A massive asteroid once believed to be on course to hit Earth, then largely ruled out, could be back on course to wreak destruction.
When it was discovered in 2004, initial calculations showed 99942 Apophis had almost a 3 percent chance of slamming into Earth in 2029. More measurements showed while it will pass close enough to be seen with the naked eye, it'll miss.
Another close encounter in 2036 will remain just that - close - but no one's managed to rule out a collision in 2068. Until recently, the chances were estimated at one-in-150,000 - but a new discovery about the 370m-wide asteroid has astronomers and physicists racing back to their calculators.
At a recent American Astronomical Society presentation, David Tholen of the University of Hawaii said Apophis is drifting away from the orbit used to do the previous calculations - it's speeding up, and diverting from its predicted course by about 170m a year.
"Basically, the heat that an asteroid radiates gives it a very tiny push," he said. "The warmer hemisphere would be pushing slightly more than the cooler hemisphere, and that causes the asteroid to drift away from what a purely gravitational orbit would predict."
The new data was collected earlier this year using a telescope atop Maunakea in Hawaii.
While 170m a year doesn't sound like much, over time it adds up - and opens up the asteroid to other influences that wouldn't have been part of astronomers' calculations in the past.
"The 2068 impact scenario is still in play," Dr Tholen said in the online presentation. "We need to track this asteroid very carefully."
When Apophis flies by in 2029, astronomers will get valuable data they can use to hopefully rule out 2068
"It will pass within the belt of communications satellites, and it will be as bright to be seen with the naked eye," said Dr Tholen. "If you're familiar with the Big Dipper, that star which connects the handle to the dipper... that's how bright the asteroid will become in 2029 on, of all dates, Friday the 13th of April."
Another asteroid with a chance of hitting Earth at a future date, Bennu, recently hosted a NASA probe which collected rock for study back here on Earth. It has a one-in-2700 chance of hitting Earth between 2175 and 2199, NASA says, with 78 potential close encounters.
Apophis, named for the Egyptian god of chaos, is bigger than Bennu. If it hit Earth at a 45-degree angle, it would explode with the energy of 150 Kaikoura quakes or 25 Tsar Bombas, the largest nuclear weapon ever detonated, according to Imperial College London's impact calculator. It would create a crater about 5km across.
It wouldn't end the world, however - the asteroid that killed off the dinosaurs was at least 30 times wider.
