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

[marcus]

*off topic*
footnote for anyone unfamiliar with that 1985 movie about time travel 30 years forward to 21 Oct 2015
https://www.washingtonpost.com/news...ure-ii-got-right-and-wrong-about-oct-21-2015/

 

[mfb]

The descent will take a while, but 4 times better resolution is certainly worth the effort. In addition, we get spectroscopy!

 

[marcus]

Rayman says the HAMO topographical mapping is complete:
==quote http://dawn.jpl.nasa.gov/mission/status.html ==
October 21, 2015 - Dawn Completes Topographical Mapping

This morning Dawn conducted its final observations from its current orbital altitude of 915 miles (1,470 kilometers). Following the 12th flight over Ceres' sunlit hemisphere in its sixth mapping cycle in this orbit, the spacecraft rotated to aim its main antenna to Earth. It is now beaming its pictures and spectra to NASA's Deep Space Network.

Next week's Dawn Journal will include a summary of this extremely productive third mapping campaign, which began on Aug. 17.

Ion thrusting to spiral down to the fourth and final orbital altitude will begin on Oct. 23.
==endquote==

 

[marcus]

Descent starts at 3:30 PM pacific time, today:
==quote==

October 23, 2015 - Dawn To Begin Final Spiral Descent

Dawn has finished transmitting its extensive observations of Ceres to Earth.

About 3:30 p.m. PDT today, the spacecraft will fire up ion engine #2 to start maneuvering to its final orbital altitude. It will take more than seven weeks to spiral down from 915 miles (1,470 kilometers) to less than 235 miles (380 kilometers).

The August 2014 Dawn Journal provided an overview of the plans for the explorer's final mapping orbit at Ceres.
==endquote==

 

[marcus]

http://neo.jpl.nasa.gov/orbits/fullview2.jpg
Simview as of 7 PM pacific on 23 Oct showed Dawn retrothrusting as it passed over Ceres north pole from night to day side.

The simulator has switched from 30 degree to 50 degree view, giving the angular size of Ceres room to grow.

 

[marcus]

Rayman's status report for today:
==quote==

October 26, 2015 - Dawn Maneuvering to Lower Orbit

On Oct. 23, when Dawn was orbiting at an altitude of ... (1,470 kilometers), it started more than seven weeks of ion thrusting to lower its orbit to less than ... (380 kilometers). Today the spacecraft 's average altitude is ... (1,345 kilometers).
==endquote==

So it has come down by over 100 km in the past 3 days or so. And it still has about 1000 km to descend.

As a rough overview, the probe has to lose some 1000 km of altitude in the next 50 days (i.e. ~7 weeks), so average descent 20 km/day.

It gets progressively harder to shrink the orbit, the more the orbit shrinks. So it will probably descend faster than 20 km/d at first, and much slower and more laboriously than 20 km/s towards the end. We'll see. Rayman's upcoming October JOURNAL entry could well show a plan of the spiral-in trajectory.

Or he could reference a projected HAMO to LAMO spiral trajectory that was posted back in 2014.

To me it's exciting to see solar-powered ion drive used this way and accomplishing this ambitious a maneuver. I'm in suspense as to whether the probe can make it (but it has shown remarkable resiliency and has had remarkable success so far.)

From an earlier Dawn Journal entry by Rayman.
He will probably give a link to it in his October DJ entry.

 

[marcus]

Hi Om!
we should start keeping an eye on
http://dawn.jpl.nasa.gov/mission/journal.asp
to see when the October DJ is posted.

For me this is the best part of the mission so far
It will be really nice if the probe gets within range (e.g. 375 km) where it can use gamma and neutron spectroscopy to detect the chemical elements in the surface material! This is getting to be my favorite picture about the mission:

The ratio of thermal (slow) to fast neutrons tells the fraction of HYDROGEN (i.e. water) in the soil down to depth of 1 meter. Because hydrogen nuclei are by far the best nuclei at "moderating" (i.e. slowing down). The more of the neutrons detected by Dawn are slow, the "juicier"/icier is the planetto

 

[OmCheeto]

Hi Om!
we should start keeping an eye on
http://dawn.jpl.nasa.gov/mission/journal.asp
to see when the October DJ is posted.

For me this is the best part of the mission so far
It will be really nice if the probe gets within range (e.g. 375 km) where it can use gamma and neutron spectroscopy to detect the chemical elements in the surface material! This is getting to be my favorite picture about the mission:
View attachment 90871
The ratio of thermal (slow) to fast neutrons tells the fraction of HYDROGEN (i.e. water) in the soil down to depth of 1 meter. Because hydrogen nuclei are by far the best nuclei at "moderating" (i.e. slowing down). The more of the neutrons detected by Dawn are slow, the "juicier"/icier is the planetto

Hi Marcus!
I check Dawn's Twitter and Facebook pages daily. Hopefully they'll announce when the new journal comes out.
I also need to remember to check "Nature" magazine for the top secret article. (Still not out yet)

That's nice that LAMO is scheduled to start around the 15th of December. We should have some very nice images for Christmas.

 

[OmCheeto]

Hi Marcus!
I check Dawn's Twitter and Facebook pages daily. Hopefully they'll announce when the new journal comes out.
...

And they did not announce that the new Journal is out.
But, it's out.

Dawn Journal | October 30 [2015]
by Marc Rayman
Chief Engineer/ Mission Director, JPL

 

[marcus]

==status update from Rayman==
November 2, 2015 -Ion Thrusting to Lower Orbit Continues

Dawn has reduced its average altitude today to about 680 miles (1100 kilometers) as it maneuvers to its final orbit. Because lower orbits require higher velocity (to balance the stronger gravitational pull), the spacecraft is now orbiting the dwarf planet at about 450 mph (725 kilometers per hour). (Orbital velocity in the third mapping orbit, which concluded on Oct. 23, was about 400 mph, or 645 kilometers per hour.)
==endquote==

I'm wondering what the current estimate is of Ceres' mass. Wikipedia now says 939e18 kilograms, but Google says 896e18 kg.
We are getting a little object lesson in basic orbital dynamics this month.
Simview says altitude 1050 km and speed 202.5 m/s
Let's use the Wikipedia numbers which have a July 2015 source:
http://webcache.googleusercontent.com/search?q=cache:http://nesf2015.arc.nasa.gov/sites/default/files/downloads/pdf/05.pdf&gws_rd=cr&ei=nmYGVoOdHIKhacahn_AC
That document is the source of Wikipedia's mean radius of 473 km, which makes the current orbit radius
1523 km (1050+473)
(G*939e18 kg/1523 km)^(1/2)
which gives 202.9 m/s, close enough for a rough calculation.

 

[AgentSmith]

Great posts guys, with lots of technical information.

 

[marcus]

Simview is showing altitude 984 km and speed 207 m/s.
That's as of about 20 hours on 5 November, which IIRC is 12 noon pacific time.

That altitude would be 1457 km radius (984+473). Circular orbit speed at that radius:
(G*939e18 kg/1457 km)^(1/2) which checks pretty well---207.4 m/s
Simview seems to be suggesting that the inspiraling path is fairly regular---a nearly circular spiral---which I guess we expected. (Last time, from survey orbit down to HAMO, it was not all that circular, but this time the orbit speed is already so high that the ion drive can make only a very gradual small percentage change at first, preserving the circularity.)

Thanks for your expressed approval, Agent Smith

 

[marcus]

We've been told to expect a final altitude of about 375 km which means final orbit radius (473+375) 848 km, when Dawn reaches LAMO
which translates to
(G*939e18 kg/848 km)^(1/2)
which google calculator says is 272 m/s
That's not a heck of a lot faster than it is going now---namely 207 m/s--- but it will be a lot closer to the ground. Within range for neutron and gamma ray spectroscopy to sense the chemical composition of a meter-thick surface layer.
So there is a lot riding on this current descent---the next 5 weeks or so of ion retrothrust.

Another status update from the mission director, Marc Rayman:
==quote==
November 5, 2015 -Dawn Descending on Course and on Schedule

As Dawn spirals to lower orbits, its average altitude today is about ...(1000 kilometers). Each week, controllers update the complex flight plan for ion thrusting, so Dawn will pause thrusting this afternoon, turn to point its main antenna to Earth to receive its new instructions, and resume thrusting tonight. Tomorrow the spacecraft will reach to below ...(966 kilometers).
==endquote==
Here's a simulated view as of around 7:30 AM pacific 6 Nov, to aid in visualizing what Rayman wrote. The probe has resumed (retro)thrusting. It is moving roughly upwards in the picture and will soon cross over the north polar region and start south over the dayside of Ceres.

 

[marcus]

New update from Rayman confirms that Dawn's altitude is now < 900 km:
==quote==
November 9, 2015 -Dawn Lowering its Orbit

More than three times as far from Earth as the Sun is, Dawn is using its ion propulsion system to maneuver to its final orbit around Ceres. The spacecraft 's average altitude above the alien world today is about 550 miles (885 kilometers).

As Dawn descends, the time to complete one revolution gets shorter, both because the velocity increases and because the distance around an orbit decreases. Today it is 11 hours. In the third mapping orbit, each revolution required 19 hours.
==endquote==

Simview agrees fairly well, says (as of 1:24 pm pacific) altitude 868 km

 

[mfb]

As Ceres' diameter is ~900-950 km, Dawn is now less than 1 diameter away.
Ceres appears as large as a football seen from 1 football away. Replace by your favorite sports, as long as it has a ball. Handegg does not count (not to be confused with Handegg).

 

[marcus]

Dawn speeds up as it spirals in closer. Simview estimates the speed at present to be about 220 meters per second.
The probe is now estimated to be within 500 miles of surface (800 km)

Emily Lakdawalla has a long article about the most recent Ceres findings, that came out at this week's DPS meeting
http://www.planetary.org/blogs/emily-lakdawalla/2015/dps15-1112-ceres.html
(American Astronomical Society Division for Planetary Sciences)
It's really interesting. Ceres outer mantle may be more like "icy dirt" than "dirty ice".
Pingo formation was mentioned.
Spectroscopy suggests ammonia-bearing minerals at the surface which is very strange. To some it suggest an outer solar system origin and Ceres somehow being moved into the asteroid belt by gravitational interaction with other bodies.

 

[marcus]

http://aas.org/meetings/dps47
The DPS meeting runs 8 - 13 November.
It is in the Washington DC area.
I want to quote some of Lakdawalla's coverage.
==quote Emily http://www.planetary.org/blogs/emily-lakdawalla/2015/dps15-1112-ceres.html ==
First here are some global properties of Ceres, as reported in a poster by Ryan Park and coauthors. The mass is slightly higher than previously thought, so the density is also slightly higher, but the difference is minor. The density is significantly less than that of silicate rock, so the Dawn team knew they were approaching a world that contained a significant quantity of water ice.

• Volume: 434 ± 2 x 10^6 km^3
• Mass: 9.38463 ± 0.00008 x10^20 kg
• Density 2161 ± 9 kg/m^3
• No observed offset between center of figure and center of mass
• Shape is consistent with hydrostatic equilibrium
• No moons observed despite very deep searches.

Carol Raymond gave an overview talk, discussing many of the surprises revealed about Ceres after Dawn's first complete global survey of the dwarf planet. If Ceres has a lot of water ice, and if it differentiated into a rockier interior and icier crust upon formation, you would expect Ceres' surface to be very smooth, with little topography. That's because ice is not structurally strong enough to hold up tall mountains over geologic time at the temperatures that prevail in the main asteroid belt. In fact, ice isn't stable over most of the surface of Ceres -- it gets warm enough to sublimate -- so there has to be at least a thin layer of rock-rich material at the surface to shield ice from the temperature variations that would make it go away. And the oblate shape of Ceres has confirmed that Ceres has at least some mass concentration toward its center; it is differentiated. So they expected to see craters like those on Enceladus or Ganymede, where the bowl-shaped topography was replaced by a flat floor with only a ring to mark the original location of the crater.

When Dawn approached, the team was surprised by how much topography there was. Michael Bland reported topographic variations of plus or minus 7 kilometers. This is much more than on Ganymede or Enceladus. It's more like Iapetus, which has been much colder for much longer than Ceres. Bland showed that you just can't support such topography without a substantial amount of rock in the upper layer -- more than 60% rock. "The takeaway message," he said, "is that Ceres interior is more like icy dirt than dirty ice."
==endquote==

 

[marcus]

More excerpts from Emily Lakdawalla's report from the DPS. Hopefully most of us will read the whole thing at her website.
==excerpts from Emily http://www.planetary.org/blogs/emily-lakdawalla/2015/dps15-1112-ceres.html ==
...
The highly variable morphology of the craters on Ceres is another puzzle. Raymond said this was evidence for a crust whose composition and structure varies from place to place. She proposed a model to explain that: Ceres began with a water ice layer at the surface (including some fine silicate material and salt impurities) over a layer of a less-differentiated, hydrated-silica core. Impact gardening -- whereby holes of different depths and sizes are dug randomly over the surface with variable depths, tossing various proportions of ice and rock to the surface at various distances from the crater -- has churned the crust into a mix of ice, rock, salt hydrates, and frozen brines that varies from place to place.

... Instead, the Dawn team has concluded that they're looking at ammonia-bearing clay minerals, which is just weird. I've never heard of such things mentioned as the component of a planetary surface before.

The problem with ammonia on Ceres is that it's thought to need an outer solar system source. In other words, either Ceres is covered with gunk from outer solar system impactors, or Ceres itself originated in the outer solar system and was transported to its current orbital position by the same solar system kablooie that scattered most of the trans-Neptunian objects. I spent fifteen minutes at the poster session listening to Pieters and Tom McCord argue amicably about whether the ammonia really required an outer solar system source (McCord thinks it can be explained through petrology with an outer main belt origin, while Pieters thinks you can't explain ammonia being observed globally without an outer solar system origin). In his talk, Simone Marchi explored whether an outer solar system origin for Ceres -- and a late capture into the asteroid belt -- could be used to explain the relative lack of large basins, but he said even that is not sufficient to explain the low numbers of craters.

At the poster session, I asked dynamicist Bill Bottke what he thinks of a potential outer solar system origin for Ceres. It's apparently not out of the question; it is possible to start with a large body beyond Neptune and transport it inward in all the wild events that happened during solar system formation. You can even end up with a relatively circular orbit, as Ceres has. But he said it's hard to do that without capturing a lot more stuff from the outer solar system while you're at it. Which would imply that a lot of the dark objects in the asteroid belt didn't actually form in the asteroid belt. It's an interesting area for future work, he said.

There were a couple of talks and posters on the interesting geomorphology of Ceres' surface. Jennifer Scully gave Britney Schmidt's presentation on flow-like features on Ceres. ..
... Both types of flows are not found on Vesta, so Scully suggested that Ceres' crustal material is weaker, flows more rapidly, and melts more easily. It requires about 30 to 40% [ice?]for these sorts of flows to happen -- which is nicely consistent with previous talks. The flow in the 3D image below is the example of a steep-toed, likely ice-creep flow that they showed.
[see the photograph in original]
Jennifer Scully and Debra Buczkowski presented adjacent posters on preliminary geologic mapping of Ceres. I talked to them for a bit about how strange Ahuna mons is and why aren't there any other features like that on Ceres? Scully told me there's another candidate feature similar to it close to the north pole, but it's hard to see because the illumination is poor.
Hanna Sizemore had a poster looking at smaller mounds found all over Ceres, and showed they likely had a variety of origins -- some are just preexisting topography embayed by impact melt (like you see in mounds on the Moon), but others could have a pingo-like origin, and some could conceivably be volcanic. ...
...

By now I know several of you are probably asking: what about the bright spots? They haven't been easy for Dawn to investigate, because Ceres is so very dark; exposure settings that allow the visible-light instruments to see most of Ceres' surface were too long for the bright spots, and they saturated the detectors, making compositional data useless. Pieters said they had to get special data in order to get the bright spots onto the scale. Even so, it's still not obvious what they are. ...
...It could be salt, but it's not a lot of evidence to go on. Paul Hayne showed that although water ice isn't stable on the surface of Ceres over geologic time, it can last tens of thousands of years, so in fact Ceres has less exposed ice from recent impact craters than he expected. This is yet another argument for a rock-rich crust for Ceres.

I think Ceres is an intriguing little world, and it's fun that it's generating puzzles for so many different kinds of scientists: cratering people, dynamicists, structural geologists, petrologists, spectroscopists. This is going to be a very productive mission, and they're just getting started!
==endquote==

 

[OmCheeto]

Good article!
I like that she makes up her own words to describe new things: "lobelettes" and "floe toes"

One looks a lot like Martian rampart craters; they originate at crater rims, are very thin (tens of meters thick), run to 25 or 30 kilometers in length, and have "lobelettes" at floe toes. They interpret these to be localized fluidized ejecta formed during or after high-velocity impacts, "because you need high energy to mobilize material in this way, and we think they require a lot of ice." At the other end of the spectrum are flows that originate from slumps, are thicker (hundreds of meters thick), shorter (about 10 kilometers long), often have parallel furrows on their surfaces, have a distinctive, steep toe, and no lobelettes. ​

 

[rootone]

Those slumps seem interesting too.

 

[marcus]

Those slumps seem interesting too.

Yes, is it your understanding that an example of a slump would be what's seen in that photograph she included? She introduced it this way:
"The flow in the 3D image below is the example of a steep-toed, likely ice-creep flow that they showed."
The caption read, in part:
"A landslide in Ceres' far north in 3D
Near Ceres' north pole, an asteroid crashed into the steep rim of a hexagonal-shaped crater, causing a large landslide. This 3D image has been rotated to place north down in order to make the lighting more intuitive,..."

You can see, at the bottom of the picture, the impact of a small asteroid, right exactly on the rim of a larger crater. Is the landslide that caused an example of what is meant by a "slump" or have I misunderstood.
She talks about two types of flows, one is "steep-toed" and involves I think stiffer slower creeping material. The other involves more runny material and may spread out into small lobes. The result is flatter (not so steep). Let me know if there's something I'm missing.

BTW Rayman posted a status update yesterday. I turned in too early to see it at the time:

==quote==

November 13, 2015 -Dawn Progressing to Lower Altitude

Today Dawn's average altitude is about 490 miles (790 kilometers).

Following its weekly pattern, Dawn stopped ion thrusting yesterday afternoon to aim its main antenna at distant Earth. The flight team at JPL transmitted an updated flight plan for the descent spiral, and Dawn sent a detailed report on its activities and health during the previous week. In addition, accurate tracking of the radio signal as the spacecraft flew around Ceres provided navigators with new data to calculate the orbit. They will incorporate the results into next week's update.

The only probe from Earth ever to take up permanent residence in the main asteroid belt between Mars and Jupiter, Dawn has now been in that part of the solar system for six years.
==endquote==
As of now, 10am pacific on 14 Nov, simview shows Dawn thrusting, at altitude 766 km and a speed 223 m/s which is just a bit slower than what we saw much of yesterday. Presumably the spiral is just slightly elliptical now, rather than perfectly circular, so the probe swings out a percent or so now and then (at correspondingly diminished speed) before falling back in closer again and speeding back up. We saw ellipticity develop earlier in the descent from survey orbit (4400 km) to HAMO (1470 km)

 

[marcus]

As of 5:30 pm pacific on 14 November, simview showed Dawn at slightly below 750 km altitude with speed of 226 m/s
That's substantially different from the numbers for 10am pacific today suggesting that at least in simview the spiraling orbit is definitely a bit elliptical.
Here again is the link to Emily Lakdawalla's report from the Division for Planetary Sciences (DPS) AAS conference held this week.
ily http://www.planetary.org/blogs/emily-lakdawalla/2015/dps15-1112-ceres.html
As Om remarked, it's a good article. Sums up the latest findings on Ceres.
=====================================

AS OF 7pm pacific 15 Nov, simview showed Dawn at altitude 717 km and speed 229 m/s
just as it passed over the south pole from the dayside to the nightside of Ceres.
So it's a reasonable bet that by Monday late afternoon or evening she'll be within 700 km of surface.

Not for nothing did Rayman speak of Dawn's "average altitude" in the last update---the spiral is definitely elliptical so there's noticeable altitude variation in the course of one orbit.

 

[marcus]

Rayman updated today!
==quote==
November 16, 2015 -Spiral Descent Continuing Smoothly

Dawn's ion engine is continuing to push it to lower orbits. Today the spacecraft 's average altitude is about... (715 kilometers).

All of Dawn's ion thrusting throughout its interplanetary journey of more than eight years has now provided the equivalent of ...(39,400 kilometers per hour), far more than any spacecraft has achieved with its own propulsion system. Because of the principles of motion for orbital flight, whether around the sun or any other gravitating body, Dawn is not actually traveling this much faster than when it launched. See here for an explanation of this curious phenomenon.

As Earth and Ceres (carrying its sole companion, Dawn) travel on their own independent orbits around the sun, the distance between them is constantly changing. This morning they were pi astronomical units (... 469.9 million kilometers) apart. They are separating at almost ... (83,000 kilometers per hour).
==endquote==

As of 1:00pm pacific 16 Nov, simview gives the altitude as 699 km and the speed as a bit over 230 m/s

The craft is crossing over the south pole to Ceres' night side.
There are only 325 more kilometers to descend to reach the target altitude 375 km of Dawn's final orbit. This is the hardest part of the descent and is expected to take over 3 weeks.

 

[marcus]

What is the orbit frequency now? I estimate 8.4 hours.
average radius of Ceres 473 km
mass 938e18 kg
average altitude about 658 km today
so orbit radius currently about 1131 km
2 pi ((1131 km)^3/(G*938e18 kg))^(1/2) gives 8.39 hours
Apoapsis seems to come around passage over N pole
with periapsis over S pole. That should be in about 4.2 hours from now, or about 6 pm pacific time or 2am UTC.
So I expect the altitude to dip below 650 km sometime around 6 pm pacific time today when the probe is over the S pole.

 

[marcus]

Yeah. As of about 1:30 am UTC on 19th, or 5:30 pm pacific on 18 Nov the altitude was, as expected, less than 650 km. According to simview : ^)
It was 644 km and Dawn was crossing the terminator at Ceres S pole, which seems to be where periapsis (nearest approach) happens. Speed was 236 m/s. As a very rough guess I'd say the the range of variation of altitude this time around might be from 642 km to 658 km.
Yes, as of 2:03 am UTC the altitude was given as 642.77 km and she had just flown over the S pole.

 

[mfb]

16:21 UTC, 14 hours or 5/3 orbits later, Dawn will cross the equator soon (going south, extrapolated from your post). 645.1 km, close to the value it had over the south pole. 528 mph or 236m/s.

 

[marcus]

Thanks for making a projection, did you notice how closely it turned out? If it was right, it represented progress. Altitude over the equator is a rough estimator of the average altitude, in the simplified picture I have in mind. Currently altitude over the equator is about 640 km.

I see Earth (the tiny dot in the box over Ceres' northwest horizon) and Orion's belt to the southeast.
I suppose in about 4 hours---5pm pacific on 19 Nov, or 1 h UTC on 20 Nov---Dawn will be crossing equator going south on the day side. By then the average alt. should be slightly less than 640. This is all just guesses and rough estimates, but seems to be working out for now, more or less.

Getting to be time to re-calculate the approximate orbital period, though. It keeps shrinking. : ^)

 

[mfb]

640.93 km, halfway on the way to the equator as of 0:08 UTC. Dawn stopped thrusting.
527 mph or 235.6 m/s.

 

[marcus]

I was away from computer and didn't check but just now 1:21 UTC simview said 633 km. Dawn was shown bit less than halfway from equator to S pole.
We could split the difference and guess it was about (641+633)/2 = 636 when it crossed equator
So that would be our new guess as to the average altitude.
average radius of Ceres 473 km
mass 938e18 kg
average altitude about 636 km today
so orbit radius currently about 1109 km
2 pi ((1109 km)^3/(G*938e18 kg))^(1/2) gives 8.15 hours for the new orbital period.

Update: crossing equator northwards at 5 h UTC (or 9pm pacific 19 Nov) with altitude given as 639 km.
Engine still off. Roughly onsistent with what mfb said which suggested the southward crossing was around 1 h UTC, or 4 hours earlier at much the same altitude (because the ion engine was off--average alt. shouldn't change). DSN shows Canberra antenna #43 talking with Dawn.

Update: Simview 20 Nov 17:18 UTC shows Dawn crossing equator southwards at altitude 629 km and speed 238 m/s

 

[marcus]

Rayman update:
==quote==

November 20, 2015 -Dawn Reaching Ever Closer to Ceres

Dawn's average altitude today is about ... (635 kilometers). The spaceship is orbiting Ceres at ...[237 m/s]

Once a week during its spiral descent, Dawn stops ion thrusting so it can point its main antenna at Earth. When it did so on Nov. 19, the JPL flight team transmitted the latest flight plan, which incorporated updates using the navigation data collected one week earlier. (See the Nov. 13 status update.)
==endquote==

as of 19 h UTC simview Dawn is passing over S pole at altitude 618 km
Update: now crossing S pole again, alt. 611 km 21 Nov 3 h UTC, speed put at 240 m/s

So orbital period is currently about 8 hours, as we thought.

Update: later I happened to see simview for 21 Nov 6:50 hours, altitude given was 628 km

 

[marcus]

Simview for 21 November 15:47 hours UTC showed Dawn just having passed over N pole, at altitude 619 km.
N pole crossing is what I've been taking for apoapsis---this can change but seems to be working well so far at least as a rough approximation.
I'll hazard a guess that around 19:30 UTC it will show passage over S pole at about altitude 605 km.
Would be surprised if the probe doesn't get below altitude 600 km sometime on 22 November.
Update: Simview Dawn crossed equator on dayside at altitude 609 km, 21 Nov 17:30 UTC.
Altitude over equator has proven reasonable indicator of average altitude, for the time being.

average radius of Ceres 473 km
mass 938e18 kg
average altitude about 609 km today
so avg. orbit radius currently about 1082 km
2 pi ((1082 km)^3/(G*938e18 kg))^(1/2) gives 7.9 hours for the new orbital period.

Update: alt = 600 km as probe approaches S pole at 18:40 hours UTC.
My guess was wrong, off by 5 km. It was too conservative.

598.44 km at 19:30 hours UTC

Further update, at 22 Nov 1:15 hours UTC, altitude over dayside equator 602 km.
So avg. altitude for now is estimated at that 602 km.
Currently, it seems to go down about 6 km each time around.

 

[marcus]

Simview updates
Passing over S pole at alt. 579 km, on 22 Nov at 18:48 hours UTC
Crossing S pole with alt. 574 km, on 23 Nov at 02:34 hours UTC
Crossing N pole with alt. 588 km, on 23 Nov at 06:40 hours UTC

So the orbit period is still just bit less than 8 hours. The spread between lowest and highest altitude is about 15 km (?). The descent each time around tends to be around 5 km. Periapsis (closest approach) still seems coincide with passage over the S pole.

Crossing S pole with alt. 562 km, on 23 Nov at 17:42 hours UTC (note orb. period diminished slightly)
Crossing S pole with alt. 556 km, on 24 Nov at 1:03 hours UTC

 

[marcus]

Rayman (mission director) update:
==quote==

November 23, 2015 -Dawn's Maneuvering Proceeding Well

As Dawn continues to lower its orbit, today its average altitude is about ... (570 kilometers). At this height, each revolution around Ceres takes about 7.5 hours.
==endquote==
So Simview is still staying reasonably consistent with the less frequent real world updates from JPL.
E.g. he could have been talking about the S pole low point (periapsis) of 562 km, because the average alt. has tended to be about 7 or 8 km higher than the lowest. That would put the average right around 570 km, as per Rayman.

Simview "Crossing S pole with alt. 562 km, on 23 Nov at 17:42 hours UTC" is not too bad a match.

BTW at the moment I see Simview saying alt 547 km, Dawn recently having passed over S pole. (16:36 hours UTC on 24 Nov) Speed = 246 m/s

An interesting commentary on the efficiency of solar powered xenon ion drive, from the October 2015 Dawn Journal:
==quote==
In its eight years of interplanetary travels, the spacecraft has thrust for a total of 1,976 days, or 68 percent of the time ... While for most spacecraft , firing a thruster to change course is a special event, it is Dawn’s wont. All this thrusting has cost the spacecraft only...(396 kilograms) of its supply of xenon propellant, which was ... (425 kilograms) on Sep. 27, 2007. The spacecraft has used 66 of the 71 gallons (252 of the 270 liters) of xenon it carried when it rode its rocket from Earth into space.

The thrusting since then has achieved the equivalent of accelerating the probe by ...(39,200 kilometers per hour). ... Having accomplished 98 percent of the thrust time planned for its entire mission, Dawn has far exceeded the velocity change achieved by any other spacecraft under its own power. (For a comparison with probes that enter orbit around Mars, refer to this earlier log.) The principal ion thrusting that remains is to maneuver from the present orbit to the final one from late October to mid-December.
...
==endquote==

 

[marcus]

So there's less than 200 km more to descend, to reach the target 375 km altitude! Orbit radius will then be 473+375 = 848 km.
And orbit period will be 2 pi ((848 km)^3/(G*938e18 kg))^(1/2) or around 5.45 hours. Something to compare with Rayman's current figure of 7.5 hours : ^)

 

[anorlunda]

So there's less than 200 km more to descend, to reach the target 375 km altitude! Orbit radius will then be 473+375 = 848 km.
And orbit period will be 2 pi ((848 km)^3/(G*938e18 kg))^(1/2) or around 5.45 hours. Something to compare with Rayman's current figure of 7.5 hours : ^)

How elliptical is the orbit?

Thanks for sharing Marcus, this stuff is way cool.