The on-again-off-again question of whether the asteroid Apophis poses a threat to the Earth this century is back on again. Don't panic though: The risk isn’t just small, it’s 48 years away, leaving plenty of time for us to fix it, if we can divert some energy from the disasters of our own making.
The asteroid Apophis will make several very close passes to the Earth over the next century. Its discovery in 2004 sparked fears it might actually hit us on one of these close approaches. However, as more detailed observations have improved our understanding of its orbit, fears of collisions in 2029 and 2036 subsided.
However, at the American Association of Science Planetary Science Division Conference, the University of Hawaii's Dr David Tholen presented a talk outlining how Apophis’s orbit is being changed by the Yarkovsky effect. Yarkovsky acceleration occurs when one hemisphere of an asteroid is warmer than the other. Radiation from the warm side pushes on the asteroid as the photons released carry momentum in the opposite direction. The force is tiny, but with nothing to counteract it, over time it can nudge the asteroid out of its orbit. Astronomers expected Apophis would be affected in this way, but did not know by how much.
Stunningly precise measurements taken by the Subaru telescope revealed the effect is shifting Apophis's orbit by 170 meters (550 feet) a year. Small as that is, over 50 years it could change a near miss into a collision, particularly when multiplied by the two intervening encounters with Earth’s gravity.
Dr Tholen said the movement observed may just be enough that the close approach on April 13, 2029 (another Friday 13th) could put Apophis on a path to hit Earth in 2068. The effects of the close approaches in 2029 and 2036 on Apophis’s orbit depend so much on its exact distance that we can’t model them perfectly, so no one yet knows for sure. In Tholen's words, “2068 is still in play.”
The good news is we will know a lot more in nine years' time after Apophis passes within the orbits of communication satellites, close enough to be visible to the naked eye. “The 2029 close approach is critical,” Dr Tholen said. If the risk remains we will have 39 years to work out how to prevent disaster. That could be blowing the threat up in a Hollywood-approved manner, nudging it sideways with nuclear weapons, or even wrapping half in mylar to supercharge the same effect moving it into range.
Apophis is 300 meters (1,000 feet) wide, too small to create a “dinosaur-killer” scale event, but enough to kill millions of people directly from the energy released on impact. Worse still, the dust thrown up could hinder crop growth for years and tip a fragile civilization into disaster. An ocean landing would unleash tsunamis that could drown cities that house much of humanity.
Redirecting an object this large at the last minute would be hard, but just as the Yarkovsky effect builds up over time, small forces applied early could prove sufficient to do it.