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Asteroid Impact

Hi there! I’m an incoming asteroid!

It’s a scenario we’ve seen played out in countless movies and TV shows: an astronomer discovers an asteroid on a collision course with the Earth. In these stories, the initial discovery is usually followed with random meteoric destruction to prove that the situation is serious, a romance or two, until finally, in the nick of time, the world is saved by an unlikely band of wisecracking heroes.

Never in fiction does an asteroid impact provoke a response of “Wait–why didn’t I hear about that?” Yet that is exactly what has happened in real life, most recently this past New Year’s Day, when the very first asteroid discovered in 2014 slammed into the Atlantic Ocean about 1,900 miles off the coast of South America.

This discovery sequence of four images shows the motion of asteroid 2014 AA over the course of about 24 minutes this past New Year’s Eve. Credit: Catalina Sky Survey, Lunar & Planetary Laboratory, University of Arizona

This discovery sequence of four images shows the motion of asteroid 2014 AA over the course of about 24 minutes this past New Year’s Eve. Credit: Catalina Sky Survey, Lunar & Planetary Laboratory, University of Arizona

Asteroid 2014 AA was first detected on New Year’s Eve by observer Richard Kowalski during the course of the enormously successful Catalina Sky Survey, which uses a consortium of telescopes in Arizona to hunt for potentially hazardous asteroids. Because they orbit in our own solar system, asteroids like 2014 AA are found by their motion relative to the background stars: simply take a sequence of images of the same part of the sky, and look for “stars” that move. Detailed measurements of asteroid positions are sent to the Minor Planet Center at the Harvard-Smithsonian Astrophysical Observatory, the official global clearinghouse for asteroid and comet observations, where they are used to calculate orbits and ultimately flag any that might be of particular interest.

The vast majority of asteroids don’t ever get very close to the Earth. Some do – the “near-Earth” asteroids – and some of those have orbits that are known so poorly that a future collision can’t completely be ruled out. The odds of an impact actually occurring are almost always miniscule, typically less than one in a million, with decades of warning time. For asteroid 2014 AA, as soon as they calculated its trajectory, the Minor Planet Center found that its probability of collision was close to 100%, with impact occurring only about 19 hours after the object was first reported – a prediction confirmed by Steve Chesley of the Jet Propulsion Laboratory and independent orbital analyst Bill Gray. On the face of it, this is worrisome. After all, there’s an asteroid on its way with our name on it! But everyone involved already knew there was nothing to fear: since 2014 AA was so close to the Earth when it was observed, and because it was still quite faint, that meant that the asteroid was really small, probably only about 2 to 3 meters across. Asteroids this size almost completely burn up during atmospheric entry. This is only a little bigger than the object that broke apart in the atmosphere above the south suburbs of Chicago in 2003, dropping hundreds of small stony meteorites on Park Forest and Olympia Fields.

Our ability to detect incoming asteroids and chart their trajectories within hours of discovery is impressive! Less inspiring, perhaps, is that news of the collision with 2014 AA was only sent out after its predicted impact. After all, we can’t count on the universe respecting our human holidays! Still, I think we can rest assured that if the asteroid had been large enough to pose any threat to Earth, not only would we have seen it with more warning time, but also the response would have been a little more timely.

Written by Mark Hammergren, astronomer at the Adler Planetarium.

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