How Do You Figure the Odds of an Asteroid Hitting the Earth?

A newly discovered asteroid, dubbed 2002 NT7, has a 1-in-200,000 chance of striking the Earth on Feb. 1, 2019. How did scientists figure out those odds?

The hazards posed by Near-Earth Asteroids are assessed by Sentry, a computer system developed by the Near-Earth Objects Group at NASA’s Jet Propulsion Laboratory in Pasadena, Calif. The software factors together a cosmic rock’s coordinates, distance, velocity, and gravitational influences to calculate its trajectory. 2002 NT7, for example, rounds the sun once every 837 days, and our fragile planet will almost certainly pass through its orbital path on Feb. 1, 2019. The big question is where the asteroid will be on that potentially fateful date—behind us, beyond us, or smack dab in the middle of Kansas.

The long odds are figured from the wildly inaccurate data provided to Sentry. It’s easy enough to gauge an asteroid’s longitude and latitude (ascension and declination in astro-speak), but figuring out its current celestial position is tricky. The only way to judge distance and velocity is by tracking the object’s movement relative to the stars in the background. This painstaking process requires months or years of observation in order to be accurate. But 2002 NT7 was only discovered on July 9, which means all positional estimates so far are likely to be off by tens of millions of miles. In the coming months, as Sentry is fed additional data regarding 2002 NT7, its guess as to the asteroid’s speed and location will become more precise. Chances are the 1-in-200,000 impact odds will become much, much longer as that occurs.

For the moment, however, 2002 NT7 sits atop JPL’s “Current Impact Risks” chart, which lists the 37 Near-Earth Asteroids most likely to dinosaur the human race. Along with 1997 XR2, it is one of only two asteroids to earn a “1” on the Torino scale, which ranks collision likelihood from 1 (“merits careful monitoring”) to 10 (cash in your IRA now). If the 2-kilometer-wide 2002 NT7 were to strike, its impact would release 1 million megatons of energy, about the strength of 20,000 H-bombs exploding simultaneously.

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Explainer thanks Dr. Paul Chodas at NASA’s Jet Propulsion Laboratory and Perry Gerakines at the University of Alabama at Birmingham.