Asteroid’s comet-like tail is not dust, solar observatories show

A strange asteroid just got a little weirder.

We’ve known for some time that asteroid 3200 Phaethon behaves like a comet. It brightens and forms a tail when it is close to the Sun, and is the source of the annual Geminid meteor shower, although comets account for most meteor showers. Scientists blamed Phaethon’s comet-like behavior on dust ejected from the asteroid as it was ignited by the Sun. However, a new study using two of NASA’s solar observatories has found that Phaethon’s tail is not dust at all but is actually made up of sodium gas.

“Our analysis shows that Phaethon’s comet-like activity cannot be explained by any type of dust,” California Institute of Technology Ph.D. student Qicheng Zhang, who is the lead author of the paper published in Journal of Planetary Science Reporting the results.

Asteroids, which are mostly rocky, do not normally form tails when they approach the Sun. Comets, however, are a mixture of ice and rock, and typically form tails as the Sun evaporates their ice, blasting material from their surface and leaving trails along their orbits. When Earth passes through the debris trail, those comet fragments burn up in our atmosphere and produce a swarm of shooting stars – a meteor shower.

After astronomers discovered Phaethon in 1983, they realized that the asteroid’s orbit matched that of the Geminid meteors. This points to Phaethon as the source of the annual meteor shower, even though Phaethon was an asteroid and not a comet.

In 2009, NASA’s Solar Terrestrial Relations Observatory (STEREO) observed a short tail extending from Phaethon as the asteroid reached its closest point (or “perihelion”) to the Sun in its 524-day orbit. Regular telescopes have not seen the tail before because it only forms when Phaethon is too close to the Sun to observe, except with solar observatories.

Stereos observed Phaethon’s tail develop on subsequent solar approaches in 2012 and 2016. The appearance of the tail supports the idea that dust was being ejected from the surface of the asteroid when the Sun heated it.

However, in 2018, another solar mission imaged part of the Geminid debris trail and was surprised. Observations from NASA’s Parker Solar Probe show that Phaethon in the trail contains much more material than it can throw up during close approaches to the Sun.

Zhang’s team wondered if something other than dust was behind Phaethon’s comet-like behavior. “Comets often shine brightly through sodium emission when they are close to the Sun, so we suspect that sodium may also play a key role in Phaethon’s luminosity,” Zhang said.

Previous studies based on models and laboratory tests suggested that intense heat from the Sun during Phaethon’s close solar approaches could vaporize sodium inside the asteroid and drive comet-like activity.

Hoping to learn what the tail is actually made of, Zhang traced it again in 2022 during Phaethon’s latest perihelion. It used the Solar and Heliospheric Observatory (SOHO) spacecraft—a joint mission between NASA and the European Space Agency (ESA)—which has a color filter that can detect sodium and dust. Zhang’s team also looked at archival images from STEREO and SOHO, looking for the tail during Phaethon’s 18 close solar approaches between 1997 and 2022.

In SOHO’s observations, the asteroid’s tail appeared bright in the sodium-detecting filter, but not in the dust-detecting filter. Additionally, the shape of the tail and the way the phaeton glowed as it passed the Sun matched exactly what scientists would expect if it was made of sodium, but not if it was made of dust.

This evidence suggests that Phaethon’s tail is composed of sodium, not dust.

“Not only do we have a really nice result that looks back 14 years on a well-observed object,” said team member Carl Battams of the Naval Research Laboratory, “but we did this using data from two heliophysics spacecraft—SOHO and STEREO— There was absolutely no intention to study a phenomenon like this.”

Zhang and his colleagues are now wondering if some of the comets discovered by SOHO—and by citizen scientists studying SOHO images as part of the Sungrazer Project—are not comets at all.

“Many of those other sunskirting ‘comets’ may also not be ‘comets’ in the sense of a normal, icy body, but instead rocky asteroids like Phaethon heated by the Sun,” Zhang explained.

Still, an important question remains: If Phaethon doesn’t throw up much dust, how does the asteroid provide the material for the Geminid meteor shower we see every December?

Zhang’s team suspects that some kind of disruptive event a few thousand years ago—perhaps a part of the asteroid that broke off under the stress of Phaethon’s rotation—caused Phaethon to eject an estimated billion tons of material to form the Geminid debris flow. But what that event was remained a mystery.

More answers may come from an upcoming Japan Aerospace Exploration Agency (JAXA) mission called DESTINY+ (short for Space Technology Demonstration and Experimentation for Interplanetary Voyage Phaethon Flyby and Dust Science). Later this decade, the DESTINY+ spacecraft is expected to fly past Phaethon, imaging its rocky surface and studying any dust that may exist around this enigmatic asteroid.