The appearance of a comet in the sky has long been seen as a herald of doom and catastrophe. This belief persists even now, when we know what comets are and where they come from, and Comet ISON — the supposed 'Comet of the Century' that's due to swing by us and the Sun later this year — apparently has people worried again.
There's certainly a good reason to be worried about the possibility of a comet hitting Earth. The impact would be devastating, likely resulting in the deaths of billions of people, the end of our civilization (at least as we now know it), and Earth being plunged back into an ice age. Just read Lucifer's Hammer, by Larry Niven and Jerry Pournelle, to get a fairly good idea of what would happen on that fateful day.
Having just read that book myself, there are a few similarities between the fictional Comet Hamner-Brown and the very real Comet ISON. Both were discovered by two amateur astronomers (Hamner-Brown by Americans and ISON by Russians). Both are very large and very bright. Hamner-Brown gave people a spectacular show on its way towards the Sun and ISON promises to do the same. Both make their closest approach to the Earth after swinging around the Sun, while on their way back out towards the edge of the solar system.
There's one big difference between Hamner-Brown and ISON, though. Comet Hamner-Brown's closest approach to Earth caused it to score a glancing blow on the planet, pummeling it with several massive chunks of ice and rock. When Comet ISON reaches its closest approach to us, on December 26th, 2013, it's going to be over 63 million kilometres away, both above us and ahead of us in our orbit at that time. By the time Earth reaches the point where we're passing through the plane of ISON's orbit, on January 16th, 2014, the comet will be roughly halfway between the orbits of Earth and Mars.
So, there's no way that Comet ISON is going to be a danger to us, here on Earth.
This video, by the good people at the Hubble Space Telescope site, give a good demonstration of ISON's orbit:
Also, for anyone who got worried again, right around 2:44 in the video, when it looks like Earth crosses ISON's inbound path, there's no need. Even if Comet ISON survives its encounter with the Sun, and even if it actually does return to the inner solar system at some point (and it's not certain that it will), and even if the timing works out that Earth is at that same point right when ISON is at that point in its path, the comet will still be far above the planet at that point. So, there'd be no danger. We'd just get an even more spectacular show than we're already being promised.
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Right now, Comet ISON is on the other side of the Sun from us, roughly one-third the distance between Mars and Jupiter. Pictures from the Hubble Space Telescope show that it's already looking quite nice.
The real show is likely to start in October, as we swing around closer to the comet in our orbit and the comet gets brighter. It'll still mostly be hidden by the light from the Sun, but there's a chance to see it in the very early morning, in the eastern sky, before Sunrise.
As it approaches the Sun, it will get brighter, but we'll mostly lose it in the Sun's glare, and the future of ISON (and whether we get the truly incredible view of this comet) will be decided when it makes its closest approach to the Sun on November 28th. 'Sungrazing comets' like ISON can very easily get torn apart by the Sun. Since ISON comes within about 1.1 million kilometres, the Sun's heat and gravity may doom it. However, if it exits around the other side intact, we'll be treated to the sight of it outshining the Moon, and likely being visible during the day, for the month of December.
Also, if Comet ISON does survive its trip around the Sun, be ready on New Years Eve, since the comet will be nearly above the Earth (but at a distance of over 65 million kms).
As a bonus, when we do pass by ISON's path on January 16th, we may be treated to a special kind of meteor shower. Since Earth passes underneath ISON's path, we probably won't get the normal kind of meteor shower that we typically see throughout the year. This is because we won't be passing directly through Comet ISON's debris trail. However, scientists are predicting that we'll get a different kind.
Since the comet will leave behind a haze of very fine dust, this dust will drift down into our atmosphere. It won't burn up like the larger grains of dust and ice that make meteors streak by, but instead they'll seed clouds in the upper atmosphere and create spectacular noctilucent clouds.
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