Megacatastrophes

An asteroid roughly 50 meters across will definitely miss the Earth next year — but not by much. 2012 DA14 will pass by on February 15, 2013, at a distance of about 24,000 kilometers, which is closer than satellites in geosynchronous orbit.

If an asteroid of this size entered the atmosphere, the likeliest outcome is that it would explode high above the ground unleasing the energy of a nuclear weapon. Such an explosion did occur in 1908 over the Tunguska river in Siberia and laid waste to hundreds of square kilometers of forest. Over a heavily populated area the effects would have been devastating.

Radar image of 2005 YU55

I’m writing this just a few hours before asteroid 2005 YU55 is scheduled to miss us by a “mere” 201,700 miles — less than the distance of the Moon. What’s special about this close encounter is the size of the asteroid. It’s 1,300-feet wide, bigger than an aircraft carrier, and a lot more massive. If it were smash into a city like London or New York it would demolish it. The last time an object this large passed so close was back in 1976, and it won’t happen again until 2028.

Worried about an asteroid suddenly barrelling into the Earth and putting an end to civilization as we know it — or, at least, making a mess of your neighborhood? Asteroids do occasionally smash into our planet, causing local, regional, or even global devastation. A particularly big one (or maybe more than one) came down about 65 million years ago and wiped out the last of the dinosaurs along with roughly half of the other species of animals and plants around at the time.

The Earth is going to be hit again at some point in the future, unless we put some kind of defense system in place to deflect or destroy any space rocks that have us in their sights. The good news is that we’re getting a lot better at knowing what asteroids pose a conceivable threat to us — so called “potentially hazardous asteroids” (PAHs).

Check out currently identified PAHs by going to the Minor Planet Center’s web page on this subject, listing all the predicted encounters by PHAs to within 0.05 AU (about 4.7 million miles, or 7.5 million km) of the Earth from the start of this year through 2178.

NEOs, as their name suggests, are objects that in their travels around the Sun can occasionally come close to the Earth or even, sometimes, ram straight into it. There are three main types: asteroids, meteoroids, and comets. According to the definition used by NEO scientists, near-Earth asteroids (NEAs) range from 50 metres to a few tens of kilometres across and have orbits that never take them more than 1.3 astronomical units from the Sun. (An astronomical unit is the mean distance of the Earth from the Sun – about 150 million kilometres.) Asteroids are made of rock but often not just a single chunk of rock. Many of the smaller ones seem to be “rubble-pile” objects, composed of rocky debris held together loosely by gravity. A surprisingly high fraction of near-Earth asteroids are binaries (two objects close together, orbiting around each other, some actually in contact) Near-Earth meteoroids are, in NEO terminology, anything smaller than 50 metres across.

Near-Earth comets are a slightly different kettle of fish. Comets are made of more volatile stuff than asteroids; in other words, as well as chunks of rock they contain lots of materials, like water ice and frozen methane, that vaporise if the comet gets into a path that brings it close to the warming rays of the Sun. That’s why comets grow tails when they enter the inner parts of the Solar System, and also why comets that repeatedly come close to the Sun eventually burn out so that all that’s left behind is a dark porous rocky husk.

With the growing realization that NEOs have caused “extinction-level” events in Earth’s history, and that even much smaller ones can have devastating local and regional consequences, scientists, politicians, and the public at large have become concerned with what to do about this danger. The Earth will be hit again, many times, in the future by everything from city-buster Tunguska-type missiles to extinction-class behemoths, unless we do something about the threat.

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