It’s officially “official” now that Asteroid 2007 TU 24 will not impact or have any affect on Earth. Astronomers have obtained the first images of the near earth asteroid using high-resolution radar data. "With these first radar observations finished, we can guarantee that next week's 1.4-lunar-distance approach is the closest until at least the [...]

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Fraser - what type of impact would a 250m diameter asteroid have on the earth's atmosphere etc?

It depends on the relative velocity. If it were something in a fairly Earth-like orbit, it would have much less impact than if it were on a highly elliptical orbit.

The answer to your question is something you can calculate yourself. The kinetic energy transfered to the Earth would be half the mass of the object times the square of the relative velocity (including the extra 11 km/sec it would gain falling into the Earth's potential well).

Assume the asteroid has two or three times the density of water, and a velocity of 20 km/sec.

Add that energy to the surface of the Earth in a 250 meter diameter spot, and compare that to the energy of nuclear weapons.

Spoiler Alert: It will make a very big mess, but won't obliterate the human race.

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