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Asteroid Phaethon, cause of the annual Geminid meteor shower, has an unexpected kind of tail [1]

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Date: 2023-04-26  

Asteroid 3200 Phaethon, which gets closer to the Sun than any other named asteroid, has a cometlike tail - but what is it made of?

Most annual meteor showers are known to be caused by comets. When comets get close to the Sun, ice from their surfaces vaporizes and carries along some dust particles. When Earth later crosses the comet’s debris trail, those dust particles hit the Earth’s atmosphere and burn up, giving us “shooting stars”.

But the Geminid meteor shower we see every December is different. It seems to be caused by the asteroid 3200 Phaethon, whose orbit lines up just right to account for it. Most asteroids don’t actively shed material and show a tail, but a group of 40 or so of them, including Phaethon, are known to do that. These are called the “ active asteroids ”.

Phaethon gets closer to the Sun than any other named asteroid. Here’s an animation of the orbit of Phaethon (magenta) that shows a close approach to Earth (blue) on Dec. 17, 2017:

The orbit of asteroid Phaethon (don’t worry; it won’t be a threat to Earth for at least 400 years)

Good old Arecibo Observatory in Puerto Rico did some radio imaging of it on that 2017 close approach (within 6.4 million miles of Earth), and here you can see it spinning counterclockwise:

The problem with Phaethon, though, is that it was only found in 2009 to have a tail and emit any material at all, and what it does emit isn’t nearly enough to account for the debris trail that causes the Geminids. Phaethon’s closest approach to the Sun heats its surface to about 1200°F, and it does brighten and show a small tail on those approaches, like a comet. But that’s much too hot for ice, even in Phaethon’s interior, to have survived all this time — it’s only 3.5 miles wide — so it’s not quite behaving like a comet.

But if not dusty ice, then what is it emitting? It had been suggested starting in about 2010 that maybe Phaethon can eject dry dust because the heat gets so intense when it nears the Sun that it breaks rocks on Phaethon’s surface apart, and then solar radiation pressure pushes the pieces away. That’s a convenient idea, but we now know that it is not correct.

We get the answer to at least this part of the mystery surrounding Phaethon in an April 25 report in The Planetary Science Journal by a collaboration among Caltech, the U.S. Navy, the University of Maryland, and Boston University.

They were able to schedule observation time on the Large Angle and Spectrometric Coronagraph (LASCO), hosted by the Solar and Heliospheric Observatory ( SOHO ). This mini-observatory keeps an uninterrupted eye on the Sun and its environs from its stable orbital spot between the Earth and Sun, at the L1 Lagrange point:

Lagrange points are positions in space where objects sent there tend to stay put. At Lagrange points, the gravitational pull of two large masses precisely equals the centripetal force required for a small object to move with them. These points in space can be used by spacecraft to reduce fuel consumption needed to remain in position.

They knew Phaethon would reach its closest approach to the Sun on May 15, 2022, so they observed it through different filters. Here’s a particularly illustrative pair of filtered images:

LASCO images of asteroid Phaethon through different filters as the asteroid passed near the Sun on May 15, 2022. Left: the sodium-sensitive orange filter; Right: the dust-sensitive blue filter. In the upper right corner, Ꙩ indicates direction to the Sun, while +v indicates the direction of Phaethon’s motion.

First of all, they were able to rule out dust grains of any kind with these observations, because those should be clearly visible through a blue filter, and you can see in the frame at right that they are not at all.

But what does show up clearly (at left) is light that makes it through an orange filter that is designed to capture and enhance the light from glowing sodium. Without this filter, it’s very hard to see the tail, much less figure out what it is, and that’s why it’s gone unnoticed until very recently.

You might remember from a chemistry class that different elements glow with different colors in a flame, and that sodium glows orange:

You’d think green comets were green because of copper, but it’s actually because of dicarbon

Every type of atom has different energy levels that electrons can occupy, and when heated, an electron can jump to a higher energy level. But then it has to come back down, and when it does, the extra energy has to go somewhere. And so it’s emitted as a photon (light).

A garden-variety sodium atom has one electron in a “3s” orbital, and when that electron absorbs some flame energy (“thermal excitation”), depending on how much energy it absorbs, it can jump up to a few different higher-energy states. When it drops back down to the “3s” orbital again, it gives off either ultraviolet (and thus invisible) emission in a couple of cases, or orange emission in another case (and that’s the one we can see):

Quantum mechanics in action. An electron can absorb discrete amounts of energy, and then it can only release those same discrete amounts of energy, which are manifested as monochromatic light. The “3p” level is actually two closely spaced levels with slightly different energy due to two possible states of angular momentum, so the emission upon falling back down to “3s” is actually two wavelengths that are very close to each other

Comets that approach the Sun usually do have at least some minor component of sodium emission in their nuclei and/or tails, and it’s often quite small compared to their dust and vapor emissions. But this kind of sodium emission has never been documented for an asteroid — even an “active asteroid” — until now. Phaethon is the first.

The timing of this finding is good, because just a couple of weeks ago, there was a viral (among astrogeeks, anyway) photo of Mercury that came out, showing its sodium tail — itself discovered only in 2001 — in terrific (I would say unprecedented) detail through a 589-nm filter (to really capture the sodium emission as efficiently as possible):

Sodium atoms are freed from Mercury’s surface by the push of sunlight and micrometeorite impacts. The sodium atoms from the surface are blasted into Mercury’s atmosphere and into space, where they form the planet’s tail.

OK, so we have our first known asteroid with a sodium tail. That’s very cool, but …. what about the Geminid meteor shower, then? This puny sodium emission can’t come anywhere close to explaining the trail of debris that hangs around Phaethon’s orbit.

Phaethon is probably not a body that recently held a lot of ice because it seems to have formed in the inner Solar System and doesn’t share physical characteristics with comets that originated out in the Oort Cloud. Comet debris trails can hang around for a few thousand years, maybe, but if Phaethon ever had any ice in it, it would have been vaporized out of there a lot longer ago than that. So it doesn’t seem like Phaethon causes meteor showers in the usual way.

That means some kind of mystery event must have happened in Phaethon’s past that we don’t know about. Did a gravitational interaction rip some of it apart? Did it collide with something?

One other wacko thing about Phaethon is that it’s one of only 11 asteroids known to have a spin that is accelerating with time. We don’t know why that is, but could an increasingly faster spin have flung some debris off of Phaethon’s surface?

The increasing capabilities of the U.S., Europe, Japan, and a good number of other countries are bringing us to the verge of an explosion of knowledge about our own Solar System. There’s so much more to come, even right here in 2023.

In fact, next year, the German-Japanese joint mission DESTINY+ launches, reaching our good friend Phaethon in 2028, ready to take a closer look at this curious fellow.

Or, if you prefer the original….

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[1] Url: https://www.dailykos.com/stories/2023/4/26/2165934/-Asteroid-Phaethon-cause-of-the-annual-Geminid-meteor-shower-has-an-unexpected-kind-of-tail

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