When you’re in an unexplored wilderness, you’d better be quiet, because you never know whether there might be dangerous predators lurking. Unfortunately, Earth has not been following this cautionary principle so far: we’ve been broadcasting radio waves into space for more than a century. If extraterrestrials eventually arrive at our doorstep, the question is: how should we respond? The chances are small, since it took ‘Oumuamua more than 10,000 years to traverse the entire solar system, and our civilization has obviously not been transmitting signals for that long. There might be plenty of small, fast-moving objects that are constantly traveling through the solar system and that we fail to recognize given the limited sensitivity of our telescopes.