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cat_named_easter
12-22-2014, 04:02 PM
Hey, I'm looking for astronomy help!

I'm about to write a short sci-fi story and was thinking of having a comet/asteroid/meteor heading for earth. My idea was to have it be something visible in the sky for years, slowly getting bigger as it gets closer. Is this realistic? Could it really be seen for years before hitting, or would a comet be super-fast and be seen for a very short time before it crashes into us?

King Neptune
12-22-2014, 05:52 PM
Hey, I'm looking for astronomy help!

I'm about to write a short sci-fi story and was thinking of having a comet/asteroid/meteor heading for earth. My idea was to have it be something visible in the sky for years, slowly getting bigger as it gets closer. Is this realistic? Could it really be seen for years before hitting, or would a comet be super-fast and be seen for a very short time before it crashes into us?

For years? No. For months, sure. You probably should look for a site that describes the approach of a comet in great detail, a series of photos, how it swept around the sky, etc. Most comets are not naked-eye visible at all, and the ones that are are visible for a fairly short time.

Larry M
12-22-2014, 06:16 PM
The comet Hale-Bopp back in 1997 was visible by eye for 18 months. It was quite a spectacular sight, and I recall taking time every night after work to go outside to see it. It had two visible tails, one yellow and one blue.

Dennis E. Taylor
12-22-2014, 08:11 PM
I guess you want it to be a one-timer, not a recurring comet (does it have to be a comet? Could it be an asteroid?)

There is an asteroid that's coming back around 2032 or 2036 or somewhere around there, that's going to be coming damned close. That's about all they can say that far in advance.

If you're watching it approaching on this cycle, you can predict within maybe a few tens of thousands of miles, so they wouldn't know for sure until maybe the last few weeks to months (there's wiggle room there, so you can use whichever extreme you need).

In terms of time, the big question would be how narrowly hyperbolic the orbit is, and whether it's coming close on the way in or the way out. The more narrow the orbit, the closer it will be to the sun at perihelion, and the harder it will be to accurately predict. A nice lazy rounded approach will give you more advance warning and more accuracy. However, you're less likely to see it well in advance because it wouldn't have as much apparent motion.

The point is, there's lots of leeway. Go with a lazy rounded orbit that doesn't come too close to the sun, an early discovery by an amateur astronomer (there are so many of them), and choose how much in advance you want them to be certain that it'll hit.