OSIRIS-REx helps scientists model the orbit of hazardous asteroid Bennu

The half-a-kilometer-wide asteroid Bennu is already one of the most well-studied asteroids prior to the OSIRIS-REx mission.

By using positional data collected over the course of the two-year sample return mission, however, scientists were able to improve their knowledge of Bennu’s trajectory by a factor of 20, NASA scientists said at a press briefing.

“The OSIRIS-REx mission collected positional data for Bennu to a level never captured before on any asteroid,” Lindley Johnson, planetary defense officer at NASA’s Planetary Defense Coordination Office at NASA Headquarters, said during the briefing on Wednesday.

The OSIRIS-REx mission’s primary scientific objective was to collect and return rock samples from the surface of Bennu.

But because asteroids conducive to sample-return missions tend to have Earth-like orbits — making them potentially hazardous objects — scientists used the OSIRIS-REx mission as an opportunity to collect vital information about the asteroid’s trajectory.

Before, during and after OSIRIS-REx’s trip around Bennu, NASA continuously pinged the spacecraft with radio signals.

By measuring the time those signals took to reach the spacecraft and then bounce back, scientists were able to precisely measure the its position in relation to Earth.

In addition, OSIRIS-REx used its many instruments to map and measure the asteroid from a multitude of vantages as it bobbed and weaved its way around Bennu, swooping down for closeups of various nooks and crannies and ascending for wide-angle views of the entire rubble pile.

“The trajectory of the spacecraft was really amazing, I like to compare it to a hummingbird,” said Dante Lauretta, study co-author and OSIRIS-REx principal investigator at the University of Arizona.

The plethora of data collected over the course of two years allowed scientists to constrain the position of Bennu in relationship to the spacecraft.

In addition to acquiring loads of positional data, researchers were also able to more precisely characterize the impacts of solar radiation on Bennu.

“After a lot of fancy modelling, we have a much, much better model for the trajectory of Bennu,” Davide Farnocchia, scientist with the Center for Near Earth Object Studies at NASA’s Jet Propulsion Laboratory in Southern California.

Farnocchia is the lead author of a new paper on the trajectory of Bennu and the risk it poses to planet Earth.

Currently, scientists peg the odds of Bennu striking Earth between now and 2300 at about 1 in 1,750, or 0.057%.

According to Johnson, If Bennu were to strike the Eastern Seaboard, the devastation would stretch up and down the coast.

“But we should remember that the risk carried by Bennu is smaller than the risk coming from undiscovered objects of a similar size,” Farnocchia said.

There is still uncertainty about Bennu’s future orbit. That’s because an asteroid’s trajectory can be altered by what are called gravitational keyholes.

If Bennu passes by another planetary object at just the right moment, a gravitational push or pull might alter the asteroid’s trajectory enough to put it on a collision course with Earth.

By constraining Bennu’s trajectory, researchers were able to rule out dozens of potential gravitational keyholes, but several still pose a small but real threat.

It may be decades before scientists can be certain whether Bennu will hit one of those keyholes or not, the scientists caution.

In total, Farnocchia and his colleagues accounted for 343 solar system objects that could potentially perturb the trajectory of Bennu in their model.

“Right now the greatest source of uncertainty is related to the mass of all the other asteroids that we’ve including in the model,” he said.

As NASA and the scientists at the Planetary Defense Coordination Office continue to search for new and potentially hazardous objects, the space agency can have confidence that sample return missions provide vital information for gauging the collision risk posed by a target asteroid.

“OSIRIS-REx has been a pathfinder mission and it can be used to inform future missions to newly discovered hazardous asteroids,” Johnson said.

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