3D shape, cartography, and geoid of Comet 67P C-G |
3D shape, cartography, and geoid of Comet 67P C-G |
![]()
Post
#1
|
|
Solar System Cartographer ![]() ![]() ![]() ![]() Group: Members Posts: 10196 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) |
|
|
![]() |
![]()
Post
#2
|
|
Senior Member ![]() ![]() ![]() ![]() Group: Members Posts: 2346 Joined: 7-December 12 Member No.: 6780 ![]() |
Rotation rate is 12.7 hours. The circumference drawn by the 4km length of the comet ( a 2km radius ) is 12.6 km So very roughly - it's doing 1km/hr or 0.28m/sec. V^2/r is thus 0.000039 m/sec^2 Surface gravity is approximated as 10^-3 m/sec^2 3 orders of magnitude higher than the centripetal acceleration due to rotation. Hmm, according to the Wikipedia version, the escape velocity is estimated to 0.46 m/s, corresponding to about 0.33 m/s for a circular orbit. So I'd say within the current uncertainty, respecting the rotation, the resulting surface gravity at the parts most distant to the center of mass is about zero. A significantly more compact body with the same angular momentum would be torn apart. This opens a scenario almost opposing the contact binary approach, meaning head and body could have been broken apart already by centrifugal pseudo-force, and kept together by the stretched "neck", which would give the "rubber" duck metaphor more sense than originally anticipated. This way the inner of the comet would be exposed at the neck. Additional momentum could have been provided by impacts or by YORP. |
|
|
![]()
Post
#3
|
|
Newbie ![]() Group: Members Posts: 4 Joined: 2-April 14 Member No.: 7160 ![]() |
Hmm, according to the Wikipedia version, the escape velocity is estimated to 0.46 m/s, corresponding to about 0.33 m/s for a circular orbit. So I'd say within the current uncertainty, respecting the rotation, the resulting surface gravity at the parts most distant to the center of mass is about zero. A significantly more compact body with the same angular momentum would be torn apart. This opens a scenario almost opposing the contact binary approach, meaning head and body could have been broken apart already by centrifugal pseudo-force, and kept together by the stretched "neck", which would give the "rubber" duck metaphor more sense than originally anticipated. This way the inner of the comet would be exposed at the neck. Additional momentum could have been provided by impacts or by YORP. Back of the envelope: Gravity /centripetal ~ GM/r^2//V^2/r ~ GpP^2 where G = grav constant of 6.67x 10^-11 (mks), p = density ~ 10^3 (mks) and P = period ~4.6x10^4 sec. Ratio ~ 10^2 Gravity wins hands down. Tidal forces are insufficient. Adjustments to Fg due to odd shape may cause ratio be somewhat less, but increasing density will go the other way. |
|
|
![]()
Post
#4
|
|
Senior Member ![]() ![]() ![]() ![]() Group: Members Posts: 2346 Joined: 7-December 12 Member No.: 6780 ![]() |
Back of the envelope: Gravity /centripetal ~ GM/r^2//V^2/r ~ GpP^2 where G = grav constant of 6.67x 10^-11 (mks), p = density ~ 10^3 (mks) and P = period ~4.6x10^4 sec. Ratio ~ 10^2 Gravity wins hands down. Tidal forces are insufficient. This would be bad for Philae, so I've looked, where the discrepancy may come from. One reason is -- if I calculated correctly -- a factor of 1 / 3π for the density calculation in the spherical case. The other one is the estimated density, which has been estimated as 10^2 kg/m³. Together we get a discrepancy factor of 30π, about 94. Now add the non-spherical shape, and things are open again. -- This shows, how important the ongoing gravimetric measurements are. |
|
|
![]() ![]() |
![]() |
Lo-Fi Version | Time is now: 23rd June 2024 - 02:52 PM |
RULES AND GUIDELINES Please read the Forum Rules and Guidelines before posting. IMAGE COPYRIGHT |
OPINIONS AND MODERATION Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators. |
SUPPORT THE FORUM Unmannedspaceflight.com is funded by the Planetary Society. Please consider supporting our work and many other projects by donating to the Society or becoming a member. |
![]() |