3D shape, cartography, and geoid of Comet 67P C-G |
3D shape, cartography, and geoid of Comet 67P C-G |
Aug 6 2014, 02:11 PM
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#1
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Solar System Cartographer Group: Members Posts: 10189 Joined: 5-April 05 From: Canada Member No.: 227 |
Explorer 1 said:
"A 2D map of C-G seems like a tough order; the projection math alone..." Don't worry! If you can put a grid on the surface (as we have seen already), you can warp that grid into any map projection you like. Mapping will be no huge problem - in fact I expect they have a rough one already (I've been playing with one myself). Phil -------------------- ... because the Solar System ain't gonna map itself.
Also to be found posting similar content on https://mastodon.social/@PhilStooke Maps for download (free PD: https://upload.wikimedia.org/wikipedia/comm...Cartography.pdf NOTE: everything created by me which I post on UMSF is considered to be in the public domain (NOT CC, public domain) |
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Aug 11 2014, 03:37 PM
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#2
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Senior Member Group: Members Posts: 2346 Joined: 7-December 12 Member No.: 6780 |
If you take a binary of two spherical bodies, the center of mass is in the middle of the line between those two bodies, but that's no gravitational low; it's more like a saddle; and it's a Lagrangian point (L1) at the same time, although the center of mass of two bodies doesn't need to be a Lagrangian point, in general. There are two lows at the respective centers of the two bodies.
For 67P/C-G there may also be two (or more) local gravitational lows, the deeper one near the center of the larger component. The center of mass should be between the gravitational low of the larger component and the neck. The neck should be near a Lagrangian point (a saddle in the field of gravity), which is between the local gravitational low of the smaller component and the center of mass. At the center of mass there should be a net gravitational pull towards the local gravitational low of the larger component. The center of mass is a point on the rotation axis. Start with the Earth-Moon system as an easier-to-understand example, when reading the second paragraph a second time. |
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Aug 11 2014, 04:08 PM
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#3
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Junior Member Group: Members Posts: 40 Joined: 28-July 07 Member No.: 2984 |
If you take a binary of two spherical bodies, the center of mass is in the middle of the line between those two bodies, but that's no gravitational low; it's more like a saddle; That's what I meant to express. A small perturbation from the saddle point not exactly on the upward isoline and its downhill to one of the two lobes. So escape velocity from the saddle point is likely lowest for the comet and it would dissipate at a higher rate, and any spray that doesn't make it off the comet from the neck is likely pulled to one of the lobes, adding to the saddle-ness of the saddle point. So for an asymetrical body, it seems that process is unstable and the asymmetry would grow, i.e., more necking. There's probably an unstable process between the two lobes too, with one having a higher rate of dissipation and the heavier one stealing some of the lighter one's mass. But this is more guessing on my part. |
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Aug 11 2014, 04:36 PM
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#4
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Senior Member Group: Members Posts: 2346 Joined: 7-December 12 Member No.: 6780 |
That's what I meant to express. A small perturbation from the saddle point not exactly on the upward isoline and its downhill to one of the two lobes. ... That's really hard to tell at the moment, since the local gravitational lows are probably within the nucleus. Since the surface of the larger component is farther away from its local gravitational low than the surface of the neck is away from the line between the two(?) gravitational lows, we've two opposite effects which may or may not cancel out. But I share the impression, that the surface of the nucleus isn't in perfect equilibrium between the several forces (gravity, inertial pseudo-forces, friction). If there is some mass waste from the neck or from the smaller component towards the larger one, the field of gravity, and the rotation also change. Asymmetric loss of volatiles may change the angular velocity too. This might eventually lead to ejection or collapse of some parts. As the comet gets closer to the Sun, I'd guess, we may see some dynamics. |
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