Thread by @astrotweeps

PHAETHON TIME
(a thread on asteroid (3200) Phaethon, why I like it, and how and why we can study things that look like dead comets)

I'll start with a small anecdote as to how I got introduced to Phaethon. Back in August, I mentioned to my advisor that I would be going to Hawaii to observe an asteroid sometime during that semester (fall 2017).

He asked if I was going to observe Phaethon and I said I wasn't sure and didn't know what Phaethon was. He said something like "You should look it up, it's pretty cool." Which, again, is science-talk for "this thing is nuts."

So I looked it up - it's an asteroid that also has a meteor shower that also develops a dust tail when its close to the sun - and I was *stoked*. It seemed confusing, defied easy explanation, and was going to get very close to the Earth in December, 2017. Like ~0.07 AU.

There's just one problem with this -- I wasn't going to Hawaii to observe it. The class, taught by Dr. Vishnu Reddy @moonyguy and Dr. Walt Harris, was going to Mauna Kea to observe a totally different interesting near-Earth asteroid as a NASA training exercise.

Which, as a side note, was wonderful! I learned how to operate the NASA Infrared Telescope Facility (IRTF), which is a bread-and-butter kind of telescope for studying asteroids and comets. Here's the only nice picture of me taken on the whole trip:

The telescope behind me is the University of Hawaii's 2.2 meter telescope, I think. The IRTF's main mirror is about 3.0 meters in diameter. Here's an image of that telescope's building. (@benjaminsharkey is the hatted figure, I think.)

If you're interested in hearing a little more about the October observing I did (the training exercise), take a look at this JPL press release. jpl.nasa.gov/news/news.php?…

It was awesome, honestly. There's a really visceral sense of the importance of that mountain when you're on it.

So backing up a few months, I ran back into my advisor and mentioned that I wouldn't be observing Phaethon when I was there. He would be going (to assist with observations planned and being led by Dr. Reddy) to observe Phaethon in December, however.

So, I started doing what any graduate student would do: quietly asking over and over and over again to let me help out and let me go to Hawaii to observe this super radical asteroid-thing, Phaethon. Eventually, somebody gave in, and I was allowed to come along and help out.

Which, again, was an incredible opportunity which I am supremely thankful for. You can see how thankful I am in this image of me at the IRTF in December, where I am pale and sweaty from oxygen deprivation. (The IRTF is at ~13,700 feet above sea level.)

In case that it was not abundantly clear - that was a joke. I am still super thankful and appreciative to everyone who helped me attend and help out with this cool observation.

The IRTF's mirror, you know, the actual telescope, is behind me in the image.

Telescopes with large primary mirrors - so, anything bigger than something your lucky friend would have had when you were a kid - have support structures to make sure everything stays aligned. The orange metal in the background as well as the lower bits of beige metal are that.

If you haven't been to a big research-grade (another phrase meaning 'big', really) telescope, it's hard to imagine such a large piece of equipment swiveling around to look at a particular point of the sky. But it does! It's really fun to watch.

For the observations we conducted of Phaethon in December, we had another co-observer: Dr. Tomoko Arai (@Apollotom ), who leads the future JAXA mission to asteroid Phaethon, DESTINY+. Here's a picture of us, also including @DSLauretta and @moonyguy .

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The observations went really well! Phaethon is an object that a *ton* of people like or are super interested in in the small-body-science community, so as soon as we finished looking at Phaethon, other groups immediately started looking at it as well.

I'm about to start talking about the actual science-of-phaethon, but I'd like to reiterate that Phaethon came *historically* close to the Earth in December. The best observing opportunity (closer=brighter) for a while to come.

That means that the newest and most interesting results are likely still being worked on, as the scientists have only had their data for a few months. It's definitely possible that a lot of what I am about to summarize will be inaccurate by the end of the Summer. Which is cool!

So, setting aside the fact that everything we know about Phaethon may get updated a bit (or a lot!) in the coming months, what *do* we know about it?

1.) It was discovered in 1983 by the IRAS telescope -- which was an infrared space telescope ("the infrared astronomical satellite", if you're a nerd). An asteroid by a space satellite doesn't seem so crazy nowadays, but that was a minor event then. Phaethon was the first!

2.) Phaethon was quickly noted (e.g. Whipple, 1983) that it had a *Very Similar Orbit* to the Geminid Meteor Shower. That meteor shower peaks in the middle of December, which is when the Earth passes through the meteoroid stream.

A small side note: you'll notice if you've been paying attention that the close approach of Phaethon to the Earth was in December *and* that the Geminids are seen from the Earth in December. That's because they have very similar orbits!

A lot of (extremely hard working, smart, driven) people work on trying to match meteor showers to "parent bodies" - the object (usually a comet) that made / released / is associated with the meteors. Finding the Geminid parent was an asteroid was ... unexpected.

Phaethon was identified (and is still identified) as an asteroid. Until Phaethon, all known meteor shower parent bodies were comets. And that makes sense! Comets release gas and dust along their orbit, so meteoroid streams are the trails of left behind comet dust.

So, the Geminid parent body, the thing that must have released all the dust that makes up the modern Geminid meteors ... is an inactive rock? That didn't make sense. Here's a picture of a rock. Does it look like it's making a meteor shower to you? No. That's nuts.

Worth noting that the above gif is perhaps not representative of all known rocks. The point is, this connection between the Geminid Meteor Shower and Phaethon, an asteroid, was not easy to explain initially.

3.) Phaethon's orbit takes it closer to the Sun than any other *numbered* asteroid.

Asteroids get numbers (e.g. 3200 for Phaethon) when their orbit is sufficiently well known, usually meaning that we've observed it throughout multiple orbits.

This means that Phaethon, like that comet I was mentioning yesterday (96P!), gets incredibly hot at perihelion (again, the point in its orbit closest to the Sun). If you're using Celsius, it's probably in the 1200-1275 degrees range. Very hot!

So, to say it a little more bluntly: it's so hot that some metals start melting. In meteorites, as a corollary, there are a *lot* of changes that start to happen to the structure of minerals at temperatures much colder than Phaethon.

So really, Phaethon isn't cooked, it's been deep fried. We can then ask, if Phaethon *used* to be a comet but then got put in the deep frier of the solar corona, would it start looking like an asteroid?

HYPOTHESIS: Phaethon is a 'dead comet', meaning that it used to display conventional cometary activity, but time and heating have caused that activity to dwindle to undetectable levels.

This idea explains some things easily, but doesn't do others as well.

The Good: it explains how it created the Geminids, but no longer is able to. It explains (at least qualitatively) the incredibly eccentric orbit that brings it so close to the Sun.

The Bad: The orbit it has, while eccentric like many comets, isn't really cometary. One paper that gets referenced a lot (Bottke et al., 2002) suggests that while they are *not sure* where Phaethon came from, it probably came from the Main Belt.

The Ugly: There are a few other features of understanding Phaethon that don't clearly prove or disprove this hypothesis. One of them is 2005 UD, an asteroid in a very similar orbit to Phaethon that many studies have shown probably broke off of Phaethon.

So, throughout the 1990s and 2000s, a ton of great work (which I greatly admire!) came out trying to explain Phaethon in other ways. Often, these papers seemed to take the Bottke et al. 2002 result and try to explain it as a main-belt asteroid that wandered near the Sun.

I won't try to highlight every paper (because I'd still miss a bunch of really important ones written by very smart people!), so instead I'm going to highlight two that sort of personify the normal asteroid understanding of Phaethon.

In 2007, Licandro et al. published a paper with detailed spectroscopic studies of Phaethon. ( aanda.org/articles/aa/fu… ). In short, it's a B-type asteroid that has a spectrum indicative of *hydrated minerals*. Let me explain all that:

Asteroids are classified by how they reflect light, generally. To push a lot of details under the rug, Phaethon is a B-type asteroid because B-type asteroids are classified as being better at reflecting blue light than red light. (They're also quite dark, sort of charcoal-like.)

You can compare how asteroids reflect light to how meteorites (and other substances) reflect light and see if you can put two-and-two together and try to understand what an asteroid is made out of. It's easier in some cases than others.

So when Licandro et al. (2007) say that the reflectance of Phaethon is indicative of hydrated minerals, they're saying that they could make a substance with similar reflective properties to Phaethon by combining things that include hydrated minerals.

It's worth noting that this can be a *very intensive* process. It's a lot of modeling and understanding the optical properties of materials and rocks and meteorites. If you're interested in combining lab work with telescopic data, this is an avenue for you!

The reason that you, my most esteemed reader, should care about this result is that they suggest that Phaethon is really the end product of what we call Main Belt Comets / Active Asteroids. They have asteroid-like orbits, but occasionally develop comet-like activity.

While some Main Belt Comets (MBCs) develop comet like activity because they're falling apart (e.g. "disrupting") or have been impacted, some really do have normal comet like activity, driven by the sublimation of ices.

A hydrated surface on Phaethon would imply that there was some water at some point to change the mineral structure of things on the surface of Phaethon. Lots of people, @asrivkin included!, were looking to test this hydrated surface hypothesis in December.

The other paper is de Leon et al. (2010). They compare spectra of Phaethon and many other B-type asteroids (so, ones that look similar to it spectroscopically) and note that it looks pretty similar to the biggest B-type asteroid there is, (2) Pallas.

Furthermore, they do some cool dynamical work (e.g., writing a computer program to see how the orbit of an object evolves in time) to see if you can knock a chunk of Pallas off and have it slowly migrate into an orbit like Phaethon's. It can totally work.

So, they propose that (3200) Phaethon is a chunk of (2) Pallas.

Summary of this bit:
HYPOTHESIS: Phaethon is a main-belt asteroid, perhaps from (2) Pallas, that used to display main-belt-comet-like activity, maybe driven by its orbital migration inwards.

I know that 'dormant main belt comet' vs. 'dormant comet' vs 'burned up chunk of pallas' might seem like a rather silly distinction to get excited over. But! It's worth noting that Phaethon has been studied for a long time, and the trial isn't over yet. Still need more data.

Oh yeah, and then there's this result from 2010 that complicates everything even further. Take a look at this image from Jewitt and Li, 2010. It's the track of Phaethon as seen by the STEREO sun-facing spacecraft.

If you look at the image and analyze it computationally, you can measure the brightness of Phaethon as it moves through its close approach to the Sun. The result: it was twice as bright as you would expect it to be. The authors say this means that Phaethon is active *today*.

In Jewitt, Li and Agarwal (2013), they even did some crazy image analysis (that I cannot say I fully understand) to see Phaethon's dust tail. An image from that paper:

So, at perihelion, when Phaethon is at its hottest, it develops a dust tail. And we've seen Phaethon do this multiple times now. It has repeated activity driven by (we think) how close it is to the Sun. Hmmmm. Totally normal nothing to see here.

So, yeah. It's an asteroid. Which does comet things. And also has an orbit that goes through the solar corona. And has a tiny relative (2005 UD) or two (1999 YC). Phaethon has a lot going for it. Or, to put it less science-y...

ITS SO COOL OMG