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

[marcus]

How elliptical is the orbit? To make a rough guess, judging from Simview, I've noticed that periapsis and apoapsis seem to come while the spacecraft is crossing the S and N poles. and the difference seems to be around 14 or 15 km.

That can vary. The actual trajectory is an elliptical spiral, not a closed ellipse. I don't know the current ellipticity, just have a rough idea, and it can change. Final orbit (alt. ≈ 375 km) will be circular.

We could be using the polar radius of Ceres, rather than the average radius of 473 km. Out of laziness I just use 473. Easier to remember one number and not distinguish between polar and equatorial radii.
So r_a in km is about 473+altitude over N pole
and r_p in km is about 473+altitude over S pole

 

[marcus]

How elliptical is the orbit?

...

You can estimate the ellipticity by looking at http://neo.jpl.nasa.gov/orbits/fullview2.jpg
At the moment I see it showing altitude 540 km as of 24 Nov 22:42 hours. It has about 45 minutes UTC to reach the S pole. So we can come back at 23:30 UTC and see it over the pole. That will be 15:30 pacific, or 3:30 pm my time. So if I'm not busy I'll check back here and get the S pole altitude.
Then we just need the next N pole altitude and we can estimate the eccentricity.

S pole alt. tends to be the low point and N pole the high.

OK I happened to catch it going over S pole at 23:30 UTC on 24 Nov.

Altitude was 539 km.
To learn the ellipticity we need a similar reading at the N pole about 3 and 1/2 hours from now or about 7:30 pm pacific time (it's 4 pm my time)

 

[OmCheeto]

...
the difference seems to be around 14 or 15 km.
...

At least it's not as wobbly as it was this last May. It looks like it was >1000 km back then.

First posted on page 23, btw. Good god, this is a long thread!

 

[marcus]

You're right, on several counts That was the descent from initial "RC3" orbit to "survey" orbit.

Orbit    dates      altitude(km)  pixelsize(m) res/HST  period  soccerball at
RC3    April 23–May 9    (13,600)    (1,300)    24     15 days    (3.0 meters)
Survey    June 6-30      (4,400)      (410)     73     3.1 days    (1.0 meters)
HAMO    Aug 17–Oct 23    (1,470)      (140)     217    19 hours    (33 cm)
LAMO Dec 15–end of mission (375)      (35)      850    5.5 hours    (8.5 cm)

Also IIRC the descent from survey to HAMO was a lot more elliptical than the present trip from HAMO to LAMO. The current trajectory seems like a fairly trim orderly spiral. At least if we go by Simview (and it has proven reasonably reliable so far.)

Thanks for finding that wavy plot of the earlier descent.
=================
Anorlunda, it looks like 15 or 16 km is the current separation between lowest and highest altitudes. I just checked simview at 7:15 pm pacific and probe was approaching N pole at altitude 555 km. Remember the S pole altitude was 540 km---so the difference is 15 km.

Or more exactly the lower figure was 539, I rounded
So the two altitudes min and max were 539 and 555. And the difference was 16 km. That was for 24 Nov.

That, together with the average radius of the body, 473 km, should be enough to calculate a measure of the ellipticity or eccentricity---the departure from perfect circularity--rotational symmetry---of the spiral. I'm not sure what a good measure would be, you could make up one that seems to make sense and define it.
or use the conventional "e" number I posted earlier.

 

[marcus]

An update from Marc Rayman:
==quote==
November 25, 2015 -Dawn Making Good Progress to Lower Altitude

During this spiral descent from the third mapping orbit to the final one, Dawn has paused ion thrusting on Thursdays so JPL mission controllers can transmit an updated flight plan. To give the flight team time off for Thanksgiving, the operations schedule has an update today instead of tomorrow. So this afternoon Dawn will stop thrusting and rotate to point its main antenna to Earth.

The tireless explorer will not take the holiday off. It will spend Thanksgiving using its ion propulsion system to reduce its orbital altitude. It will be thankful that on Nov. 26, its average altitude will be ... (537 kilometers), so it will only have to lower its orbit by about ... (160 kilometers) more.
==endquote==

Going by Simview, for 24 Nov we had min and max altitudes 539 and 555 km, with average 547.
So the semi major axis, or average orbit radius was 473+547 = 1020 km.
2 pi ((1020 km)^3/(G*938e18 kg))^(1/2)
approximate orbit period= 7.2 hours.

Today, 25 Nov, at 18:00 hours UTC the probe passed over N pole with alt. 540 km. The speed indicated was 246 m/s. Thruster was still on. The fact that the thruster will be turned off (for navigation and communication) later today could change what we should expect. Peri and apo may have shifted.

Update, crossing S pole with alt. 524 km, so the spread is still 16 km. Average comes out 532 km.

 

[OmCheeto]

I knew I should not have started this latest project...

I title this graph; "How not to do science, unless you are REALLY dedicated enough to take measurements for more than 24 hours every 30 minutes..."

x-axis: hours since Nov 24 2332 utc
y-axis: altitude in km
black vertical bars: period

Conclusion: Science was much simpler, in the olden days, when it took Dawn two weeks to get around Ceres.

 

[marcus]

Heh heh,
to me it reads as a tongue-in-cheek comment on my own efforts the past few days.

 

[marcus]

DSN does not show any antenna talking with Dawn, as yet. Simview shows thruster off and Dawn oriented for communication. So I guess what Rayman was talking about as planned for today is in progress. Since we've turned a page I'll bring forward a couple of images that encapsulate what this is all about.

Because of its density (2 g/cc) Ceres is assumed to have a large component which is water, making it similar to a number of outer solar system bodies, but unique in the inner solar system. It's shape indicates some degree of differentiation---with a denser core and a less dense outer layer (mantel) covered by some type of crust. The outer mantel may be a rock+ice mixture---could vary in consistency---a 60%-40% mix was conjectured recently and reported by Lakdawalla. I don't know of a cutaway diagram for Ceres that reflects current thinking. The less oblate the more differentiated, and Ceres is comparatively UN-oblate.

The next image is a reminder of the analysis of surface chemistry (down to depth of one meter) that is planned for the final stage, starting in a little over 2 weeks. This involves neutron and gamma ray spectroscopy.

Optical and IR spectroscopy have already indicated some stuff about the surface material, reported at a recent conference (Lakdawalla's account). Ammonia-bearing clays.

Update: Now, 3:12 pm pacific, DSN shows Goldstone antenna #14 assigned to Dawn, but not transmitting or receiving yet. So that business is presumably just getting started.

I can't use the N and S pole crossing to estimate max and min altitudes now. They were good markers but the apsides have shifted. I'll try 30 degrees before you get to the N pole, for max, and 30 degrees before you get to S pole, for min.

 

[marcus]

...
I can't use the N and S pole crossing to estimate max and min altitudes now. They were good markers but the apsides have shifted. I'll try 30 degrees before you get to the N pole, for max, and 30 degrees before you get to S pole, for min.

That's right, apsides have shifted (could have something to do with shutting off thrust Wednesday for navigation etc.)
Altitude got a max (around 535 km) at some 30 degrees before getting to N pole. So I'm expecting min to come about 30 degrees before S pole.

 

[OmCheeto]

That's right, apsides have shifted (could have something to do with shutting off thrust Wednesday for navigation etc.)
Altitude got a max (around 535 km) at some 30 degrees before getting to N pole. So I'm expecting min to come about 30 degrees before S pole.

I should be in full relax, data collecting mode by 6 am tomorrow.
I'll see if I can't capture the new apsides.

But I'm off to dinner now.
Happy Thanksgiving, Marcus!

 

[marcus]

Happy Thanksgiving to you, Om! I was just helping in the kitchen---somehow the mashed potatoes were boiling over. Slowly and slurpily.

514 km at 60 degrees before S pole, (27Nov 0012?? UTC)
512.66 km at 45 degrees before S pole 27 Nov 0036 UTC

529 km at 90 degrees before N pole 27 Nov 0327 UTC
526 km at 30 degrees before N pole 27 Nov 0442 UTC507 km at 45 deg before S pole 27 Nov 0732 UTC
506.81 km at 30 deg before S pole 27 Nov 0759 UTC

 

[mfb]

509.26 at 16:07:28 UTC.
The coordinate system is in the shadow right now, but Dawn is at ~45 degrees after crossing the South pole.

 

[marcus]

Mfb, thanks for helping with this! A figure of 509 or 510 could be a reasonable estimate for the average altitude this time around.

I suspect that periapsis (min) is about 45 deg before S pole, and apoapsis (max) is about 45 deg before N pole. So that halfway between the apsides would be somewhere around 45 deg after crossing S pole. Or after crossing N pole (where the coordinate system would be more visible).

 

[marcus]

Just noted:
27Nov17:36 519.31 at what looks like 65 deg before NP
27Nov18:10 517.8 at what looks like 50 deg before NP

That suggests I underestimated the shift in apsides. Doesn't it seem more likely that apoapsis is now closer to 60 degrees before NP
and periapsis more like 60 deg before SP?

 

[OmCheeto]

Sorry for not paying attention, but are the latitude lines 30° apart?

 

[marcus]

I think so, Om.
Moreover it may be a sign of your astute good judgement that you have not been paying attention
For the better part of a month the apsides were approximately stable (or so it seemed) at or around the poles. But now they seem to be adrift and it may be a wild goose chase trying to keep track of them.

Even a slight change of the angle of thrust could do that. Make them differ day by day. The navigation team may be trying now to circularize the spiral.
27Nov20:38 500.16 at what looks like 80 deg before SP
27Nov21:02 498.03 at what looks like 60 deg before SP (that might be as close an estimate of the periapsis that we can get for now)
27Nov21:26 496.67 at what looks like between 40 and 45 deg before SP (so it's not 60 deg anyway)
27Nov22:15 498.41 right on the SP (I got absorbed with something else and neglected to check earlier.)

 

[OmCheeto]

I think so, Om.
27Nov20:38 500.16 at what looks like 80 deg before SP

Yay!

I'm in full nerd mode this morning, and very happy to see that our eyeballs are calibrated.

Doh! I'm a minute late for my next reading...

an: after north
bn: before north
np: north pole
as: after south
bs: before south

 

[mfb]

500.61 at 60° after S pole, 13:00:24 UTC, 564 mph.
Ceres doesn't fit into the picture any more.
Om, do you have that table in a format that allows to copy it?

 

[OmCheeto]

500.61 at 60° after S pole, 13:00:24 UTC, 564 mph.
Ceres doesn't fit into the picture any more.
Om, do you have that table in a format that allows to copy it?

Let me know if this doesn't work for you.
You should be able to import this as a csv file.
If not, I can export it as csv, excel, or numbers, and upload the file.

eyeballer, eyeballed °, dir, mo, day, hr, min, , km, m/s, mph, time
, , , Nov , 27, 15, 43, utc, 505.46, 251, 562, 71.12
mfb, 45, as, Nov , 27, 16, 7, utc, 509.26, 251, 561, 71.52
, , , Nov , 27, 16, 34, utc, 513.85, 250, 560, 71.97
, , , Nov , 27, 16, 58, utc, 517.26, 250, 559, 72.37
, , , Nov , 27, 17, 22, utc, 519.19, 249, 558, 72.77
marcus, 65, bn, Nov , 27, 17, 46, utc, 519.31, 249, 557, 73.17
om+marcus, 47.5, bn, Nov , 27, 18, 10, utc, 517.80, 249, 556, 73.57
om, 10, bn, Nov , 27, 18, 34, utc, 515.17, 249, 556, 73.97
om, 0, np, Nov , 27, 19, 1, utc, 511.66, 249, 557, 74.42
om, 25, an, Nov , 27, 19, 25, utc, 508.53, 250, 560, 74.82
om, 50, an, Nov , 27, 19, 49, utc, 505.62, 252, 563, 75.22
om, 70, an, Nov , 27, 20, 12, utc, 502.94, 253, 566, 75.60
om+marcus, 80, bs, Nov , 27, 20, 39, utc, 500.16, 254, 569, 76.05
om, 60, bs, Nov , 27, 21, 3, utc, 498.03, 254, 569, 76.45
om, 40, bs, Nov , 27, 21, 26, utc, 496.67, 254, 568, 76.83
om, 20, bs, Nov , 27, 21, 52, utc, 496.68, 253, 566, 77.27
om, 0, sp, Nov , 27, 22, 16, utc, 498.41, 252, 564, 77.67
om, 20, as, Nov , 27, 22, 40, utc, 501.71, 252, 563, 78.07
om, 40, as, Nov , 27, 23, 6, utc, 506.47, 251, 562, 78.50
om, 60, as, Nov , 27, 23, 30, utc, 510.69, 251, 561, 78.90
om, 80, as, Nov , 27, 23, 54, utc, 513.79, 250, 560, 79.30
om, 80, bn, Nov , 28, 0, 18, utc, 515.11, 250, 559, 79.70
om, 50, bn, Nov , 28, 0, 42, utc, 514.51, 249, 558, 80.10
om, 30, bn, Nov , 28, 1, 9, utc, 512.03, 249, 557, 80.55
om, 10, bn, Nov , 28, 1, 33, utc, 508.92, 249, 558, 80.95
om, 5, an, Nov , 28, 1, 57, utc, 505.61, 250, 560, 81.35
om, 30, an, Nov , 28, 2, 21, utc, 502.52, 251, 562, 81.75
om, 50, an, Nov , 28, 2, 45, utc, 499.81, 253, 566, 82.15
om, 80, an, Nov , 28, 3, 11, utc, 497.13, 254, 569, 82.58
om, 80, bs, Nov , 28, 3, 35, utc, 494.95, 255, 570, 82.98
om, 55, bs, Nov , 28, 3, 59, utc, 493.12, 255, 570, 83.38
om, 35, bs, Nov , 28, 4, 25, utc, 492.04, 254, 569, 83.82
om, 10, bs, Nov , 28, 4, 49, utc, 492.51, 253, 567, 84.22
om, 10, as, Nov , 28, 5, 13, utc, 494.62, 253, 565, 84.62
om, 30, as, Nov , 28, 5, 37, utc, 498.14, 252, 564, 85.02

It looks like this when graphed:

x-axis: hours
y-axis: km, altitude

My eyes were getting exhausted last night, so I took a break.
I decided to collect two full cycles to see what the overall descent rate was.

I also plotted back to Nov 24th, to see if I could remove the descent from the graph and see if the ellipticity has been changing.

upper graph is actual altitude vs time
lower graph has the slope removed

 

[mfb]

Thanks. Looks like a sine and a smooth slope fit quite well.

Just crossing a pole again (maybe 5-10° later), S I guess. 485.13 km, 568 mph, 18:42 UTC.

70° after crossing the pole: 497.37 km, 565 mph, 19:56:46 UTC.

That is a very fast increase in height.

Right at the equator: 499.72km, 564 mph at 20:20:53 UTC.

 

[marcus]

40 deg before Spole, 477.3km, 29Nov 00:26
20 deg before Spole, 477.64km, 29Nov 00:50
==========at Spole, 479.82km, 29Nov 01:14
20 deg after Spole, 483.58km, 29Nov 01:38

I suspect the mission controllers would be circularizing the orbit somewhat now. I don't expect the spread this time to exceed 20 km. That is, I'm thinking min = around 477 and max = no more than 497. Don't really know, haven't been paying much attention today, just how I imagine it. May be optimistic.

Om made a beautiful plot a couple of posts back. That was for yesterday (and before).

Let's check back after the craft passes the equator going north. Should be starting to approach max then.
Max might come around 29Nov 04:00? what is half the orbit period now?

Let's say the average of max and min is 487km and so radius is 473+487 = 960 km
2 pi ((960 km)^3/(G*938e18 kg))^(1/2) = 6.56 hours, half would be 3.28 hours

Update: crossing equator at 02:53, 495.72km.
10+ deg north at 03:17, 496.41
30 deg north at 03:41, 494.98 (my goodness! it maxed out at 496.4 km!)
over Npole at 4:31, 487.84 (I missed a couple of check points)

And the maximum came way before I expected---80 degrees before Npole.
That means I don't have an accurate figure for the minimum, it would have occurred (unless I'm mistaken) around 80 degrees before Spole. There could be some kind of error, I should keep recording a while longer.

BTW the Dawn mission was canceled in 2006, and then re-instated by NASA some weeks later. It almost didn't fly.
https://www.physicsforums.com/threads/dawn-back-on-2007.115809/
http://www.nytimes.com/2006/03/28/science/space/28asteroid.html?_r=0
The NYT article gives the underlying reasons for the initial decision and for its later reversal.

30 deg north, 29Nov 05:43, 478.26 km
===========equator, 06:09, 475.84
40 deg before Spole, 06:33, 474.08
Maybe it's just a fact about continuous retrothrusting spirals that if they are a bit elliptical the peri and the apo are not 180 degrees apart. Maybe it is more than 180 deg from apo to peri, and then a lesser angle going from peri to apo. It's that downwards sloping sine curve, as in Om's plot.

 

[marcus]

Same thing happened today as happened yesterday. Altitude peaked at around 80 deg before N pole.
This is curious, at least to me, because the minimum does not come at the opposite point---i.e. at 80 deg before crossing S pole.
Today (watching carefully) the max came at 29Nov 16:22, 80 deg bef N pole, 487.17 km
Yesterday we saw it at 29Nov 03:17, 80 deg bef N pole, 496.41 km

Incidentally the two times differ by about 13 hours which is consistent with a 6.5 hour orbit period.
In the course of two orbits the max altitude was reduced by about 9 km.
That's more or less expected. The surprise for me was that the min seemed to come quite a bit closer to the S pole. Something like 40 deg before S pole. Instead of 80 degrees.
This could have a simple explanation related to the fact that the curve (e.g. in Om's plot) is sloping down.

Yes!
Today (watching carefully) the min came at 29Nov 20:04, 45 deg bef S pole, 464.11 km
Yesterday I recorded it coming at 29Nov 00:26, 40 deg bef S pole, 477.3 km, close enough to 45 degrees.

Indeed it does have a simple explanation in terms of basic diff. calculus. The extrema come where the derivative is zero. For a sine superimposed on a down-slope the extrema come where the sine has a positive slope, to cancel---before the sine's own max and after its min.
So the max is advanced and the min is delayed in the down-slope sine.

The max-to-min leg of the cycle has more than its share of the 360 degrees, and the min-to-max part has less

80 deg before N            40 deg before S           80 deg before N
               220 degrees               140 degrees

 

[marcus]

This time around the minimum altitude is 460 km. We just got the simview 30Nov 02:34UTC, 45 degrees before S pole, 460.67 km
Since we just turned a page I'll bring these forward:

Orbit    dates      altitude(km)  pixelsize(m) res/HST  period  soccerball at
RC3    April 23–May 9    (13,600)    (1,300)    24     15 days    (3.0 meters)
Survey    June 6-30      (4,400)      (410)     73     3.1 days    (1.0 meters)
HAMO    Aug 17–Oct 23    (1,470)      (140)     217    19 hours    (33 cm)
LAMO Dec 15–end of mission (375)      (35)      850    5.5 hours    (8.5 cm)

This is basically how Dawn will be able to identify and estimate abundances of various chemical elements in the surface material (to a depth of about 1 meter). Neutron and gamma spectroscopy under bombardment by cosmic rays.

Ceres' density (2 g/cc) indicates a substantial portion of water ice, possibly mixed with rocky minerals. Ammonia-bearing clay has been detected according to reports at a recent conference
http://www.planetary.org/blogs/emily-lakdawalla/2015/dps15-1112-ceres.html

Finally have a better understanding of the apsides.
29Nov 16:22, 80 deg bef N pole, 487.17 km
30Nov 18:33, 85 deg bef N pole, 470.32 km
Maximum altitude comes around 80 before N.
So in 26 hours the apoapsis went down by 17 km.

Let's see how the periapsis is doing. Yesterday minimum altitude came at:
29Nov 20:04, 45 deg bef S pole, 464.11 km
I may not be able to record today's since have some errands to do.

Just missed it, wasn't back in time. Got the reading right after the minimum altitude one.
30Nov 22:13, 25 deg bef S pole, 448.05 km

but 464 - 448 = 16, so that's close enough to say that the periapsis went down by about the same amount as the apoapis, in that 26 hour interval. And according to simulation, the craft has come within 450 km of Ceres' surface.

 

[OmCheeto]

The new journal is out!​

 

[marcus]

For comparison:
30Nov 22:13, 25 deg bef S pole, 448.05 km
01Dec 17:01, 25 deg bef S pole, 436.61 km
I missed checking earlier when the minimum altitude would have occurred around 40 deg before S. But the change from that and the next report at 25 deg before S pole is not great, so this gives a rough idea of the minimum---which someone else already noted was 435 km.

The new target altitude for the final orbit is 385 km (changed from 375, according to the recent DJ)

So 435 - 385 = 50 km left to go.

 

[OmCheeto]

A couple of notable notes I noticed in the new journal:

Note 1:

November 2015 Journal

"...
It requires 118 spiral revolutions around Ceres to reach the low altitude (and additional revolutions to prepare for and conduct the trajectory correction maneuver described below). Compare this to the previous spiral. (Readers with total recall will note that this is fewer loops than illustrated last year. The flight team has made several improvements in the complex design since then, shortening the time required and thus allowing more time for observing Ceres.)
..."​

For those of us without total recall:
April 2014 Journal

"...
HAMO to LAMO: Dawn will complete 160 revolutions in two months
...
"​

160 previously planned, to 118 now planned, seems somewhat significant.Note 2, from yesterday's journal:

"...
Finally, we round the altitude numbers to the nearest multiple of five (both for miles and kilometers), because, as we will discuss in a subsequent Dawn Journal, the actual orbit will vary in altitude by much more than that.
..."​

So, it appears that Dawn will maintain an elliptical orbit.

 

[marcus]

01Dec 22:44, 45deg before S, 431.67 km
That must be the new minimum altitude for this time around.
For greater exactitude I decided to use the ion engine vent as a mark to sight along : ^)
as Dawn gets closer the angular size of its silhouette grows so some choice like that is needed.

Om, those are both interesting points! Part of the variation in altitude may be due to the difference between polar and equatorial radii. Rounding to the nearest 5 kilometers would be reasonable simply on that account, I think. And the orbit may, in addition, be elliptical.

Wikipedia (you may be able to find something more recent or reliable) give the diameters as
(965.2 × 961.2 × 891.2) ± 2.0 km and the average radius as 473 km.
I guess the equatorial radius is something like 482 km and the polar radius is something like 446 km
So even if you had a perfectly circular polar orbit the altitude above surface would change substantially

 

[mfb]

If oblateness would be taken into account in MYSTIC, the equatorial values should be significantly smaller than the polar ones. I don't see such a pattern.

Currently over the pole, 435.63 km, 582 mph, 23:32:10 UTC

 

[marcus]

I agree! The simulation does not take account of the difference between polar and equatorial radii, in listing altitudes.
But it remains to be seen what Marc Rayman meant by these words:
"because, as we will discuss in a subsequent Dawn Journal, the actual orbit will vary in altitude by much more than that."
I guess he plans to explain in a subsequent Journal entry. Actual orbit could mean something else besides MYSTIC orbit.

02Dec 05:17, 30deg before S, 429.12 km

 

[mfb]

02 Dec 17:56:55 UTC, south pole, 425.84 km, 584 mph.

An exactly polar orbit will reduce changes in the orbit from the inhomogeneous gravitational field, but it won't remove them completely. I don't think they plan to adjust the orbit frequently, so it will vary a bit.
Related post

 

[marcus]

In his Journal entry, Rayman suggested the main descent might be complete by 7 December--just four more days. After that they will consider a "TCM" or trajectory correction maneuver, if the orbit needs correction. A big effort is clearly being made to conserve hydrazine. Essential for keeping sensors trained on the surface, and then turning antenna towards Earth to transmit the data.

The thruster has been off (according to simulation) for many hours while DSN showed no communication.
Don't know why but it may have something to do with subtleties of timing and attitude control.

Last time I checked, the min altitude was 416.41 km at roughly 110 deg before S pole, that was 3Dec 22:09.
Max was around 435 km, a couple of times that same day. With the ion engine off, one assumes the apsides are 180 degrees apart. What the new ones will be when they restart the thruster will presumably depend on when they resume thrust how it's directed. In any case, if we follow Rayman's example, and round to nearest 5 km then the simulated craft is down to 415 km of Ceres' surface.
Leaving 30 km left to go.
==quote from Journal==
When ion-thrusting ends on Dec. 7, navigators will measure and analyze the orbital parameters to establish how close they are to the targeted values and whether a final adjustment is needed to fit with the intricate observing strategy. Several phenomena contribute to small differences between the planned orbit and the actual orbit. (See here and here for two of our attempts to elucidate this topic.) Engineers have already thoroughly assessed the full range of credible possibilities using sophisticated mathematical methods. This is a complex and challenging process, but the experienced team is well prepared. In case Dawn needs to execute an additional maneuver to bring its orbital motion into closer alignment with the plan, the schedule includes a window for more ion-thrusting on Dec. 12-14 (concluding on Dawn’s 3000th day in space). In the parlance of spaceflight, this maneuver to adjust the orbit is a trajectory correction maneuver (TCM), and Dawn has experience with them.

...
...
Now the mission lifetime is limited by the small supply of conventional rocket propellant, expelled from reaction control system thrusters strategically located around the spacecraft . When that precious hydrazine is exhausted, the robot will no longer be able to point its solar arrays at the sun, its antenna at Earth, its sensors at Ceres or its ion engines in the direction needed to travel elsewhere, so the mission will conclude. The lower Dawn’s orbital altitude, the faster it uses hydrazine, because it must rotate more quickly to keep its sensors pointed at the ground. In addition, it has to fight harder to resist Ceres’ relentless gravitational tug on the very large solar arrays, creating an unwanted torque on the ship.

...
...
Dawn will measure the energies and numbers of neutrons and gamma rays emanating from Ceres as soon as it arrives in its new orbit. With a month or so of these measurements, scientists will be able to determine the abundances of some of the elements that compose the material near the surface.
...
==endquote==

Most recent max: 4Dec 01:26 435.19 about 115 deg before N
(the readings just before and just after were 433.91 and 433.8)
Most recent min: 3Dec 22:09 416.41 about 110 deg before S

Thruster is still off, according to simulation.
Apsides seem about 180 deg apart. Spread still around 19 km. Something MFB said may apply. Irregularities can cause a polar orbit to evolve over time. The Dawn team may be intentionally letting the probe coast because this does something to the orbit that they want to happen. (without costing any hydrazine to orient the thrust, or to orient antenna for communication ). It's hard to guess the reason for this long period of coasting (if indeed the simulation showing it is correct).

 

[marcus]

According to simview, the ion engine is back on.
4 Dec 15:43 UTC, 413.29 km altitude, around 140 deg before S pole.

That was a long pause, part of Wednesday and all or much of Thursday. With no communication AFAICS on Thursday. Maybe Rayman will explain in a status update. It could have to do simply with the Thanksgive holiday. Earlier they always paused (for review and recalculation) on Thursday. But they wanted to give the team a break for the holiday, so they did all that work on Wednesday. But then they didn't want to start up on Thursday because that would throw everything off, so they continued the pause Thursday---now they are back on a regular Thursday pause schedule. One possible explanation. TGIF (thank god it's Friday) and Dawn has its blue-green ion tail again.

Everytime they do the weekly pause there's an opportunity for the apsides to shift, depending on where in the orbit they shut engine off and where they turn back on. So now I'm guessing the minimum altitude might be coming at what we just now see: maybe around 140 deg before S. We can check.

Nope, min seems to be more like 80 or 110 before S. Just saw this:
4 Dec 16:32 UTC 413.06 km, 80 bef S
Also this:
4 Dec 19:25, 429.93, 100 bef N

 

[mfb]

22:39 UTC: 410.01 km, 597 mph, 75° before N.
1:32 UTC: 423.72 km, 587 mph, equator going S.

Assuming the longitude markers are 30 degrees apart as well, Dawn is about 45 degrees away from the terminator. At that distance, it needs sqrt(2) times the Ceres radius as orbital radius to stay in sunlight, roughly 680 km or an orbital height of 200 km. So it has some significant safety margin.
At the height of 385 km (assuming it is above the equator), the maximal angle is ~56 degrees. That is 100 degrees away in terms of the Ceres' orbit, or 1.0 years. I don't expect hydrazine to last that long, but even if it does this will be the end of the mission.

 

[marcus]

Nice analysis!
Rayman just posted this update:
==quote==
December 4, 2015 -Dawn Closing in on Final Mapping Orbit

Dawn has now reduced its orbital altitude to 270 miles (435 kilometers). The probe is continuing to make excellent progress to its fourth and final mapping orbit.
==endquote==

As best I can tell, for now, apoapsis is coming around 110 deg before N
and periapsis around 80 deg before S

 

[marcus]

According to simview, Dawn crossed Ceres N pole today at altitude < 400 km

5 Dec 15:01 UTC, 395.71 km, 267 m/s, 15º past N pole.
5 Dec 15:26 UTC, 394.52 km, 45º past N pole ( must be the periapsis, i.e. min altitude, this time around)
5 Dec 15:51 UTC, 395.33 km, 75º past N pole
5 Dec 16:15 UTC, 397.11 km, 105º past N pole

5 Dec 17:53 UTC, 408.14 km, 20º past S pole
5 Dec 18:18 UTC, 411.06 km, 45º past S pole
5 Dec 18:43 UTC, 412.08 km, 70º past S pole (the max this time around)
5 Dec 19:06 UTC, 410:59 km, 90º past S pole