AEONS ago, perhaps long before Earth itself existed, a hunk of rock circling a star somewhere in the Milky Way was thrown out of its orbit so violently that it was ejected from its natal system. Thus began a journey that would, in time, take it within an astronomical hair’s breadth of humanity’s home planet. On October 19th this visitor was spotted by Rob Weryk of the University of Hawaii in pictures produced by Pan-STARRS 1, a telescope on Haleakala. It thus became the first interstellar interloper into Earth’s solar system to be spied by astronomers.
Its origin is clear from its speed. When spotted, it was travelling at 25.5km per second. That is too fast for it to have a closed, elliptical orbit around the Sun. Nor could its velocity have been the result of an encounter with a planet giving it an extra gravitational kick, for it arrived from well above the ecliptic plane, close to which all the Sun’s planets orbit. Dr Weryk’s object, now named A/2017 U1 (the “A” stands for...Continue reading
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