NASA published this original article on April 25, 2023. Edits by EarthSky.

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Asteroid Phaethon just got a little weirder

We’ve known for a while that asteroid 3200 Phaethon acts like a comet. It brightens and forms a tail when it’s near the sun. And it’s the source of the annual Geminid meteor shower, even though comets are responsible for most meteor showers. Scientists blamed Phaethon’s comet-like behavior on dust escaping from the asteroid as it’s scorched by the sun. However, a new study using two NASA solar observatories reveals that Phaethon’s tail is not dusty at all. In fact, it’s made of sodium gas.

Qicheng Zhang, a California Institute of Technology Ph.D. student, is the lead author of a paper published in the peer-reviewed Planetary Science Journal reporting the results. Zhang said:

Our analysis shows that Phaethon’s comet-like activity cannot be explained by any kind of dust.

The difference between comets and asteroids

Asteroids, which are mostly rocky, do not usually form tails when they approach the sun. However, comets are a mix of ice and rock and typically form tails as the sun vaporizes their ice. The vaporization blasts material off their surfaces and leaves a trail of debris along their orbits. When Earth passes through a debris trail, those cometary bits burn up in our atmosphere and produce a meteor shower.

Asteroid Phaethon produces the Geminid meteor shower

After astronomers discovered 3200 Phaethon in 1983, they realized that the asteroid’s orbit matched that of the Geminid meteor shower. This indicated Phaethon was the source of the annual meteor shower, even though Phaethon was an asteroid and not a comet.

The tail was spotted at perihelion in 2009

In 2009, NASA’s Solar Terrestrial Relations Observatory (STEREO) spotted a short tail extending from Phaethon. At the time, the asteroid reached its closest point to the sun (or “perihelion”) along its 524-day orbit. Regular telescopes hadn’t seen the tail before because it only forms when Phaethon is too close to the sun to observe. However, solar observatories can observe it then. STEREO also saw Phaethon’s tail develop on later solar approaches in 2012 and 2016. The tail’s appearance supported the idea that dust was escaping the asteroid’s surface when heated by the sun.

Then, in 2018, another solar mission imaged part of the Geminid debris trail and found a surprise. Observations from NASA’s Parker Solar Probe showed that the trail contained far more material than Phaethon could possibly shed during its close approaches to the sun.

Zhang’s team wondered whether something else, other than dust, was behind Phaethon’s comet-like behavior. Zhang indicated:

Comets often glow brilliantly by sodium emission when very near the sun, so we suspected sodium could likewise serve a key role in Phaethon’s brightening.

An earlier study, based on models and lab tests, suggested that the sun’s intense heat during Phaethon’s close solar approaches could indeed vaporize sodium within the asteroid and drive comet-like activity.

Color filters were able to detect sodium and dust

Hoping to find out the composition of the tail, Zhang looked for it again during Phaethon’s latest perihelion in 2022. He used the Solar and Heliospheric Observatory (SOHO) spacecraft – a joint mission between NASA and the European Space Agency (ESA) – which has color filters that can detect sodium and dust. Zhang’s team also searched archival images from STEREO and SOHO, finding the tail during 18 of Phaethon’s close solar approaches between 1997 and 2022.

In SOHO’s observations, the asteroid’s tail appeared bright in the filter that detects sodium. But it did not appear in the filter that detects dust. In addition, the shape of the tail and the way it brightened as Phaethon passed the sun matched exactly what scientists would expect if it were made of sodium, but not if it were made of dust.

The tail of Phaethon is made of sodium

Team member Karl Battams of the Naval Research Laboratory said:

Not only do we have a really cool result that kind of upends 14 years of thinking about a well-scrutinized object, but we also did this using data from two heliophysics spacecraft – SOHO and STEREO – that were not at all intended to study phenomena like this.

Below are two images of the asteroid Phaethon. In the orange image on the left, Phaethon appears as a bright, blurry object with a faint tail below it. In the blue image on the right, nothing appears.

Zhang and his colleagues now wonder whether some comets discovered by SOHO – and by citizen scientists studying SOHO images as part of the Sungrazer Project – are not comets at all. Zhang explained:

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.

How does Phaethon shed enough dust for the Geminids?

Still, one important question remains. If Phaethon doesn’t shed much dust, how does it supply the debris for the Geminid meteor shower every December?

Zhang’s team suspects that some sort of disruptive event happened a few thousand years ago. Perhaps a piece of the asteroid broke apart under the stresses of Phaethon’s rotation. That would cause Phaethon to eject the billion tons of material estimated to make up the Geminid debris stream. But what that event was remains a mystery.

More answers may come from an upcoming Japan Aerospace Exploration Agency (JAXA) mission called DESTINY+. That’s short for Demonstration and Experiment of Space Technology for Interplanetary voyage Phaethon fLyby and dUst Science. Later this decade, the DESTINY+ spacecraft will fly past Phaethon. So it’ll image its rocky surface and study any dust that might exist around this enigmatic asteroid.

Bottom line: The asteroid Phaethon is the parent object of the annual Geminid meteor shower. It forms a comet-like tail made of sodium, not dust.

Via NASA