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SYFY WIRE Bad Astronomy

Largest Fireball Since Chelyabinsk Falls Over the Atlantic

By Phil Plait

On Feb. 6, at about 14:00 UTC, a tiny chunk of interplanetary material plunged into Earth’s atmosphere and burned up—likely exploding—about 30 kilometers above the Atlantic Ocean. The energy released was equivalent to the detonation of 13,000 tons of TNT, making this the largest such event since the (much larger) Chelyabinsk blast in February 2013.

OK, so first, off: Don’t panic! As impacts go, this was pretty small.* After all, you didn’t even hear about until weeks after it occurred. Events this size aren’t too big a concern. Had it happened over a populated area it, would’ve rattled some windows and probably terrified a lot of people, but I don’t think it would’ve done any real damage.

For comparison, the Chelyabinsk explosion, which was strong enough to shatter windows and injure more than 1,000 people (due to flying glass), had an equivalent yield of 500,000 tons of TNT, 40 times the energy of this more recent impact.

The event was reported on the NASA/JPL Near-Earth Object Fireball page, which lists some of the brightest such things.

I talked about events like this for Crash Course Astronomy:

A little background: The Earth is bombarded by debris from space to the tune of about 100 tons every day. Most of this stuff is quite small, like the size of a grain of sand or smaller, and burns up 100 kilometers or so off the ground. We call the solid bit of debris a meteoroid, the bright phenomenon a meteor, and, if it hits the ground, a meteorite.

If the piece is bigger, it can get deeper into our atmosphere before burning up. Moving at orbital speeds, they can enter our atmosphere from roughly 10–100 kilometers per second. For comparison, a typical rifle bullet moves at 1 kps. As they plow into the air, they compress the gas in front of them violently, heating it up. This in turns heats up the meteoroid, which starts to glow. Material can vaporize and blow off (this is called “ablation”), and usually within seconds the meteoroid is either slowed so much it no longer glows, or it vaporizes entirely.

If it’s much bigger, centimeters or more across, it can start to disintegrate as the air in front of it imparts enormous pressure on it. It flattens (called “pancaking”) and breaks up. Now we have several smaller pieces, and each starts to burn up; the increase in surface area means more heating and glowing, then those pieces break up and get smaller, and you get a runaway cascade. This happens very rapidly on a human time scale; the Chelyabinsk asteroid broke up as it came in and this was seen as a series of very bright pulses of light. It can happen so rapidly that it may as well be called an explosion, a huge amount of energy released all at once. In the end, the huge energy of motion (the “kinetic energy”) is converted into light and heat (and also to break up the meteoroid).

Given the explosive energy of the Feb. 6 meteoroid, if it were made of rock like the Chelyabinsk asteroid then it was very roughly 5–7 meters across, the size of a large living room, say. I calculated that by a straight comparison to Chelyabinsk: We know that was from a rock about 19 meters across; the energy released scales as the mass, and the mass increases with radius cubed for a sphere. So this is all approximate with a few guesses thrown in, but it’s probably close.

It would’ve been a dramatic sight to say the least. But, it happened about 1,000 kilometers off the coast of Brazil, ESE of Rio de Janeiro. That’s far enough out over the ocean that it’s unlikely anyone saw it. So how do we know about it?

Good question. The report came to the JPL folks via the U.S. government; as you might imagine, various arms of the military are curious indeed about atmospheric explosions. However, not much information is revealed by the source; just the time, direction, explosive yield, and things like that. I can think of three ways to detect a big fireball in this case: Satellite observations, which would image them directly; seismic monitors, which can detect the explosion as the sound wave from the blast moves through the ground; and atmospheric microphones, which can detect the long-wavelength infrasound from an event. This may have been detected by any combination of these (though since it was over the open ocean, seismic monitors seen unlikely).

Impacts like this happen several times per year on average, with most going unseen. The Earth is mostly water, and even where there’s land, it’s sparsely populated overall. Chelyabinsk was both relatively energetic and happened over a populated area (the city of Chelyabinsk has more than 1 million people). Still, I would assume the military sees most if not all events this size but chooses not to report them for their own reasons. I understand the desire for them to keep their technology and capabilities secret. It would be nice scientifically to have this data available, but then again they don’t have to release any of it at all, so even having this much is better than nothing. And it’s useful.

And as usual, all of this underscores the need to be on the lookout. A rock this small is almost impossible to see more than a few hours before impact, but the flip side is that it’s also really unlikely to do any damage. But once they get into the 20–50 meter range that changes; explosions from impacts like that rival nuclear bombs. Happily, they’re very rare—here we’re talking fewer than once per century, statistically speaking—but it would be nice if we knew they were coming. It’s hard to say just what we would do if we saw one, but right now we don’t even have that option.

Tip o’ the Whipple Shield to Ron Baalke.

*Astronomers call anything that hits our planet an impact, even if it burns up high in the atmosphere.

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