Bennu is a 500-meter-wide asteroid that orbits our sun and follows an orbit that periodically brings it alarmingly close to planet Earth. NASA scientists have used new data and complex computer models to predict the exact path of the Bennu asteroid over the next 300 years, according to Icarus. Of particular interest is the probability of an asteroid impact, which for a Bennus-sized asteroid could leave a 3-6-mile wide crater and pack the energy from more than 1.1 billion tons of TNT.
As NASA’s planetary defense officer Lindley Johnson told reporters, “An object the size of Bennus hitting the states on the east coast would pretty much destroy things up and down the coast.”
Low chance of asteroid impact
Fortunately, the probability of Bennu colliding with Earth is small: approx. 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 (approximately 0.037%) chance of hitting Earth.
Although these numbers are slightly higher than previous estimates, any increased anxiety should be offset by the improved ability to predict the movement of objects near Earth accurately. This also makes it far more likely that humans will eventually be able to change the motion of such objects if necessary.
OSIRIS-REx provides unprecedented precision
As reported by NASA, accurate predictions for the Bennu asteroid were possible due to 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 almost 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 orbit. 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 locate Bennus’ position within a few meters. This is quite an achievement, given that the radius of its orbit is about 168 million kilometers, according to NASA.
Small forces can change asteroid orbits
Asteroids are much smaller than planets, so their orbits are affected by a wide range of small forces, including heat from the sun. The Bennu asteroid rotates around its axis and constantly changes which areas are exposed to the sun. When areas heated by the sun rotate into darkness, they cool and release infrared energy, which generates a small amount of pressure.
Steve Chesley, a senior researcher involved in the study, says: “The effect on Bennu is equivalent to the weight of three grapes constantly acting on the asteroid – small, yes, but significant when it comes to determining Bennu’s future impact over decades and centuries to come. ” That power has caused Bennu to operate at about 934 feet a year.
Other factors incorporated in 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 air resistance caused by solar winds and the effect of Bennus’ habit of expelling particles from the surface, which NASA describes as unexpected.
Finally, scientists considered the effect of OSIRIS-REx collecting a sample from Bennus’ surface in October 2020. This last effect was negligible, which is encouraging news for future asteroid emissions.
Close encounters with the earth
The data collected by OSIRIS-REx have allowed scientists to calculate the orbit of the Bennu asteroid with a very high degree of certainty up to 2135, which is improved over previous estimates by a factor of 20. There is no chance of a asteroid impact from Bennu before after kl. 2135, where calculations for the course become less reliable due to a larger event.
By September 2135, Bennu will be temporarily closer to Earth than the Moon (approximately 123,000 miles from the Earth’s surface compared to 238,900 miles from Earth to the Moon). Depending on exactly when and where Bennu makes this close approach, it is possible that Earth’s gravity will change Bennus’ orbit around the sun just enough for it to collide with Earth at a later date.
Computer simulations have identified the small areas of space that Bennu had to pass through in September 2135 to establish a future impact on Earth. These 26 “gravity 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 that Bennu could pass through 24 of these keyholes. Two of the keyholes remain an option.
Astronomers will closely monitor this event in September 2135 and revise estimates of potential future impacts. It will give humanity nearly 50 years to prepare for a possible impact in September 2182. The information and technology available at the time should be spectacular. Alas, that event is two lives away.
Nevertheless, there will be exciting events to enjoy well in advance before then. OSIRIS-REx left Bennu in May 2021 and is about to throw a 4.5 billion-year-old rock sample into the Utah Desert in September 2023. In 2022, NASA will try to push the Didymos asteroid enough to change its orbit, as It writes the New York Times. Didymos poses no immediate threat and is only about a third as wide as Bennu, but this mission will test whether humans can prevent an asteroid impact.
Scientists have cataloged more than 26,000 terrestrial asteroids to date, including about half of the asteroids the size of Bennus (larger than ~ 140 m). Until more work is done, we can only imagine what remains to be discovered.
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