This strange space rock has long confused scientists with its non-asteroid-like behavior, and they just discovered another interesting feature. In 2009, NASA’s Solar Terrestrial Relations Observatory (STEREO) discovered that asteroid 3200 Phaethon was doing something strange in orbit, brightening up and forming a large tail when it reached its closest point to the Sun on its 524-day orbit.

This was already a strange phenomenon, as comets have tails due to their icy composition, not rock and metal asteroids. For a long time it was believed that the dust ejected from the surface of the asteroid was dust. Now scientists have discovered that there is no dust in the field of view; It is a magnificent tail composed mainly of sodium gas.

“Our analysis shows that the comet activity of Phaethon cannot be explained by any type of dust,” said lead author Qicheng Zhang, a postdoctoral researcher at Caltech. “Comets often shine brightly due to sodium emission when they are very close to the Sun, so we suspected that sodium may also play an important role in the illumination of Phaethon.”

Previous research has shown that sodium is the cause of the “hiss” on the asteroid’s surface. While Phaethon is close to the Sun with a diameter of about 3.9 miles (6.3 km), its surface temperature is about 750 °C, causing some matter to evaporate.

The discovery was made with the help of two NASA solar observatories. The Solar and Heliospheric Observatory (SOHO) spacecraft was used to observe the asteroid through two filters that could identify sodium and dust, respectively. The researchers then consulted the STEREO and SOHO image archives and found the presence of Phaethon’s tail in 18 approaches to the Sun between 1997 and 2022.

SOHO’s filters confirmed their suspicions: the asteroid’s tail was clear and bright when using the sodium-detecting lens, and scientists could see its shape and density change as it moved along its path. No trace of dust was found in the other filter.

“Not only did we have a really cool result that overturned 14 years of thinking about a well-studied object, but we also did it using data from two heliophysical spacecraft – SOHO and STEREO – that were not designed to study the phenomenon. Team member Carl Battams of the Naval Research Laboratory “This is it,” he said.

The discovery adds to the mystery of Phaethon, which orbits closer to the Sun than other named asteroids, is one of the few asteroids to appear blue on the surface and is the source of the annual Gemini meteor shower. Comets are usually responsible for meteor showers.

Described in 1983 and named after the son of the Greek mythology sun god Helios, Phaethon passed by Earth in 2017 but was still 6.4 million miles (10.3 million km) from the planet. It won’t be here until 2093.