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

[OmCheeto]

Om thanks for spotting that. The picture of the bright spots is considerably sharper and shows more detail than I had see before. I can understand Chris Russell guessing that the spots are reflection from ice.
http://dawn.jpl.nasa.gov/news/news-detail.html?id=4619

If the spots are small ice-fields somehow pushed up from within then that seems even more interesting than that they might be dry salt flats. There are a remarkable number of them in just that one crater.
I see about 8 small separate speckles besides the two larger splotches.

I'm now leaning towards salt flats...

...

Ok. I don't really care which it is. This is just freakin' fun!

 

[OmCheeto]

A few new images were posted yesterday:

Ceres' Southern Hemisphere in Survey

A large crater in the southern hemisphere of dwarf planet Ceres is seen in this image taken by NASA's Dawn spacecraft on June 6, 2015. This image shows many different surface structures associated with impacts.
This is among the first snapshots from Dawn's second mapping orbit, which is 2,700 miles (4,400 kilometers) in altitude. The resolution is 1,400 feet (410 meters) per pixel.
​

Ceres' Northern Hemisphere in Survey

...image taken by NASA's Dawn spacecraft on June 6, 2015​

VIR Image of Ceres, May 2015

Images from Dawn's visible and infrared mapping spectrometer (VIR) show a portion of Ceres' cratered northern hemisphere, taken on May 16, 2015. From top to bottom, the views include a black-and-white image, a true-color view and a temperature image. The true-color view contains reddish dots that are image artifacts, which are not part of Ceres' surface.
These images were taken at a distance of 4,500 miles (7,300 kilometers) from Ceres. They have a resolution of 1.1 miles (1.8 kilometers) per pixel.
The temperature image is derived from data in the infrared light range. The lightest areas are the hottest and the darkest are the coolest.​

 

[Dotini]

A few new images were posted yesterday:

Ceres' Southern Hemisphere in Survey

By casual inspection, the northern hemisphere seems to differ markedly from the southern hemisphere. I believe Mars and Vesta share this characteristic.

 

[mfb]

Craters all the way down.

The illumination seems to be different for the two images, so I would be careful with comparisons.

 

[OmCheeto]

The two upper pictures I posted this morning were from an altitude of 4400 km.
The other day I determined that the framing cameras wouldn't have full Ceres filling images until it was down to 3400 km.
Did I screw up my maths again, or are these cropped pictures? [I screwed up the maths]

[edit: corrected!]

Field of vision     5.5°    vertical & horizontal (θ)
1/2 Field of vision 0.048 radians
distance            6800    km
1/2 field            327    km   tan(θ) * distance
full field           653    km
1/2 diagonal         462    km   √ (2 * half field^2)
full diagonal        924    km
polar diameter       891    km   96% of full diagonal
equatorial diam      963    km   104% of full diagonal

hmmm...

6/6/2015 images
0.410    km/pixel
1024     pixels h&v
419.84   km h&v captured
927      km (Ceres average diameter)

It would appear that my previous calculations were off.
I will stop any further attempts at doing math.

I came up with 0.413 km/pixel for an altitude of 4400 km, so I'm sticking with my new numbers.

 

[OmCheeto]

More confusion. Dr. Rayman responded to my last email that I sent him:

June 4, 2015
Hi Marc,
...
As I suspected, everyone [here at PF] looked at the elliptical orbital graph you sent last time, and said it was “obvious”. I was of course, totally confused, as always.

[Om]

ps. I’ve started another argument, on Twitter this time……..

Peter Fries ‏@Peter_Fries Jun 2
@NASA_Dawn @b0yle Does Dawn have the capability to send back 'natural color' images?

NASA's Dawn Mission ‏@NASA_Dawn Jun 2
@Peter_Fries @b0yle yes, I can take data with which to make color images, but the team has not yet released any yet

OmCheeto 17 minutes ago
@NASA_Dawn @Peter_Fries @ridingrobots @b0yle I'm confused. http://blogs.egu.eu/geolog/2015/04/15/findings-from-nasas-dawn-mission-shed-new-light-on-ceres/ …​

June 12, 2015
Hi [Om],

I didn’t mean to confuse you with my plot. Never hesitate to let me know if I send you something unclear or unhelpful.

The image you refer to in your Twitter exchange is not natural color but rather false color. Dawn not only has the capability to take natural color images, but we released them for Vesta. So far, the only one we have released for Ceres is here but both the science camera and the visible mapping spectrometer can do this. It’s a surprisingly difficult process, however. Human vision is complex, and making natural color images is a little tricky. Still, we surely will release more.

Marc

I have just informed Marc that it is fortunate for him, that I am surrounded by the smartest people in the universe, here at PF.
I also asked for clarification on his response:

When you say the image is “false color”, do you mean the colors aren’t real, or are they real, but just over-saturated?

 

[OmCheeto]

For the record, I warned him, to not continue, to correspond with me...

Hi [Om],

On Dawn (and in general) false color does not mean oversaturated. Rather, certain wavelengths in the original data are assigned different (you could even say false) wavelengths in the picture. As an example, you might have data on a scene in near ultraviolet, red, and infrared. We can’t even see the first and last of those, so it would look black to us. But we can synthesize an image in which we assign the near ultraviolet data to, say, blue. That’s false, and in fact the real scene might look different in blue light, because the materials reflecting the light have different optical properties in blue light from ultraviolet light. Similarly, the other wavelengths may be given different colors. That’s OK. We label it as false color and never pretend it is anything else. We might assign the actual red wavelengths to green and the infrared wavelengths to red. So, now we have views in blue, green, and red, and we can construct a picture from that. It’s colorful, but it’s false color.

Some observations may be conducted at wavelengths that have special meaning. In astronomy, for example, one might measure the wavelength at which certain interesting atomic species emit radiation. It might be near the visible or it might even be far away. It could be gamma ray, X-ray, UV, IR, microwave, etc. Measure several of those, put them together with false color, and it’s a convenient method to see where the action is that you’re interested in.

It can be more complicated. Sometimes the false colors don’t correspond to any wavelength range in the original scene. The choice of the colors is usually meant to help our eyes pick our salient features. In some cases, it is more powerful to assign ratios of colors in the original data to individual colors in the final image. (Ratios of reflectivity at different wavelengths can be very diagnostic of certain mineral types.) So, for example, you might take the ratio of infrared to green and assign that to blue. If the ratio is high (much more IR than green light reflected from the object), it would be bright blue. If the ratio is low, it would be dark blue.

In all these cases, it is the underlying quantitative data that tell the real story, but the false color images help guide us to the interesting places. As visual creatures, such imagery can be very useful.

I hope that’s helpful.

If I had more time, I might enjoy the physics forum. I have wide ranging interests. All my degrees are in physics, and I greatly enjoy cosmology, astrophysics, particle physics, laser and atomic physics, relativity, and more. Working on a planetary mission is great fun, but my principal scientific interests lie elsewhere. Alas, Dawn keeps me much too busy to follow any popular discussions, much as I enjoy communicating with the public and sharing interesting ideas.

Marc

I don't know about you all, but I have the best pen pal in the world.

 

[OmCheeto]

But then, I yelled at him;

in the future, instead of using the term; “enhanced colour view”, could you say; “trans-human compressed electromagnetic spectrum view”.​

I have no idea why he continues to correspond...

 

[OmCheeto]

The reaction of human eyes is also highly nonlinear. A camera needs tricks like multiple images with different exposure times to get the same range.

After 7 days of study, I think I've figured it out. At the most basic of levels, of course.
I had to re-read Dr. Rayman's last email at least 5 times before I could understand what he was trying to get across.
Unfortunately, I responded after reading it only once.
But fortunately, after re-reading my response, it appears that I did not let on that I didn't really fully understand what he was saying.

I also re-read Drakkith's response in the CCD thread, and your reference. After accumulating much needed missing data, those responses too made sense.

As Mr. Spock would say; "Fascinating..."

 

[OmCheeto]

No, please don't do it! Not the obelisk!

Still no obelisks. But they did find a pyramid.

NASA's Dawn Mission ‏@NASA_Dawn 11 hours ago
A "pyramid"-like feature stands out in today's #Ceres image http://go.nasa.gov/1GP9ogi​

I'm guessing this is what they are talking about.

It looks more like a "limpet" snail to me. hmmm... They can call it Mt. O'Limet.
In a later tweet, they claim it is 5km (3 miles) high.

Is this our first mountain?Interesting "fracture" structure in the bottom left hand:

 

[johnnymorales]

That pyramid looks just like a cinder cone or stratovolcano on Earth. Though on Ceres is could very well be a cryovolcanic cinder cone.


Here is a photo of Mt. Mayon on Earth it's close to a perfect match when you take into account the likely effects of much lower gravity on Ceres.

https://en.wikipedia.org/wiki/File:Mt.Mayon_tam3rd.jpg


It's pretty surprising they'd call it a pyramid.

It's egging on the people who live and breath conspiracies as an explanation for all that they see.

It's also pretty surprising that in place of valid scientific speculation they let loose with a nonsense comparison.

Pyramids are not cones. So why inspired them to make such a brain dead comparison.

All and all par for the course regarding the Ceres team. They apparently lack the media savvy of the New Horizons team.

You can bet anything that looks vaguely volcanic or tectonic on Pluto will be declared as such immediately with validation to come later.

The fact that the Ceres team simply refuses to engage in the type of scientific speculation that makes space exploration so exciting for the public is extremely disappointing.

Suggesting the obvious in a news release would dramatically increase the public's interest and support of this mission.

The fact that it is much larger than any Earthly cinder cone can easily be explained by Ceres having just a tiny fraction of Earth's gravity. With such low gravity a cinder cone could grow to immense heights on Ceres.

 

[OmCheeto]

Ceres in the news:

Dawn over Ceres, a mission of humankind (PBS News Hour)

I'm not sure if I missed it, or forgot it, but the first image is not familiar:

THE DWARF PLANET — This image, taken by NASA's Dawn spacecraft , shows dwarf planet Ceres from an altitude of 2,700 miles (4,400 kilometers). The image, with a resolution of 1,400 feet (410 meters) per pixel, was taken on June 6, 2015. Image by NASA/JPL-Caltech/UCLA/MPS/DLR/IDA​

Here's the video imbedded in the write-up:

Dawn mission director describes the dwarf planet Ceres
​

[edit]

By casual inspection, the northern hemisphere seems to differ markedly from the southern hemisphere. I believe Mars and Vesta share this characteristic.

From the above article, it appears you are correct:

"The southern hemisphere is less densely cratered than the northern."
​

I think that's weird.

 

[Dotini]

"The southern hemisphere is less densely cratered than the northern."
​

I think that's weird.

Weird things are fun, and mean there's more to learn.

 

[OmCheeto]

Weird things are fun, and mean there's more to learn.

I can think of no mechanism inside our solar system which would cause such a thing.
So yesterday I spent about 3 hours researching the origins of asteroids.
I found tons of conflicting information.

One person here at PF said they were formed by vacuum welding. (Sounds funny to me, and no reference was listed.) [ref] Actually, it sounded like everyone was guessing in that thread. And given the level of knowledge at PF, my guess is, that know one knows.

One website article said they were formed during the big bang. (I sense that this is totally wrong. Most references list only Hydrogen and Helium production during that period) [ref]

So I researched a bit more about novas and supernovas, wondering if anyone knew if their heavy end stage nucleosynthesized elements, nickel and iron, were distributed atomically, or in big molten blobs. I found no answer. But looking at the Crab Nebula, it looks filamentized, so I thought there might be some merit to the idea.

Anyways, my theory was, that a local star, somewhere between 1 and 4 billion years ago. in the general direction of Polaris, might have gone nova, showering us with asteroids.
I was going to do the math, on the probability, given the rate of stellar explosions, local star density, and the age of our galaxy, but I was hoping that someone knew the answers to these things, off the top of their head. I hate doing needless maths. If no one knows, I will, obligingly, do the maths.

ps. Wouldn't it be fun, if the Dawn mission, gave us the answer.

 

[marcus]

Hi Om, I've never been too good at telling when my leg was being pulled but I think if an math explanation needs to be given it would be to explain why Jupiter's gravity would interfere with all that belt of rock coalescing into a planet.

I think at one time the band of near-circular orbits at 1 AU (plus or minus) where the Earth is now probably looked like Asteroid Belt only if anything much much thicker.

But the junk and rubble around 1 AU was able to start gathering and coalescing and snowballing together into one big heap called the Earth. Once one significantly bigger lump collects then it gradually pulls the rest in. But when our planet was forming it did not have the disruptive presence nearby of a planet like Jupiter to deal with.

The interesting question, for someone who wants to get into the math detail of orbital mechanics, would be to explain exactly how the proximity of Jupiter would disrupt the coalescence.

Jupiter could also have ejected asteroid matter from the belt. Pull chunks of it out of circular into eccentric orbits and then give them a gravity slingshot boost altogether out of there. That could be part of the reason there is not all that much mass in the Belt. Ceres alone represents 1/3 of the mass of the Belt. 3 x Ceres mass is really not what we expect for a usual planet.

 

[OmCheeto]

Hi Marcus,

I think I'd better slow down and start from scratch, and explain what I was looking for.
By reading Dotini and mfb's posts in sequence, I misinterpreted what Dotini had actually said.

By casual inspection, the northern hemisphere seems to differ markedly from the southern hemisphere. I believe Mars and Vesta share this characteristic.

Craters all the way down.

The illumination seems to be different for the two images, so I would be careful with comparisons.

All Dotini had said was, that they were different. I unfortunately added the two comments together and came up with; "Ceres is more heavily cratered in the north, which is true of Mars and Vesta", which is obviously not true. Unfortunately, I didn't figure that out until just now. So the original question, isn't even valid.

But I think a valid question still exists; "Why is Ceres more heavily cratered in the northern hemisphere?"

I've just spent about 6 hours trying to put the pieces together, but I think it's all just a bunch of very bad science on my part.

But all is not lost. I think I ran across the answer to your following question:

The interesting question, for someone who wants to get into the math detail of orbital mechanics, would be to explain exactly how the proximity of Jupiter would disrupt the coalescence.

Kirkwood gaps
A Kirkwood gap is a gap or dip in the distribution of the semi-major axes (or equivalently of the orbital periods) of the orbits of main-belt asteroids. They correspond to the locations of orbital resonances with Jupiter.

 

[OmCheeto]

An underexposed image of the bright spots published today:

image taken: June 15, 2015
altitude: 2,700 miles (4,400 kilometers).
resolution: 1,400 feet (410 meters) per pixel.
​

This is a heavily magnified shot from the TIFF image at JPL.

​

Well, ok. I can't see anything either. But at least they're reading my mind now.

 

[marcus]

the new Dawn Journal is out!
http://dawnblog.jpl.nasa.gov/2015/06/29/dawn-journal-june-29/

 

[marcus]

The 8th and last observation cycle in survey orbit should be completed today. So we can expect descent to the third mapping orbit (which used to be called "HAMO") to start soon.
I say that because status update http://dawn.jpl.nasa.gov/mission/status.html said the last cycle started 27 June, and a cycle lasts 3.1 days (half the time on the dayside, observing, and the rest of the time on the nightside, transmitting).

We should be seeing Dawn talking with DSN, either transmitting the data from the last cycle or getting programmed for the descent, which will take about 5 weeks if I remember right. I just checked and Canberra #45 is receiving data (it is 2AM there).
My guess is Dawn will turn on the ion thruster sometime (pacific) tomorrow, 1 July, if not earlier.

The simulated view http://neo.jpl.nasa.gov/orbits/fullview2.jpg shows the craft crossing over the N pole to the dayside. I'll bring that timetable forward, since we've turned a page. What I'm reading into it is that descent to the next orbit starts tomorrow and will take until the first week of August.

Orbit    dates      altitude(km)  pixelsize(m) res/HST  period  soccerball at
RC3    April 23–May 9    (13,500)    (1,300)    24     15 days    (3.0 meters)
Survey    June 6-30      (4,400)      (410)     72     3.1 days    (1.0 meters)
HAMO    Aug 4–Oct 15     (1,450)      (140)     215    19 hours    (33 cm)
LAMO Dec 8–end of mission  (375)      (35)      850    5.5 hours    (8.5 cm)

EDIT: Rayman's "Mission update" for 30 June includes this:
==excerpt==
...
Tonight Dawn will power on its ion propulsion system to begin a spiral descent to the third mapping orbit. It will take about five weeks to complete the maneuvering to an altitude of about 900 miles (less than 1,500 kilometers).
==endquote==
So if that went as planned propulsion is now on. I see from DSN that Dawn is not engaged in communication so I conclude the descent is in progress.
http://dawn.jpl.nasa.gov/mission/status.html

 

[OmCheeto]

Ha ha! The "bright spots" have their own wiki page:

Bright spots on Ceres

as does the mountain:

Pyramid-shaped mountain on Ceres

I discovered these this morning, when I was trying to verify a few posts on Facebook this morning; "These bright features have an albedo of about 40% (4 times brighter than the rest of Ceres's surface).^[3]"

Reference [3] is the 90 minute lecture that Dr. Rayman gave back in April.
Having already watched it, I was listening to it, while surfing this morning. I obviously missed it.

I guess it doesn't really matter what the actual albedo is, as ice/snow has a "proclaimed" variability of from 30 to 85%. [per wiki, and therefore, unreliable ]

 

[rootone]

The mountain:
"Although origin hypotheses for the mountain include volcanism, impacts, and plate tectonics, clear evidence backing any of these is currently lacking."

My bet is on it being a central 'rebound' mountain of the kind often associated with large impacts.
If this impact was very ancient, other more recent and smaller impacts could have eliminated other evidence of it - such as a discernable crater rim.

Plate tectonics and volcanism seem to me less likely on a body the size of Ceres, and if either were the case it's hard reconcile with there being just the one isolated mountain, as tectonics usually produce ranges of mountains, and volcanism usually produces several volcanos within in a well defined area..

 

[OmCheeto]

The mountain:
"Although origin hypotheses for the mountain include volcanism, impacts, and plate tectonics, clear evidence backing any of these is currently lacking."

My bet is on it being a central 'rebound' mountain of the kind often associated with large impacts.
If this impact was very ancient, other more recent and smaller impacts could have eliminated other evidence of it - such as a discernable crater rim.

Plate tectonics and volcanism seem to me less likely on a body the size of Ceres, and if either were the case it's hard reconcile with there being just the one isolated mountain, as tectonics usually produce ranges of mountains, and volcanism usually produces several volcanos within in a well defined area..

Someone, some time back, described low gravity, extra-terrestrial objects, as, "rubble piles".

It may just be a big pile of rubble.

 

[OCR]

[per wiki, and therefore, unreliable ]

Ohhhhh, my !

 

[marcus]

The 8th and last observation cycle in survey orbit should be completed today. So we can expect descent to the third mapping orbit (which used to be called "HAMO") to start soon.

Orbit    dates      altitude(km)  pixelsize(m) res/HST  period  soccerball at
RC3    April 23–May 9    (13,500)    (1,300)    24     15 days    (3.0 meters)
Survey    June 6-30      (4,400)      (410)     72     3.1 days    (1.0 meters)
HAMO    Aug 4–Oct 15     (1,450)      (140)     215    19 hours    (33 cm)
LAMO Dec 8–end of mission  (375)      (35)      850    5.5 hours    (8.5 cm)

EDIT: Rayman's "Mission update" for 30 June includes this:
==excerpt==
...
Tonight Dawn will power on its ion propulsion system to begin a spiral descent to the third mapping orbit. It will take about five weeks to complete the maneuvering to an altitude of about 900 miles (less than 1,500 kilometers).
==endquote==
So if that went as planned propulsion is now on. I see from DSN that Dawn is not engaged in communication so I conclude the descent is in progress.
http://dawn.jpl.nasa.gov/mission/status.html

It turns out that did NOT go as planned. The craft did briefly turn on ion propulsion to start the descent, but detected something was wrong and shut down the thruster, putting itself in safe mode to allow the engineers to check things out. Rayman's status update of 6 July:
http://dawn.jpl.nasa.gov/mission/status_2015.html
==quote==

2015

July 6, 2015 - Dawn Holding in Second Mapping Orbit

NASA's Dawn spacecraft is healthy and stable, after experiencing an anomaly in the system that controls its orientation. It is still in its second mapping orbit 2,700 miles (4,400 kilometers) above dwarf planet Ceres.

On June 30, shortly after turning on its ion engine to begin the gradual spiral down to the next mapping orbit, its protective software detected the anomaly. Dawn responded as designed by stopping all activities (including thrusting), reconfiguring its systems to safe mode and transmitting a radio signal to request further instructions. On July 1 and 2, engineers made configuration changes needed to return the spacecraft to its normal operating mode. The spacecraft is out of safe mode, using the main antenna to communicate with Earth.

Dawn will remain at its current orbital altitude until the operations team has completed an analysis of what occurred and has updated the flight plan.

Because of the versatility of Dawn's ion propulsion system and the flexibility of the mission's plan for exploring Ceres, there is no special "window" for starting or completing the spiral to the third mapping orbit. The plans for the third and fourth mapping orbits can be shifted to new dates without significant changes in objectives or productivity.
==endquote==

 

[OmCheeto]

...
The plans for the third and fourth mapping orbits can be shifted to new dates without significant changes in objectives or productivity.
...

As I've suspected, all along, the project is in good hands.

 

[marcus]

Emily Lakdawalla commented on the 6 July status report (quoted in full two posts back)
=== http://www.planetary.org/blogs/emily-lakdawalla/2015/07091718-updates-dawn-osiris-rex-juno.html ===
The wording of this statement is a little bit ambiguous, because Dawn has already lost the use of two of its four reaction wheels, and as a result there is not one but actually two systems that work together to control its orientation: the two remaining reaction wheels and its hydrazine attitude control thrusters. So either there is a problem with one of the remaining reaction wheels (which wouldn't be too surprising at this point, nor does it jeopardize the completion of the mission, but it would be an inconvenience) or there is a problem with its thrusters (which would be much worse news). The fact that it is out of safe mode and "using the main antenna to communicate with Earth" means that they do have three-axis control of the spacecraft , which makes me inclined to think that it's another reaction wheel problem. Dawn is fully capable of completing its science mission at Ceres using only thrusters to control its orientation in space, so if this is a serious reaction wheel problem and not just a brief glitch, the mission will be just fine.
==endquote==

 

[OmCheeto]

Mission Status Updates
- from Chief Engineer/ Mission Director, Marc Rayman (JPL)

July 13, 2015 - Dawn Healthy as it Orbits Ceres

Dawn remains healthy and functioning normally in its second mapping orbit at Ceres. Engineers are continuing their investigation into the anomaly that caused a discrepancy in the orientation on June 30.

When I went to check the DSN about 10 minutes , Canberra 34 & 35 were in standby mode with Dawn, just a moment later, Dawn started transmitting to #35.

hmmmm... Perhaps this is a sneaky way of JPL to get us to pay some attention to Pluto.

[edit] ps. No top secret information from the Top Ramen on the matter.

 

[marcus]

Now we know that the anomaly was with the orientation (hydrazine jets, reaction wheels, and their control) and not with propulsion.
Emily L was speculating that it could be with one or the other and trouble in the propulsion department would be more serious.
Thanks for relaying this word from the Top Ramen.

I think the long delay in restarting the attempt to spiral down to lower orbit may actually have to do with Pluto and DSN priorities. Just a guess.
Ceres and Pluto are in the same sector of the sky, so in case of emergencies the two missions would be competing for the same antennas.

It could make sense for Dawn team to have delayed until after Pluto flyby so that IF Dawn made a try to start spiraling down, and encountered another anomaly in the orientation it would be able to get an antenna assigned to it immediately and start recovery.

 

[Dotini]

The Dawn team seem to be slowly coming around to the bright spot salt explanation mentioned long ago by marcus.

http://www.washingtonpost.com/news/...te-spots-on-ceres-might-not-be-ice-after-all/
According to Christopher Russell of the University of California at Los Angeles, the Dawn mission's principal investigator, the team is "shying away from there being ice on the surface."

"The general consensus on the team right now is that water is definitely a factor on Ceres, but that the spots themselves are more likely to be just highly reflective salt, rather than water," Russell told The Post.

 

[marcus]

Judging from simview, they fixed the problem.

 

[marcus]

Turns out that announcement may have been premature! Today the Top Ramen reported that they are conducting a test to make sure the probe is ready to start the next descent. The "test" may have actually been to try a brief descent, with the thruster on, and see how things went.
http://dawn.jpl.nasa.gov/mission/status_2015.html
But I reckon it's approximately right in any case. Dawn may indeed have already started spiraling down
Simview for 4PM pacific on 15 July shows the altitude at 4.22 kkm down from 4.4 and over the past months simview has turned out to be a reasonably good approximation of reality.

 

[marcus]

Simview now shows thruster off, and DSN shows two Madrid antennas #65 and #54 devoted to Dawn, one in two way communication and the other in standby.
I think this is what constituted the TEST referred to in status report. Namely run the thruster for a day or two and then get a fix on the probe and check out how everything went.

Simview shows the altitude already diminished from 4400 to just under 4400. If that checks out and everything has behaved normally, it's good news.

 

[marcus]

Simview now shows thruster on (as of 6 am Pacific 17 July) and DSN shows no talk with Dawn.
I infer that they made the test---tried thrusting for a day or so, then shut off propulsion for something like 12 hours while they made sure everything checked out.
And I guess as a result the craft was declared OK, so they have now turned back on ion propulsion and resumed descent.

It's possible they changed which ion engine is being used. IIRC there are 3 units.

Simview gives the altitude as 3910 km now (down from 4400) and the speed as 269 mph

 

[marcus]

in fact they did switch to another ion engine
==quote http://dawn.jpl.nasa.gov/mission/status.html ==

July 17, 2015 - Dawn Maneuvering to Third Science Orbit

NASA's Dawn spacecraft is using its ion propulsion system to descend to its third mapping orbit at Ceres, and all systems are operating well. The spiral maneuvering over the next five weeks will take the spacecraft to an altitude of ...(less than 1,500 kilometers) above the dwarf planet.

The spacecraft experienced a discrepancy in its expected orientation on June 30, triggering a safe mode. Engineers traced this anomaly to the mechanical gimbal system that swivels ion engine #3 to help control the spacecraft 's orientation during ion-thrusting. Dawn has three ion engines and uses only one at a time.

Dawn's engineering team switched to ion engine #2, which is mounted on a different gimbal, and conducted tests with it from July 14 to 16. They have confirmed that the spacecraft is ready to continue with the exploration of Ceres.

By the end of the day on July 17, Dawn will have descended to an altitude of about ... (3,900 kilometers). After arrival at its next mapping orbit -- called the High-Altitude Mapping Orbit, or HAMO -- in August, Dawn will begin taking images and other data at unprecedented resolution.
==endquote==

 

[marcus]

The simview has proven reasonably reliable and it gets regularly updated. It says Dawn's altitude is now 3200 km.
Dawn started descending from the second orbit (altitude 4400 km) on 30 June, but there was that glitch. Apparently some problem with the gimbal mounting of ion engine #3 which it was using. So they stayed in orbit at altitude 4400 km for a couple of weeks while they changed over to ion engine #2 and tested that out, and reprogrammed everything.

Then around 15 July they started descent again and already have made quite a lot of progress. Of course the probe speeds up as it goes lower. I forget what the orbital speed at the 3rd orbit (altitude 1450 km) will be. Simview gives the current speed as 0.13 km/s. This is as of 10AM pacific on 21 July.

For some reason they give the speed in higher precision in mph---currently 293 mph

 

[marcus]

Official status report:
==quote==
July 20, 2015 - Dawn's Spiral Descent Continuing Smoothly

Dawn is continuing to lower its orbital altitude. Today the spacecraft will descend from ... (3,500 kilometers) to ... (3,400 kilometers).

In its previous mapping orbit, the explorer was 2,700 miles (4,400 kilometers) above the alien world. When it completes its spiral to the third orbit, it will be about 900 miles (less than 1,500 kilometers) high.
==endquote==
Official: http://dawn.jpl.nasa.gov/mission/status.html
Unofficial: http://neo.jpl.nasa.gov/orbits/fullview2.jpg

 

[OmCheeto]

Regarding the "bright spots" crater:

Dawn at Ceres: A haze in Occator crater?
Posted by Andrew Rivkin
2015/07/21 22:54 UTC
...
Russell said that a “haze” was visible in this crater at certain times of day, and that it was seen more than once. A follow-up question, the only one there was time for, established that the haze was confined to the crater itself.
...

Apparently they've named some of the craters.

 

[marcus]

Interesting about that haze in the crater! Another status update came out today.
==quote http://dawn.jpl.nasa.gov/mission/status.html ==

July 22, 2015 - Dawn, Ceres and Earth Closest To Each Other

As Ceres (with its new permanent resident, Dawn) and Earth follow their own independent orbits around the sun, today they are at their closest since June 2014. The dwarf planet and Dawn today are 180 million miles (290 million kilometers) from our home. (For more details and a diagram, see the June Dawn Journal.)

Meanwhile, Dawn is continuing to thrust with its ion propulsion system to shrink its orbit. Today it will descend from ...3,200 kilometers to ...3,100 kilometers.
==endquote==
Simview agrees reasonably well with official update. It says as of 8pm pacific on 22 July the altitude is 3.06 kkm or about 3060 km. Fairly close to the official 3100.

Official: http://dawn.jpl.nasa.gov/mission/status.html
Unofficial: http://neo.jpl.nasa.gov/orbits/fullview2.jpg

So Ceres is on the meridian (the sky centerline) at midnight, now. that means that prime DSN antenna time is 9pm to 3am or thereabouts. Maybe anything from 7pm to 5am local time, Ceres would be in the sky and the antenna could make contact .

 

[marcus]

New update today!
==quote==
July 23, 2015 - Dawn Maneuvering to Lower Orbit

As Dawn maneuvers closer to Ceres, today it is reducing its altitude from ... 3,100 kilometers to ... 2,900 kilometers.

In lower orbits, the spacecraft circles Ceres more quickly, not only because the distance around the orbit is shorter but also because it travels faster in the dwarf planet's tighter gravitational grip. (For more on this, see this Dawn Journal explanation.) In the previous mapping orbit ... 4,400 kilometers, it took slightly more than three days to complete one revolution. Now the orbital period is a little less than two days. When Dawn is in its next mapping orbit at... less than 1,500 kilometers, each loop will take about 19 hours.
==endquote==
http://dawn.jpl.nasa.gov/mission/status.html
http://neo.jpl.nasa.gov/orbits/fullview2.jpg

Simview as of 12:20 PM pacific 23 July give the altitude as 2920 km and the speed as 0.14 km/s.

 

[marcus]

The Wikipedia on Ceres has been updated from [6] Marc Rayman's Dawn Journal of May 2015 to give latest figures on the mass, radius etc.
Mean radius
469 km
Equatorial radius
481.5 km[6]
Polar radius
445.5 km[6]
Mass
9.39×10²⁰ kg[6]
So let's calculate the orbit period at radius of 1450+470 = 1920 km
When I put in for circular orbit speed at that radius I get 180 m/s
and when I put in the circumference divided by that speed
2pi*1920 km/(G*9.39e20 kg/1920 km)^(1/2)
I get 18.55 hours.
So that agrees with what Marc Rayman says namely 19 hours for the period of the next orbit.

Just to see what the current orbit period would be, if it were circular, I put in
2pi*3470 km/(G*9.39e20 kg/3470 km)^(1/2)
and got 1.9 days
That would be for an altitude of 3000 and a radius of 470km. It agrees with today's status update (in the preceding post#540) which says "a little less than 2 days"
https://www.physicsforums.com/threa...-to-1450-km-orbit.793140/page-27#post-5177268

 

[marcus]

Marc Rayman's Dawn update says altitude 2900 km by the end of the day 23 July (see post #540 for additional info)
2900+470=3370
Because of ellipticity the descent trajectory doesn't stay radius ≤ 3370 km at first. It still has to settle down and circularize. But just for practice I'll calculate the orbit period for circular at radius 3370 km.

2pi*((3370 km)^3/(G*9.39e20 kg))^(1/2)

The googly calculator says 1.8 days.

 

[marcus]

Dawn down around altitude 2600 km now. Here's simview as of late Sunday 26 July
http://neo.jpl.nasa.gov/orbits/fullview2.jpg

It's about time for a new Dawn Journal to appear, they tend to come out around the end of the month:
http://dawn.jpl.nasa.gov/mission/journal.asp
That should give a revised schedule for when the probe will arrive at third science orbit (altitude 1450 km) and when it will start descent to the fourth and lowest orbit.

We might also get a status update soon. The last one was 23 July.
http://dawn.jpl.nasa.gov/mission/status.html

I'll bring the old schedule forward for eventual comparison. The 30 June glitch (possibly together with some other considerations) caused a two week delay.

Orbit    dates      altitude(km)  pixelsize(m) res/HST  period  soccerball at
RC3    April 23–May 9    (13,500)    (1,300)    24     15 days    (3.0 meters)
Survey    June 6-30      (4,400)      (410)     72     3.1 days    (1.0 meters)
HAMO    Aug 4–Oct 15     (1,450)      (140)     215    19 hours    (33 cm)
LAMO Dec 8–end of mission  (375)      (35)      850    5.5 hours    (8.5 cm)

 

[marcus]

There was a brief status update yesterday, noting that the spacecraft was down to 2600 km altitude. (agreeing with simview for yesterday)
==quote==
July 27, 2015 - Dawn Orbiting Closer to Ceres

Today the spacecraft is orbiting ...2,600 kilometers above the ground. As with all of Dawn's complex maneuvers from each mapping orbit to the next, the spacecraft is not taking a perfect spiral path for technical reasons. The altitude does not change as much over the course of the day today as it does some other days. Nevertheless, the probe is accurately following its carefully designed course.
==endquote==
Today, 28 July, according to simview she seems to be down to 2500 km. So as a rough estimate there is about 1000 km of descent left to go and it's progressing down at the rate of 100 km per day, roughly speaking.

It's harder to descend the closer in one gets---more momentum has to be blown off by the ion engine, with each step. So as a crude guess I'll estimate two weeks to close in the rest of the way. Dawn might be settled in at the 1450 km orbit by 14 August.

In any case I hope very much we get a Journal entry from the mission director by Friday---the journal has been coming out monthly around the end of each month.

When you think about it, it's actually pretty fantastic that the spacecraft (now in its eighth year) is still functioning. Two of the four reaction wheels are worn out. Either one or two of the three alternate ion engines are no longer usable, it has been hit and temporarily stunned by high energy cosmic ray particles on several occasions, extraordinary measures are being taken to conserve hydrazine (used for attitude control) and it still has a lot of hard work ahead of it.

Om, mfb, and others please correct me here if I'm off on any of these details.

 

[marcus]

Recent (27 July) space.com article about Ceres
http://www.space.com/30054-dwarf-planet-ceres-bright-spots-atmosphere.html
new figure for equatorial radius 481 km.
haze seen over bright spots suggests they are ice, actively subliming, rather than dry salt residue.

Large areas where craters seem partially erased suggest geological activity

Color-coded topographic map (brown and red are high, up to +6 km, purple and deep blue are low, to -6km below surface average level) projected on a sphere in this animation:

The YouTube has no sound track but there is some annoying crosstalk from something else so I turn the sound off while watching. It takes only about one minute to view.

Current simview says altitude 2420 km (11:56 PM UTC = 4 PM pacific) and speed 148 m/s
(G*9.39e20kg/2900 km)^(1/2) --> 147 m/s

 

[marcus]

The new Dawn Journal
http://dawn.jpl.nasa.gov/mission/journal.asp
http://dawnblog.jpl.nasa.gov/2015/07/29/dawn-journal-july-29/
gives a revised schedule for when the probe will arrive at third science orbit (altitude 1470 km) and when it will start descent to the fourth and lowest orbit.

HAMO dates changed from Aug 4 - Oct 15 to Aug 17 – Oct 23, five fewer days will be spent in HAMO

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)

Here are links describing what the probe does at the different orbits. Science at LAMO was detailed in the August 2014 Journal entry
RC3
Survey
HAMO
LAMO
== http://dawnblog.jpl.nasa.gov/2014/08/31/dawn-journal-august-31/ ==
The spacecraft will use its sophisticated gamma ray and neutron detector (GRaND) to determine the atomic constituents of the material on the surface and to a depth of up to about a yard (a meter). .. it also measures the energy of each kind...
Most of the gamma rays and neutrons are byproducts of the collisions between cosmic rays (radiation from elsewhere in space) and the nuclei of atoms in the ground. ...In addition, some gamma rays are emitted by radioactive elements near the surface. Regardless of the source, the neutrons and the gamma rays that escape from Ceres and travel out into space carry a signature of the type of nucleus they came from. When GRaND intercepts the radiation, it records the energy, and scientists can translate those signatures into the identities of the atoms.
==endquote==

The way I picture it. Cosmic ray particles are typically protons. They collide with atomic nuclei on Ceres and in effect make the nuclei radioactive. The nuclei decay emitting characteristic energy gamma, from which you can tell what type nucleus it was.

Cosmic rays can penetrate down a meter or so below surface, so they can activate subsurface material. So we can be learning about the chemical elements comprising material on and slightly below Ceres surface, if all goes as planned. This does not start in earnest until 15 December.

Marc Rayman gave new figures in his journal for Ceres' radius:
equatorial 482 km
polar 447 km
average (482²447)^1/3 = 470 kmhttp://dawn.jpl.nasa.gov/mission/status.html

 

[marcus]

A new status report is out, for the 31 July.
http://dawn.jpl.nasa.gov/mission/status.html
I found a Wikipedia discussion of how gamma spectroscopy is able to detect some 20 different elements in the soil
https://en.wikipedia.org/wiki/Gamma_ray_spectrometer#Planetary_gamma-ray_spectrometers

Have to go help with supper, no time to finish, very interesting so will finish later

==quote==
How are gamma rays and neutrons produced by cosmic rays? Incoming cosmic rays—some of the highest-energy particles—collide with the nucleus of atoms in the soil. When nuclei are hit with such energy, neutrons are released, which scatter and collide with other nuclei. The nuclei get "excited" in the process, and emit gamma rays to release the extra energy so they can return to their normal rest state...
==endquote==

==quote==
These surfaces are subjected to a continual bombardment of high-energy cosmic rays, which excite nuclei in them to emit characteristic gamma-rays which can be detected from orbit. Thus an orbiting instrument can in principle map the surface distribution of the elements for an entire planet. Examples include the mapping of 20 elements observed in the exploration of Mars, the Eros asteroid and the Moon. [1] They are usually associated with neutron detectors that can look for water and ice in the soil by measuring neutrons. They are able to measure the abundance and distribution of about 20 primary elements of the periodic table, including silicon, oxygen, iron, magnesium, potassium, aluminum, calcium, sulfur, and carbon. Knowing what elements are at or near the surface will give detailed information about how planetary bodies have changed over time.
==endquote==
This is what is supposed to start in earnest around 15 December, at 375 km

 

[marcus]

The article says that neutron detection is one of the ways of detecting hydrogen in the soil---which would most likely be in the form of water.
Not sure this is relevant:
https://en.wikipedia.org/wiki/Small-angle_neutron_scattering#Technique
==quote==
The following table shows the scattering lengths for various elements (in 10⁻¹² cm).[1]

    H      D      C      N     O      P      S
-0.3742 0.6671 0.6651 0.940 0.5804 0.517 0.2847

Note that the relative scale of the scattering lengths is the same. Another important point is that the scattering from hydrogen is distinct from that of deuterium. Also, hydrogen is one of the few elements that has a negative scatter, which means that neutrons deflected from hydrogen are 180° out of phase relative to those deflected by the other elements.
==endquote==

 

[mfb]

I don't have access to the source, but 10^-12 cm are 10 femtometers, not much more than the size of a nucleus. That does not look right.

It fits to the bound coherent scattering length here, but we have unbound neutrons here.

 

[marcus]

Thanks, mfb--so that's not likely to be relevant. I'm curious about this statement in
https://en.wikipedia.org/wiki/Gamma_ray_spectrometer#Planetary_gamma-ray_spectrometers
==quote==
They are usually associated with neutron detectors that can look for water and ice in the soil by measuring neutrons.
==endquote==
Might you know or have a guess as to how detecting (and presumably measuring energies of) neutrons is a way of seeing water and ice?
I guess water is a good moderator
especially the hydrogen atom (which would most likely be occurring in water) because of its low mass is a good moderator.
And the cosmic rays probably make fast neutrons
so seeing a lot of slow ("thermal"?) neutrons might be an indicator of water.

 

[mfb]

I think so. The MESSENGER page agrees as well.