Expert: Jayendra Upadhye - 8/18/2008

Question
QUESTION: Hi again Jayendra,

I've been wondering about the orbits of comets. Do they orbit fractionally closer each orbit?

Thanks Robert

ANSWER: Hi,
Robert, To my knowledge, Cometary orbits are as ubder:-
1 - The first is a class of orbits with known periodicity, such as the halleys comet, and its periodicity was established by Halley based on previous well kept records. Since their orbits start far out in the kuiper belt, they are of a very high eccentricity.
2 - Comets that have parabolic "opene ended" orbits. Talking of periodicity is meaningless for these.

The main problem is that closer to perihelion, they tend to have close fast flybys around the sun, and as a result of heating, loose mass (gas ices and water ices on their outer layers, formed in the outer solar system, quickly melt / ablate) with each perhihelion pass. (The evidence is the tail).
Some times gases built up under the surface, cause "cold volcanism" and erupt as geysers of water / gas and break away large chunks (like iceburgs dropping off the ice shelfs).

This disturbs the orbital parameters (especially at perhilion), as centripetal acceleration -m*v*v/r - s a function of mass, and linear velocity, and equals solar 'g' at all times).

Comets are also subject to tidal break up (roche's limit more or less severe), and non-tidal (outgassing) breakup. The alteration of individual momenta of the fragments decide their orbits. Ideally if fragmentation is slow, the fragments can continue to orbit in "same lane" for some time. Outgasssing happens along the "day side of the comet, and actually gives a "rocket like" push to the comet "away from the sun"! But it is a gentle nudge only, at all times, as momentum of gas carried away is small comapred to the mass of the main body which can be in millions of tons.

Though i could not come up quickly with pages in support, mechanics compells me to say that there will be circularising influence over time as outgassing progresses, and a reduction in orbital eccentricity.

Again rotation can affect fragmentation.

refer:-
1 - From Cradle To Grave: The Rise and Demise of the Comets:-
www.lpi.usra.edu/books/CometsII/7012.pdf
2 - effects of outgassing:-
http://www.lpi.usra.edu/meetings/lpsc2007/pdf/1694.pdf

Hope that suffices,
regards,
Jayen

---------- FOLLOW-UP ----------

QUESTION: Thanks Jayendra, for all this info. But I'm still a little puzzled. Does the orbital period reduce(very fractionally) every orbit? Or is it so close to call nobody knows. I just thought through time the Sun would eventually pull it in closer. The heat and tidal pulling eventually breaking it up. I am assuming the outgassing isn't strong enough to push it into a larger orbit. Please let me know if my thinking is right here.

Thanks Robert

ANSWER: Robert,

If your mind is made up, nothing will change that!

>> just thought through time the Sun would eventually pull it in closer. The heat and tidal pulling eventually breaking it up<<

well what can i say? you have already decided about that!

Any other questions are welcome.

I will put my POV, and you take it or leave it at that.

As an expert, (for whatever i am worth), I do grant considerable time and attention to your questions, but i cant go on repeating this cycle for too long.

Hope you understand.

please do try the other experts out!

regards
Jayen




---------- FOLLOW-UP ----------

QUESTION: thanks Jayendra, but my mind isn't made up. I suspect that any reduction in forward velocity would result in a reduction in the orbital period. I don't know if your answer is yes, no, or can't be determined. Sorry I don't know what POV is either

best regards Robert

Answer
Hi Robert,
1 - POV is Point Of View.
2 - It is NOT reduction in forward velocity.
Even if a comet is rotating, only the side facing the sun goes thru the outgassing phase. (surface ablation + sub-surface outgassing).
Look up wikipedia for knowing what is ablation.

Consequently, the velocity component of the rapidly sunward exiting mass gives the comet a momentum away from the sun.

This slows it down during approach (slightly only), and accelerates it during post perihelion phase when it is moving away from the sun.

However, once the gas has exited, THAT gas is then independently (un-connected momentum wise to the comet), pushed radially away from the sun by the hydrostatic pressure of the solar wind. (the wind  conctitutes a continuous "hydrostatic" flow that can exert pressure on tenuous gas even at 1 astronomical unit and more.

The overall effect should be to circularise slightly with each pass, the very very eccentric orbital path of the comet.

I think that should suffcie.
Robert, i will stop this thread here if you dont mind.

As pointed out earlier, there are more capable experts on this site than me, who may explain things better.

For correspondence if you so wish, my mail id is jupadhye_99@yahoo.com.

Do send in mails.

regards
Jayen