Closer Encounters

5 February 2013

A 50 meter rock is heading straight for us. On February 15, it will miss us by a hair’s breadth in cosmic terms, coming within 3 Earth radii. At its nearest approach it will be closer to the Earth than a geosynchronous satellite. It will pass through the spot Earth was just 20 minutes earlier. While that sounds like a dangerous close call, it poses no risk to us. While this particular rock will get fairly close, it will still miss us entirely.

Close approach asteroids such as this one are not uncommon. Most of them don’t get closer than the moon, but when one does it generally makes the rounds in the news. So far none of them have had any chance of hitting Earth. That doesn’t mean the chance is always zero. Asteroids have struck Earth throughout its geologic history, and some (such as the 10km one that struck 66 million years ago) even cause mass extinctions.

Fortunately NASA has a list of possible impact objects, so you can check up on your risk of death by asteroid. There is also a risk scale known as the Torino scale. It summarizes the risk as a number from 0 - 10 based on the object’s size and chance of impact within the next century. At 0 the risk is non-existent, while at 10 it’s up to Bruce Willis to save us. The highest any object has ever rated on the scale is 4. This was for a 400 meter asteroid known as Apophis with an estimated 1 in 42 chance of hitting Earth. It was downgraded to level 1 a couple of years after its discovery, and has since been downgraded again to 0.

So no risk this time, nor in the foreseeable future. Just one more close encounter.