First detected in 2004, Apophis caused some heart-stopping moments when early calculations suggested a 2.7% – or one in 45 – chance of hitting Earth in 2029, the highest probability seen for an asteroid. A collision with Apophis, named after the ancient Egyptian demon of darkness and destruction, would be equivalent to the impact of 25,000 Hiroshima bombs.
Thankfully, further observations swiftly downgraded any probability of a collision. Nonetheless, when Apophis does flyby in 2029, it will be a close shave at 30,000km. By comparison, the Moon orbits Earth at 385,000km and communications satellites at 36,000km.
Apophis will pass by at 14.5 million kilometres (nine million miles). As well as the Slooh telescopes in the Canary Islands, scrutinising the flyby closely will be NASA’s deep-space radars at Goldstone, in California’s Mojave Desert, and at Arecibo in Puerto Rico.
13 April 2036: “tiny chance of impact”
That’s not because they’re at all concerned about tomorrow’s flyby. Rather, they want to clarify – and hopefully downgrade – the calculated risk of another close encounter, on 13 April 2036. According to NASA’s Jet Propulsion Laboratory “there is still a tiny chance of an impact” on this date, which it places at about one in 250,000.
Specifically, astronomers hope to clear up a big unknown called the Yarkovsky effect. In short, a slowly rotating body that orbits close to the Sun experiences heating on one side of its body that then cools at ‘night’ as it turns over. This alternate heating and cooling can cause a tiny momentum, depending on the body’s spin and amount of area that warms. The question is whether, over time, the Yarkovsky effect is accelerating Apophis, thus skewing estimates for future approaches.
“Using new measurements of the asteroid’s distance and line-of-sight velocity, we hope to reduce the orbital uncertainties and extend the interval over which we can compute the motion into the future,” JPL’s Lance Benner said.
“It’s possible that the new measurements improve the orbit to the point that we can completely rule out an impact.”
99942 Apophis flyby live streamed by the SLOOH Space Camera.