Discovered in September of 1999, asteroid 101955 Bennu is a 500-meter (1,640 foot) asteroid that crosses Earth’s orbit once every six years. Because it has been observed for 17 years, astronomers have been able to plot its orbit very accurately, and have found that it will make a series of extremely close passes to the Earth between 2169 and 2199, but they calculate that the chance of an impact is only 1-in-2,700.
Unfortunately, this possibility of an impact has fueled the circulation of a great deal of misinformation on the internet, with most articles illustrating a civilization-ending impact to take place in 2135, with the equivalent energy of 3 billion tons of TNT — but this figure appears to be grossly erroneous.
"We’re not talking about an asteroid that could destroy the Earth," explains University of Arizona’s Dante Lauretta, the principal investigator for the OSIRIS-REx program. "We’re not anywhere near that kind of energy for an impact."
While at 500 meters (1,640 feet) across, Bennu is a sizable rock, but the potential destructiveness for most meteors comes mostly from their impact speed, and Bennu isn’t traveling fast enough to cause widespread damage: the difference in Bennu’s orbital speed and that of the Earth’s is only 6,400 km/h (3,977 mph) — that might seem to be fast, but as an example, that’s a mere one-tenth of the relative speed of the meteor that exploded over the Chelyabinsk region in Russia in 2013.
Bennu’s size, however, means that it would have an energy release 60 times more powerful than the Chelyabinsk meteor, at roughly 30 megatons. While this could cause massive regional damage, it is far from the Earth-shattering kaboom that is feared. The 2135 date is also inaccurate, as these close passes aren’t calculated to begin until 2169.
In the meantime, NASA plans to send the OSIRIS-REx space probe to Bennu, to study the asteroid’s composition and surface features, and to return collected samples of the ‘roid back to Earth for study. The information gathered during the mission is expected to help researchers learn more about the formation of the early solar system, and also to help refine the plotting of Bennu’s orbit. The probe is scheduled to be launched on September 8 of this year, will rendezvous with Bennu in 2018, and (hopefully) return to Earth with its samples by 2023.