Asteroid 3200 Phaethon, a space oddity, is even stranger than astronomers thought, NASA researchers said Tuesday. 

The asteroid acts like a comet and astronomers had thought its tail was made of dust, but a new study published in The Planetary Science Journal found the tail is actually made of sodium gas.

Qicheng Zang, a California Institute of Technology PhD student who is the lead author of the study, used the Solar and Heliospheric Observatory spacecraft to examine Phaethon and its tail. Most asteroids, made of rock, do not usually form tails as they approach the sun, according to NASA. Comets, which are made of rock and ice, usually form tails. 

Based on the study of Phaethon, Zhang and other scientists wonder whether some comets may not actually be comets. 

“A lot of those other sunskirting ‘comets’ may also not be ‘comets’ in the usual, icy body sense, but may instead be rocky asteroids like Phaethon heated up by the sun,” Zhang said in a NASA post.

Phaethon is also the source of the annual Geminid meteor shower, even though comets cause most meteor showers. When astronomers thought Phaethon’s tail was made of dust, this made sense because burning bits of the debris trail produce meteor showers. Now experts are left looking to answer how Phaethon, with its sodium gas tail, provides material for the Geminid meteor shower each December. 

There may have been a disruptive event a few thousand years ago that caused Phaethon to eject the material making up the Geminid debris stream, Zhang’s team said. 

Researchers may get more answers later this decade. An upcoming Japan Aerospace Exploration Agency mission called DESTINY+ is expected to fly past Phaethon, imaging its rocky surface and studying any dust that might exist around the asteroid.

Phaethon also caught NASA’s attention in 2017 when it came close enough to Earth that it was classified as “potentially hazardous” by the International Astronomical Union’s Minor Planet Center.