Science and Space

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How sciencetists calculate the chance of an asteroid hitting Earth

Earth has already been hit by asteroids many times throughout history. A recent example, called the Tunguska event, occurred in 1908, when a meteor or comet exploded over the wilderness of Siberia, damaging farmland and leveling trees for miles around. Because most of the earth is covered by oceans, there may also be many small impacts that go unnoticed.

Although the odds of any one particular asteroid ever impacting Earth are quite low, it is still likely that one day our planet will be hit by another asteroid. At the current rate of impacts, we would expect about one large asteroid to impact Earth every 100 million years or so.

When a new asteroid is discovered, astronomers analyze it to determine whether its orbit around the sun could bring it close to the Earth. They take successive images of the asteroid over the course of days after its discovery in order to predict its probable orbital path for the near future. The predicted orbit is then compared to the orbit and position of Earth to check for any times when they might pass close to each other.

Although scientists can calculate a most-likely orbit from these early observations, each single observation of the asteroid’s position contains some uncertainty. Most asteroids are small objects, a few meters to a few tens of meters across, and even the resolving power of a large telescope cannot determine their positions exactly. The uncertainties in an asteroid’s position lead to uncertainties in how well we can determine its speed and direction of travel. As a result, a large number of possible orbits for an asteroid can be predicted within these windows of uncertainty.

Careful computer simulations are used to calculate the future orbital path of the asteroid, with randomly chosen initial positions and velocities that fall within the margin of error of the telescopic observations to date. A large number of these simulations are generated for each asteroid. The probability that any particular one will actually hit Earth is given by the fraction of the extrapolated paths that leads to an impact. For example, if one million different possible orbits are calculated, and one of those leads to an impact, then we say that the odds of the asteroid hitting our world are one million to one.

posted by: kyawoo at 10:57 | link | comments (4) |
asteroids

Comments:

#1  26 December 2004 - 18:20

 

Very good blog. I hope you don't mind that a writer who knows little about astronomy but is interested in it has bookmarked your work.

Best wishes for the new year!

Devon

Anonymous

#2  27 December 2004 - 15:15

 

Interesting, but I'm sure Jesus and George W. Bush will protect us, and if not, it's all part of their mysterious plan.

Anonymous

#3  28 December 2004 - 12:40

 

Of course, the chance we'll be struck by a major asteroid is 100%... given enough time. The only question is, how much? :)

Anonymous

#4  29 December 2004 - 11:03

 

yay.. that's scaring!! Bush? how could him hel us? when he helped someone?..

Anonymous

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