When a fast-moving fireball streaked through the sky in northern Thailand on Monday before apparently exploding, many wondered why nobody saw it coming.
While there are systems in place to track space debris, comet fragments and meteorites, an inbound daytime meteorite is hard to spot and warning the public is nearly impossible in this type of situation, according to Mike Hankey, American Meteor Society (AMS) operations manager.
“For sporadic fireball meteor events like this one, there is no warning sign,” writes Mr. Hankey. “They happen too quickly. In the case of this meteor, since it happened during the day, it was traveling away from the sun, which would make it impossible to detect with telescopes, since it would only be visible during the day time.”
Also, he says, this was not “space junk,” but more likely a minivan-sized meteorite.
“An object the size of a golf ball could make a bright fireball at night time. An object like this that is bright enough to be visible during the day would have to be significantly bigger,” writes Mr. Hankey in an email interview. “The asteroid over Russia in 2013 was estimated to be 50 feet wide ... Based on the videos, I don't think the object over Thailand was this big, though. The Sutter's Mill daytime fireball over California in 2012 was estimated to be about 10 feet wide. For this event, my guess would be somewhere around that size, but it’s impossible to say for sure without more data. A good guess would be about the size of a car or minivan.”
He adds that, “Size is relative, especially in space. So, in terms of asteroids, the Russian fireball of 2013, while the biggest recorded event in 50+ years was still relatively small in terms of destructive power. Larger than 50 feet wide and it starts to get potentially dangerous. One hundred to one hundred fifty feet wide and it could be disastrous.”
A fireball that is visible during the day over a densely populated area is very rare, despite the fact that several thousand meteors of fireball magnitude fall into the Earth’s atmosphere each day, according to the AMS. However, most of these events take place in uninhabited areas or over the ocean and are masked by daylight.
Meteors entering the earth’s atmosphere are under such enormous atmospheric drag and thermal stress that larger meteors (particularly those made of stone) usually break up between seven and 17 miles above the surface as they travel at speeds ranging from 11 to 72 km/sec (25,000 to 160,000 m.p.h.), according to the AMS.
Hankey explains that “There are lots of resources and efforts put into finding near-Earth objects (NEOs) in space, plotting their orbits and determining if they will cause a threat to Earth.” However, these surveys prioritize the largest objects. Smaller objects like the one that fell over Thailand are often too small to even see.
“This event was most certainly not space trash based on the speed of entry,” Hankey says. “Until a rock on the ground is found related to the event, no one can say with 100% certainty exactly what it was, but it was either a comet fragment or a piece of an asteroid."
The NASA Orbital Debris Program Office, at the Johnson Space Center in Texas, keeps track of "space trash," including derelict spacecraft components, debris intentionally released during spacecraft separation, debris created as a result of spacecraft explosions or collisions, solid rocket motor effluents, and tiny flecks of paint released by thermal stress or small particle impacts.
Hankey concludes, “Smaller objects like this one do not pose a threat because they are effectively destroyed in the atmosphere on entry. Hence, the telescope sky survey efforts [and] asteroid detection efforts are focused on the larger objects.”