Expert: Philip Stahl - 2/19/2007

Question
Mr. Stahl- I have recently been hearing about this asteroid called Apophis which I understand has a slight chance of hitting the Earth in about 30 yrs. How worried should we be? If it did strike us how bad do you think the damage would be? What can we do to prevent this? I heard that blowing it up would not be effective. Why? I also hear that there is a plan to use gravity to move it away from the Earth. Is this true? I know these are alot of questions but we are talking about our existence here. Thank you for your time.
Michael

Answer
Hello,

At about 400 meters diameter, the asteroid Apophis - if it struck Earth- would carry roughly the same impact as the Tunguska meteroid that hit Siberia in 1908. That event devastated approx. 10,000 sq. kilometers, killing crops, humans, animals, and had an explosive release of perhaps 100 megatons TNT equivalent.

While such an event would surely be no "walk in the park" it is still vastly short of the worst event that could occur - a planet-sterilizing impact arising from an object 3 km or more in diameter (e.g. roughly two miles or more across).  Such an event would carry an explosive release of 100,000 megatons TNT equivalent and annihilate an area: > 50 x 10^6 sq. km.

Probably 30% of the planet's population would be wiped out in the initial impact, and the remainder would perish within 2 years after a nuclear winter set in - doubtless because of the death of all green plants, crops, etc.

Fortunately we aren't talking about this scenario but a far diminished event that barely ranks '1' on the asteroid impact Torino Scale. Further - the chances are currently only about 1 in 45,000 that any impact will be made at all. Those are pretty decent odds.

Thus on a 'worry scale' of 1 to 10, where 10 denotes maximum worry and sleepless nights, I would put it at about a 1. The major worry will obviously be for those that experience direct impact.

Fortunately again, a sizeable area fraction (~ 3/4) of the planet is covered by ocean, so the odds are probably better than 3 to 1 - even if impact occurs, it will be somewhere in the ocean and not atop New York City. The primary concern then will not be atomic blast type destruction, but rather a tsunami that will probably reach 100' height for some coasts, depending on the coastal geology, reefs etc.

If the thing DID strike a populated area, obviously the damage would be horrendous. Given how much area was wiped out in the 1908 Siberian strike, we can fairly well conjecture the asteroid would take out a city the size of Milwaukee, and not leave much behind except for a 3-4 mile crater. Again, let me remind you the odds of this are so extremely small as to be not worth losing sleep over!

Right now there's precious little that can be done to prevent it. It isn't that blowing it up, say with a nuclear missile (of at least 100 megatons payload) wouldn't be effective - but rather such an impact would likely splinter Apophis into 10-12 objects of perhaps 30 m - 50 m each diameter. What you do then is spread the destruction to maybe a dozen places - whereas if you leave it alone, you get maybe just an ocean strike and everyone ready to deal with the tsunami.

Is this a gamble? Yes, but it's the best we can do for now, given our technology. Maybe in 100-200 years a ship can be designed with a gravimeteric field capability that can alter the gravitational dynamics and the orbit. But not today.

If there is a plan to move it from Earth I haven't heard anything about it, and have a difficult time visualizing just how it could be done (other than the gravimetric dynamic fix noted above, but which our technology still can't provide!)

In the end, though Apophis does pose some minuscule risk, I would not lose sleep over it. I *would lose sleep* and majorly so - if an asteroid like Ceres was headed our way!!