2029

Apophis Near Miss

Dedicated telescopic surveys in the 1990s and 2000s have identified hundreds of thousands of new asteroids zipping around the solar system. Most are in the main asteroid belt between Mars and Jupiter, but many are in other populations as well, such as Jupiter’s Trojan asteroids and three different populations of near-Earth asteroids (NEAs)—the Atens (orbiting closer to the Sun than Earth), Amors (orbiting farther out from Earth), and Apollos (with orbits that cross Earth’s orbit). All three classes of NEAs pose potential impact hazards for Earth.

One of the most closely watched members of the NEA population is a small asteroid named 99942 Apophis. First discovered in 2004, its orbital parameters were calculated from follow-up telescope observations, including Arecibo radar measurements. Then, like hundreds of other NEAs, its parameters were entered into an automated computer program developed by astronomers to predict the future trajectories and Earth-impact probabilities of these asteroids. Apophis quickly set off some alarms because the calculations came back showing an approximately 1 in 37 chance that the asteroid would impact Earth on April 13, 2029. Apophis set the record for greatest impact risk yet identified, with a Torino Scale score of 4 out of 10.

The detail shows a zoom of how close Apophis is predicted to get to our planet (the white bar represents the trajectory uncertainty).

Astronomers quickly organized more observing campaigns to refine the predictions of Apophis’s orbit. The new data showed that the asteroid will get very close to Earth, only about two to three Earth diameters away—well within the orbits of geosynchronous satellites—but that it will not actually impact our planet. Apophis will pass close to Earth again in 2036, but its odds of hitting Earth then are down to 1 in 250,000, and its Torino Scale score has been downgraded to a 0.

Still, prudence is advised: an impact by a rocky asteroid 1,000 feet (300 meters) in diameter would not be devastating globally, but it would be bad news locally (as would, for example, a giant impact-generated tsunami). Apophis is named after the Egyptian god of destruction—let’s hope that this hazardous little asteroid’s name doesn’t turn out to be accurate.

A trajectory plot, to scale, of the positions of the Earth and Moon and the predicted path of the near-Earth asteroid 99942 Apophis during its April 13, 2029, close approach.