Two years ago, scientists spotted a cigar-shaped object hurtling through the solar system. Dubbed 'Oumuamua, the entity is considered by most to be an interstellar comet flung out from around another star. But close observations showed that 'Oumuamua was accelerating, as if something were propelling it, and scientists still aren't sure why. Avi Loeb, a Harvard University astrophysicist has proposed that, instead of a comet, the interstellar visitor could have been an alien probe pushed by a lightsail — a wide, millimeter-thin piece of material that accelerates as it's pushed by solar radiation. Other scientists have thrown cold water on Loeb's idea, pointing out that hydrogen ice could have melted off the object in a way that was similar to a rocket engine or other propulsion method. But in August, Loeb fired back, writing in a study stating that hydrogen ice is very easily heated, even in the cold depths of interstellar space, and should have sublimated away before 'Oumuamua reached our system. It seems the debate might go on for a little longer at least.
