Armageddon averted in 2036.

Asteroid’s disastrous collision with Earth no longer on the cards.

The most dangerous asteroid discovered yet looks like it’s not going to impact Earth after all — but if it did it would create a bigger crater.

In 2004, astronomers discovered Apophis and initially deemed it to have a 2.7 per cent chance of slamming catastrophically into Earth in 2029, when it flies within 36,000 kilometres (22,000 miles) of Earth, inside the ring of geostationary satellites. That danger was quickly ruled out as its orbit was plotted more precisely, but a further menace lurked in the future; if during the 2029 encounter Apophis flew through a particular position in space known as a gravitational keyhole, where Earth’s gravity could perturb its orbit, there was a slim chance that the asteroid would hit Earth on 13 April 2036.

However, over the new year Apophis came close enough, 14.5 million kilometres (9 million miles), for telescopes to get a good look at it. The Goldstone radar in California has been able to make measurements allowing its orbit to be calculated with more precision, suggesting that the closest it will come to Earth in 2036 will be 14 million kilometres (8.7 million miles).

The European Space Agency’s Herschel Space Telescope also took the opportunity to observe Apophis in January 2013. Herschel was able to detect the thermal emission from the asteroid — the amount of infrared light emitted as heat at different infrared wavelengths. The level of infrared light radiated by the giant space rock assisted astronomers in determining how big Apophis is. Prior estimates suggested it was 270 metres (885 feet) across, whereas the Herschel observations indicate a diameter of 325 metres (1,065 feet). It’s a good job Apophis isn’t going to crash into Earth — the extra 55 metres (180 feet) would dramatically increase the damage it could have caused, either by carving out a bigger crater on land or by creating a larger tidal wave were it to slam into the sea. NASA has estimated that if Apophis ever did collide with Earth, it would do so with the force of around 500 megatons, ten times the energy of the largest nuclear bomb ever detonated.

Another important observation made by Herschel was how reflective the asteroid is. It measured Apophis reflecting 23 per cent of sunlight that falls on it, as opposed to the previous estimate of 33 per cent reflectivity. The reason this is important is because the way in which the asteroid heats up in sunlight and then cools down by radiating energy in infrared as it rotates can affect its orbit through something called the Yarkovsky effect. Having a greater understanding of the thermal properties of the asteroid will allow astronomers to better calculate its true orbit. Furthermore, if Apophis or any other asteroid were to be found to be on a collision course, we could potentially use the Yarkovsky effect to deflect it, by splashing its surface with highly reflective paint fired in capsules from a spacecraft.

Despite being shown to be safe, Apophis still holds a great deal of interest to astronomers. «Although Apophis initially caught public interest as a possible Earth impactor, which is now considered highly improbable for the foreseeable future, it is of considerable interest in its own right, and as an example of the class of near Earth objects,» says Goran Pilbratt, who is the Herschel Project Scientist at the European Space Agency.

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