Sometime around the year 1837, a strange object passed an invisible cosmic mile marker: 1,000 astronomical units from the Sun. (One astronomical unit, or AU, is the average Earth-Sun distance.) For more than a century, it continued undetected toward our star. Finally, on Oct. 19, 2017, humans noticed the visitor. That night, a faint, thinContinue reading "Oct. 19, 2017: The discovery of ‘Oumuamua"

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