Earth has been bombarded by space rocks throughout its history, but we’re lucky no large ones have slammed into the planet lately. Astronomers keep a careful watch on the skies, hoping to spot potential impactors far enough in advance that we can do something about it, and one of the most worrying objects is 99942 Apophis. This skyscraper-sized asteroid might still hit Earth in 2068, according to a new analysis from the University of Hawaii and NASA’s Jet Propulsion Laboratory.
Scientists discovered Apophis in 2004, sounding the alarm when initial observations suggested it had a worryingly high 2.4 percent chance of hitting Earth in 2029. Thankfully, further study lowered that probability to zero. Still, astronomers have been keeping an eye on Apophis ever since — it’s currently considered the third-highest impact threat to Earth, behind 101955 Bennu and 29075 (1950 DA). However, the highest impact risks for those objects are centuries out.
NASA’s Sentry Risk Table shows a 1 in 150,000 chance of Apophis hitting Earth in 2068, but that doesn’t take into account a phenomenon known as the Yarkovsky effect. As asteroids tumble through space, they absorb energy from the sun. That energy is radiated back into space as heat, but the process is not uniform over the object’s entire surface. The result is a small but measurable push that alters the object’s orbit. Davide Farnocchia at NASA and Dave Tholen from the University of Hawaii used data from the Subaru Telescope to try and pin down how much the Yarkovsky effect changes our odds.
Tholen says the true impact risk is probably closer to 1 in 530,000, a number used by the NEODyS impact monitor service that includes the Yarkovsky effect. The new observations will probably push NASA’s Sentry risk to a similarly low level. So yes, it’s probably less likely Apophis will hit Earth in a few decades, but astronomers will need to monitor its orbit over time to make sure. There is still a very real, non-zero chance that Apophis will get caught in Earth’s gravity in 2068.
You don’t want to take any risks with an object like Apophis. While it’s not quite “mass extinction” big, an impact would be catastrophic. It’s a simple matter of physics — Apophis hitting Earth results in an explosion equivalent to 1,151 megatons of TNT. By comparison, the largest nuclear weapon ever detonated by humans was around 57 megatons. The 1883 eruption of Krakatoa clocked in at about 200 megatons. Apophis could level a small country, cause massive tidal waves, and spark widespread wildfires. All in all, a pretty bad day for Earth.
In the event Apophis is ever on a collision course, astronomers should be able to tell us well in advance. Maybe it’ll even be early enough to try one of those pie-in-the-sky asteroid deflection systems we always hear about.
