Comet ISON Plays Coy With Astronomers


Hubble Space Telescope got its first look at comet ISON back in April and found that it exhibited some curious properties. Credit: NASA/Hubble Space Telescope
Hubble Space Telescope got its first look at comet ISON back in April, and the newly released findings show that that the icy interloper exhibited some curious properties. Credit: NASA/Hubble Space Telescope

The latest observations from professional and amateur astronomers around the world show that the much-hyped comet ISON is still very much alive, and new data suggests that it may very well survive its close swing by the Sun on November 28.

The big question on skywatchers’ minds, however, is how good of a sky show ISON will put on in the coming weeks and months. (See also Comet ISON: Pop or Fizzle?)


New studies presented at the American Astronomical Society’s Division for Planetary Sciences meeting this week in Denver suggest that ISON is not a puny comet. Its nucleus measures somewhere between 1 to 4 km across—which bodes well—meaning that it shouldn’t disintegrate from the intense heat during its sun-grazing pass.


This image shows comet ISON as it appeared to pass by the planet Mars on October 5, 2013. Credit: Michael Jäger, Weißenkirchen, Austria

Hubble observations from back in April  hint that the axis of spin (or pole) of the comet’s nucleus is continually pointing towards the Sun. If this is the case, then one hemisphere of the comet is always facing away from the sun.

This could be both bad and good news for comet watchers.  As the comet continues to plunge towards the sun, intense solar radiation and tidal forces will hit its sun-facing side,  possibly pulling the comet apart. But because one side of the comet has been in the dark, it has remained frozen and in pristine condition.

As of early October comet ISON  has formed a distinct tail and shines with a green glow, but is still a telescopic object shining at magnitude 11.

According to Hubble data, ISON  should continue to possess the same axis of rotation until it rounds the sun at the end of November, at which time the other, untouched hemisphere will suddenly get blasted with scorching heat as it starts its journey back to the outer solar system.  The sun’s fire may possibly cause an outburst of gas and dust as the comet’s hitherto unexposed surface-ice sublimates at high rates.

“We measured the rotational pole of the nucleus.  The pole indicates that only one side of the comet is being heated by the Sun on its way in until approximately one week before it reaches it closest point to the Sun,” said Planetary Science Institute scientist Jian-Yang Li, who led the team that imaged the comet with Hubble in news statement.
“Since the surface on the dark side of the comet should still retain a large fraction of very volatile materials, the sudden exposure to the strong sunlight when it gets closer to the Sun than Mercury could trigger huge outbursts of material,” Li said.

If these predictions hold true, then the best sky show for ISON may come in December and into the new year. However because these observations date back to April, the comet may have changed its rotation since then. Hubble is scheduled to do follow-up observations this month and the new data will surely help narrow down comet ISON’s potential performance.

For now it’s anyone’s guess what ISON will do—so we’ll just have to wait and see.


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Changing Planet

Meet the Author
Andrew Fazekas, aka The Night Sky Guy, is a science writer, broadcaster, and lecturer who loves to share his passion for the wonders of the universe through all media. He is a regular contributor to National Geographic News and is the national cosmic correspondent for Canada’s Weather Network TV channel, space columnist for CBC Radio network, and a consultant for the Canadian Space Agency. As a member of the Royal Astronomical Society of Canada, Andrew has been observing the heavens from Montreal for over a quarter century and has never met a clear night sky he didn’t like.