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.
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.
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.
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.
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.
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.
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.
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.
- Earth Impact Effects Program
Robert Marcus, H. Jay Melosh, and Gareth Collins; Imperial College (London), Purdue University
- "News, Good and Otherwise: and Billions of Worlds"
A Catholic Citizen in America (January 11, 2013)
- "Asteroid Misses Earth, Good News; It's Not Unique, Not-So-Good News"
(June 29, 2011)
- "Asteroid Apophis: Probably Won't Hit Earth in 2036 or 2068"
(March 10, 2011)
- "Dinosaurs, Three Major Asteroid Impacts, and Massive Volcanic Eruptions"
(August 29, 2010)
- "Asteroids and Earth: Time for the Space Patrol?"
(May 2, 2010)