APOD: Hale Bopp: The Great Comet of 1997 (2013 Oct 13)

[Anthony Barreiro]

I've had the impression that because Comet ISON's orbit is hyperbolic, this will be it's one and only pass through the inner solar system. Have I been laboring under a misapprehension?

Yes, you have been somewhat under a misapprehension. In fact, most comets with hyperbolic orbits are actually periodic (although all long period comets which haven't been observed before are called non-periodic). So-called hyperbolic orbit comets typically have eccentricities only very slightly greater than one, and those eccentricities drop below one again once they are some distance from the Sun. The result is a comet with a period of hundreds of thousands or more years (there's no way to reliably determine the period of such comets), but definitely an orbit that will remain closed around the Sun. Only a very few comets have been observed on obvious ejection orbits.

C/2012 S1 is almost certainly periodic.

Thanks Chris.

With an eccentricity so close to 1 and a small, volatile nucleus, can we really predict whether or not this particular bit of space fluff will ever return to inner solar system? If it does, hopefully there will still be human beings with astronomical records hundreds of thousands of years in the future, so they can correct our error and call this Comet Nevski-Novichonok.

[sandij12]

I have been disappointed with Ison so far, I've not seen it and now there may be nothing left to see. I remember Hale Bopp though, I was pregnant with my daughter at the time and it lit up the sky night after night for weeks on end and is the only comet I've seen.
I suppose for scientists Ison might be more interesting, they probably have better equipment and can learn much more now than in '97 hence the excitement which I haven't experienced yet.

[JohnD]

I've had the impression that because Comet ISON's orbit is hyperbolic, this will be it's one and only pass through the inner solar system. Have I been laboring under a misapprehension?

Yes, you have been somewhat under a misapprehension. In fact, most comets with hyperbolic orbits are actually periodic (although all long period comets which haven't been observed before are called non-periodic). So-called hyperbolic orbit comets typically have eccentricities only very slightly greater than one, and those eccentricities drop below one again once they are some distance from the Sun. The result is a comet with a period of hundreds of thousands or more years (there's no way to reliably determine the period of such comets), but definitely an orbit that will remain closed around the Sun. Only a very few comets have been observed on obvious ejection orbits.

C/2012 S1 is almost certainly periodic.

Revival of this thread, to ask about the difference between "non-periodic" comets, AKA never seen before, comets with very long orbits, and Oumuamua. Chris, you said above that some long orbit comets cannot have their period determined, but that some have been on ejection orbits. That was five years ago.
Oumuamua was unique in that it was clearly rocky, with little or no tail but was announced as definitely coming from outside the Solar System and heading away, never to return. Has orbit calculation improved since then, to allow such confidence? Or could previous icy, tailed comets have been extra-solar?
John

[Chris Peterson]

I've had the impression that because Comet ISON's orbit is hyperbolic, this will be it's one and only pass through the inner solar system. Have I been laboring under a misapprehension?

Yes, you have been somewhat under a misapprehension. In fact, most comets with hyperbolic orbits are actually periodic (although all long period comets which haven't been observed before are called non-periodic). So-called hyperbolic orbit comets typically have eccentricities only very slightly greater than one, and those eccentricities drop below one again once they are some distance from the Sun. The result is a comet with a period of hundreds of thousands or more years (there's no way to reliably determine the period of such comets), but definitely an orbit that will remain closed around the Sun. Only a very few comets have been observed on obvious ejection orbits.

C/2012 S1 is almost certainly periodic.

Revival of this thread, to ask about the difference between "non-periodic" comets, AKA never seen before, comets with very long orbits, and Oumuamua. Chris, you said above that some long orbit comets cannot have their period determined, but that some have been on ejection orbits. That was five years ago.
Oumuamua was unique in that it was clearly rocky, with little or no tail but was announced as definitely coming from outside the Solar System and heading away, never to return. Has orbit calculation improved since then, to allow such confidence? Or could previous icy, tailed comets have been extra-solar?
John

Orbit calculation methods haven't changed, and won't. The problem is that you're trying to figure out the precise shape of a massive orbit from just a handful of measurements that are very close together along that orbit's total path length. The tiniest measurement errors, at the level of noise, significantly affect the results of the calculation. And it's not a simple two-body problem. The passage of a comet through the inner system alters its orbit, which in the case of a long period comet might alter its period by thousands of years. That can be modeled using numerical integration, but you run into limitations from the finite numerical precision of the methods.

The difference between Oumuamua and all the other near-hyperbolic comets we've seen is that their eccentricities were very different. All near-hyperbolic comets we've observed have had e very close to one, numbers like 1.01. Oumuamua's eccentricity was 1.2. That's well outside the measurement error range and is certainly hyperbolic, which means it must have originated outside our system. And the nearness to one of all the other comets strongly argues that they came from inside the system, given how nearly matched they are to a solar orbit.

[JohnD]

Thank you, Chris!
And thank you for again for stimulating my education. I had to go and find out about "orbital eccentricities" of between 0 and 1 (Solar) and greater than 1 (Hello! Goodbye!)

JOhn