debris that were compressed together by gravity. It is as big as the Empire State Building, with a mean diameter of 490 m (1,610 ft), compared to the Empire State Building's height of 443 m (1,454 feet). It was likely formed in the Main Asteroid Belt between Mars and Jupiter, but because of gravitational interactions with massive planets and the pushing force caused by photons emitted from the Sun (known as the Yarkovsky effect), it has drifted into near-Earth space. Interestingly, the asteroid also appears to be full of holes inside or made up of a loose collection of rocks, like a pile of rubble, with 20 to 40 perfect of its volume thought to be just empty space. This means that it could fly apart if it rotates much faster or interacts closely with another planetary body.

The ancient asteroid was discovered by the Lincoln Near-Earth Asteroid Research survey on September 11, 1999, and was named 1999 RQ36. In 2013, Michael Puzio, a third-grade student, won a contest called Name that Asteroid!, a collaboration between the mission, the Planetary Society, and the LINEAR asteroid survey.

Puzio also gave his reasons for the name Bennu - he suggested that the spacecraft’s Touch-and-Go Sample Mechanism (TAGSAM) arm and solar panels looked like the neck and wings in the illustrations of Bennu, an ancient Egyptian deity depicted as a gray heron. The deity was also linked with the Sun, creation, and rebirth. Considering that the asteroid dates back to the creation of the Solar System - the name Bennu fits rather well.

Bennu is classified as a B-type asteroid - it has a lot of carbon along with various minerals. Since it hasn't undergone any drastic change, its 'deeper-than-pitch-black' surface could have chemicals and rocks dating back to the birth of the solar system. Bennu is likely to contain organic molecules that are similar to those that could be found at the start of life on Earth.

Will Bennu hit Earth?

Bennu has a close approach to Earth every six years, within 0.002 AU.

Scientists used NASA's Deep Space Network and state-of-the-art computer models to determine its total impact probability through the year 2300 and found that Bennu has a 1 in 1,750 chance of impacting the Earth during one of its close approaches to the Earth in the late 22nd century. Researchers singled out September 24, 2182, as the likeliest date in terms of a potential impact, with a probability of 1 in 2,700.

if Bennu hit Earth, the destruction would be enormous. It would be around two million times the explosive energy that rocked Beirut in the port explosion tragedy on August 4, 2020.

Though the asteroid will not pose any danger to the planet, scientists feel it is imperative to figure out Bennu's exact trajectory to predict how Earth's gravity will alter the asteroid's path around the Sun, and how this would affect the hazard of impact with our planet. The asteroid's size (~500-meter diameter), primitive composition, and close orbit have also made it a very accessible near-Earth object and the perfect target for OSIRIS-REx.

Before leaving Bennu, OSIRIS-REx spent more than two years close to the asteroid. It monitored and gathered information about Bennu's size, shape, mass, composition, spin, and original trajectory. The sample of rock collected from the asteroid's surface will be delivered to Earth on September 24, 2023, for further scientific investigation.

"The OSIRIS-REx data give us so much more precise information, we can test the limits of our models and calculate the future trajectory of Bennu to a very high degree of certainty through 2135," said study lead Davide Farnocchia, of the Center for Near Earth Object Studies (CNEOS), which is managed by NASA’s Jet Propulsion Laboratory in Southern California. "We’ve never modeled an asteroid’s trajectory to this precision before."

Could help trace the history of the Solar System

The spacecraft's navigation camera also noted that Bennu was emitting streams of particles a couple of times every week. It turns out that Bennu is not just a rare active asteroid but also among the first of its kind observed by humans from a spacecraft. The spacecraft also revealed that Bennu has pieces of another asteroid scattered across its surface - thought to be at least in part as a result of a collision with the asteroid Vesta.

This vital piece of information could help scientists estimate the time it takes boulders on asteroids like Bennu to break down into smaller pieces. They may either eject into space or stay on the asteroid's surface.

"The thermal fractures on Bennu are quite similar to what we find on Earth and on Mars in terms of how they form," said Christophe Matonti, a co-author of the paper at Université Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, Géoazur, Sophia-Antipolis, Valbonne, France. "It is fascinating to see that they can exist and are similar in very ‘exotic’ physical conditions [low gravity, no atmosphere], even compared to Mars."

"Keep in mind, the topography of Bennu is young, but the rocks on the asteroids are still billions of years old and hold valuable information about the beginning of the solar system," said Jason Dworkin, OSIRIS-REx project scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland.

Once OSIRIS-REx returns with the sample, more pieces will be added to Bennu's puzzle, creating a clearer picture of the solar system's evolutionary history.