A Tumbling Apophis: Good News for Earth

Asteroid 99942 Apophis isn't a huge threat to Earth — at least not for the next century. Telescopic observations made in early 2013 show that it is both elongated and tumbling, characteristics that help predict its future location.

Asteroid 99942 Apophis has been worrisome ever since its discovery in 2004. Estimated to be a bit more than 1,000 feet (300 m) across, it would deliver the kinetic-energy equivalent of 500 megatons of TNT were it to strike Earth — which it might well do some day. At first, dynamicists predicted a dangerously close pass in 2029 and a 3% chance of collision. More observations eased those fears — but triggered worry instead about a threatening flyby in March 2036.

Radar views of asteroid Apophis
A series of radar maps acquired in January 2013 barely resolve 99942 Apophis, a threatening near-Earth asteroid. These views, acquired with NASA's big radio dish Goldstone, California, have a horizontal resolution of about 250 feet (75 m).
As I reported three months ago, radar observations made last January have seemingly ruled out any chance of an impact in 2036. Barely resolved views made with NASA's big radar dish at Goldstone, California, even show Apophis to be quite elongated (as many near-Earth asteroids are). Mechanical problems at the Arecibo radio telescope prevented radar astronomers there from obtaining even better views.

Yet some dynamicists weren't ready to dismiss a 2036 collision completely because of a computational wild card known as the Yarkovsky effect. This gentle but persistent nudging arises when sunlight is absorbed by a rotating object and then reradiated as heat in some other direction. In particular, if Apophis were spinning retrograde (opposite the way Earth does), then over time its orbit would change in a way that increases the chance of impact in 2036.

A spinning body radiates the most heat from its afternoon side, creating a slight thermal imbalance called the Yarkovsky effect. Over time an asteroid rotating in the same sense as its motion around the Sun is gradually accelerated and pushed into a wider orbit. Conversely, a retrograde spinner is doomed to spiral inward toward the Sun. Click on the image for a larger version.
Sky & Telescope illustration.
But now we can rest easy, because Apophis appears to be tumbling as it orbits the Sun. That's the conclusion reached by a team of telescopic observers who monitored the asteroid's light curve as it passed near Earth in January. Apophis is spinning around two axes at the same time, implying that any Sun-warmed surfaces are radiating heat in all directions, not just one in particular.

"This greatly reduces Yarkovsky drift as a dynamical consideration for Apophis," concludes Jon Giorgini, one of the NASA dynamicists who've been focused on this body's orbit for a decade. As the team reports on its Apophis page, this new development "eliminat[es] any impact chance in 2036."

It also simplifies the task of computing collisional probabilities down the road. One date of interest is a close flyby of Apophis on April 14, 2068, which currently has a 1-in-430,000 chance of impact.

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