[meteorite-list] Largest Fireball Since Chelyabinsk Falls Over the Atlantic

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Tue, 23 Feb 2016 16:09:31 -0800 (PST)
Message-ID: <201602240009.u1O09VSX005322_at_zagami.jpl.nasa.gov>

http://www.slate.com/blogs/bad_astronomy/2016/02/20/fireball_over_atlantic_ocean_on_february_6_2016.html

Largest Fireball Since Chelyabinsk Falls Over the Atlantic
By Phil Plait
February 20, 2016

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.

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).

The statistics for the impact reported on the NASA/JPL fireball page.

http://neo.jpl.nasa.gov/fireballs

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).

[Map]
The location of the event. It was pretty far out over the water.
Google Maps

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.
Received on Tue 23 Feb 2016 07:09:31 PM PST