Fireball: Couch-Sized Rock Lights Up the Sky Over Arizona

Last Thursday, June 2, 2016, at 10:56:32 UTC (03:56 local time), a rock the size of a typical living room recliner burned up over Arizona, bright enough to light up the sky and spark a lot of social media chatter and 911 calls.

There are quite a few videos of it online. Here’s a typical one:

The bolide (the technical term for a bright meteor) was probably several times brighter than the full Moon, and as you can see in the video it brightened rapidly, flashed several times (this happens as the solid part breaks apart, releasing energy much more rapidly), then disappeared. It also left a long, thick “smoke trail”, actually vaporized rock that can glow for quite some time. It lasted long enough to get lit up by the rising Sun, and get whipped around by upper level winds, creating an eerie, twisted train:

[In that video you can see the old crescent Moon, too.]

There were enough witnesses and good data from the event that scientists were able to backtrack its path into space, and even get an orbit for it! It was on an elliptical path, tilted to the plane of the planets, going out past Mars, and just to Earth’s orbit.

NASA created a very cool animation showing its path, and an asteroid-eye-view of what it would’ve looked like to be sitting on it as it approached our planet. Watch this!

This looks to be a pretty typical event, one that happens fairly often, maybe once per month or so somewhere over the planet (most go unseen; the Earth is mostly water, and even much of the land surface is only sparsely inhabited). What made this one different to me is that JPL put it up on their fireball page, listing the location, time, and most importantly energy released. They get this info from the government, though exactly who is not clear. As I wrote about it recently:

…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.

The explosive yield —the energy released by the event—is what interests me. From that we can determine roughly how big the meteoroid, the solid bit that burned up, might have been. On Twitter I said that the rock was probably a couple of meters across, and I got some questions how I got that number. Let me show you!

In this case, the explosive yield was 0.5 kt, or the equivalent of half a kiloton of TNT going off. This energy comes from the motion of the object; it’s moving fast, at interplanetary speeds, and when it burns up in our atmosphere it releases all that energy as light and heat. The velocity of this particular object was not reported to JPL, unfortunately, but a typical value is about 20 kilometers per second.

That energy of motion of an object, called its kinetic energy, is equal to

energy = 1/2 x mass x velocity²

Therefore, the mass of the object is

mass = 2 x energy / velocity²

A number I happen to know is that a megaton (a million tons) of TNT exploding is equivalent to 4 x 10¹⁵ Joules (a Joule is a unit of energy). The energy of the Arizona fireball was equivalent to 500 tons, or 2 x 10¹² Joules.

Plugging and chugging (and making sure I do everything in the right units):

mass = 2 x 2 x 10¹² Joules / (20,000 meters/sec)² = 10,000 kilograms or 10 tons.

So the object had a mass of 10 tons or so. How big is that? Let’s assume it was a sphere, and made of rock. Rock has a density of very roughly three tons per cubic meter. Knowing that the volume of a sphere is 4/3 x pi x radius³, and density = mass/volume, we can solve for the radius. I’ll save you the algebra and just write down the result:

radius = cube root(3/4 x mass x 1/density x 1/pi)

Plug and chug, and you get the radius is very close to one. So if the fireball was made of rock, it was about two meters across. Sofa sized! NASA reported roughly the same size for it as well. The biggest assumption was the velocity, but it turns out 20 km/sec was pretty close.

Note that the JPL report lists the total radiated energy as much smaller than the yield, only about 10 percent of the total. That’s because not all the energy of the impact goes into light; in general a few percent is about right. The rest goes into heat, sound, and so on.

I’ll note that this chunk of asteroid probably burned up dozens of kilometers over the ground; ones this small don’t hit the ground unless they’re made of iron (in which case this would’ve been smaller than two meters across because iron is much denser than rock). I know some folks are on the lookout for any pieces that might have made it down. I hope someone finds some; it’s rare to get meteorites from an observed fall. There’s a lot of ground to cover! But such meteorites tend to be far more valuable.

I’m glad to see this get so much coverage, too. The more observers who report fireballs to the American Meteor Society, and who get video or pictures, the better we can understand the paths and therefore the orbits of these objects. It also helps us see how they behave as they slam into our air at hypersonic velocities. The physics of that is still not well understood, so the more data we have, the better!

Tip o’ the Whipple Shield to Ron Baalke.