A space rock explosion earlier this month over an island region of Indonesia is now being viewed as perhaps the biggest object to tangle with the Earth in more than a decade.
On Oct. 8, reports from Indonesia told of a loud air blast around 11 a.m. local time. One report indicated a bright fireball, accompanied by an explosion and lingering dust cloud, as the origin of the air blast.
According to experts at the NASA/JPL Near-Earth Object Program Office in Pasadena, Calif. — Don Yeomans, Paul Chodas, Steve Chesley — the blast is thought to be due to the atmospheric entry of an asteroid more than 30 feet in diameter. Due to atmospheric pressure, the object is thought to have detonated in the atmosphere, yielding an energy release of about 50 kilotons (the equivalent of 110,000,000 pounds of TNT explosives).
"My understanding is that this may have been the largest object to strike the Earth since the fireball near the Marshall Islands in the South Pacific on February 1, 1994," said Clark Chapman, a noted specialist in asteroids and a planetary scientist at the Southwest Research Institute in Boulder, Colo.
"Although the Indonesian object was large and the resulting atmospheric explosion may have been the equivalent of several Hiroshima bombs, it is not unexpected for our planet to be hit every decade or so by such an object," Chapman told SPACE.com.
A preliminary look at the incident has been performed by Canadian researchers Peter Brown and graduate student Elizabeth Silber, of the Meteor Infrasound group in the department of Physics and Astronomy at the University of Western Ontario in London, Ontario.
The researchers made a detailed examination of all International Monitoring System infrasound stations of the Comprehensive Nuclear-Test-Ban Treaty Organization. An initial look found that a total of 11 stations showed probable signals from a large explosion.
Based on their scrutiny of the infrasound records, the Canadian research team reported that a large (40-50 kilotons of TNT) bolide detonation occurred near the coastal city of Bone in South Sulawesi, Indonesia. The infrasonic geolocation is not precise enough to determine if the bolide was over water or land, but it was relatively near the coast, the team reported.
Follow-on observations from other instruments or ground recovery efforts, the Canadian team added, would be very valuable in further refining this unique event. Their analysis corresponds to an object some 16-33 feet in diameter. Based on the earlier work by Brown, such objects are expected to impact the Earth on average every two to 12 years.
"We are trying to coordinate with some local scientists to secure more local data, but that will likely take several weeks," Brown told SPACE.com.
Brown said a YouTube video that was aired a few days after the event convinced them it was a bolide.
"Had this happened over the ocean we would only have known that there had been a big explosion...we would presume it was a fireball, but it could be anything producing a large impulsive shock in the atmosphere," Brown said.
More data is expected from U.S. military space assets that likely detected the event. From their vantage point in space, multiple sensor systems would have seen the huge explosion and there surely is a rich dataset of measurements to be plumbed relating to the detonation.
Wall of secrecy
Why wasn't this asteroid observed before it hit?
Chapman of the Southwest Research Institute said he was not aware that the object was seen before it plowed into Earth's atmosphere.
"The body was large enough that some of the current Spaceguard Survey telescopes might have detected it a couple of days before it hit, were it coming from the night sky. But it struck during daytime and probably could not have been seen by those telescopes," Chapman explained.
A second question is whether it was detected by military satellites that monitor bright flashes in the Earth's atmosphere for defense and security purposes.
"Almost certainly it was detected and presumably immediately identified as an explosion of a large meteoroid rather than, say, an explosion of a human-made device in the atmosphere," Chapman figures. "But these satellites are secret and, in the past, the establishments controlling them have delayed releasing the data, for weeks or months."
Earlier this year, Chapman added, a change in previous policy led the U.S. military to withhold the data from the public.
"Scientists hope that they will reverse that policy. This event will demonstrate whether the wall of secrecy is coming down again, or not," Chapman noted. "Evidently, because of the passage of weeks since the event, there has been no decision to release the data promptly."