# ISON, Oort, Hyperbolic Trajectory question.

**URL:** <https://boards.straightdope.com/t/ison-oort-hyperbolic-trajectory-question/674784>\
**Category:** Factual Questions\
**Created:** [November 25, 2013, 10:52pm UTC](https://boards.straightdope.com/t/ison-oort-hyperbolic-trajectory-question/674784 "2013-11-25T22:52:58Z")\
**Posts on this page:** 7\
**Page:** 2

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**Author:** ![Esox\_Lucius](https://avatars.discourse-cdn.com/v4/letter/e/848f3c/32.png) [@Esox\_Lucius](https://boards.straightdope.com/u/Esox_Lucius)\
**Post date:** [November 29, 2013, 5:44am UTC](https://boards.straightdope.com/t/ison-oort-hyperbolic-trajectory-question/674784/21 "2013-11-29T05:44:36Z")

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I’m just realizing how close ISON is to the sun, less than a million miles away. It doesn’t sound that close, but when Earth is 93 million miles away, and Mercury is anywhere from 28 to 43 million miles away in its orbit, it’s no wonder that a puny, fragile comet wouldn’t survive. So much for the Great Comet of 2013.

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**Author:** ![waterj2](https://avatars.discourse-cdn.com/v4/letter/w/858c86/32.png) [@waterj2](https://boards.straightdope.com/u/waterj2)\
**Post date:** [November 29, 2013, 6:54am UTC](https://boards.straightdope.com/t/ison-oort-hyperbolic-trajectory-question/674784/22 "2013-11-29T06:54:28Z")

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Just minutes ago, I was baffled by [the most recent xkcd](http://xkcd.com/1297/). Then I came across this thread. So, I’m going with curiosity as the reason for an Oort Cloud object to head toward the sun.

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [November 29, 2013, 4:04pm UTC](https://boards.straightdope.com/t/ison-oort-hyperbolic-trajectory-question/674784/23 "2013-11-29T16:04:38Z")

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Nah, Curiosity is way down in the inner Solar System, on Mars.

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**Author:** ![74westy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/74westy/32/3950_2.png) [@74westy](https://boards.straightdope.com/u/74westy)\
**Post date:** [November 29, 2013, 10:12pm UTC](https://boards.straightdope.com/t/ison-oort-hyperbolic-trajectory-question/674784/24 "2013-11-29T22:12:30Z")

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> [@Chronos](#):
>
> Nah, Curiosity is way down in the inner Solar System, on Mars.

Where it’s wreaking havoc on the Martian cat population.

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**Author:** ![Musicat](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/musicat/32/20189_2.png) [@Musicat](https://boards.straightdope.com/u/Musicat)\
**Post date:** [November 30, 2013, 12:28am UTC](https://boards.straightdope.com/t/ison-oort-hyperbolic-trajectory-question/674784/25 "2013-11-30T00:28:34Z")

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> [@ftg](#):
>
> I don’t get why you posted this. ISON was (relatively) fresh in from the Oort cloud. While it no doubt has had various interactions in the long ago past, those don’t really matter. It’s the recent one that counts. The one that significantly changed it’s path into a death dive towards the Sun.
> 
> Just ask yourself: if ISON has always gone thru just tiny interactions, what was it doing just before the last little one, when it was still humming more-or-less around the Oort cloud?

Who knows? Do you?

> [@](#):
>
> It is extremely unlikely, as I pointed out earlier, that this major change was spread out over 2 or more interactions given the incredible sparsity of the Oort cloud objects.

I never said how many interactions there were. Maybe it was merely a chance encounter in a long-ago, forgotten time.

The [NY Times](http://www.nytimes.com/2013/11/27/us/comet-nears-sun-offering-planetary-clues.html?src=un&feedurl=http%3A%2F%2Fjson8.nytimes.com%2Fpages%2Fscience%2Fspace%2Findex.jsonp) seems to agree with me:

> [@](#):
>
> ISON was probably dislodged a few million years ago by the gravitational nudge of a passing star, sending it on a trajectory to the inner solar system for the first time and quite possibly the last.

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**Author:** ![Chronos](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/chronos/32/134_2.png) [@Chronos](https://boards.straightdope.com/u/Chronos)\
**Post date:** [November 30, 2013, 2:08am UTC](https://boards.straightdope.com/t/ison-oort-hyperbolic-trajectory-question/674784/26 "2013-11-30T02:08:32Z")

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> [@](#):
>
> Where it’s wreaking havoc on the Martian cat population.

Eh, don’t worry, those things breed like tribbles.

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**Author:** ![ftg](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/ftg/32/2801_2.png) [@ftg](https://boards.straightdope.com/u/ftg)\
**Post date:** [November 30, 2013, 8:45pm UTC](https://boards.straightdope.com/t/ison-oort-hyperbolic-trajectory-question/674784/27 "2013-11-30T20:45:03Z")

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It was asked earlier about how well we knew for sure that ISON was (?) on a hyperbolic trajectory, and I had assumed that this was worked out long ago.

Not so fast.

While some sites give a number above 1, e.g., [1.000003457](http://www.cometison2013.co.uk/ison-fact-sheet/), and therefore hyperbolic, others give a number just under 1, and therefore elliptical. E.g., 0.9999947 at [this](http://www.minorplanetcenter.org/db_search/show_object?object_id=C%2F2012+S1) page.

While many comets have eccentricities such as 0.995, it is having 5 9s that’s making things tricky.

In particular, the fact that ISON and the Sun aren’t an isolated 2-body system creates issues. The Sun is wobbling a bit, mainly due to Jupiter. So the idealized position of the Sun isn’t the actual position of the Sun. Apparently it was going to take some observations after perihelion to nail things down.

In any case, from what I’ve seen, those sites with e\<1 give periods in the 100+k years range. Sometimes much larger.

Aphelion distance, if the e\<1 figure was correct varies as well. E.g., near 5000 AU at the above page. A bit over .07 light years. A factor of 10 lower than the average number given for the “middle” of the Oort cloud. And quite a bit less still than the distance to Alpha Centauri. (And which is nothing compared to the infinity of a hyperbolic path.) Curious.

(Explaining that something was caused by some events but not able to suggest what the state was before the last event or what it was is basically the same as not giving an explanation.)

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