Discovery of three large, near-Earth asteroids from 1 mile to 12 miles in diameter and research indicates small asteroid strikes are ten times more common than previously thought

Three large, near-Earth asteroids, two of which measure about 12 miles in diameter — are the largest near earth asteroids to have been discovered in 23 years. The smallest of the three asteroids measures little more than a mile across, but it may pass within 3.4 million miles of Earth, making it a “potentially hazardous asteroid.”

When an asteroid exploded over the Russian city of Chelyabinsk in February, shattering windows for miles and injuring well over 1,000 people, experts said it was a rare event — of a magnitude that might occur only once every 100 to 200 years, on average. But now a team of scientists is suggesting that the Earth is vulnerable to many more Chelyabinsk-size space rocks than was previously thought. In research being published Wednesday by the journal Nature, they estimate that such strikes could occur as often as every decade or two

Nature – The trajectory, structure and origin of the Chelyabinsk asteroidal impactor.

Nature – A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors

“One kilometer is more than just dangerous,” said Edward T. Lu, a former NASA space shuttle astronaut who heads the B612 Foundation, a private effort to launch a space telescope that could find smaller asteroids. “One kilometer is end-of-human-civilization kind of dangerous.”

The Chelyabinsk asteroid was just 60 feet wide. Speeding around 40,000 miles per hour, it released energy equal to 500,000 tons of TNT. A larger asteroid, perhaps two or three times the diameter of the Chelyabinsk one, exploded over Siberia in 1908 and is estimated to have released energy equivalent to 5 million to 15 million tons of TNT, flattening millions of trees.

The proposed B612 telescope, to be called Sentinel, is intended to find asteroids about 450 feet wide, although it will also find many that are smaller. Dr. Lu said the mission would cost $450 million — $250 million to build the spacecraft and $200 million to operate it for a decade.

A 450-foot-wide asteroid, Dr. Lu said, would be equivalent to 150 million tons of TNT. “You’re not going to wipe out humanity,” he said, “but if you get unlucky, you could kill 50 million people or you could collapse the world economy for a century, two centuries.

Smooth sections on asteroid Itokawa are shown. Image: ISAS/JAX

This object is coming from out beyond Pluto, from the region we call the Kuiper Belt. And to top it off, it is also orbiting in a backward direction compared to all the planets. Nearly all of the 1,000 currently known Kuiper Belt objects reside in orbits that at all times keep them at least as far away as Neptune. This new object, “UQ4,” is on an orbit that carries it closer to the sun than Mars, meaning that it comes rather close to the Earth. From our spectral measurements, we can estimate that its composition is likely carbon-rich, meaning the surface is very dark, reflecting only about 4 percent of the sunlight that hits it. Even though this object does not reflect very much light, the fact that we can see it in our telescopes implies that it must be rather large. From our measurements, we deduce it is nearly 20 kilometers, or 12 miles, across.

These newly found objects are in orbits that usually keep them rather far from the sun, meaning they are too faint to see. In addition to being far from the sun, they also spend most of their time way above or way below the plane where the Earth and other planets orbit — thus they are far from where astronomers concentrate most of their searches. So it has been a combination of these objects just happening to be getting close enough to the sun and the ongoing diligence of search teams that has revealed them to be there.

ABSTRACT – A 500-kiloton airburst over Chelyabinsk and an enhanced hazard from small impactors

Most large (over a kilometre in diameter) near-Earth asteroids are now known, but recognition that airbursts (or fireballs resulting from nuclear-weapon-sized detonations of meteoroids in the atmosphere) have the potential to do greater damage than previously thought has shifted an increasing portion of the residual impact risk (the risk of impact from an unknown object) to smaller objects. Above the threshold size of impactor at which the atmosphere absorbs sufficient energy to prevent a ground impact, most of the damage is thought to be caused by the airburst shock wave, but owing to lack of observations this is uncertain. Here we report an analysis of the damage from the airburst of an asteroid about 19  metres (17 to 20 metres) in diameter southeast of Chelyabinsk, Russia, on 15 February 2013, estimated to have an energy equivalent of approximately 500 (±100) kilotons of trinitrotoluene (TNT, where 1 kiloton of TNT = 4.185×10^12 joules). We show that a widely referenced technique of estimating airburst damage does not reproduce the observations, and that the mathematical relations based on the effects of nuclear weapons—almost always used with this technique—overestimate blast damage. This suggests that earlier damage estimates near the threshold impactor size are too high. We performed a global survey of airbursts of a kiloton or more (including Chelyabinsk), and find that the number of impactors with diameters of tens of metres may be an order of magnitude higher than estimates based on other techniques. This suggests a non-equilibrium (if the population were in a long-term collisional steady state the size-frequency distribution would either follow a single power law or there must be a size-dependent bias in other surveys) in the near-Earth asteroid population for objects 10 to 50 metres in diameter, and shifts more of the residual impact risk to these sizes.

ABSTRACT – The trajectory, structure and origin of the Chelyabinsk asteroidal impactor

Earth is continuously colliding with fragments of asteroids and comets of various sizes. The largest encounter in historical times occurred over the Tunguska river in Siberia in 1908, producing an airburst of energy equivalent to 5–15 megatons of trinitrotoluene (1 kiloton of trinitrotoluene represents an energy of 4.185 × 10^12 joules). Until recently, the next most energetic airburst events occurred over Indonesia in 2009 and near the Marshall Islands in 1994, both with energies of several tens of kilotons. Here we report an analysis of selected video records of the Chelyabinsk superbolide of 15 February 2013, with energy equivalent to 500 kilotons of trinitrotoluene, and details of its atmospheric passage. We found that its orbit was similar to the orbit of the two-kilometre-diameter asteroid 86039 (1999 NC43), to a degree of statistical significance sufficient to suggest that the two were once part of the same object. The bulk strength—the ability to resist breakage—of the Chelyabinsk asteroid, of about one megapascal, was similar to that of smaller meteoroids and corresponds to a heavily fractured single stone. The asteroid broke into small pieces between the altitudes of 45 and 30 kilometres, preventing more-serious damage on the ground. The total mass of surviving fragments larger than 100 grams was lower than expected

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