- #736
mfb
Mentor
- 37,125
- 13,967
21:10 UTC, 390.63 km, 45° past N pole, 601 mph. Another 4 km of periapsis in a single orbit. Two days left to lower the periapsis a tiny bit more and to make the orbit a bit more circular.
marcus said:!
I rounded his 390.63 to 391. We both reported the same "45º past N pole" for the min position.mfb said:21:10 UTC, 390.63 km, 45° past N pole, 601 mph. Another 4 km of periapsis in a single orbit. Two days left to lower the periapsis a tiny bit more and to make the orbit a bit more circular.
Still no update on MYSTIC.marcus said:Mystic simulator has been shut off---the last view was dated 6 December 7:22 UTC. That was Saturday 5 December at 11:22 PM pacific time.
So here it is 10 PM pacific Sunday the 6th, almost a full 24 hours that the simulation has been off. I have no idea why. Maybe at this point getting into the final orbit is too complicated for the person running Mystic to want to simulate it. Or it is not yet decided how to proceed. Maybe it got too far out of synch with real Dawn and needed to be restarted. Perhaps the simulation will resume when something is resolved. Or maybe it's off for good.
marcus said:Thanks for alerting us! Now I see Madrid #63 is inactive and Goldstone #25 is receiving signal from Dawn. So I would guess the probe has turned off engine after the long (nearly two month) descent.
If that's right then and it's according to the plan Rayman described, the probe is at least approximately in the desired orbit (LAMO) and they are now considering whether they need it to perform a "TCM" (trajectory correction maneuver) to in effect fine-tune the orbit----for stuff like optimal observation, stability, low hydrazine cost, or other.
Yay!marcus said:Judging by simulation, the TCM is complete. Simview shows the probe at 385 km with ion engine turned off.
12Dec 15:34 UTC, 385.48 km, 60º past S pole
12Dec 15:57 UTC, 381.97 km, 80º past S pole
Now the long awaited gamma ray spectroscopy of Ceres surface material can begin.
Canberra #35 was also in standby @ 15:34 & 15:57.
Yes, it does appear that way, but I'm still stuck on homework problems from 2 weeks ago.marcus said:Om, it looks like Simview thinks periapsis of the new orbit is right at the N pole. (see post#756). That would be neat, no?
12Dec 19:25 UTC, 385.33 km, 45º bef S polemarcus said:Judging by simulation, the TCM is complete. Simview shows the probe at [edit: or near] altitude 385 km with ion engine turned off.
12Dec 15:34 UTC, 385.48 km, 60º past S pole
12Dec 15:57 UTC, 381.97 km, 80º past S pole
12Dec 16:20 UTC, 377.11 km, 75º bef N pole
12Dec 16:44 UTC, 371.71 km, 50º bef N pole [edit: I added some more data points, still seems close enough]
12Dec 17:06 UTC, 367.77 km, 24º bef N pole
12Dec 17:30 UTC, 366.54 km, right over N pole*
12Dec 17:53 UTC, 368. 50 km, 30º past N pole
...
12Dec 18:39 UTC, 377.96 km, 85º past N pole
specific energy = v^2/2 - mu/(altitude + imm.rad.)mfb said:How did you calculate the energy?
I need a different brain...Maybe you need a different value for Ceres' radius?
Where are the pole crossings in that graph?
The given height values don't look like MYSTIC takes the oblateness of Ceres into account. Does the agreement get better if you use a single value for the radius? I would also try different values for the radius to see if others fit better.OmCheeto said:specific energy = v^2/2 - mu/(altitude + imm.rad.)
imm.rad. = immediate radius of Ceres, where r(theta)=(a*b)/sqrt((b*cos(theta))^2+(a*sin(theta))^2)
a = major axis = 481,500 meters
b = minor axis = 445,500 meters
theta = angle in radians from the equator heading ccw
mu = G * (mass of Dawn + mass of Ceres) = 6.67e-11 * (1240 + 9.38e18)
For none circular orbits, the first equation still applies for any point of the orbit. The second equation becomes -(1/2)GM/a, where a is the semi-major axis of the orbit.marcus said:Let's compare formulas we use.
specific potential energy at radius R: -GM/R
specific kinetic energy in circular orbit at radius R: (1/2)v2 = (1/2)GM/R
It seems like the specific total would be KE+PE = -(1/2)GM/R
so the deeper down in the well you go, the faster the orbit so the KE increases, but the PE gets more negative, and it's twice as big, so the total gets more negative. The smaller R gets, the more negative the specific orbital energy.
You probably have gone beyond this, to non-circular orbits, and have a more complicated formula. Can you write conveniently write down the formula you are using?
mfb said:The given height values don't look like MYSTIC takes the oblateness of Ceres into account. Does the agreement get better if you use a single value for the radius? I would also try different values for the radius to see if others fit better.
I'm still scratching my head about this, as "a" is a constant.Janus said:For none circular orbits, the first equation still applies for any point of the orbit. The second equation becomes -(1/2)GM/a, where a is the semi-major axis of the orbit.