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SYFY WIRE Bad Astronomy

The trail of Phaethon

By Phil Plait
The optical dust trail shed by the asteroid Phaethon (between the two red arrows) seen by the Parker Solar Probe. The position and size of the Sun is shown to the left, and the fuzz in the middle is the Milky Way.

Did you catch any of the Geminid meteors in mid-December? It's one of the best meteor showers of the year, occurring when the Earth plows through a debris trail left behind by the asteroid Phaethon as it orbits the Sun and tiny bits of asteroidal material burn up in our atmosphere. Sadly, this year was seriously hampered by the nearly full Moon, so the show from Earth wasn't as good as it could've been.

But from space, the story is different. Really different: A spacecraft designed to investigate the Sun (which, I'll note, is significantly brighter than your typical meteor) saw the Geminids …. Kinda. Not the meteoroids themselves, but the combined glow of their dust trail orbiting the Sun millions of kilometers away.

This is so cool.

Most meteor showers are due to comets. When a comet gets near the Sun, the ice in it turns to gas, releasing tiny bits of dust and gravel that then form a long debris trail that orbits the Sun in more or less the same orbit as the parent comet. If that orbit crosses Earth's we get a shower.

The Geminids are different. Their parent is 3200 Phaethon, an asteroid. It has an orbit which brings it extremely close to the Sun, just 21 million kilometers from that stellar furnace. The rock gets so hot it vaporizes, so in this case it acts a little bit like a comet (astronomers call it an "activated asteroid"). We know Phaethon is the parent because its orbit is essentially identical to that of the meteors coming in (the streaks they leave in our air as they burn up can be traced backwards, and an orbit found from that motion).

STEREO observation of 3200 Phaethon, showing a stream of debris that will eventually become Geminids.

Although Phaethon only sheds particles when it's near perihelion (closest approach to the Sun), we know those bits of junk spread out along its orbit, because we get the Geminids every year, no matter where Phaethon is in its orbit.

This dust reflects sunlight, making it visible … in principle. In practice, from Earth, it's nearly impossible. It's just too faint (worse, it's brightest near the Sun, which poses an obvious issue). But from space it certainly is possible.

The Parker Solar Probe launched in August 2018 into an elliptical orbit around the Sun, currently getting as close as about 20 million km (it passes close to Venus, using the planet to change the spacecraft's orbit, dropping it close to the Sun over time). During its November 2018 pass of the Sun, the wide-field camera on board took images in the right direction to see the Phaethon Geminid dust trail. And after a lot of complex image processing, the trail is indeed visible!

The optical dust trail shed by the asteroid Phaethon (between the two red arrows) seen by the Parker Solar Probe. The position and size of the Sun is shown to the left, and the fuzz in the middle is the Milky Way.

The trail can be seen very faintly between the two red arrows. The match between it and the path of the asteroid leaves no doubt that's what this is.

Making a few reasonable assumptions, the astronomers were able to estimate the total mass of the dust orbiting the Sun, and found it to be very roughly a billion tons. That may sound like a lot, but it's a small fraction of the total mass of Phaethon itself, which is about 6 km wide. If you could gather up all the dust in the trail it would make a sphere only about 800 meters across — only about 0.2% of the volume of the asteroid.

Interestingly, they also found that Phaethon loses about 100,000 tons of material every pass of the Sun (which it does every 1.4 years). That's far less than what's seen in the dust trail, which means what's seen is likely old (it takes a long time to shed that much material). Other estimates of the total mass of the trail (based on the mass accumulated by the meteors themselves every year) are larger, and it's not clear why this one is lower, though it may be due to the assumptions the astronomers had to make to do the calculation (like the size of the dust grains).

The orbit of the Parker Solar Probe (dashed black line) and Phaethon (blue) as seen from above the solar system looking down (top) and from the side (bottom). The Sun is marked by the star, the Earth by the green dot.

This isn't the first time the dust trail has been spotted; along with trails from several comets it was seen extremely faintly in data from the Cosmic Background Experiment after some careful reprocessing. In 2013, another space-based solar observatory, STEREO, also saw Phaethon and spotted it losing dust. That's not exactly the same as seeing the trail itself, but it showed for the first time the asteroid actively shedding material.

These sorts of observations are tricky because the trails are so faint, but they help us understand how comets — and, in this one case, an asteroid — lose material as they orbit the Sun. This can help us understand just how that material is lost and what happens to it over time as it orbits on its own, and perhaps could lead to better predictions of meteor showers. And, of course, it just helps us understand comets and asteroids better, which is cool.

And we'll get lots more chances; Parker orbits the Sun every 88 days, so it can potentially see this trail 4 times per year. There will be plenty more data to analyze in the coming months and years to look for the faint, ghostly debris from this Sun-diving asteroid.

P.S. I'll note that the lead author on the paper, Karl Battams, is a pal of mine, and we've had many fun discussions about comets that plunge toward the Sun (called Sun-grazers) over the years. He commonly tweets about this, and if you're on Twitter you should follow him.