Last year, astronomers were fascinated by a runaway asteroid passing through our Solar System from somewhere far beyond. It was moving at around 68 kilometres per second, just over double Earth's speed around the Sun. Imagine if it had been something much bigger and faster: A black hole travelling at more like 3,000 km per second. We wouldn't see it coming until its intense gravitational forces started knocking around the orbits of the outer planets. This may sound a bit ridiculous – but in the