![]() `Oumuamua didn’t show any signs of such comet-like activity.Īnother is to track how its brightness changes over time. ![]() One key observation was to determine whether the object was surrounded by a fuzzy coma of dust and gas, the signature of a comet heating up and releasing gas as it approaches the Sun. But “extraordinary claims require extraordinary evidence, so people across the planet went into a frenzy to get more observations and lock things down.” Tell-tale observations This immediately indicated `Oumuamua could be something novel, according to Jonti Horner, an astrobiologist at the University of Southern Queensland. “It is the only object seen so far with a strongly hyperbolic orbit, meaning that it is travelling so fast that the Sun’s gravity cannot hold it back,” explains astronomer David Jewitt at the University of California, Los Angeles. Though it passed through our solar system, it was not captured by the gravitational pull of the Sun. So what can we find out about such alien objects? The first thing that stood out about `Oumuamua was its orbit. The only reason we have any chance of spotting interstellar objects is thanks to new automated surveys like Pan-STARRS, the Catalina sky survey and the ATLAS survey, which scour the sky for moving objects. It’s hard to interrogate a faint, fast pin-prick of light in the vast blackness of space. So if an alien probe does turn up in our corner of the galaxy, how would we recognise it? Identifying an interstellar visitor Other intelligent, spacefaring civilisations – if they exist – would surely come up with the same elegant idea. One possibility, proposed by mathematician John von Neumann in the 1940s, would be to send robotic probes that clone themselves using raw materials mined from asteroids and then spread out across the galaxy. One day our probes will explore nearby star systems, and they might do more than just beam back data. New Horizons reached Pluto in July 2015 and is on its way to rendezvous with an object way out in the Kuiper belt by 2019. In 2012 the Voyager 1 spaceship made history when it became the first man-made object to enter interstellar space, and Voyager 2 is close on its heels. We know there are at least a few spaceships exploring outer space because we sent them. But what if it had been an extraterrestrial spaceship? What tell-tale signs would give it away? Chasing far-flung stars Even so, astronomers were able to figure out it was no spaceship it was a weirdly-shaped asteroid they christened `Oumuamua, which in Hawaiian means “a messenger from afar arriving first”. It had been passing through for years and was on its way out at high speed, having slingshotted around the Sun, when the Hawaii-based Pan-STARRS telescope noticed the interloper. In October 2017 the first interstellar visitor ever spotted by human astronomers passed through our solar system.
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