Expert: Tom Whiting - 6/20/2010

Question
Thanks Tom, I guess I used the wrong words, if you took my previous question and replaced it with high density asteroid, what would the difference be? Remember, I'm only doing this for a story I'm writing, and would appreciate any help you can give me. Thanks again.

Answer
Hi Chris,
Well there is no large Earth-crossing asteroid right now... most are just a few feet across up to about 40 miles long.  The very large asteroids are out in the asteroid belt between Mars and Jupiter, the largest being the first one discovered on 01/01/1801 which is 1 Ceres at about
600 miles in diameter, see the Big 4 Asteroids at
http://library.thinkquest.org/J0112388/asteroids.htm

But these are all in relatively stable orbits in the Belt.  It would take a near Act-of-God to send one of those hurtling inward.
Of more importance is the newer Kuiper Asteroid Belt out by Pluto... Pluto in fact is simply one of the largest of the Kuiper Belt members, thus the real reason for the downgrade from planet
status.  (if you want the real truth). Eris is actually larger than Pluto! See
http://en.wikipedia.org/wiki/Kuiper_belt
But if perchance there was a collision way out there, it would take centuries for one of the
Kuiper Belt members to make it all the way into the inner Solar System, and currently none has
been observed heading inward.... all the bigger ones are in pretty stable outer orbits.
Even if a relatively large asteroid headed inward, the odds on even a close approach to the Earth are pretty minimal.  We're a very fast moving, small target. At 18.5 miles per second around the sun, we move our own diameter every 430 seconds, or 7 min 10 seconds. So a potential
projectile would not only have to be either crossing up through, or down through... the ecliptic
(plane of the Earth-Sun), and at the correct spot of 365 days of revolution, (otherwise it would
pass either above or below the Earth), it has to also be ON TIME to within about 7 minutes, otherwise it passes in front of or behind the Earth. So there are 3 perfect requirements for interception with the Earth. That's why the odds are so low... if any one of the requirements are off just a little, it's a near miss and not a hit. And that's why we don't have to lose any sleep over it because it only happens once every 50 million years or so. (I'm talking about one large enough to do some major damage, like an asteroid 5 miles across or bigger.
(It's not the mass of the body that's the killer, it's the velocity, that 40 miles per second appraoch speed. Recall your kinetic energy formula, KE = 1/2 m  V squared... it's that velocity
squared... that does you in, not the mass. Again, on a near miss, the effects on the small asteroid and it's orbit are drastic, but the Earth hardly budges due to it's much higher mass,
in fact, on a 10 mile wide near miss, the effects on the earth would be hardly measureable, let
alone catastrophic.
You have a real challenge from a real astronomy standpoint, because it just ain't going to happen
in real life. What WE humans do to our own home planet is far more devastating...with our bombs and our pollution. Of course in a few thousand years, it won't matter because we are way overdue
for the next Pleistocene Ice Age to occur... we've been on borrowed time for the past 1000 years for the onset of the next ice age. So burn more fuel... help to delay it even longer.
Clear Skies,
Tom