The asteroid with a surprising tale

With the Geminid meteor shower reaching its peak this weekend – as the Earth passes through the debris trail left by the asteroid Phaethon – here’s an interview I did at EPSC 2013 with Professor David Jewitt about his discovery of the rocky body’s unusual behaviour.

(3200) Phaethon is responsible for the Geminid meteor shower (illustration). Credit: Will Gater

What prompted you to study Phaethon?

We were interested in the Geminid meteoroid stream and where the Geminid meteors come from. When the parent of the Geminid meteors, (3200) Phaethon, was discovered and found to be an asteroid it was interesting to ask why would an asteroid produce dust particles. Phaethon was identified in 1983, but until recently it showed no evidence of any strange behaviour. It just looked like an asteroid, which it is, but it didn’t seem to produce any material or show any obvious mechanism for producing dust. The breakthrough was to use a solar telescope [STEREO] to look at it when it’s near the Sun, when it’s very very hot, instead of using a big night-time telescope when you tend to look away from the Sun. And apparently Phaethon is active when it’s near to the Sun but not far away from it.

Artist’s impression of one of the STEREO spacecraft. Credit: NASA/Goddard Space Flight Center Conceptual Image Lab

So these were relatively recent observations?

Yeah, they were taken with a couple of spacecraft [STEREO A and B] that look at the Sun all the time. Phaethon just happens to pass through their fields of view. It’s been doing that for some years, but we just started to look at the data recently and we found this kind of interesting result. We took the individual images from STEREO and we shifted them and added them together, basically to build up the signal to noise and to make a kind of super image with all of the motion removed. Then we could see the elongation [of the tail] pretty clearly.


And that tail only emerges around the time when Phaethon is close to the Sun?

Yeah, when Phaethon is at its closest to the Sun – 0.14AU, so it’s really close.

Phaethon’s elongated tail appears in the processed STEREO images. Credit: Jewitt, Li, Agarwal/NASA/STEREO


Is the tail material that you’ve detected around Phaethon responsible for the Geminid meteors?

The particles that we see are small and we can tell that because they are pushed very strongly by radiation pressure from the Sun. Radiation pressure is very weak but it can accelerate these dust particles strongly. That’s only possible if they are really tiny, like a thousandth of a millimetre. But the rocks that make the meteors that you can see at night time are more like a millimetre in size so they are individually much bigger than the ones that we see. But on the other hand it’s quite possible, in fact likely, that they are there [in Phaethon’s tail]. You have a distribution of particles sizes – lots and lots of little tiny ones and not so many big ones. The big ones cause the meteors but if you’re just looking at reflected light it comes mostly from the small ones. So we naturally detect the small ones.

So in amongst the small particles you’ve seen there must be bigger ones?

Right, and so the issue is can we go the next step which is to find those bigger guys and prove that those are the Geminid meteoroids being produced now. That would answer the question “is Phaethon still producing the Geminid meteoroids?” Or was there some historical event that was like an impulsive explosion of stuff that came out and that made this ring of debris that the Earth goes through every year. Or is it an active process and this body is continuing to decay.

How can we see those larger particles?

We can’t see them with the solar observing spacecraft, we’ll have to use night time telescopes. We have to see Phaethon in a dark sky and try to see these comparatively rare big particles just by looking very carefully at night time with some big telescope somewhere.

What’s the mechanism releasing the small particles?

We think they’re just being fractured off the surface because the surface is so hot. If you take a rock and heat it up it’s going to expand; a typical rock is made of little grains of different minerals and they’ll all expand at slightly different rates. So if you just heat up a rock it will tend to fracture itself because one part is going to expand more than the other. And when it fractures some of the energy of the expansion goes into the kinetic energy of the fragments. So we think they’re almost like popcorn, bursting on the surface of the object and popping off. And as soon as they leap off the surface then this weak pressure from sunlight can blow them away into a tail.

How long is the tail?

The piece that we see is 350,000km long and it appears in one day. So it goes from nothing to 350,000km in one day. To get up to that speed in such a short time you need a big acceleration. And that’s how we infer the particle size – they have to be small and light to be accelerated quickly to go into the tail.

How hot does Phaethon get when it’s close to the Sun?

At midday on the surface of Phaethon with the Sun directly overhead you have the peak temperature, which is about 1000K [over 700°C]. We know that if you take rocks that we find on the Earth and heat them up to those same temperatures they do crack. Some rocks contain water and they dry out, shrink and crack at temperatures even lower than that.

And that’s hot enough to rule out water-ice helping form the tail?

Yeah. There’s no way to keep water ice at the surface – it sublimates so quickly.

Finally, you’ve also ruled out the tail being a sodium tail. Why’s that?

Well if you look at comets, there are some that go close to the Sun. Some of those comets show a sodium tail. The sodium seems to come from rocks; it is boiled out and gets blown away, making a distinct sodium tail. So I thought maybe that could be the case also for Phaethon. But, in fact, it can’t be because we have a filter on the camera that excludes light from sodium so we couldn’t, even if it were there, see it.

You can read the paper announcing the discovery of Phaethon’s tail here: http://arxiv.org/abs/1306.3741

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