On Oct. 19, 2017, a telescope in Maui detected something that had entered our solar system from elsewhere in the galaxy. Astronomers named it Oumuamua, Hawaiian for “scout” or “messenger,” because it was the first interstellar object they had ever recorded — the only known traveler to have crossed the vast distance between another star system and our own. Where it came from was only part of its mystery. Oumuamua fit none of the well-understood astronomical categories. If it was a rock — an asteroid — it was an extremely strange one. Researchers estimated that it was at least the length of a football field; its shape was hard to determine, but it seemed to be long and thin, like a cigar. “No known objects in the Solar System have such extreme dimensions,” wrote the group of astronomers who discovered the object.
The more that scientists studied Oumuamua, the weirder it seemed. Analysis of its trajectory showed that, in the weeks before its detection, Oumuamua sped up as it approached the sun, and its acceleration couldn’t be explained by the sun’s gravity alone. That extra kick would be normal for a comet. Comets are rocky snowballs, and when they get close to the sun, ice within them turns to vapor, releasing gas and giving them a boost. But Oumuamua lacked a comet’s signature tail, and none of the telescopes that observed it detected water vapor, carbon monoxide or other telltale signs of sublimating ice. Scientists started inventing wild ideas to explain Oumuamua’s observed characteristics, things like hydrogen icebergs and gigantic dust bunnies less dense than air. They were reaching.
Avi Loeb, a theoretical astrophysicist at Harvard University, followed the news about Oumuamua for months. Then one morning in the fall of 2018, he had an idea. For Oumuamua to accelerate as it did, something had to have given it a push. What if that thing was sunlight? For years, scientists have theorized that sunlight, properly captured in the vacuum of space, could exert enough force to boost an object to incredible speeds. Nature doesn’t make anything that harnesses light quite so well, but Loeb thought he might have the answer. “One possibility,” he and a postdoctoral researcher wrote in a paper, “is that Oumuamua is a light sail.” Light sails have long been proposed as a method of space travel, though as of now they are mostly hypothetical. (Japan’s space agency successfully tested one in 2010.) The idea is that a superthin metallic sheet could catch sunlight the way a ship’s sail catches wind, propelling a craft through space. Loeb’s hypothesis could explain some of Oumuamua’s strange behavior, but if he was right, it meant the object was not a natural phenomenon. It was an extraterrestrial artifact.
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