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

[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.

 

[marcus]

Thanks! It makes it clear---you compare the number of slow neutrons with the number of fast ones. You get an estimate of the proportion of light elements (like H) to heavier ones
==quote MESSENGER page==
The lighter the nuclei that it collides with, the more energy a neutron loses. The lightest nucleus (consisting of a single proton) is that of the element hydrogen. Since hydrogen is particularly effective at slowing the neutrons down, a large increase of slow-moving neutrons or a large decrease of neutrons of higher energy (due to this slowdown) can indicate the presence of hydrogen-rich materials such as water ice. So the numbers of fast and slow-moving neutrons detected by GRNS are clues to the relative abundance of light and heavier nuclei on the planet's surface.
==endquote==

 

[marcus]

The last time I looked, simview gave the altitude as 2150 km, probe just crossing south pole into Ceres night side. Call it 2200 km.
The target HAMO orbit altitude is 1470 km, so roughly 700 km of descent remains to be achieved.

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)

So we can expect Dawn to arrive at HAMO by 17 August---in a little over 2 weeks! For me the main excitement now has to do with the GRaND (gamma ray and neutron detector) instrument which only gets turned on in HAMO for purposes of calibration, to establish background levels etc. It will not get used in earnest until LAMO starts in December.

But that won't be for a while, so what can we anticipate from the time spent in HAMO (high altitude mapping orbit)? Marc Rayman discussed the activity planned for this orbit in his June 2014 Journal:
http://dawnblog.jpl.nasa.gov/2014/06/30/dawn-journal-june-30-2/

One thing will be a more precise topographical map. Better understanding of the geology probably.

 

[mfb]

We also get a factor 3 better resolution for all images.
And hopefully spectroscopic data from the bright spots and other areas.

 

[marcus]

Good point, better resolution and hopefully spectroscopic data.
As of 6PM pacific on 3 August, simview says the altitude is 2020 km, which leaves about 550 km left to descend (to reach the target 1470 km).

Simview also gives the speed as 158 m/s.

Remember Rayman's new figure for average radius is 470 km, so 2020 translates to 2490 km
(G*9.39e20kg/2490 km)^(1/2) ---> 158.6 m/s

So according to Simview the speed IS what it would have to be for circular orbit at that altitude. It wouldn't be surprising, though, for Simview to be slightly off. We may get another status update from Rayman to compare.
http://dawn.jpl.nasa.gov/mission/status.html
http://neo.jpl.nasa.gov/orbits/fullview2.jpg

 

[mfb]

The ion engine becomes weaker and weaker relative to the orbital parameters, so the descent orbit becomes more circular - acceleration within a single orbit becomes a small effect.

 

[marcus]

That makes sense! The spiral should be more circular this time even though (for navigation) they have shut off the ion engine for part of a day on a couple of occasions.
Mission director posted a status update for 3 August which confirmed what Simview indicated---that by the end of the day the probe was down to altitude 2000 km

 

[marcus]

A reminder of one of the reasons Ceres is an especially interesting solar system body. Its shape is only slightly oblate, indicating that it has differentiated into layers, by density. Its low density suggests it contains a huge amount of water (with various other chemical compounds dissolved in it). This would be primarily as ice--although I suppose there could be subsurface patches of liquid. Here's a projected cutaway diagram:

I suspect humanity may eventually want to set up chemical manufacturing industry in Ceres ice mantle.
Subsurface liquid water, circulated, could provide cooling for power generation.
Bulk chemicals already in orbit, in low gravity so easy to move, are potentially of high economic value.

Simview as of 10PM pacific on 4 August gave the altitude as 1930 km and speed just over 160 m/s---the target altitude for the next orbit is 1470 km, which means an orbit radius of 1940 km.
(G*9.39e20 kg/1940 km)^(1/2) ⇒ 180 m/s

 

[mfb]

You don't need the very deep natural water (if it exists at all) for cooling. A massive amount of ice is sufficient for cooling - dumping low power into it will melt some water which is a better conductor and allows to dump even more power into it.
Nuclear power (fusion or fission) is the only relevant application that could generate more heat than required. Some heating is needed for a station (even unmanned), and solar panels don't provide that.

 

[marcus]

... A massive amount of ice is sufficient for cooling - dumping low power into it will melt some water which is a better conductor and allows to dump even more power into it.
Nuclear power (fusion or fission) is the only relevant application that could generate more heat than required. Some heating is needed for a station (even unmanned), and solar panels don't provide that.

I agree. I wasn't thinking of naturally occurring liquid water but of this application. Waste heat could melt a limited volume of ice and make a subsurface cooling pond. If the water volume then dispersed heat widely enough into the ice, it could be conducted away by the ice without further melting. An application for nuclear power.

I suspect humanity may eventually want to set up chemical manufacturing industry in Ceres ice mantle.
Subsurface liquid water, circulated, could provide cooling for power generation.
Bulk chemicals already in orbit, in low gravity so easy to move, are potentially of high economic value.

Simview as of 10PM pacific on 4 August gave the altitude as 1930 km and speed just over 160 m/s---the target altitude for the next orbit is 1470 km, which means an orbit radius of 1940 km.
(G*9.39e20 kg/1940 km)^(1/2) ⇒ 180 m/s

As of 5 PM pacific 6 August, simview gave the altitude 1820 km and speed 164 m/s

 

[marcus]

New animation. 2 minutes. Vertical relief is 5x exaggerated.

Mountain's height is given as 4 miles which would be over 6 km. Interesting dark streaks down one side and light streaks down the other, like a volcano with two colors of lava

The probe is now spiraling down to its next-to-final orbit, altitude 1470 km. Should be there is about 10 days and start mapping and spectroscopy on 17 August.

DSN shows it is in radio contact with Goldstone antenna #24---a regular navigation break. Simview shows the ion engine temporarily turned off.

The craft is at altitude 1810 km, so it has about 340 km left to descend. Simview shows the current speed as about 165 m/s.

 

[marcus]

As of 11:30 PM pacific on 8 August, simview says altitude 1740 km
speed 169 m/s
270 km more to descend

EDIT: At 9:00 AM pacific on 9 August, altitude 1680 km and speed 170 m/s

 

[marcus]

Simview as of 11 am pacific on 11 August gives the altitude as 1600 km (which means 130 km more to descend) and the speed as 173 m/s.

Rayman's status update for 10 August said that the probe was instructed to simply coast for 23 hours starting sometime 9 August because its descent was getting ahead of schedule. They had allowed slack in case discrepancies developed and additional maneuver was required but descent went more smoothly than anticipated. Rather than advance the schedule which would have been very complicated---just to gain a day---they let Dawn coast with its ion engine off, to use up the slack.

Since we've turned a page I'll bring forward the schedule for to have it handy:

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)

LAMO (low altitude mapping orbit) is where gamma spectroscopy and thermal neutron detection will play a major role in discovering the chemical composition of Ceres surface material ("regolith").

Most recent Simview altitude was 993.37 miles translating to 1598.67 km, so FWIW just barely under 1600 km. Simview sometimes gets ahead of the official status report, but it has been doing rather well as a tracker, so I'd say worth consulting.

 

[marcus]

Also for convenient reference, since we turned a page, here are reminders of how gamma and neutron spectroscopy works, and the projected structure based on observed density, rotation and oblateness.

Nuclei hit by cosmic ray protons release fast neutrons. Nuclei hit by neutrons emit distinctive frequency gamma---so one can "hear" what types of atoms, what chemical elements, are in the regolith. Fast neutrons are slowed ("moderated") most effectively by repeated collision with hydrogen nuclei so the percentage of slow ("thermal") neutrons the probe detects, compared with fast neutrons, will indicate how much hydrogen is in the regolith. This will give a handle on the water content.

 

[marcus]

As of 10 pm pacific on 11 August, Simview gives the altitude as 1560 km, just 90 km from the HAMO target, and speed 175 m/s. At this rate Dawn will have to spend another day or two coasting with the engine off, just to keep from achieving HAMO ahead of schedule.

 

[marcus]

This is what HAMO (the next-to-final orbit) is supposed to look like. Simview indicates Dawn is already there. Officially Dawn is not expected to start mapping the planetino until 17 August. There was probably slack in the schedule which would now be awkward to compress.

You can see 1470 km altitude at speed 180 m/s which are the HAMO (high altitude mapping orbit) target parameters.
Update Simview now (9:15am pacific) gives altitude 913.26 miles and 402 mph figures, which translate:
slightly less than 1470 km and just under 180 m/s

Just checked again (as of 9:37am pacific) 912.67 and 402 which translate to 1469 km and 180 m/s
Simview shows the planetto rotating, so you can see the headlights coming around in this shot:

 

[marcus]

Mission director Marc Rayman made it official. http://dawn.jpl.nasa.gov/mission/status.html
==quote Dawn status update 13 Aug==

August 13, 2015 - Dawn Arrives in Third Mapping Orbit

Dawn completed the maneuvering to reach its third mapping orbit and stopped ion-thrusting this afternoon. This was a little ahead of schedule because the spiral descent went so well that some of the allocated thrusting time was not needed. Since July 14, the spacecraft has reduced its orbital altitude from 2,700 miles (4,400 kilometers) to approximately 915 miles (1,470 kilometers). The orbit period has correspondingly decreased from 3.1 days to 19 hours.

Dawn is scheduled to begin its new observations on Aug. 17 and continue for more than two months. First, however, the mission control team will measure the actual orbit parameters accurately and transmit them to the spacecraft
==endquote==

 

[marcus]

Dawn is taking pictures!
==quote Rayman's Dawn status update==

August 17, 2015 - Third Mapping Campaign to Begin Tonight

The mission control team has now provided Dawn with accurate knowledge of its orbit parameters. They have also completed transmitting all of the other information it needs and confirmed that the explorer is ready for its new Ceres mapping campaign.

Dawn has been pointing its main antenna to Earth since Aug. 13. Shortly after 9:00 pm PDT today it will start rotating to point its camera and other sensors at the landscape below and will begin taking pictures over the north pole less than an hour later.
==endquote==
If this mapping sequence goes like the previous two, the craft will take pictures (and IR spectral data etc) while on the day side, on the way from the Npole to Spole, and then rotate around and transmit the pictures and data while it is on the night side, returning back to Npole. Then it has to rotate around again to point its cameras etc back at the surface to start taking pictures on the next pass over the day side. Each orbit cycle takes about 19 hours. And she will be in this orbit (1460-1470 km altitude) for about 2 months. So that's a lot of cycles and a lot of pictures. Sort of 60*24/19 ≈ 76 rounds as a rough estimate.

 

[marcus]

New status update:
==quote http://dawn.jpl.nasa.gov/mission/status.html ==
August 21, 2015 - Dawn's New Mapping Phase Off to a Smooth Start

Dawn is performing flawlessly as it takes pictures and collects other data in its new orbit. The spacecraft 's view is now three times as sharp as in its previous mapping orbit, which concluded in June.

At this orbital altitude, it takes Dawn 11 days to photograph all of Ceres and transmit the data to Earth. The probe is scheduled to map Ceres six times over the next two months. The latest Dawn Journal includes a description of the plans for this phase of the exploration of Ceres.
==endquote==
As the update notes, Marc Rayman has just posted the Dawn Journal for August:
http://dawnblog.jpl.nasa.gov/2015/08/21/dawn-journal-august-21/

 

[marcus]

Parts of the August Journal struck me as so interesting that I decided to quote. This excerpt describes how Dawn will actually be taking 3D pictures of Ceres landscape by making successive passes with the camera aimed at different angles---not always straight down, sometimes off to the side in various directions to get perspective on the various mountains, planes, and craters.
==quote August Journal==
In its first mapping cycle, which is taking place now, the explorer aims its instruments straight down. For the second, it will keep the camera pointed a little bit back and to the left, making another full map but with a different perspective. For the third, it will look a little back and to the right. The fourth map will be viewing the scenery ahead and to the left. The fifth map will be of the terrain immediately ahead, and the sixth will be farther back than the third but not as far to the right.
==endquote==
Rayman also reminds us of the other sensors operating concurrently with the main camera:

==quote==

In addition to the stereo pictures and the many spectra (which reveal the nature of the minerals as well as the surface temperature), Dawn will use the color filters in its camera to record the sights in visible and infrared wavelengths.
...
...
The probe also will continue to acquire spectra both of neutrons and of gamma rays. It is unlikely to detect more than a whisper of neutrons from Ceres at this height, but the radiation coming from elsewhere in space now will serve as a useful calibration when it measures stronger nuclear emanations from one quarter the altitude starting in December, allowing scientists to inventory Ceres’ atomic constituents.

Precise measurements of Dawn’s radio signal will reveal more details of the dwarf planet’s gravitational field and hence the distribution of mass within. When the spacecraft is not aiming its main antenna at Earth, it will broadcast through one of its three auxiliary antennas, and the Deep Space Network will be listening (almost) continuously throughout the 84 orbits.
==endquote==

The idea of continuous monitoring a carrier signal (even though no data is being transmitted) is that temporary Doppler shifts show the craft momentarily speeding up and slowing down in its orbit as it approaches and passes over subsurface concentrations of mass.
Mapping irregular variations in subsurface density could give clues as to the planetino's interior composition and geological history.

 

[mfb]

No pictures yet :(.