There are hundreds of high-velocity gas clouds moving at hundreds of km/s through the outskirts of our galaxy, mostly in stable orbits that keep them out of the galactic plane. They're typically irregularly shaped, thousands of light years across, and contain many millions of times the mass of our Sun. However, one such cloud, known as the "Smith Cloud" (above), is very different from all the others. It's much more distant, and it's moving towards us incredibly rapidly: at about 310 km/s, or around 700,000 miles per hour. And when I say it's moving towards us, it's projected to collide...