How many house-sized NEOs?

A new study shows house-sized NEOs – Near-Earth Objects – to be 10 times fewer than studies had indicated. Still, there are some 3.5 million NEOs larger than 10 meters across.

Vapor trail left by the Chelyabinsk meteor, as captured by Flickr user Alex Alishevskikh.

Many people were driving and startled to see the now-famous Chelyabinsk meteor hurtling through Earth’s atmosphere on the morning of February 15, 2013, shortly before it exploded over the Russian city of Chelyabinsk. The explosion shattered windows, and sent more than a thousand people to medical centers for injuries, mostly from flying glass. It’s thought that, when it was in space, the Chelyabinsk meteoroid was in the range of 10 to 20 meters across (30 to 60 feet across), about as big as a house. A new study whose lead investigator is the director of the Kitt Peak National Observatory, astronomer Lori Allen, looked at how many house-sized rocks – similar to the Chelyabinsk meteor – have orbits that bring them close to Earth. The study found these objects to be rarer than previously thought. Allen said:

There are around 3.5 million NEOs larger than 10 meters, a population 10 times smaller than inferred in previous studies. About 90% of these NEOs are in the Chelyabinsk size range of 10-20 meters.

Near-Earth Objects (NEOs) are asteroids or comets whose orbits bring them close to Earth’s orbit. Their close approach makes them a potential Earth-impact hazard capable of causing destruction on the scale of cities. The astronomers’ statement explained:

While very large (10 km-sized) impactors can induce mass extinction events like the event that led to the demise of the dinosaurs, much smaller impactors can also wreak havoc. The meteoroid that exploded in Chelyabinsk unleashed a powerful shock wave that destroyed buildings and blew people off their feet. Relatively petite at a ‘mere’ 17 meters in diameter, comparable to the size of a 6-story building, the impactor, when it exploded, released about 10 times the energy of the Hiroshima atomic bomb.

A dashboard camera caught the bright fireball from the Chelyabinsk meteor – February 15, 2013 – as it was exploding in the atmosphere.

To carry out their study, these astronomers directly surveyed NEOs with a wide-field CCD imager called DECam on the 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory in Chile.

The study has been accepted for publication in the peer-reviewed Astronomical Journal.

The astronomers say it is:

… the first to derive, from a single observational data set with no external model assumptions, the size distribution of NEOs from 1 kilometer down to 10 meters. A similar result was obtained in an independent study that analyzed multiple data sets (Tricarico 2017).

While the surprising results do not alter the impact threat from house-sized NEOs, which is constrained by the observed rate of Chelyabinsk-like bolide events, they do lend new insights into the nature and origin of small NEOs.

Astronomer David Trilling of Northern Arizona University is the study’s first author. He explained how the study reconciled the surprisingly small number of house-sized NEOs with the observed rate of Chelyabinsk-like events:

If house-sized NEOs are responsible for Chelyabinsk-like events, our results seem to say that the average impact probability of a house-sized NEO is actually 10 times greater than the average impact probability of a large NEO. That sounds strange, but it may be telling us something interesting about the dynamical history of NEOs.

Trilling speculates:

… that the orbital distributions of large and small NEOs differ, with small NEOs concentrated in bands of collisional debris that are more likely to impact Earth. Bands of debris could be produced when larger NEOs fragment into swarms of smaller boulders. Testing this hypothesis is an interesting problem for the future.

Read more about the study from NOAO

Another shot from a dashboard camera of the Chelyabinsk meteor seen over Russia on February 15, 2013.

Bottom line: Astronomers surveyed NEOs with a wide-field CCD imager on the 4-meter Blanco telescope at Cerro Tololo in Chile, to learn that house-sized NEOs – similar to the Chelyabinsk meteor that exploded over Russia in 2013 – may be 10 times fewer in number than previously thought.

Deborah Byrd