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AK
2003-May-07, 12:09 AM
Heh, no, this isn't a Planet X question. I was thinking more of an object like Nereid. I'm hardly a physics genius, but it seems to me an orbit like that would be highly unstable to say the least...

daver
2003-May-07, 12:55 AM
Heh, no, this isn't a Planet X question. I was thinking more of an object like Nereid. I'm hardly a physics genius, but it seems to me an orbit like that would be highly unstable to say the least...

I think you're right. It's distant, it's highly elliptical. If i'm reading the site right, it's not retrograde, which is kind of curious.

On the other hand, it's pretty far away from any major perturbing elements.

Tim Thompson
2003-May-07, 01:00 AM
Orbits are only unstable if there is something else out there to make them unstable. If it were simply a two body problem, nothing but Nereid (http://www.nineplanets.org/nereid.html) & Neptune (http://www.nineplanets.org/neptune.html), then all possible orbits would be equally stable (in the not always realistic assumption that Neptune & Nereid are both point masses). Nereid remains in a stable orbit because there is no reason for it to do otherwise; there is no other substantially massive body in the vicinity, that could release it from Neptune's grasp, even at a apaneptune distance of 9,623,700 km.

While Nereid has the largest orbital eccentricity as any planet or satellite (e = 0.7512), there are several comets that outclass Nereid in the eccentricity department. Comet Halley (http://www.nineplanets.org/halley.html) has an orbital eccentricity of 0.967, and comet Encke (http://cometography.com/pcomets/002p.html) is 0.847. The great comet Hyakutake (http://nssdc.gsfc.nasa.gov/planetary/hyakutake.html) comes in with an eccentricity of about 0.9998, an orbital period about 40,000 years, and an apahelion distance of about 1165 AU. But the Oort cloud should still be stable even 10,000 to 100,000 AU from the sun. So even with an eccentricity over 0.999, Hyakutake is probably on a stable orbit, until something really big comes along.

Donnie B.
2003-May-07, 01:06 AM
There's nothing about an elliptical orbit that's fundamentally unstable, no matter how elongated it is. For the two-body problem, any ellipse is as stable as any other.

OTOH, I suppose that such orbits are somewhat more prone to being perturbed by third bodies, since they spend a long time far from their primary. Still, large objects are few and far between out there, especially for highly inclined orbits.

kurtisw
2003-May-07, 02:54 AM
OTOH, I suppose that such orbits are somewhat more prone to being perturbed by third bodies, since they spend a long time far from their primary. Still, large objects are few and far between out there, especially for highly inclined orbits.

That's certainly part of it. On a highly elliptical orbit, the object spends a lot of near
the most distant point, where a small perturbation can have a large effect.

For comets, their elliptical orbits also bring them near the giant planets once in a
while. That sudden jolt of gravity can change the orbit quite a bit. (Look at poor Shoemaker-Levy 9!)

Gsquare
2003-May-07, 02:55 AM
I was thinking more of an object like Nereid. ... it seems to me an orbit like that would be highly unstable to say the least...

"Stability" implies conservation of orbital angular momentum. As long as there is no change in orbital angular momentum, Kepler's law says the period,T, and semi-major axis, R, remain constant.

T^2/R^3 = Constant

G^2

Gsquare
2003-May-07, 03:06 AM
That's certainly part of it. On a highly elliptical orbit, the object spends a lot of near
the most distant point, where a small perturbation can have a large effect.

For comets, their elliptical orbits also bring them near the giant planets once in a
while. That sudden jolt of gravity can change the orbit quite a bit. (Look at poor Shoemaker-Levy 9!)

Good point; but more often a comet's eccentricity is so high that its perihelion is so close to the solar surface that it is destroyed by solar radiation (and tidal forces), or its perihelion is inside the solar radius, and so crashes into the sun!

G^2

kurtisw
2003-May-07, 03:58 AM
[quote=kurtisw]

Good point; but more often a comet's eccentricity is so high that its perihelion is so close to the solar surface that it is destroyed by solar radiation (and tidal forces), or its perihelion is inside the solar radius, and so crashes into the sun!

G^2

Absolutely, but this isn't because the orbit is unstable, it is because something
was in the orbital path.

Likewise, just because my dog runs into the wall doesn't mean that quadrupedal
locomotion is unstable. His mental stability is a separate issue.

tracer
2003-May-07, 06:25 AM
even at a apaneptune distance of 9,623,700 km.
"Apaneptune." Heh. I wonder if "Apneptune" would sound better.

Oh, wait, with the peri- and apo- derivatives, you're supposed to use the Greek name for the body in question, not the Roman name, right? E.g. "aphelion" for the sun, "apogee" for the Earth, "aposelene" for the moon, etc.. That would mean it should be called "apoposeidon," not "aponeptune."

Glom
2003-May-07, 09:14 AM
That doesn't always apply. The term perilune and apolune are used in the Apollo presskits. Perijove and apojove are used to describe Jovian orbits. For orbits around other stars, periastris and apoastris are used. They both sound stupid anyway. Just stick to the general term for conics, periapsis and apoapsis. They apply to anything.

tracer
2003-May-07, 03:12 PM
I thought orbits around the moon had "periselene" and "aposelene." I know that orbits around other stars have "periastron" and "apastron."

But, yeah, "perijove" I have heard in use, and it doesn't fit the pattern. It ought to be "perizeus." ;)

Glom
2003-May-07, 04:04 PM
I thought orbits around the moon had "periselene" and "aposelene."

There are three terms that are used exclusively for Luna. Cynthian is used to describe the orbit of a spacecraft that has been launched from Earth, while selene as lune are interchangeable to mean general orbits. Don't ask me what the deal with cythian is, I don't know. But it sounds nice.


I know that orbits around other stars have "periastron" and "apastron."

Yeah, you're right there. I thought astris didn't sound right. But either way, that's still latin isn't it?

ToSeek
2003-May-07, 04:37 PM
My former colleagues at APL working on the MESSENGER Mercury mission were using the terms periherm and apoherm.

Gsquare
2003-May-07, 04:50 PM
Absolutely, but this isn't because the orbit is unstable, it is because something
was in the orbital path.


You missed my point; you were looking at the FAR end of a highly elliptical (cometary) orbit for a perturbation. My point was that more often, before that has a chance to occur, the short end of the stick 'destroys' the orbit.



...because my dog runs into the wall doesn't mean that quadrupedal
locomotion is unstable. His mental stability is a separate issue.

I'm sorry to hear about your dog....try taking him to a eye doctor> :lol:

G^2

tracer
2003-May-07, 07:14 PM
I know that orbits around other stars have "periastron" and "apastron."

Yeah, you're right there. I thought astris didn't sound right. But either way, that's still latin isn't it?
Actually, no, it's Greek. The Latin "astra" never declines such that it would have an "-on" on the end, but the Greek "asteri" (ancient Greek "asteros") does.

Glom
2003-May-07, 07:20 PM
I did ancient Greek for a short while and that ending sounds familiar. What case is being used? Are all gee, selene, helion, chron, all in that case? Astra is also a latin word though, isn't it?

Eta C
2003-May-07, 07:39 PM
Don't forget that Greek (ancient and modern) also has genders. The words we're looking at here have the following nominative form in modern (apologies for the transliteration).

sun: o helios masc
earth: i gis fem
moon: i selene fem
time: o chronos masc
star: to astron neut

hope that helps.

edited for correction

Glom
2003-May-07, 08:04 PM
Is the case used accusative then? I ask that because if so, astron would be the same in the nominative as in the accusative and since it's a neuter word, I think that's supposed to be the case (no pun intended). It certainly is in latin.

Eta C
2003-May-08, 12:40 PM
Sorry for the delay, had to go check & be sure. I understand Greek well enough, but when speaking I always get the declensions & conjugations wrong. Anyway, yes. For neuter nouns the accusative and nominative have the same form. It's also true for feminine, but not masculine. For them, drop the s at the end. So o helios (nom) ton helio (acc) for sun.

Glom
2003-May-08, 04:40 PM
I asked my former Classics teacher and he confirmed the accustive was being used. Apparently peri means around and apo means towards. So perihelion means around the Sun. In latin, these kinds of prepositions take the accusative as well.

traztx
2003-May-12, 05:54 PM
Heh, no, this isn't a Planet X question. I was thinking more of an object like Nereid. I'm hardly a physics genius, but it seems to me an orbit like that would be highly unstable to say the least...

For N-body systems with more than 2 bodies, all orbits vary.

xriso
2003-May-13, 05:05 AM
For N-body systems with more than 2 bodies, all orbits vary.

However, more than 2 bodies can be made /stable/. Witness the various resonances of orbits in our solar system.