Space Rock Turning into Comet
© HEATHER ROPER/UNIV. OF ARIZONASpace rocks called centaurs could someday become brilliant comets, like the one shown in this artist’s illustration. Astronomers have spotted a centaur that is expected to become a comet in about four decades.
Like the mythical half-human, half-horse creatures, centaurs in the solar system are hybrids between asteroids and comets. Now, astronomers have caught one morphing from one type of space rock to the other, potentially giving scientists an unprecedented chance to watch a comet form in real time in the decades to come.

"We have an opportunity here to see the birth of a comet as it starts to become active," says planetary scientist Kat Volk of the University of Arizona in Tucson.

The object, called P/2019 LD2, was discovered by the ATLAS telescope in Hawaii in May. Its orbit suggests that it's a centaur, a class of rocky and icy objects with unstable orbits. Because of that mixed composition and potential to move around the solar system, astronomers have long suspected that centaurs are a missing link between small icy bodies in the Kuiper Belt beyond Neptune and comets that regularly visit the inner solar system (SN: 11/19/94).

These "short-period" comets, which are thought to originate from icy objects in the Kuiper Belt, orbit the sun once a decade or so, and make repeat appearances in Earth's skies. (Long-period comets, like Halley's Comet, which visits the inner solar system once a century, probably originate even farther from the sun, in the Oort cloud (SN: 10/25/13).)