Death by asteroid later this century?

Friday 13th asteroid a close call - National - smh.com.au

Most readers have probably heard of the asteroid that is due to zip past earth tomorrow, fortunately at a safe distance. However, as the short story above notes:

But astronomers fear Earth could be in very real danger on Friday, April 13, 2029, when another 400-metre-wide asteroid, called Apophis, is predicted to pass at a distance of just 35,000 kilometres.

"If an asteroid [that big] hit Earth it could destroy an area half the size of NSW," Dr Nick Lomb of Sydney Observatory said.

Apart from direct destruction, the effect on the climate would presumably be enormous for some years.

Wikipedia has a pretty detailed entry about Apophis:

As of February 2005 it is predicted that the asteroid will pass just below the altitude of geosynchronous satellites, which are at 35,786 km (22,300 mi).

Now that is close. But it could get worse:

A pass on April 13, 2036 still carries some risk.

Apophis remains at level one on the Torino scale because of a very low but non-zero probability of impact in 2036. However, the close approach in 2029 will substantially alter the object's orbit, making predictions uncertain without more data. "If we get radar ranging in 2013 [the next good opportunity], we should be able to predict the location of 2004 MN4 out to at least 2070." said Jon Giorgini of JPL [3].

So I guess we have some years notice. What worries me is if the close visit in 2029 causes enough of a change in orbit for it to hit in 2036, that is only 7 years to do something about it.

And if it hits Earth, how much energy will it release?:

NASA initially estimated the energy that Apophis would have released if it impacted Earth as the equivalent of 1480 megatons of TNT (114,000 times the energy from the nuclear bomb Little Boy, dropped by the United States on Hiroshima, Japan). A more refined later NASA estimate was 880 megatons, which is still around 65,500 times the energy of the bomb mentioned above. The impacts which created the Barringer Crater or caused the Tunguska event are estimated to be in the 10-20 megaton range. The 1883 eruption of Krakatoa was the equivalent of roughly 200 megatons.

So about 40 times bigger than Tunguska, which according to Wikipedia, caused damage over an area of 2,150 square kilometers.

Start building your kid's asteroid shelters now. I'm sure a good one takes a lot of time.