NASA/Goddard

In 2135, asteroid Bennu will make a close flyby of Earth. Our planet's gravity will tweak Bennu's path, making it a challenge to calculate its future trajectory.

While the University of Arizona-led OSIRIS-REx mission studied and sampled the asteroid Bennu, it was also helping scientists calculate if — or when — the half-kilometer body will one day slam into the Earth.

Their findings appear in the journal Icarus. 

Sept. 24, 2182: In the next 300 years, that's when Bennu has the best chance of striking Earth: roughly .05%.

To figure that out, the team accounted for everything from hundreds of gravitational nudges to minute thrusts from surface heating and particle ejection. 

"Just like the photons from the heat transfer momentum to the asteroid, the ejection of particles from its surface also can have an effect," said Dante Lauretta, the University of Arizona planetary scientist who leads the mission.

That matters because even a small variation in where or when Bennu passes between the Earth and the moon in 2135 could exert a large influence on its 2182 return path.

"The cases where Bennu lines up perfectly to become an impactor, we call that a keyhole. So if it's exactly at the right place at exactly the right time, the Earth's gravity will basically slingshot it around. Fifty years later, it's coming back to hit," said Lauretta. 

Bennu is only a fraction of the size of the body that wiped out the dinosaurs. It is also unclear whether the loosely-packed conglomerate, or "rubble pile," asteroid would break up as it entered the Earth's atmosphere. But either way, the total energy would remain the same.   

"I like to say, 'Would you rather get hit by a cannonball or by a shotgun blast?' Because that's what's going to happen when Bennu desegregates in the atmosphere: The pieces are going to spread out over hundreds of miles, and you'll have smaller impacts, but spread over a much larger area," said Lauretta.

Launched in 2016, OSIRIS-REx (the Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer) reached Bennu in 2018, where it studied the makeup, mass, structure and temperature of the planetary fragment. In 2020, the craft performed a touch-and-go sampling of one kilogram of surface material, which it will deliver via parachute to the Utah desert in September 2023. 

After that, Lauretta's team hopes to extend the craft's operations.

"Once the spacecraft releases the sample return capsule, that vehicle is still very capable. All the science instruments are functional for the most part. And so we were thinking, where else could we go?" said Lauretta.

One possible destination: 99942 Apophis, another asteroid once thought to be on a collision course with Earth. Scientists now know that Apophis is destined for a flyby, not a fly-into, but the 340-meter asteroid will swing very close to the Earth in 2029.

Flight dynamics calculations allow for OSIRIS-REx to be in the neighborhood then too. 

"We can design our gravity assist so that a couple of weeks later the spacecraft will rendezvous with the asteroid," said Lauretta.

The plan must first pass NASA senior review, in which the teams will detail their plans and the agency will decide whether to fund an extension to the year 2030.