an asteroid
Examples
Noise proves nothing. Often a hen who has merely laid an egg cackles as if she laid an asteroid.
I've seen Australia and I've lived on an asteroid and I'd take the asteroid.
This planet is 15 million years overdue for an asteroid strike like the one that killed the dinosaurs.
A hybrid human-robot mission to investigate an asteroid affords a realistic opportunity to demonstrate new technological capabilities for future deep-space travel and to test spacecraft for long-duration spaceflight.
We can't say that when x happens we get a mass extinction. To the extent we understand mass extinction, one has been caused by glaciation event, one has been caused by a massive climate change, and one has been caused by an asteroid. These events turn out to have no precedent.
An asteroid or a supervolcano could certainly destroy us, but we also face risks the dinosaurs never saw: An engineered virus, nuclear war, inadvertent creation of a micro black hole, or some as-yet-unknown technology could spell the end of us.