Friday, January 11, 2013

Asteroid Apophis: No "Earth Shattering Kaboom"

Asteroid 99942 Apophis was something of an anticlimax this week, instead of Marvin the Martian's "earth shattering kaboom," all we got was some fascinating data and a few fuzzy pictures.

That's just as well, since 99942 Apophis is almost a quarter-mile wide. Compared to Earth's (roughly) 8,000 mile diameter that's not much: but it's still big enough to stir up a spot of unpleasantness.

Asteroid Impact: 'What If?'

Let's see what would happen if New York City's Greenwich Village got in the way of an asteroid about the same size as 99942 Apophis. That part of New York City's built on nice, solid, Manhattan schist. ("J. Hood Wright Park," City of New York Partks and Recreation)

The Lemming's assuming that the rock in question is porous, 1066 feet across, traveling 17 kilometers a second (38,028 miles an hour), and hits at a 45 degree angle. That's a fairly typical speed and angle for asteroids falling on Earth.

Hello Asteroid, Goodby Greenwich Village

Our first point of view is Hoboken, about two miles west of Greenwich Village. That should be close enough for a good view, provided our observer is in a tallish building.

Quite a lot of energy is going to get released when that asteroid hits. Maybe our observer is a little too close.

The asteroid started breaking up at about 238,000 feet. Earth's atmosphere is noticeable, even 45 miles up: particularly for a rock zipping along at 38,028 miles an hour.

By the time it hits Greenwich Village, pieces of our space rock are spread out over an oval about three quarters of a mile long and a little over a half-mile wide. The pieces are still going pretty fast, and they're concentrated in a fairly small area. When they hit, all that kinetic energy will be released. It amounts to what you get by setting off about 697 megatons of TNT.

At this point, any wealth invested in Greenwich Village real estate disappears: along with Greenwich Village.

For a moment, there's a hole in Manhattan Island almost two miles across and a bit over 3,600 feet deep. Never mind those numbers, though. There's an explosion in progress, and that crater is changing: fast. By the time rock stops flowing, our crater will be 2.31 miles across, but only 1,440 feet deep.

Meanwhile, in Hoboken - - -

Our observer in Hoboken is okay, for the moment. The impact didn't release much heat, and the earthquake-like shaking shouldn't damage well-built structures.

That's the good news. The bad news is that our observer, along with the building and quite a lot of Hoboken, is sliding into the crater.

Let's say that our observer got into a helicopter as the building collapsed. Just under 10 seconds after Greenwich Village disappeared, the air blast hits, along with a 4,670 mile per hour wind.

Newark: 'Ideal for Remodeling'

Hoboken was definitely too close. Let's try another observer, on the west site of Newark, about 10 miles from the center of Greenwich Crater. At that distance, the ground won't start shaking for about three and a quarter seconds.

The air blast takes almost 49 seconds to arrive, and has lost quite a bit of speed: the wind is only about 629 miles an hour. That's enough to blow over a highway truss bridge, knock down most buildings, and generally wreak havoc.

Atlantic City: Pretty Good Odds

In Atlantic City, about 100 miles away, nobody's likely to notice what happened. At first.

About 32 seconds after impact, dishes will start rattling, and folks inside will probably hear walls and doors make odd sounds: sort of like a truck hitting the building.

Just over three minutes after Greenwich Village land value dropped to zero, bits and pieces of the asteroid and Manhattan start start falling. They're about one quarter of an inch across, on average.

About eight minutes and eight seconds after impact, the air blast will arrive. By now it's about as loud as heavy traffic: annoying, maybe, but not a threat.

Why Worry?

Scientists are fairly sure that rocks the side of the Lemming's hypothetical example hit Earth every 65,000 years or so: with emphasis on "or so." We could see two in a matter of weeks, and then wait 130,000 years for the next one. It's an average, not a schedule.

The odds are that none have hit during recorded history. Or maybe one did, and left no surviving witnesses. We just don't know.

From a 'big picture' point of view, there's very little to be concerned about. That hypothetical rock wouldn't have a noticeable effect on Earth's climate, and wouldn't do more than rearrange part of an island near the mouth of one river. 'No big deal.'

The Lemming's pretty sure that folks living on North America's east coast wouldn't agree, though. Aside from humanitarian considerations, New York City's important to the regional economy: and, arguably, much farther away.

New York City might be rebuilt, but that would take time: a lot of time.

'Asteroid Patrol'

Happily, some folks have been tracking asteroids. Humanity has the start of a sort of 'Asteroid Patrol,' that might at least spot an incoming rock months before it hit.

Humanity even has some ideas for how to push dangerous rocks into less-dangerous orbits. In the Lemming's opinion, technology isn't the issue: it's convincing national leaders that there's a problem, and that something can and should be done.

And that, as the Lemming often says in another blog, is another topic.
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