There was no Palomar Observatory when the object now known as comet C/2022 E3 (ZTF) made its last close approach to Earth 50,000 years ago. There were mastodons and woolly mammoths, and great swaths of glaciers covering portions of North America and northern Europe, as the planet went through its last ice age. But there were no trained skygazers.

Now, the ghostly green comet, spotted last year by two modern-day astronomers working at Palomar, in southern California, is swinging back our way.