An asteroid the size of the Empire State Building has a slim chance of hitting Earth.
Do not worry. You’ll be dead long before this has a chance. So will your children. Probably all of your grandchildren too.
At a news conference on Wednesday, NASA scientists said the chance that an asteroid called Bennu, which is slightly wider than the Empire State Building, could collide with Earth between now and 2300 is 1 in 1,750.
That’s actually a bit higher than an earlier estimate of 1 in 2,700 over a shorter period of time, between now and 2200.
“It’s not a significant change,” said Davide Farnocchia, a scientist at NASA Jet Propulsion Laboratory in California and lead author of an article published in Icarus magazine. “I’m no more worried about Bennu than I was before. The likelihood of an impact remains very low. “
In any case, Bennu’s trajectory is known so precisely that the probability of a collision for the next century is exactly zero. But the crystal ball becomes more blurry in 2135. This year Bennu will still miss Earth, but it will get pretty close, at about 125,000 miles or so, or about half the distance from Earth to the Moon.
The exact distance is crucial, because the gravity of the earth will shoot Bennu with a sling as he passes. When it passes a certain distance at a certain time – what planetary scientists call a “gravitational keyhole” – it is sent on a trajectory that could actually cross the earth about half a century later.
The most worrying day would be September 24th, 2182, but the probability that it would be a bad day is only 0.037 percent. Bennu is about a third of a mile or half a kilometer wide. That’s not nearly big enough to cause global extinction, but it could wreak considerable havoc.
“Usually, as a rule of thumb, the crater size is 10 to 20 times the size of the object,” said Lindley Johnson, NASA’s Planetary Defense Officer. “An object half a kilometer in size will create a crater with a diameter of at least five kilometers that can be up to ten kilometers in diameter. But the area of devastation will be much, much wider, up to 100 times the size of the crater. So an object the size of Bennu affecting the states on the east coast would devastate things along the coast quite a bit. “
The improved probabilities come from NASA’s OSIRIS-REX spacecraft, which Bennu studied at close range for two years. The spacecraft left Bennu three months ago and is now on its way back to Earth to release rock and dirt samples it collected from the asteroid for detailed laboratory research by scientists.
When the spacecraft was in orbit around Bennu, Dr. Farnocchia and his colleagues pinpoint the asteroid’s orbit. This enabled them to improve the estimate of Bennu’s position in 2135 by a factor of 20.
The observations made by OSIRIS-REX also made it possible to calculate the small forces imposed on Bennu by the heating and cooling of the surface.
Mission leaders chose Bennu in part because it appears loaded with the types of carbon molecules that may have provided the building blocks of life on earth. But they also chose it in part because Bennu is what is known as a near-Earth asteroid. Its orbit crosses that of Earth, and once it was discovered in 1999, planetary scientists were able to see that it posed a potential threat to our planet.
But a greater danger could come from the asteroids that have not yet been seen. Dr. Johnson said about 40 percent of near-Earth asteroids the size of Bennu have yet to be found.
If an asteroid could collide with Earth, humanity could try to put it into an orbit that would miss the planet. NASA will conduct an experiment with this technique on an asteroid named Didymos in 2022 and send the Double Asteroid Redirect Test spacecraft to push it out of orbit. The spacecraft is scheduled to launch later this year.
Dr. Johnson said if there were decades of warning, such a diversion would work for a wandering asteroid the size of Bennu, although multiple impactors would be required.