Dawn taking chem makeup data at 385 km

[Marshall]

Well one way to find out is to go ahead
http://en.wikipedia.org/wiki/Ceres_(dwarf_planet)
Longitude of ascending node
80.3293°
Argument of perihelion
72.5220°
http://en.wikipedia.org/wiki/Argument_of_periapsis
So the longitude of the perihelion is 153 degrees.
So the longitude of the aphelion is 333 degrees.

I was guessing somewhere between 270 and 360 just by looking at the diagram. Turns out it's 333.

Ceres is right now around longitude 270 so it is approaching aphelion, and it is definitely in the SLOW part of its orbit. It would not be going its average speed of 17.9 km/s

It's speed at aphelion, for example, would be roughly 17.9 divided by 1.0758 which is what Wikipedia gives for 1+eccentricity. 1.0758 is ratio of perihelion to semi-major. 17.9/1.0758 = 16.638
Wow! I didn't think it would be that slow.

Also that "ascending node"thing. The descending node must be around longitude 260. and Ceres is just a little past that point now. so it must be sloping down by about 10 degrees, from the ecliptic plane.
We can kind of see that in some of the current status simulated views, where you see it move against the background of stars over the course of a few days. I don't judge any of this precisely, but there are suggestions of it.

miscellaneous handy converter:
https://frostydrew.org/utilities.dc/con ... ordinates/

[Marshall]

I'll bring forward the current status view, to have it handy

Ceres' motion against the background of stars (watched over several days) is to the left and slightly down. Dawn is thrusting so as to catch up and match speeds, as it approaches. We see them roughly from sunwards direction.
Since we just turned a page I'll also bring forward the large-scale orbit diagram to have it handy for reference as well.

In this picture solar system "longitude" has 0 degrees correspond to "East" or rightwards of the sun. To a good approximation, 90 degrees longitude is up. Ceres and Dawn are both at about 270 degrees longitude now.
This longitude handle on direction is useful because sources like Wikipedia give us information in those terms. For example Ceres is closest to the sun when it is at longitude 153 degrees on its orbit.
And its orbit plane is inclined slightly compared with the earth's orbit plane, so that Ceres is above our orbit plane while its longitude angle is between 80 and 260 degrees, and below the rest of the time. Technically its "ascending node" is at 80 degrees.

[Darby]

Hopefully we'll finally get to see those photos from 2/12 today.

[Marshall]

I sure hope so! It's been a long wait. I'll bring forward that table of unofficial projected coordinates for the next few days.
Current distance 54.54 kkm seems consistent.

X Y Z are coordinates relative to Ceres, which is (0,0,0), measured in kkm---thousands of km.
X is directed out from sun, in Ceres orbit plane
Y is directed perpendicularly up off the orbit plane, in Ceres' north pole direction
Z is directed forwards in Ceres orbit plane, the direction Ceres is moving, a negative shows the probe trailing behind.

Code: Select all

date      X          Y          Z        distance from Ceres
F17   -45.9972    6.4086    -27.2882    53.86                 
F18   -38.555    9.71627    -28.2185    48.75
F19   -32.3324    12.4392    -29.202    45.30                 
F20   -26.169    14.8491    -29.9728    42.46
F21   -19.6171    17.2648    -30.4689    40.14
F22   -13.2794    19.4975    -30.6993    38.71
F23   -6.73346    21.6416    -30.593    38.07
F24   -0.502056    23.4431    -30.212    38.24
F25     5.62894    25.0851    -29.7158    39.29
F26     11.407    26.4613    -29.1488    40.98
F27     17.2899    27.6663    -28.1919    43.11
F28     22.8583    28.5286    -27.0313    45.46
M1      27.9985    29.1842    -25.6846    47.90
M2      32.8862    29.7513    -24.1873    50.51
M3      37.6439    30.1647    -22.7166    53.31
M4      41.9734    30.4246    -21.3167    56.05
M5      45.8274    30.5605    -19.8726    58.55
M6      49.5028    30.6491    -18.2955    61.02
M7      52.8252    30.4896    -16.7451    63.24
M8      55.7681    30.3242    -15.1946    65.27
M9      58.5427    30.0761    -13.6441    67.21
M10     58.5427    30.0761    -13.6441    67.21
M11     63.2886    29.4796    -10.5963    70.61
M12     65.1256    29.1961    -9.32939    71.97



These projected coordinates, with minor changes, are copied from some prepared by L. Petrich. I believe they were derived from figures shown in Marc Rayman's Journal. The dates/distances can differ fractionally from those in the journal, as if a different time zone or time-of-day sampling might have been used But by and large they seem to match up.

[Darby]

Bingo ...

http://dawn.jpl.nasa.gov/multimedia/Two ... _Ceres.asp



I'm still thinking the bright reflectivity in some of those crater bottoms is CO2 ice.

[Darby]

Ok, Thursday marks RC2, which should generate a small deluge of photos, since it appears to be one of the main points in the approach where the main camera is used to verify relative position and help finesse the coming approach and capture phase.

By coincidence, New Horizons is wrapping up the same phase as it approches Pluto. I kicked off a thread for it over here.

[Darby]

I wonder if there's a betting pool over at JPL on how much fuel they'll have left when they complete the capture phase maneuvers. Ah, to heck with wondering, you KNOW someone over there has one. Perhaps a question to forward to the MD ? ;)

[Marshall]

Ok, Thursday marks RC2, which should generate a small deluge of photos, since it appears to be one of the main points in the approach where the main camera is used to verify relative position and help finesse the coming approach and capture phase.

By coincidence, New Horizons is wrapping up the same phase as it approches Pluto. I kicked off a thread for it over here.

Here's that table from Rayman's January journal that gives the percent illumination. I'm wondering if it makes much difference that the RC2 photos will have Ceres only 87% illuminated.
If they take the time to photograph a large part of one full rotation they can get every sector illuminated.

The photos we just saw, from 12 Feb, did not look like they were taken under 98% illumination, but presumably they were.

Relevant column headings:
Distance from Dawn to Ceres in (kilometers)
Ceres diameter in pixels
Resolution in (kilometers) per pixel
Resolution compared to Hubble
Illuminated portion of disk

Code: Select all

Jan 25   (237,000)   43     (22)   1.3   96%   
Feb 3   (146,000)   70    (14)     2.2   97%
Feb 12   (83,000)   121    (7.8)    3.8   98%
Feb 19   (46,000)   221    (4.3)   7.0   87%
Feb 25   (40,000)   253    (3.7)   8.0   44%
Mar 1   (49,000)   207    (4.6)    6.5   22%   
Apr 10   (33,000)   304     (3.1)    9.6   18%   
Apr 15   (22,000)   455    (2.1)    14   50%   


[Marshall]

The most recent status report is as of 6PM pacific on 17 Feb. The range is 52.37 kkm making Ceres appear 208% moon-size from the probe's standpoint. Dawn is approaching the planet at 83.6 m/s.

the next photo shoot is planned for Thursday 19 Feb, from a distance of 46 kkm

[Darby]

That means unless the MD stays late Friday, we wont see photos until Mon 23rd.

Rats.

[Darby]

OpNav images of a narrow crescent won’t contain enough information to warrant the expenditure of hydrazine in all that turning. Moreover, the camera’s precision optics and sensitive detector, designed for revealing the landscapes of Vesta and Ceres, cannot tolerate looking too close to the sun, even as far from the brilliant star as it is now. Therefore, no pictures will be taken in March and early April when Dawn is far on the opposite side of Ceres from the sun. By the end of April, the probe will have descended to its first observational orbit (RC3), where it will begin its intensive observations.

No pics in March and most of April either.

[Marshall]

Heh heh,
We are being taught patience.
As you point out, the real fun only begins two months from now, around end of April! I'm glad you started a Pluto thread BTW. When there are long waits it's good to have several interests.
We can switch back and forth between topics.

It's true that I'm especially interested in Ceres---because inner solar system accessible and (according to one model) representing a huge supply of water. Your idea of CO2 is also interesting but in any case we'll be learning more.
the grimy crust over the mantle does seem to be fairly thin, a couple of people have pointed to the "ghostly" outlines of a large impact crater on one side, as if the crust is thin enough for markings to subside and disappear with time.
And you are pointing to implications of the white spots.

[Darby]

If I'm reading the distances right, it looks like the capture point mentioned in this diagram ...



Will happen sometime next week. I'd guestimate that after the next round of photos, they'll turn arse-backwards and fire up the ion thruster to cancel out enough of the approach delta to let Ceres take hold.

[Marshall]

Yes, but not exactly next week, capture right around March 5 or March 6.

We will be able to check the speed on the current status page, and compare it with escape velocity at the indicated distance, to verify capture. Mass is estimated at 943 billion billion kg.


I'm also interested in something currently that has nothing directly to do with space exploration---I started a thread about it in Philosophy of Science. It's the Montevideo interpretation of quantum mechanics. I have no idea if this is the kind of thing you are interested in, maybe not, it's pretty academic looked at from one angle at least.

[Marshall]

If you look back a few posts you see the distance given for 5 March:

Code: Select all

date      X          Y          Z        distance from Ceres
...
M1      27.9985    29.1842    -25.6846    47.90
M2      32.8862    29.7513    -24.1873    50.51
M3      37.6439    30.1647    -22.7166    53.31
M4      41.9734    30.4246    -21.3167    56.05
M5      45.8274    30.5605    -19.8726    58.55
M6      49.5028    30.6491    -18.2955    61.02
M7      52.8252    30.4896    -16.7451    63.24
M8      55.7681    30.3242    -15.1946    65.27
M9      58.5427    30.0761    -13.6441    67.21
M10     58.5427    30.0761    -13.6441    67.21


Distance is 58.55 kkm and escape velocity is square root of 2GM/R so paste this into google:
(2G*943e18 kg/58550 km)^.5
and we get 46.4 meters per second.
The speed relative to Ceres is now 84 m/s so somehow, by 5 March the speed has to get down to around 46 m/s. Then we have capture.

On the current status page they give the speed in "miles per hour" to 3 digit accuracy so we need to convert it to m/s if we are keeping track but that's easy enough.
Capture might not happen then, might be a day or so later.

[Marshall]

A good time-table:

[Darby]

Thanks for correcting me on the orbital diagram, and posting much better diagrams.

The ion drive is low thrust, so I'm guessing it'll start decelerating right were it says "new approach trajectory" in the upper left, or somewhere a little further long, like around the 24th.

Very cool !

the grimy crust over the mantle does seem to be fairly thin, a couple of people have pointed to the "ghostly" outlines of a large impact crater on one side, as if the crust is thin enough for markings to subside and disappear with time.

Yes, it will be interesting to see if there are any large impactor traces once we see a full spherical map, or it will be a fairly evenly spaced smattering of tiny, small and medium sized craters.

Some more thoughts:

Composition: Ceres is BIG ... far more than big enough to crush itself into a proper sphere under its own mass, even if all the material that it formed from was a mix of dust, silicates, rock, metallic asteroid fragments, yadda yadda, and some frozen gasses and dirty snow. As the sheer mass of all that compacts under its own weight, plus several billion years of additional gradual accretion, any solidified gas/liquid residues and water ice that might have been present would have long since been squeezed out towards the surface, and slowly sublimed/ionized away by the solar wind, leaving a dead cold core of mixed solid matter and dust, which eons of compaction would slowly turn into rock. Any remaining icy material would be limited to the outermost layers. Doesnt seem cold enough to be able to hold on to some of the colder gaseous materials we see out on Titan or the ice giants, so those have likely long since sublimed away well. What sort of ice is left depends on the thermal readings. I think I read somewhere that the Herschel readings didnt show enough water vapor to account for much, so that would seem to lean towards CO2, which could have degassed from comets travelling further inward.

I'm also saying a big fat NO to cryovolcanos. Yes, yes and maybe on ice moons like Enceladus, Titan and Europa, but no friggin way on someplace much closer sunwards Ceres, which has neither tidal flexion nor a molten core to keep rhythmic state changes going. No no no no NO. Yeah, sure ... perhaps there's some radioactives present somewhere deeper inside, but ceres doesnt have enough mass for all those materials to melt and stratify, so even if present, itll be a buckshot mess that's probably not compact enough to generate meaningful heat over a large enough area to matter. Did I say no already ? Yes I did ... No crovolcanos.

I'm not a planetary geologist, that's just my own speculation.

[Darby]

For some spiffy Saturnian cryovolcano goodness, go to the link below for Cassini, and click on the following slides:

http://saturn.jpl.nasa.gov/photos/halloffame/

Slide 71: Tiger Stripe fissure (Enceladus)
Slide 86: Sotra Facula (Titan)

All 87 slides are pretty amazing, but only these two are relevant here.

Gotta love those shots of the hexagonal polar storm on Saturn though ... slides 16, 21, 36, 53 and 57.

Ok, back on topic.

[BurtJordaan]

Thanks for correcting me on the orbital diagram, and posting much better diagrams.

The ion drive is low thrust, so I'm guessing it'll start decelerating right were it says "new approach trajectory" in the upper left, or somewhere a little further long, like around the 24th.

I think the ion drive is already 'reverse-thrusting', but gravity is still speeding Dawn up and will do so until periapsis in about 5 days time. As Marshall worked out, its relative speed is still way above escape speed and it will have to keep on throwing out ions in the 'forward' direction in order to get under escape speed for its 'then' altitude, in order to be captured.

Fascinating approach orbit at present. Still wish I had tools to plot it cylindrically* from the data supplied. Faradave, Dave, anyone?

--
Regards
Jorrie

PS: I have something like this in mind, but zoomed in to the position of Ceres:

[Darby]

Yes, the current capture maneuver is not quite what I was expecting.

If I had to speculate, it's a max low consumption alternative method that they settled on after leaving Vesta and discovering they had both fuel and maneuvering thruster issues. In this method, they they appear to use the iondrive as much as possible in order to minimize the load on the maneuvering thrusters as they drop backwards from the same (rather than the opposite) side.

It's really very clever. I like it.

[Darby]

I think I read somewhere that the Herschel readings didnt show enough water vapor to account for much, so that would seem to lean towards CO2

It looks like I was wrong about Herschel ruling out water as the main candidate for the spot reflectivity.

http://www.esa.int/Our_Activities/Space ... anet_Ceres

Water is still in the running (pun intended).

BTW, whoever wrote that article is not using their reference to cryovulcanism correctly. There is a big difference between simple degassing of volatiles, and a decidedly more geologic process of standing volcano/rift like structures whos innards constantly vary in their changes in state. Without that, the physical metaphor of a volcano goes away.

[Watson]

I think these are more recent pictures, but I'm not sure if they are noticeably cleared.

[Watson]

This was just posted so I think it current. It is getting down to highway speeds.

Dawn is <50 K km from Ceres (13% of the Earth-moon distance) & approaching at a relative velocity of ~45 meters/second (100 mph).

[Darby]

Time to roll down the windows and smack those fuzzy dice then. :)

[Marshall]

Watson, thanks for the update! You reminded me to check current status:
http://neo.jpl.nasa.gov/orbits/fullview2.jpg
This is as of 1PM pacific or 21 h UT on 18 February, so it is nearly current.
Current status says distance 49.02 kkm
and speed relative to Ceres is 185 mph (when I put that into google it says 83.7 m/s)

2 arcsin(475 / 49 020) = 1.1104007 deg, so 222% of moon's angular size.

You point out a good thing! Not all of Dawn's 84 m/s is aimed directly at Ceres!
You quote the recent Tweet
https://twitter.com/NASA_Dawn/status/568133976151363585
which says the spacecraft is getting closer at rate of about 45 m/s.

I've got to figure out better language to use.

[Marshall]

So we can say by what angle Dawn's course is OFF from directly aimed at Ceres
arccos(45 / 83.7) = 57.4772525 deg

It is off by about 57.5 degrees.

Relative to Ceres, the probe is GOING 83.7 m/s

but its velocity instead of being directly at Ceres has some component UP and some drifting BACK so not all of that 83.7 m/s is aimed at Ceres. It is only getting nearer to the planet at speed of 45 m/s.

We know from Rayman's diagrams copied in this thread that the probe is also heading up (out of the orbit plane, in the Ceres north pole direction ) and is also falling behind Ceres. Eventually Ceres gravity will help cancel out that lag and excess rise.

[Marshall]

Distance 41.59 kkm. Angular size 2arcsin(475 / 41 590) = 1.31 deg, speed relative Ceres is around 83.15 m/s, call it 83.

Current status shows the probe still in picture-taking or communicating attitude. Not yet having resumed thrusting.

That could just be a delay in updating the graphic view.

It has finished communicating---earlier today it was talking with Earth via Deep Space Network antennas in Canberra, Madrid, and Goldstone California.

EDIT, Dawn has now reoriented itself and resumed thrusting.
41. 46 kkm.

[Marshall]

Range 38.85 Nkkm speed 80 m/s as of 6PM pacific on 22 Feb.

today the solar panels hardly show because they are turned facing the sun and we see them edge on.

this is about as close to Ceres as the probe will get for a while. It is passing Ceres "astern" of the planet and needs to slow down more in order to gain capture by Ceres gravity. So it will take a kind of detour for the next few weeks.

Gradually slowing down by means both of Ceres gravity and its own solar electric thruster.

[Darby]

With any luck, the photos from 2/19 will get released by CoB** tomorrow.

----------------
** Close of Business

[Marshall]

It wouldn't surprise me if you were the first one of us to see them :^)

BTW looking back I see I calculated that the speed relative to Ceres would have to be down to around 46 m/s
by 6 March, when the distance is predicted to be about 61 kkm in order to achieve capture.

We only seem to get the speed in mph (to three digits precision). It is 175 mph today. If the capture goal is really 46 m/s that is equivalent to 103 mph so that is kind of the target now.

To get from 175 down to 103 by 6 March.

Another BTW, Darby: what does CoB mean? there could be a joke I'm not getting. But yeah! whatever CoB is, I hope it releases pictures soon!