Nancy Huang

Jan 31st 2022

Bennu Asteroid Unlikely to Hit Earth in the Next 300 Years


Bennu is a 500-meter wide asteroid that orbits our sun, following a trajectory that periodically brings it alarmingly close to planet Earth. NASA scientists have used new data and complex computer models to predict the precise path of the Bennu asteroid for the next 300 years, according to Icarus. Of particular interest is the probability of an asteroid impact, which for an asteroid of Bennu’s size, could leave a 3-to-6-mile-wide crater and pack the energy of more than 1.1 billion tons of TNT.

As NASA’s planetary defense officer Lindley Johnson told reporters, “An object Bennu’s size impacting in the Eastern Seaboard states would pretty much devastate things up and down the coast.”

Low Chance of Asteroid Impact

Thankfully, the probability of Bennu colliding with Earth is small: about a 1 in 1,750 (or 0.057%) chance between now and the year 2300. The date with the highest probability of a collision is September 24, 2182, when Bennu has a 1 in 2,700 (about 0.037%) chance of hitting the Earth.

While these numbers are slightly higher than previous estimates, any increased anxiety should be offset by the improved ability to predict the movement of near-Earth objects accurately. This also makes it far more likely that humans will eventually be able to alter the movement of such objects if necessary.

Asteroid hits Earth

OSIRIS-REx Provides Unprecedented Precision

As reported by NASA, precise predictions for the Bennu asteroid were possible because of data from the agency’s OSIRIS-REx spacecraft, which stands for Origins, Spectral Interpretation, Resource Identification, Security-Regolith Explorer. Bennu was discovered in 1999, and ground-based telescopes were used to study close encounters with Earth in 1999, 2005 and 2011. OSIRIS-REx was launched in 2016 and reached Bennu in December 2018.

For nearly two and a half years, OSIRIS-REx flew around Bennu like a hummingbird, taking pictures and gathering information about Bennu’s size, shape, mass and composition, and monitoring its spin and orbital trajectory. During that time, it constantly sent information back to Earth using NASA’s Deep Space Network of giant radio antennas and was often able to pinpoint the position of Bennu to within a few meters. This is quite an accomplishment, considering that the radius of its orbit is about 168 million kilometers, according to NASA.

Small Forces Can Change Asteroid Trajectories

Asteroids are much smaller than planets, so their trajectories are affected by a wide variety of small forces, including heat from the sun. The Bennu asteroid rotates around its axis, constantly changing which areas are exposed to the sun. As areas warmed by the sun rotate into darkness, they cool and release infrared energy, which generates a small amount of thrust.

Steve Chesley, a senior research scientist who was involved in the study says: “The effect on Bennu is equivalent to the weight of three grapes constantly acting on the asteroid — tiny, yes, but significant when determining Bennu’s future impact chances over the decades and centuries to come.” That force has caused Bennu to drift by about 934 feet a year.

Other factors incorporated into the calculations include the effect of gravity from the sun, the planets, the moon, Pluto and more than 300 asteroids in the asteroid belt. Also included was the drag caused by solar wind and the effect of Bennu’s habit of ejecting particles from its surface, which NASA describes as unexpected.

Finally, researchers considered the effect of OSIRIS-REx collecting a sample from Bennu’s surface in October 2020. This last effect was negligible, which is encouraging news for future asteroid missions.

Close Encounters With Earth

The data collected by OSIRIS-REx has allowed scientists to calculate the trajectory of the Bennu asteroid with a very high degree of certainty through 2135, improving on previous estimates by a factor of 20. There is no chance of an asteroid impact from Bennu until after 2135, when calculations for the trajectory become less certain because of a major event.

In September 2135, Bennu will temporarily be closer to Earth than the moon (about 123,000 miles from the Earth’s surface, compared to 238,900 miles from the Earth to the moon). Depending on exactly when and where Bennu makes this close approach, it’s possible that Earth’s gravitational pull will change Bennu’s orbit around the sun just enough that it will collide with Earth at a later date.

Computer simulations have identified the small regions of space that Bennu would have to pass through in September 2135 to set up a future impact with Earth. These 26 “gravitation keyholes” range in size from several hundred feet to a few miles wide. With data from OSIRIS-REx, researchers were able to eliminate the possibility of Bennu passing through 24 of these keyholes. Two of the keyholes remain a possibility.

Astronomers will be closely watching this event in September 2135 and revising estimates for potential future impacts. That will give humanity nearly 50 years to prepare for a possible impact in September 2182. The information and technology available by then should be spectacular. Alas, that event is two lifetimes away.

Nonetheless, there will be exciting events to enjoy well before then. OSIRIS-REx left Bennu in May 2021 and is on track to drop a 4.5-billion-year-old rock sample into the Utah desert in September 2023. In 2022, NASA will attempt to nudge the Didymos asteroid enough to alter its trajectory, as The New York Times reports. Didymos poses no immediate threat and is only about one-third as wide as Bennu, but this mission will test whether humans can prevent an asteroid impact.

Scientists have cataloged more than 26,000 near-Earth asteroids to date, including about half of the asteroids of Bennu’s size (larger than ~140 m). Until more work is complete, we can only imagine what remains to be discovered.

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