Earth's Asteroid Risk Downgraded

xanthines-R-yummy writes "Relax, everyone - the risk of a gigantic asteroid colliding with Earth just got smaller! Nature reports: "A new survey revises down the likelihood of a massive asteroid hitting the Earth by 20-30%. We're only due to collide with rocks larger than one kilometre across roughly once every 600,000 years, it concludes." Whew! What a relief!"

Comments

Re:down 20-30%...?

[EulerX07]

Not really. Even if statistically one should hit every 600 000 years, it is statistically possible to go 2 million years without any impact.

Flip a coin. You've got 50% chance of getting heads or tails. But it is possible to get heads 10 times in a row. The odds of this combination happening is 0.5^10*100 (in percentage = 0.09765% chance of happening).

An event that has 1/600 000 years chance of happening does not have a 100% chance of happening every 600 000 years.

Been a while since my stats course and I'm still trying to erase the painfull memory, but someone probably has the knowledge fresh enough in his head to calculate the probability of this happening after 1 year or 2 million years if the odds are 1 in 600 000 years. It would probably look like :
1 year : 1X10^-xx %.
2 000 000 year : 99%.
200 000 000 years : 99.9999999%.

It just never would be 100% except with an infinite time.

Actually same risk, reduced consequences

[Evil Pete]

The article points out that rather than there being 20-30% fewer rocks out there which could hit us, they are 20-30% smaller. So the chances of being hit are not less, just the chances of of it being over the magic size 1 kilometre (claimed to be the size required to knockout civilisation or whatever).