A little weirder: Asteroid Phaethon's comet-like tail is made of sodium, not dust

A new study has found that asteroid 3200 Phaethon's comet-like tail is not made of dust, as had been previously thought, but is instead composed of sodium gas.

Discovered in 1983, Phaethon is an unusual object because it behaves like both an asteroid and a comet. When it gets close to the Sun, it brightens and forms a tail - a behaviour of comets. Phaethon is also the source of the annual Geminid meteor shower. Most meteor showers are associated with comets, but the Geminids originate from an asteroid rather than a comet.

"Our analysis shows that Phaethon's comet-like activity cannot be explained by any kind of dust," said California Institute of Technology PhD student Qicheng Zhang, who is the lead author of a paper published in the Planetary Science Journal reporting the results.

To determine the composition of Phaethon's tail, Zhang looked for it during the asteroid's latest perihelion in 2022. He used the NASA/ESA Solar and Heliospheric Observatory (SOHO) spacecraft that has color filters capable of identifying sodium and dust. Zhang's team examined archived images from NASA's Solar Terrestrial Relations Observatory (STEREO) and SOHO and managed to locate the tail during 18 of Phaethon's close approaches to the Sun from 1997 to 2022.

In the observations made by SOHO, the tail appeared luminous in the filter that detects sodium, but was absent in the dust-detecting filter. Additionally, the tail's shape and the way it brightened as Phaethon made its way past the Sun corresponded precisely to what scientists would expect if it were made of sodium, not dust.

These findings suggest that asteroid 3200 Phaethon's comet-like tail is composed of sodium instead of dust.

The team now wonders whether some comets discovered by SOHO and by citizen scientists studying SOHO images are not comets at all.

Who needs a dusty old tail? Not this asteroid! ☄️⁰Two @NASASun observatories revealed that asteroid 3200 Phaethon’s comet-like tail is made of something much more intriguing: sodium gas. More: https://t.co/v5XAt3HjTk pic.twitter.com/Ma6MbMdugV

— NASA 360 (@NASA360) April 26, 2023