One of the most famous recent cases of a devastating visit from space is the huge fireball that hit the Tunguska region of Siberia in 1908.
Trees within about a 14 kilometre radius were incinerated and those within a 40 kilometre radius were knocked down.
Many scientists believe the fireball was caused by an exploding comet, around 100 metres wide, says Francis. Others believe a meteor caused the devastation.
Still others have made a case that the Tunguska explosion was due to a crippled alien spacecraft, a possibility that -- while remote -- shouldn't be abandoned. I recommend "The Fire Came By," a speculative interpretation of the Tunguska mystery with a foreword by none other than arch-UFO skeptic Isaac Asimov.
(Some readers will have noted the "block quote" format above. I'm thinking of using this from now on when citing others' writing. Hard-to-read? Let me know.)