THE chance that asteroid 2002 NT7 will hit the Earth on 1 February 2019 has now been ruled out. But here is New 杏吧原创鈥檚 own asteroid prediction: expect continued warnings of doom, followed almost invariably by all-clear announcements days or years later.
Most astronomers expected the threat of an impact by the 2-kilometre asteroid to fade as they collected more data on its orbit. So despite the media frenzy, this week鈥檚 all-clear was no surprise. They say they鈥檙e not crying wolf, but doing their job cataloguing potentially perilous asteroids.
鈥淲e鈥檒l just have to get used to newly discovered objects that initially have non-zero Earth impact probabilities,鈥� says Don Yeomans of NASA鈥檚 Jet Propulsion Laboratory in Pasadena, California. 鈥淭his is exactly how the system should operate.鈥�
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The problem is that the orbits of newly discovered asteroids are inherently uncertain. No matter how many observations are collected in the first few days after discovery, the data covers only a small fraction of the asteroid鈥檚 orbit. That can give us a rough idea of its path, but leaves large uncertainties about the orbit鈥檚 exact shape and the asteroid鈥檚 future positions. These are both crucial factors in calculating the chance that an object will hit the Earth.
If the latest calculated orbit for 2002 NT7 was exact, it would miss Earth by some 54 million kilometres, says Gareth Williams of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts (see Diagram). But last week the results were uncertain because they were only based on measurements from the first 15 days after the asteroid鈥檚 discovery on 9 July. The best astronomers could do then was to estimate the asteroid鈥檚 position along its orbit in 2019 to within several tens of millions of kilometres, giving a 1 in 250,000 chance of impact on 1 February 2019.
That gave 2002 NT7 the highest ever score on the Palermo scale, a rating system developed to help astronomers categorise impact risks. It compares the danger posed by a specific known asteroid to the average chance of an impact by a body of the same size over a long period of time. The high score reflects not only the risk, but the large size of the asteroid, the comparatively short time before the event, and the likely speed of the body at the point of collision.
But on the morning of 28 July, an Austrian amateur astronomer made new observations that rule out an impact in 2019. A remote risk remains in 2060, however (see ).
The asteroid鈥檚 odd orbit will keep it in view for nearly a year, long enough for astronomers to get a much better fix on its location. Meanwhile, others are looking for old images that might show the asteroid鈥檚 position long before its discovery. That would pin down its orbit over many years, and 鈥� astronomers expect 鈥� rule out a global catastrophe on the asteroid鈥檚 next close approach in 2060.