What if ISON fragments as it shoots around the sun. Can anyone translate the deceleration of the fragments such that they could potentially hit earth?
Quoting: Anonymous Coward 47820629 First of all, if there is something you do not understand about a paper I present, send me a message about it before you go post based on your misunderstanding. I presented this paper before, I'm sure that's how you found it. You have taken an aspect unrelated to what I presented about it to try to suggest that there's a risk of impact if ISON fragments. There is no risk of impact, even if it fragments. The fragments essentially continue to follow the orbital path of the original comet but with locations along the orbit that diverge over time - the deceleration mentioned in the paper is radial, along the velocity vector of the comet.
"The deceleration Γ is a result of the momentum transfer between the two fragments due to their different outgassing rates and masses.
It is measured in radial direction only..."
"The parameters are used to constrain the fragmentation event dynamically through the time Ts when the splitting happened,
the radial Vr, transverse Vt and normal Vn components of the separation velocity of the secondary fragment relative to the primary one, and the deceleration parameter Γ of the secondary relative to the primary component.
Vr points in the direction of the radius vector of the comet, positive along the radius vector..."
Moreover, the deceleration is nowhere near enough to produce the change in velocity required to divert ISON into an impact trajectory, even if it
were oriented in a direction that could that (it isn't).
"The coarse histogram distribution in Fig. 2 suggests that
long-period and new comets tend to produce fragments
subject to decelerations Γ of 10^–4 - 10^–3 x solar gravity, while the fragments of short-period comets show on the average smaller Γ values."
10^-3 solar gravity at ISON's perihelion when solar gravity is at its greatest is about 0.038 m/s^2. At that rate, it would take 47.5 years of constant acceleration that high to achieve the necessary 57 km/s delta-V needed to alter ISON's orbit into an earth-collision trajectory at perihelion. Obviously, since the deceleration actually experienced by the fragments is a function of solar gravity, that deceleration will rapidly decrease as solar distance increases, plus the fragments don't have "47.5 years" to wait to be put onto an earth-collision trajectory; they'd be long gone by then, and last but not least the vector of the deceleration is along the comet's velocity vector which is about 90 degrees wrong for putting it into an earth-collision trajectory. To put it into an earth-collision trajectory at perihelion requires the velocity vector to point at the sun.
So to answer your question in a single word, no.
Speak English ya mutt.
It is english. It's not my problem if it's over your head.
I don't understand how they can calculate any of this as they have no idea what the comet nuclei is made of
Quoting: ACWrong. They do have a very good idea what it's made of based on previous comets. Furthermore, it doesn't much matter what it's made of as far as celestial and orbital mechanics are concerned.
and no idea what the degassing will be at or near the sun.
Quoting: ACYou mean outgassing and it does not matter.
According to this the velocity of a comet is affected by non-gravitational forces including degassing which are not constants:
[
link to www.lpi.usra.edu]
It's outgassing, not degassing. Don't bitch at me about my english when you butcher the terminology repeatedly yourself. Non-gravitational effects are minor, especially over short time scales. They are very important in determining the long-term orbits of periodic comets that constantly orbit within the solar system and are observed over hundreds of years, but they are less important to long-period comets where the effects weren't even detected until the late 1960's and 70's with high precision astrometry. Even then, because of how short the period is when the comet will actually be observable, the non-gravitational effects of outgassing are of academic interest more than anything else; they can make the difference in the long-term evolution of the comet's orbit and determine whether it will ultimately be a hyperbolic comet or not. To quote from your own paper:
"Modern astrometric positions, particularly those that are referenced to Hipparcos-based star catalogs and where the brightest pixel is employed as the true position of the cometary nucleus, are usually accurate to the subarcsecond level."
In other words, more than accurate enough even for most telescopic astrometry.
"Yet
multiple apparition orbital solutions for active short-period comets cannot often provide a root mean square (rms) residual (observed minus computed observational position) that is subarcsecond."
In other words, where it matters is with short-period comets, not long period comets like ISON.
What if it breaks in to a million pieces of metal (assuming that's at the core) as it spin around the sun.
Quoting: ACThen those pieces will generally follow the orbit of the original comet as they retain the comet's inertia.
That would cause an absolute deceleration of the the fragments as there would be no degassing which would naturally cause propulsion.
Quoting: ACWord salad from someone with no idea what he's talking about. Outgassing is not "propelling the comet around the sun" like some kind stupid B grade movie sci-fi rocket. Good lord. Study celestial mechanics before you make such ridiculous claims. If all outgassing on ISON ceased this very second, the comet's nucleus would continue to orbit around the sun exactly as predicted anyway. Deceleration of fragments during fragmentation events is along the comet's velocity vector and as I already showed is orders of magnitude too small to matter and in the wrong direction to produce a collision trajectory. Even if you assumed for a moment that nearly the entire mass of the comet was volatile and capable of outgassing, and that it was outgassing in a single consistent direction aimed exactly at the vector needed to divert it into an earth-collision trajectory at perihelion, the fraction of the mass of the comet left after the necessary delta-V has been achieved is insanely, ridiculously low.
Let me put it to you like this; the mighty Saturn V rocket, the most powerful rocket ever flown, which lifted a payload smaller than the space shuttle, had a delta-V of about 12 km/sec. The Saturn V was a skyscraper which was about 95.7% explosive fuel by mass. And all of that fuel, directed in a very controlled and specific direction, got you a mere 12 km/sec delta-V. You would need about 5 times that much delta-V to put any piece of ISON onto an earth collision course, and it would have to be done in a very, very specific and sunward way at perihelion. A comet's outgassing velocity is also not nearly as high as a rocket, somewhere in the ballpark of 900 m/s for a comet (Combi et al 1997 [
link to link.springer.com] ) vs somewhere around 4177 m/s for the upper stages of a Saturn V rocket. Let's assume ISON is 1km in diameter, and is a denser than normal comet, with 1g/cm^3. That would mean the total comet mass would be around 5.24 x 10^11 kg. In order to deliver 57 km/sec of delta-V, all but about 0.000000000000000163 kg of the comet would have to be "fuel." That is because delta V = Exhaust velocity * ln(total mass/final[payload] mass) The "payload fraction" that reaches earth would have a mass of 0.000000000000000163 kg.
Is there a scientific model for such an event? In fact when is the last time a comet sling-shot around the sun?
Quoting: ACComets go around the sun all the time. It's not a true "slingshot" or what NASA calls a "gravity assist maneuver" unless it occurs around a planet in the solar system. Contrary to Star Trek IV, you essentially can't gain momentum relative to the sun by using the sun as the body around which to "slingshot" (true situation is slightly more complex due to the difference between the sun's center and the solar system barycenter, but for all intents and purposes you can't). You have to use a planet orbiting the sun.
