In 1871 an astronomer in Europe discovers an asteroid on a unique Earth-crossing orbit. Motivated by this unusual discovery, astronomers around the world begin to search for Earth-approaching asteroids, a difficult and nearly futile task until photographic technology arises to permit effective searches.
The science and mathematics for tracking and predicting the orbits of these asteroids develops slowly, until by about 1890 it becomes clear that the first discovered near-Earth asteroid is passing ever closer to Earth on each trip around the Sun, one orbit every six years. It becomes clear that the asteroid, now named Thanatos, will pass extremely close to Earth, close enough to be a serious collision hazard, in 1938. Is this a death sentence for Earth?