I Dawn dead in Ceres orbit, ran out of fuel Oct 2018

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The Dawn spacecraft successfully observed Ceres from a distance of 238,000 miles on January 13, 2015, capturing over half of its surface at a resolution of 27 pixels. The mission aimed to enter a polar orbit around Ceres, with a planned descent to an altitude of 375 km, but faced challenges due to limited hydrazine propellant for attitude control. A cosmic ray event in September 2014 had previously disrupted the propulsion system, complicating the approach trajectory. Despite these issues, the spacecraft was expected to achieve a stable orbit around Ceres, ultimately becoming a "perpetual satellite" as it ran out of fuel. The mission's success would provide valuable data on Ceres' physical characteristics and surface mapping.
  • #751
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  • #752
Simview now shows Dawn performing TCM (trajectory correction maneuver). It is not simply retrothrusting as it was during the main descent.
Then the ion beam was in the direction of motion, to slow down. Now the beam is sideways the direction of motion.
As if the orbit was askew---not exactly over the poles---and a sideways push was needed to true it up and make the orbit more exactly polar.
TCM11Dec.jpg

Here, as I interpret it, we see the ion beam pointed at 9 o'clock while the probe is crossing the terminator at the S pole in about a 4 o'clock direction. The thrust is only partly aligned with the direction of motion.
 
  • #753
Thrusting directly over the poles (as shown in the image) doesn't make an orbit more polar, it shifts the orientation of the orbit relative to the terminator. They ruin my earlier estimate of the maximal remaining probe lifetime!
 
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  • #754
Yes, I can see how it might change your estimate of how long the probe can remain in permanent sunshine before it begins to fall into Ceres' shadow for part of each orbit. That was a pretty robust estimate though, as I recall, and not apt to change too much. The dwindling supply of hydrazine is more critical---so much so that it would be tactful to avoid mentioning it.

Still, it is a great success that Dawn is finally down in LAMO, at target altitude! I feel jubilant about this--relieved and heartened. At last some gamma spectroscopy of the chemical elements in the surface! When my wife heard the news she proposed toasting Dawn with something fizzy. :oldbiggrin:==quote Rayman==

December 11, 2015 -Dawn Ion-Thrusting to Adjust Orbit

Dawn is now using its ion engine to adjust its orbit. This maneuver (explained in the November Dawn Journal.) will synchronize the spacecraft 's orbital motion with Ceres' rotation around its axis to fit with the plan for the extensive observations that will begin next week.

Yesterday while the flight team was preparing Dawn's flight plan, the spacecraft tested its backup camera. Controllers occasionally run the camera through a series of tests to verify that it remains in good condition should the primary camera have a problem. (The test of the camera was performed eight years to the day after its first operation in space.) Although the results have not been analyzed in detail yet, all indications are that the backup is in excellent condition.
==endquote==
12Dec 3:36 UTC, 385.44 km, passing over S pole (positive inline thrust)
Judging from Simview the main function of the TCM (correction maneuver) was to raise the average altitude from around 360-370 to 385
12Dec 4:23 UTC, 384.15 km, 55º past S pole
12Dec 5:09 UTC, 376.16 km, 80º bef N pole
12Dec 5:55 UTC, 366.25 km, 30º bef N pole
12Dec 6:19 UTC, 364.65 km, passing over N pole
You can see why they are applying positive inline thrust. They want to raise the altitude into the 380s instead of having it sagging down in the 370s. Higher altitude means longer orbit period. The orbit period wants to be finely adjusted to synch with Ceres rotation to get efficient coverage of the surface.

I suspect the main reason they raised the LAMO altitude from original 375 to 385 has to do with timing.
 
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  • #755
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
Now the long awaited gamma ray spectroscopy of Ceres surface material can begin.
http://dawn.jpl.nasa.gov/mission/status.html
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
http://dawn.jpl.nasa.gov/mission/journal.asp
https://eyes.nasa.gov/dsn/dsn.html (Madrid antenna #55 is in standby)
http://dawn.jpl.nasa.gov/mission/live_shots.asp
 
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  • #756
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.
Yay!
...
https://eyes.nasa.gov/dsn/dsn.html (Madrid antenna #55 is in standby)
...
Canberra #35 was also in standby @ 15:34 & 15:57. :oldwink:

ps. Has anyone else tried to analyze the specific orbital energy? My graph comes out sinusoidal, so I'm not sure what's wrong.

specific.orbital.energy.dawn.ceres.Nov.27.28.png

x axis = time in hours ____ t0 = Nov 27, 2015 17:46 UTC
y axis = energy

I haven't really sat down to figure out why the energy gets higher the closer the orbit is to Ceres, but I checked around, and saw that's just the way it is.

[edit #1: Doh! Canberra #35 is now unassigned]
[edit #2: Madrid #65 in two way com @ 17:44 UTC, 10.00 b/sec?]
 
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  • #757
How did you calculate the energy?
Maybe you need a different value for Ceres' radius?

Where are the pole crossings in that graph?
 
  • #758
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?
 
  • #759
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?
Yes, it does appear that way, but I'm still stuck on homework problems from 2 weeks ago.
I'm trying to find the answers to mfb's questions, but I have so many goofy graphs going on at once, I have no idea what is going on.
If anyone has any questions, about how NOT to be a scientist, just ask. :oldcry:
 
  • #760
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?
 
  • #761
Since we turned a page, I'll bring forward the post summarizing some reasons one might be especially interested in studying Ceres and in the current exploration by Dawn.
==quote post #748==
Just as a reminder, what now happens has consequences for human history because Ceres is (by a wide margin) the nearest icy dwarf planet and offers an advantageous site for chemical and materials manufacture in low gravity. Whether or not this will be developed depends somewhat on Ceres layer structure and chemical composition
Thinking has changed about the layer structure (Lakdawalla report on recent AAS conference) because at least in some regions craters remain sharply defined and are slow to smooth out---suggesting they are supported on a slow-flow rock+ice mix that could be as much as 60% rock. The searchable online cutaway GRAPHICS go back to 2005 and 2006 when scientists had only Hubble space telescope images to go on. I couldn't find any updated cutaway diagram showing more recent guesses about layer structure.
There were also reports at that AAS conference of detection of ammonia-bearing clays in the hydrated minerals at Ceres' surface, by optical/IR spectroscopy, which would be important if confirmed. Nitrogen is a key chemical element for both manufacturing and biology--common in outer solar system bodies but unexpected on an asteroid belt body like Ceres.
Anyway if things work out as planned we now get to learn a great deal more about Ceres' chemical and layer composition. This kind of graphic will hopefully be revised:
View attachment 93140

The icy layer should be more indicative of a 60-40% rock+ice mix. Orbit tracking should be able to map subsurface irregularities in density. Gamma and neutron spectroscopy is to measure the abundances of various chemical elements in surface material, to a depth of about 1 meter.

The last (chemical abundances) is especially significant so I'll bring forward the graphic. Spectroscopy depends on activation by cosmic ray particles.
View attachment 93141
Impact by cosmic rays (high energy protons, mainly) causes a sparkle. The frequencies of the gamma-ray sparkle reveal the identities/abundances of the atoms giving off the sparkle. Moreover among the scattered neutrons the fraction of them which have been slowed by successive collisions with hydrogen nuclei reveals the amount of hydrogen (e.g. water ice) in the surface material.
==endquote==
marcus 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
12Dec 19:25 UTC, 385.33 km, 45º bef S pole
12Dec 19:49 UTC, 387.07 km, 24º bef S pole
12Dec 20:11 UTC, 387.88 km, right over S pole**
12Dec 20:35 UTC, 387.88 km, 30º past S pole**
12Dec 20:58 UTC, 386.84 km, 50º past S pole
 
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  • #762
mfb said:
How did you calculate the energy?
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)​

Maybe you need a different value for Ceres' radius?
I need a different brain...
Where are the pole crossings in that graph?

specific.orbital.energy.dawn.ceres.Nov.27.28.with.annotated.poles.png
 
  • #763
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)​
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.
 
  • #764

The 2 minute youtube records the moment when Laurent Fabius says he hears no objection and so
"L'accord de Paris pour le climat est accepté"
Climate day (12 December) was also "Ceres Day"---when Dawn was finally in near orbit and could get down to real business.
 
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  • #765
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?
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.
 
  • #766
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.

Along with all of the previously mentioned derogatory phrases my mother used to call me, she also called me a "doffa henna". Which I believe means; "Idiot chicken". But don't quote me on that translation.

In my previous graphs, I was off by 2 orders of magnitude for the mass of Ceres.
Inserting the correct mass of Ceres, and using: average radius + altitude = r
I come up with the following graph:

spec.orb.energy.corrected.for.mass.of.ceres.no.adj.for.ellip.of.ceres.png

This makes a tad bit more sense to me.
I spent several hours trying to figure out the gravitational field around an oblate spheroid, but could not comprehend any of the maths.
The only thing I could extract, was that the force of gravity at the poles is greater than at the equator.
I'm guessing that this might explain the shape of this new graph.

This apparently was previously such an obscure, aka hypothetical, problem, that there is little discussion about it here at PF. At least that I could find.

But it was fun looking. DH mentioned an article in one thread about "gravitational anomalies of the moon", with disastrous consequences.
Hopefully, the Dawn crew is familiar with gravitational potholes. :biggrin:

[edit]
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.
I'm still scratching my head about this, as "a" is a constant.
 
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  • #767
Okay, the equator energy is too high, so we are closer to the center than the calculation assumes (as the speed value is directly from MYSTIC). If your latest plot was done with a constant radius for r, using the adaptive radius would make it even worse. There are two symmetric oscillations per orbit, so there is no periapsis/apoapsis problem left (at least not dominant).

The gravitational force is stronger at the poles if you are on the surface (there being at the poles makes you closer to the center) - in orbit for a given height, it is stronger over the equator. We would have to estimate the quadrupole component of the gravitational potential to include that effect.
 
  • #768
Just to keep us up to date (don't let this interrupt the orbital energy discussion) according to Simview the correction maneuver is accomplished and Dawn is in LAMO, with engine off.
DSN graphic shows Madrid antenna #65 in two-way communication

It seems pretty clear (at least to me) that what Simview calls "altitude" is altitude above an idealized spherical Ceres, not above the actual surface of the body. I'm not sure what sphere radius Simview uses---maybe 473 km, which I've seen quoted as the average Rav of the true oblate dimensions. In that case, in case some notation might be helpful: Let's have H stand for the "altitude" term.

Dawn's radial distance from center = D = Rav + H = 473 km + "altitude".

In a few minutes Dawn will pass over Ceres S pole and that might roughly coincide with Dmax or apoapsis. We'll see. I'll record what Simview says at that point, and check later how well it agrees.

It's fascinating how much smoother the surface is in the wide equatorial belt, and how much sharper the craters are in polar regions. As if the proportion of ice to rock is higher in equatorial subsurface material, so that it flows more easily---e.g. 40-60% rock to ice. And as if the proportion of ice is lower in polar subsurface--e.g. 60-40% rock to ice--providing a stiffer foundation for longer-lasting craters.

13Dec 18:10 UTC, 376.33 km, right at S pole*

I think that is the Hmax of the corrected orbit (simulation) so will tentatively mark it with an asterisk. Well see if H declines at the next reading, and whether Hmin comes at N pole (if that is not too much to ask of Mystic :oldbiggrin:

Postscript: Yes! Hmax = 376 km and it came right at the S pole. I'm thinking we should be on the lookout for Hmin about 3 or 3:30 hours from Hmax: around 1 PM pacific or 21 hours UTC. Hope I don't get too busy with something else and forget.

Postscript:
13Dec 20:53 UTC, 356.27 km, right at N pole*
A spread of almost exactly 20 km. H was decreasing up to that point, so if it's the min as I guess it will increase at the next reading.
Yes it increased
13Dec 21:16 UTC, 357.70 km, 20º from N pole

It seems that Simview has fallen into a pattern where the "altitude" H is going to be between 356 and 376km, with Hmin at N pole and Hmax at S pole.

To check consistency I took a reading around Hmin again. It occurred as expected
14Dec 02:17, 356.25 km, 8º to N pole
the readings immediately before and after were larger (358.55 and 357.20)

So we can say with reasonable confidence that the semimajor axis a = 473+366 = 839 km.
as far as Simview is concerned, assuming 473 is right for the Ceres average radius
2 pi ((839 km)^3/(G*938e18 kg))^(1/2) = 5.36= 5: .36*60 = 5:22

20:53 + 5:22 = 26:15 = 24+2:15 hours, which is pretty close to 2:17!
 
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  • #769
Since I haven't checked in on Dawn much so far today, I'll take a look at the new orbit according to Simview. See if it's consistent with our perceptions of it yesterday. It looked then as if the "altitude" (probe distance from center, minus something like 473 km, the average body radius) was going to be between 356 and 376 km with (approximately at least) the min coming at Ceres' Npole and max at Spole.

I'll tag the max and min with asterisks.

Here are some fresh readings:
15Dec 01:49 UTC, 373.70 km, 70º to S pole
15Dec 02:12 UTC, 375.77 km, 45º to S pole
15Dec 02:35 UTC, 376.32 km, 24º to S pole*
15Dec 02:58 UTC, 375.88 km, right at S pole
15Dec 03:21 UTC, 374.84 km, 30º from S pole

Assuming the estimate of 5:22 hours for the orbit period (see previous post) is right the time to periapsis (min "altitude") is about 2:41, so we might expect it around 15Dec 05:16 UTC, just adding 2:35 and 2:41.
That's 9:16 pm pacific time, hope I don't forget : ^)

15Dec 05:17 UTC, 357.65 km, 30º to N pole
15Dec 05:40 UTC, 356.68 km, right at N pole*
15Dec 06:03 UTC, 358.90 km, 20º from N pole
 
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  • #771
marcus said:
...
...
15Dec 02:58 UTC, 375.88 km, right at S pole
...
Assuming the estimate of 5:22 hours for the orbit period (see previous post) ...

Three times 5:22 is 16:06. So if I add 16:06 hours to 2:58 UTC and get 19:04 UTC that should be the next S pole crossing---just as a check on that figure of 5:22 for the period.
15Dec 18:47 UTC, 378.17 km, 35º to Spole*
15Dec 19:09 UTC, 377.04 km, 12º to Spole
15Dec 19:34 UTC, 374.88 km, 15º from Spole
well, it's not perfect. I wonder if the Sun's gravity has an effect that I should be allowing for. Apoapsis seems to have increased by a couple of km and to have rotated slightly sunwards. The period is slightly more than the 5:22 found yesterday.
 
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  • #772
marcus said:
I wonder if the Sun's gravity has an effect that I should be allowing for.
Completely negligible: at a level of 10 parts in a billion.
Inhomogeneous mass distributions in Ceres are much more important.
 
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  • #773
Thanks! Maybe inhomogeneous mass distribution accounts for the seemingly chaotic jitter. After all, every time Dawn comes south across the day side it is a different dayside, because Ceres is rotating. Rayman alerted us that subsurface differences in density would be being detected at this level by the probe speeding up and slowing down. So they hope that they can map some underground bodies of ice (lower density) or rock (higher density) by this kind of "Xray vision" based on precise tracking.
So the "inhomogeneous mass distributions" must be having an effect---and maybe there are even significant departures from the oblate spheroid model. Maybe there are significantly different equatorial radii, besides a polar one, and that could have an effect.
 
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  • #774
Anyway it looks like she crossed the Spole around 15Dec 19:20 UTC today. And stubbornly holding to that 5:22 underestimate of the period, for the time being, I'll guess the Npole crossing will come 2:41 after that 19:20+2:41 = 22:01 UTC. Or around 2 PM pacific. We'll see.

According to Rayman's Update that Om linked us to, observation by the GRaND instrument began 14
December and will continue---so information is already being accumulated about elemental abundances.
==quote Rayman==
Dawn thrust with its ion engine on Dec. 11-13 to fine tune its orbit. When it finished, it pointed its gamma ray and neutron detector (GRaND) at Ceres. GRaND measures the energies and numbers of these two components of nuclear radiation, from which scientists can determine the abundances of some elements on the dwarf planet.
...
... the intensive observation campaign that will begin on Dec. 18. In the meantime, the spacecraft will collect more radiation measurements as well as conduct some bonus photography and infrared spectroscopy on Dec. 16-17.
==endquote==
http://dawn.jpl.nasa.gov/mission/status.html

Update: Earlier I estimated 2PM pacific for periapsis and here it is 20 minutes before 2. So I'll check
15Dec 21:29UTC, 357.79 km, 46º to Npole
15Dec 21:52UTC, 356.59 km, 15º to Npole*
15Dec 22:14UTC, 358.33 km, 10º from Npole

Reply to Om's question in next post: Yes, I believe I noticed the same thing you did. I had been expecting the "altitude" to stay within bounds like 356-376, and it overshot the upper bound to around 378. You showed that with your graphic. I'm just getting used to a little chaos in the numbers--as long as the overall outlines seem OK.

So if 5:22 hours is the period (underestimate) and 2:41 being half that, then the next apoapsis could be expected around 21:52 + 2:41 = 24:33 = 16Dec 00:33 UTC, which is about 4:30 PM pacific, this afternoon.
Well here it is 4:30 pacific and I see:
16Dec 00:11 UTC, 377.43 km, 38º to Spole* , and then later
16Dec 00:34 UTC, 376.56 km, 14º to Spole
16Dec 00:57 UTC, 374.92 km, 9º from Spole

I may have missed it, was busy otherwise should have gotten down and looked a little before 4:30.

16Dec 22:22 UTC, 377.34 km, 20º to Spole (probably close to "apo")
17Dec 01:04 UTC, 357.18 km, 26º to Npole (probably close to "peri")
 
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  • #775
marcus said:
Thanks! Maybe inhomogeneous mass distribution accounts for the seemingly chaotic jitter. After all, every time Dawn comes south across the day side it is a different dayside, because Ceres is rotating. Rayman alerted us that subsurface differences in density would be being detected at this level by the probe speeding up and slowing down. So they hope that they can map some underground bodies of ice (lower density) or rock (higher density) by this kind of "Xray vision" based on precise tracking.
So the "inhomogeneous mass distributions" must be having an effect---and maybe there are even significant departures from the oblate spheroid model. Maybe there are significantly different equatorial radii, besides a polar one, and that could have an effect.

My thoughts, exactly. :rolleyes:*

*Ok. My thoughts, are really, that this is all way over my head.

But did anyone else notice the deviation of the max altitude at the south pole?

intro.to.lamo.Dec.15.2015.png


x = time from 1:49 utc Dec 15 2015 (hours)
y = altitude per Mystic (km)
 
  • #776
Another new status update:

December 16, 2015 -Dawn Begins Photography and Infrared Spectroscopy

Dawn is now taking pictures and infrared spectra of Ceres...

The JPL flight team is continuing to incorporate the latest orbital parameters into the plans for the more intensive observations that will start on Dec. 18.

On Dec. 14, mission controllers activated Dawn's two operable reaction wheels ...They are performing well...

Whoop! Whoop! :smile:
 
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  • #777
Reaction wheels, fingers tightly crossed.
 
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  • #778
Great news about the reaction wheels! If both continue to work well it will save a lot of hydrazine and extend the mission. In addition to "Dawn is now taking pictures and infrared spectra of Ceres from its new average orbital altitude ..." which Om quoted, I want to quote another line from the latest status update:
"Nuclear spectroscopy and gravity measurements began last week upon arrival at this low orbit..."
We've already been anticipating the nuclear (gamma emission) spectroscopy. The gravity measurements could discover below-ground regions of higher/lower than average density---subsurface bodies that are mostly rock or mostly ice instead of simply being an average mix. As I understand it, subsurface regions of varying density are to be spotted by observing the probe speed up and slow down as it passes over a concentration of mass---IOW by highly precise tracking.

BTW:
17 Dec 06:32UTC, 357.85 km, 29º to Npole (I should have checked a bit earlier, just forgot.)
17 Dec 17:24UTC, 357.30 km, 30º to Npole
Seems fairly consistent. The difference in times is 10:52, half of which is 5:26 not too different from the orbit period estimate of 5:22 we've been using.
17 Dec 03:24 UTC, 378.04 km, 43º to Spole
18 Dec 01:08 UTC, 379.55 km, 50º to Spole (this time I remembered to check the one before and it was less, as was the one after, so this indicates where the max comes, approximately since we have only discrete rather than continuous readings.)
18 Dec 06:33 UTC, 378.65 km, 56º to Spole. (the next altitude reading was less, that's apt to've been max)
18 Dec 06:56 UTC, 377.50 km, 34º to Spole.
 
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  • #779
http://dawn.jpl.nasa.gov/mission/status.html
==quote new Rayman update==

December 18, 2015 -Dawn Ready for More Observations of Ceres

While Dawn was taking preliminary pictures and infrared spectra on Dec. 16-17, the flight team was putting the finishing touches on commands the probe will use for further observations that start later today. After the trajectory correction maneuver that completed on Dec. 13, navigators measured Dawn's orbital parameters very precisely. Combined with their latest measurements of Ceres' gravity field, they formulated a new prediction of Dawn's orbital motion over the coming weeks. The detailed plans for observing the dwarf planet then were adjusted to account for this latest information.
==endquote==
18Dec 22:50 UTC, 379.61 km, 60º to Spole* (given that readings come at discrete intervals, not continuously, this does seem to be the best guess as to apoapsis--the altitudes before and after this one are lower)
19 Dec 04:18 UTC, 380.09 km, 60º to Spole*
This compares with
18 Dec 01:08 UTC, 379.55 km, 50º to Spole*
and is consistent---given the discreteness we can't narrow it down much better than 50-60º.
Also for the periapsis I just saw
19 Dec 01:57 UTC, 358.04 km, 53º to Npole*
and altitude was larger both right before and right after so given the discreteness probably the best we can say is that for the moment the line of apsides runs through 50-to-60 degrees to Npole and around 50-60 degrees to Spole. And the range is about 358 km to 380 km. Admittedly that is just for the simulation. We'll see if it is reasonably stable.
 
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