We have lost more than 900 near-Earth asteroids. We鈥檇 seen each of these potential near-Earth asteroids once, but we didn鈥檛 continue tracking them, so we don鈥檛 know where they are or if they鈥檙e on a crash course with Earth.
Between 2013 and 2016, 17,030 potential near-Earth asteroid (NEA) candidates were added to a list maintained by the International Astronomical Union鈥檚 Minor Planet Center. Of those, about 11 per cent were categorised as 鈥渋nitially unconfirmed鈥�. This means that the few observations we had were not enough to pin down an orbit, so we don鈥檛 know where on the sky to look to find these objects again.
at the Minor Planet Center and his colleagues sifted through the data to figure out why we lost track of so many of them. They found that the main factor is time. To nail down an asteroid鈥檚 trajectory, it has to be observed more than once across a period of a few hours. 鈥淲e need to act fast,鈥� says Vere拧. 鈥淭omorrow, that object could be on the other side of the sky, and nobody really knows where it will be.鈥�
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Some telescopes take 20 hours or more to report potential NEAs, which makes them almost impossible to find again and confirm. Sometimes bad weather means that we can鈥檛 look again in the hours following the initial observation. And the objects can be moving up to tens of kilometres per second, hurtling across the solar system so fast that a few hours after we first saw them, they could be almost anywhere on the sky.
Potentially hazardous
Because we couldn鈥檛 calculate their orbits, we don鈥檛 know how close these asteroids could get to Earth, but Vere拧 says they could be anywhere from several times the moon鈥檚 distance to much closer than the moon.
The researchers used initial measurements of each asteroid鈥檚 brightness to estimate its size. The biggest of the lost asteroids appears to be a few kilometres across, Vere拧 says. For comparison, the asteroid that killed the dinosaurs is believed to have been about 10 kilometres across.
Vere拧 and his colleagues estimated that 102 of the unconfirmed NEAs have diameters above 140 metres, the line at which we define an asteroid as potentially hazardous. Most of the rest are tens of metres across or smaller.
This may be an issue for our estimates of how many NEAs there are total. 鈥淚f the models don鈥檛 take these unconfirmed objects into account, maybe they underestimate the total population by 10 or 20 per cent,鈥� Vere拧 says.
Luckily, he says, this is mainly a problem for smaller objects. 鈥淲e believe that the largest ones 鈥� planetary killers larger than one kilometre 鈥� those are basically all found.鈥�
Vere拧 says we shouldn鈥檛 be worried聽鈥� statistically, even a dangerous but non-lethal object about 20 metres across only hits Earth once every 50 to 100 years, and bigger impacts are even more rare. 鈥淚 would say the danger is coming from objects we haven鈥檛 discovered yet,鈥� he says.
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