Hubble Watches as Comet 332P Breaks Apart

Back in January, a team of observers had a hunch that Comet 332P/Ikeya-Murakami was rapidly falling apart — and they were right!

Breakup of Comet 332P

When astronomers turned the Hubble Space Telescope on Comet 332P/Ikeya-Murakami in January 2016, they found two large fragments (dubbed C and A) and more than a score of smaller pieces. To get these images, astronomers used HST's Wide Field Camera 3 and a broadband red filter (the blue tint isn't real). Total exposure: 100 minutes. The box around fragment C corresponds to the portion shown in the animation below.
NASA / ESA / D. Jewitt

Pity the unfortunate fate of Comet 332P/Ikeya-Murakami. It quietly drifted in the Kuiper Belt for 4½ billion years, minding its own business. But about 10 million years ago something happened — perhaps a close brush with another object — and this icy little body found itself redirected on a path toward the inner solar system. Over time repeated tugs from Jupiter locked it into a tight, 5½-year-long orbit around the Sun, where it was spotted on November 3, 2010, by Japanese amateurs Kaoru Ikeya and Shigeki Murakami.

Comet 332P might have lived out its days peacefully. But it was suspiciously too bright (8th magnitude) when discovered, and astronomers soon realized it must have undergone some kind of disruptive outburst. Its coma quickly faded from view.

Then, on December 31, 2015, the PanSTARRS 1 telescope captured the comet on its next return to perihelion — except this time around the comet had split into two large fragments. A team led by David Jewitt (University of California, Los Angeles) quickly obtained observing time with the Hubble Space Telescope and found that Comet Ikeya-Murakami had completely disintegrated into at least 25 pieces.

Comet 332P's slow-motion breakup

The Hubble Space Telescope tracked more than two dozen fragments of Comet 332/Ikeya-Murakami's disrupted nucleus over 3 days in January 2016. Fragment C is at far left; another bright piece, fragment A, is well off to the right of this close-up view.
NASA / ESA / D. Jewitt

Over three days in January (26–28) and two more in April (12–13), Hubble's Wide Field Camera 3 captured the comet's demise. Although it's impossible to know for sure, Jewitt's team suspects that the individual pieces must have been roughly 20 to 60 meters (65 to 200 feet) across. How and why it broke apart isn't certain, but a likely cause is that amorphous water ice in the comet's nucleus spontaneously converted to its crystalline form — a runaway reaction that not only generates heat but also releases any trapped gas. (The same scenario might have led to the amazing outburst of Comet 17P/Holmes in November 2007.)

However, as the team details in Astrophysical Journal Letters for September 20th (a full preprint is available here), there's a problem with that interpretation. "No direct evidence for amorphous ice in comets exists," they write, "and it is not clear that gas drag forces could be sufficient to expel fragments 10s of meters in size, as observed, even against the low gravity of [small] parent nucleus."

One thing seems certain: we're unlikely to see much of Comet 332P — if anything of it remains — during its next perihelion in late 2021.

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