Is an asteroid hit imminent?
ON FEB 15, a 13,000-tonne rock plunged through the skies above Chelyabinsk, Russia. It shone 30 times brighter than the sun, and hurtled at 67,600kmh towards a city of more than a million people.
As the rock broke apart during its fiery descent, it dispersed energy equivalent to 500 kilotons of TNT, shattering thousands of windows.
Some 1,500 people were injured, and shock waves caused a ribbon of damage extending 89km on either side of the meteor's path. Witnesses thought a nuclear war was upon them. Luckily, no one died.
This incident alone might not be cause to start worrying about death by asteroid. But new research suggests that space rocks as large as the one that fell over Chelyabinsk - about 19m across - are three to five times more numerous than scientists had realised.
The study, led by Peter Brown at the University of Western Ontario, also found that larger and more dangerous ones are unexpectedly abundant.
For decades, astronomers have focused on the dangers posed by very large asteroids. Starting in 1998, Nasa led an effort to catalogue "near-Earth objects" at least a kilometre in diameter - big enough to cause a global catastrophe if they collided with Earth.
About 90 per cent of these have been identified. Yet smaller, Chelyabinsk-sized objects are harder to find.
Scientists estimate there are more than a million of them nearby, and only about 1,000 have so far been found. Locating and tracking every one isn't practical. So what to do?
Astronomers at the University of Hawaii are working on one solution. Their system, called Atlas, will deploy eight small telescopes to survey the entire sky twice each night in search of incoming asteroids, starting in 2015.
Atlas won't peer as deeply into space as other detection systems - such as the Pan-STARRS facility, which hunts for "killer asteroids" that are decades from colliding with Earth - but it should find imminent threats more efficiently.
Nasa and other space watchers are making progress on their next goal of cataloguing near-Earth objects larger than 140m, but the cosmos is still full of nasty surprises.
In just the past few weeks, Nasa has discovered three huge new space rocks, two of them initially estimated to be 20km wide.
Although these won't threaten Earth any time soon, the find was alarming. If such a behemoth was on a collision course with Earth, disaster planning wouldn't be enough: We'd need a strategy for deflecting it.
Nasa has suggested several plausible options, including hitting an incoming asteroid with a spacecraft or a nuclear weapon. Unfortunately, it's hard to get people to take this idea seriously.
And Congress has yet to designate responsibility for such a mission to any agency or take any other substantial steps to ensure the United States could react quickly to a potential threat.
The United Nations is starting to take the lead on that front. It's helping countries coordinate their detection efforts, and its Committee on the Peaceful Uses of Outer Space is making plans to coordinate an international deflection mission.
In a cosmos roiling with hazardous and unpredictable threats, preparation is the best defence we have.