Breaking up is easy to do. If you’re a comet.

By Phil Plait | November 5, 2012 7:00 am

So there’s this comet named 168P/Hergenrother. It’s one of a bazillion such iceballs orbiting the Sun, but this one turns out to be more interesting than most. For one thing, it has a short period, orbiting the Sun once every 6.8 years or so. Its orbit goes out to about that of Jupiter’s, and reaches down into the inner solar system about as far as Mars. It never gets closer than about 80 million kilometers (50 million miles) to us, so it’s usually relatively faint, and you need a big ‘scope to observe it.

It was discovered in 1998, and made a second pass down our way in 2005. This year, 2012, it came by again, and folks around the world observed it as they do any comet. But then, in September, it gave us a surprise. A big one. Lots of observers were reporting that practically overnight the comet grew hugely in brightness, getting as much as 700 times brighter than expected! Not only that, but observations showed the shape of the comet had changed, going from fairly point-like to much fuzzier.

That could mean only one thing. The comet was breaking up.

The picture above is from the Faulkes Telescope North, located on the Hawaiian observatory on Haleakala. It’s a composite of lots of separate exposures that were added together; you can see the stars are trailed (actually stippled; each exposure was short but then shifted to line up on the comet). The comet is the bright fuzzy blob in the upper right, and if you look just below the main part you can see a second fuzzy blob, much fainter.

That’s proof positive the comet calved, or had a big chunk break off. In fact, observations using the huge Gemini telescope show that the main body has broken up into at least four pieces! So what does this mean?

First, don’t panic. We’ve seen this happen to comets before, and this one is so far away from us we’re in no danger at all. It literally cannot get near us.

Second, it’s very interesting scientifically. Comets are basically big frozen snowballs peppered with rock. Imagine scooping up a handful of gravel and snow and then packing into a loose ball. That’s a comet, if your snowball is several kilometers across and the ice is actually frozen water and carbon dioxide. When they are far from the Sun comets stay frozen and are exceedingly dim. When they get closer, the ice goes directly to a gas (called sublimation), and escapes from the solid part (called the nucleus). It expands and can form a big fuzzy head around the solid nucleus that can be tens of thousands of kilometers across, bigger than planets! This is also what gets blown back by the solar wind (and the pressure of sunlight) to form the tail(s) of the comet.

This means that every time a comet gets closer to the Sun and starts to sublimate, it dies a little bit. Material leaves the comet and never comes back. But that ice is what holds the comet together! So sometimes enough ice turns into a gas and escapes that the comet gets substantially weaker, and big chunks of it can dislodge, falling away. That’s what appears to have happened to Hergenrother.

While we’ve seen this before with other comets, it’s not like it happens every day, so any chance to see this occur is fascinating. In 2006 we watched as comet 73P/Schwassmann-Wachmann 3 literally disintegrated. Even more amazing, in 2007 the run-of-the-mill comet 17P/Holmes suddenly erupted, getting hugely brighter, and a huge shell of dust was seen to be expanding around it. Now we think Holmes collided with a small asteroid, and the violence of the event blasted off the material. I saw Holmes with own eyes when this happened, and even though it was past the orbit of Mars, the shell of dust was easily visible to the naked eye. It was awesome.

No two comets are ever really alike. They have different sizes, shapes, compositions, and orbits. And each will behave slightly differently as they round the Sun and head back into deep space. If there’s a lesson from Hergenrother, it’s this: it’s always a good idea to keep an eye on everything in the sky. Just because something looks routine now doesn’t mean it won’t try to pull a fast one later.

Tip o’ the Whipple Shield to AsteroidWatch NickAstronomer. Image credits: Hergenrother: LCOGT/Giovanni Sostero, Nick Howes, Alison Tripp & Ernesto Guido; Holmes: Tamas Ladanyi

Related Posts:

Ten Things You Don’t Know About Comets
The Sun ate another comet
Hubble peers into the weird heart of Comet Holmes
Rosetta: mission to land on a comet

CATEGORIZED UNDER: Astronomy, Cool stuff, Pretty pictures

Comments (9)

  1. Bill T

    “So sometimes enough ice turns into a gas and escapes that the comet gets substantially weaker, and big chunks of it can dislodge, falling away.”

    Or a crack crew of oil jockies flew up there, drilled a hole into the comet, planted a nuclear bomb and blew it apart.

    One of the two.

  2. Phil

    Cool, but no video?

  3. Chris

    Something to note. Even though it is brighter, it is still around a magnitude 8, so unfortunately you can’t see it with the naked eye.

  4. Nic

    So we are in a warm, nay hot climate! This boy has been cruising around for a long long time, and calves now..

    It is hot here.

    It’s 2C/35F outside. Blistering for a comet!

    Thanks Phil, I will continue to follow you on your new site.
    Be well.


  5. Ragutis

    So, after the icy materials are all gone, we have dark chunky clouds of rocky stuff in cometary orbits? Also, does the loss in mass comets undergo every cycle noticeably affect the period or trajectory?

  6. The fourth fragment is hard to detect in the Gemini picture (or I’m getting older faster than I wish ;-), so I tried some enhancements. Couldn’t get much but you might look for yourself.

  7. Chet Twarog

    Can we do without up/down words such as in: “Its orbit goes out to about that of Jupiter’s, and reaches DOWN into the inner solar system about as far as Mars.”
    Where the heck is “down in the inner Solar System”? It didn’t “go UP out to about that of Jupiter’s [orbit]”.
    Just getting tired of drivers blowing through STOP signs or “up Norths and down Souths; sun UP or sun DOWN.” We live on a planet rotating on its axis that’s spiral orbiting a star orbiting…. . Why are we unable to think that way?

  8. Andy

    Its worth mentioning that the split was first discovered by an amateur astronomer on his lunch break at work (Nick Howes – I think). He describes the discovery in a recent 365 days of astronomy podcast.

  9. Jon Hanford

    “Now we think Holmes collided with a small asteroid…..”

    I’ve also seen work that models the outburst from Comet Holmes as a rapid, explosive sublimation of material from just beneath the comet’s surface (and could also account for a second outburst from Holmes at discovery in 1892):

    Btw, I too was lucky enough to see Holmes’ 2007 cruise through Perseus under the light dome of Tampa. Quite a memorable sight.


    I can also remember back to high school days and observing the breakup of Comet West in 1976:


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