Asteroid Phaethon – parent of Geminid meteors – gets weirder

Asteroid Phaethon: Space rock venting glowing yellow streamers with bright sun in mid distance.
Artist’s concept of asteroid 3200 Phaethon – parent object of the Geminid meteor shower – near the sun. This asteroid is already strange because it has a comet-like tail. But the tail isn’t made of dust. As the sun heats asteroid Phaethon, its surface gets so hot that sodium inside Phaethon likely vaporizes and vents into space. Astronomers now think the sodium is what causes Phaethon to brighten – as comets do near the sun – and to form a comet-like tail. Image via NASA/ JPL-Caltech/ IPAC.

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

Help! EarthSky needs your support to continue. Our yearly crowd-funding campaign is going on now. Donate here.

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.

Bright dot moving across gray background with many random flashing white speckles.
This 2-hour sequence of images from the Solar and Heliospheric Observatory (SOHO) shows Phaethon (circled) moving relative to background stars. SOHO captured these images on May 15, 2022, when the 3.4-mile-wide (5.5 km) asteroid ventured close to the sun. It was 13 million miles (20,921,472 km) distant then. While SOHO routinely observes the sun, it also observes many objects that pass near the sun, including comets and asteroids. The random white specks are energetic particles, or cosmic rays, that constantly bombard the SOHO camera. Image via ESA/ NASA/ USNRL/ Karl Battams.

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.

Two panels. Left shows bright spot with short tail. Right is blue background only.
View larger. | The Large Angle and Spectrometric Coronagraph (LASCO) on the Solar and Heliospheric Observatory (SOHO) imaged asteroid Phaethon through different filters as the asteroid passed near the sun in May 2022. On the left, the sodium-sensitive orange filter shows the asteroid with a surrounding cloud and small tail. This suggests that sodium atoms from the asteroid’s surface are glowing in response to sunlight. On the right, the dust-sensitive blue filter shows no sign of Phaethon. Thus, the blue filter indicates that the asteroid is not producing any detectable dust. Image via ESA/ NASA/ Qicheng Zhang.

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.


April 28, 2023

Like what you read?
Subscribe and receive daily news delivered to your inbox.

Your email address will only be used for EarthSky content. Privacy Policy
Thank you! Your submission has been received!
Oops! Something went wrong while submitting the form.

More from 

EarthSky Voices

View All