THE WOODLANDS, Tex.—Back in late 2018 Earth dodged a bullet. Well, almost—a hefty space rock streaked through the upper atmosphere, detonating with the power of a nuclear bomb over an isolated stretch of the Bering Sea, between Russia and Alaska.

The blast occurred roughly 16 miles above the ocean, creating a high-altitude airburst with perhaps 40 percent of the energy released by the destructive February 2013 meteor over Chelyabinsk, Russia.

This late-breaking bombshell was unveiled here by Kelly Fast, NASA’s Near-Earth Object Observations program manager, during a media briefing on the agency’s planetary defense programs prior to the start of this week’s 50th Lunar and Planetary Science Conference.

The after-the-fact finding was confirmed by U.S. Air Force satellites, as well as the International Monitoring System, a global network of ground-based listening stations set up under the Comprehensive Nuclear-Test-Ban Treaty.

On December 18, 2018, slicing through the atmosphere at more than 71,500 miles per hour, the object broke up off the coast of Russia’s Kamchatka peninsula, discharging, in the process, an estimated 173 kilotons of energy—more than 10 times the power of the atomic bomb that devastated the Japanese city of Hiroshima at the end of World War II.

“But this is the small stuff, compared to what we are worried about in planetary defense,” says Lindley Johnson, NASA’s Planetary Defense Officer. “Objects of this size break up in the atmosphere. This is the small end of the population [of asteroids] that’s out there,” he adds.

Johnson says there is good reason for the lag in identifying and reporting on the space rock blast. “The Air Force puts together what’s called a ‘consolidated picture’ of what the event was, using a number of data sources that they have. It was seen that this was somewhat a unique event,” he says.

The object came in at a very high angle, close to the zenith, prior to its explosive disintegration, Johnson says. The resulting outburst was closer to things that Air Force sensors were originally designed to see in their space-based missile-monitoring mode—that is, the detonation of a nuclear weapon.

“It took them a while to be assured it was a natural event and something that should be reported to NASA,” Johnson says. “Also, the area in the world that it came in played on that a bit, as you can well imagine. So it took some additional analysis for them to clearly know they had a natural event to sort out, and the data could be releasable.”

Shock Footprint

Also wringing out complimentary information about the fireball detonation was Peter Brown of Western University in Ontario. He studies small bodies of the solar system with particular emphasis on bolides—large meteors that explode in Earth’s atmosphere.

Brown pored over low-frequency sonic waves recorded by Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO) stations—so-called infrasounds emitted by large explosions.

“I came across this event when looking at the CTBTO list of automated infrasound-only detections from December 2018. There are a half-dozen such events globally every day, and they usually are some form of atmospheric explosion or event, such as a rocket launch or mining blast,” Brown says.

Occasionally, every week or so, an explosive event occurs in a region where such events are unlikely, such as far out at sea. “In this case, the event was both out at sea and detected by a large number of stations, indicating an energetic explosion. This is what bolides look like in infrasound,” Brown says.

By checking one of the nearer CTBTO stations, Brown found that the dominant period of the infrasound was very long, on the order of 20 to 25 seconds. “There is a tight correlation between the dominant period of an atmospheric explosion and its yield. In this case, periods of 20 to 25 seconds correspond to energies on the order of 100 to 200 kilotons.” In contrast, he says, the one that occurred over Chelyabinsk in 2013 had a dominant period of about 32 seconds, corresponding to more than 500 kilotons.

Airburst data in hand, Brown tweeted about finding the event on March 8. And shortly thereafter, the Web site of NASA’s Jet Propulsion Laboratory’s Center for Near Earth Object Studies (CNEOS) updated to show the event as well. CNEOS is NASA’s center for computing asteroid and comet orbits and their odds of Earth impact.

Brown says he and his colleagues are working on gauging the threshold at which such airbursts can produce damage on the ground. For instance, a detonation in the range of five to 10 kilotons would likely shatter or crack windows in buildings below.

“This event certainly would have had a strong enough shock at the ground to damage windows if any were under its shock footprint, which, of course, was not the case,” Brown says. “More generally, we are looking at how well the CTBTO system detects fireballs and what sort of information we can glean from each detection.”

Tipping Point

NASA’s Johnson is a 25-year veteran of scoping out the threat of asteroids with Earth in their crosshairs, first while he was at the Air Force and then over the past 15 years that he has been at the civilian space agency.

The next city-threatening bolide could arrive at any time. Being prepared for that eventuality requires developing a total consolidated program for planetary defense, Johnson says. Doing so demands progress in ground-based optical and radar observations of near-Earth asteroids and comets, as well as a dedicated, space-based asteroid-detection telescope. NASA is presently considering just such a project, dubbed NEOCam, although critics suggest ground-based surveys could serve just as well for planetary defense. Furthermore, he says, interagency and international cooperation on the issue of Earth-threatening space rocks is blossoming.

Although the December 2018 event was small, these run-ins are a wake-up call “and help us understand what we’re facing and what the asteroid population is to put into our future plans,” Johnson says.

On one hand, the Chelyabinsk incident “got everybody excited to do something, but then they are off to some other crisis,” Johnson observes. Still, over the years, there has been a steady buildup of interest in planetary defense, he says, and much work remains to be done.

“I think we’re at that tipping point right now, getting the support to fit this all together and reach those goals that I set for such a program 25 years ago,” Johnson concludes.