Top 5 Comet ISON Questions Answered

Comet ISON, imaged by longtime amateur astrophotograper Damian Peach in the U.K. He used a 4-inch f/5 telescope for 12 minutes of combined exposures on November 15th. Credit: Damian Peach /
Comet ISON, imaged by longtime amateur astrophotographer Damian Peach in the U.K. He used a four-inch f/5 telescope for 12 minutes of combined exposures on November 15. Credit: Damian Peach/

Comet ISON has captured the imagination of astronomers and the public for the past year, and now we can’t let it go.

While the bizarre icy visitor didn’t manage to live up to the hype of “comet of the century,” it left behind a wealth of new data that will keep the scientific community busy for years.

We’ve rounded up some of the top questions many of you have been asking about Comet ISON:

How big was Comet ISON?

Earlier this year astronomers were able to get their first accurate measurements, thanks to Hubble Space Telescope observations, which showed that Comet ISON’s center, or nucleus, appeared to be much smaller than originally anticipated—around 2 to 4 miles (3.2 to 6.4 kilometers) across.

Why the surprise? Because ISON shone so brilliantly while it was still located around the orbit of Jupiter—some 373 million miles (600 million kilometers) from the sun. Researchers theorized that the comet’s brightness meant that the dirty ball of ice, gas, and dust must have been a whopper. Also, Hubble observations made at this time found that the gaseous and dusty coma that surrounded the solid body of ISON measured an incredible 3,100 miles (4,989 kilometers) across.

Will Earth pass through ISON’s remains?

There is no chance of a collision with Earth, since any kind of rubble left behind by ISON will continue to float along its original pathway back toward the outer solar system, which will take it more than 39 million miles (63 million kilometers) away from our planet at its closest approach.

However, Earth may slam into dust clouds trailing in the wake of the comet, along its orbital path.

Astrophysicist Karl Battams said in a blog post about ISON that Earth may encounter these comet remnants sometime early next year.

“The net result of this will be at least one or two, if not a handful, of extra shooting stars in the sky over a couple of nights,” Battams wrote.

“Indeed, Earth passes through numerous comet tails every year—that is what the Leonids and Perseids are, for example. So don’t worry about this, and don’t even expect to notice it.”

Why did ISON not live up to its “comet of the century” billing?

If it had held together during its fiery baptism through the sun’s outer atmosphere, Comet ISON would have had the potential to put on quite a sky show.

However, researchers now theorize that the comet suffered a catastrophic breakup a couple of days before its close passage by the sun. Basically, the intense heat and gravitational pull from the sun pulled the nucleus apart and vaporized its entire ice-water reservoir in a matter of hours.

Any chance we can still see Comet ISON in the sky?

As of December 2, the cloud of debris that ISON left behind shines no brighter than a star of approximately 8th magnitude, one that requires binoculars to view.

That’s because all that is left now is a dust cloud, which is spreading out and becoming more diffuse, with less and less light reflecting off its remaining dust grains. Experts conclude that only wide-angle shots from dark skies may pick up some of the fuzzy patch. And since there is no new source material being pumped into space, Comet ISON is now really an ex-comet.

According to, only “experienced astrophotographers might be able to capture the comet’s fading ‘ghost’ in the pre-dawn sky of early December, but a naked-eye spectacle is out of the question.”

What do astronomers hope to learn from Comet ISON?

Part of what has excited astronomers about ISON is that it originated from the Oort Cloud—a frozen reservoir of billions of comets at the outer edge of the solar system.

And because it was buzzing the sun so closely, it gave scientists a unique opportunity to see outgassing of pristine material from within the comet that originally formed when the solar system was born some 4.6 billion years ago. So expectations are that the mountains of data collected about the comet will offer clues about the chemistry of frozen visitors from the outer solar system such as ISON, and may help answer some of the questions surrounding the birth of planets like Earth.

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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.