IPB

Welcome Guest ( Log In | Register )

41 Pages V  « < 37 38 39 40 41 >  
Reply to this topicStart new topic
Rosetta - Early Orbital Operations at Comet 67P C-G, August 6, 2014 - November 13, 2014
Gerald
post Nov 4 2014, 02:13 PM
Post #571


Senior Member
****

Group: Members
Posts: 1881
Joined: 7-December 12
Member No.: 6780



The landing site has now been called Agilkia:
QUOTE
The landing site, previously known as ‘Site J’, is named for Agilkia Island, an island on the Nile River in the south of Egypt. A complex of Ancient Egyptian buildings, including the famous Temple of Isis, was moved to Agilkia from the island of Philae when the latter was flooded during the building of the Aswan dams in the 20th century.
Go to the top of the page
 
+Quote Post
mcgyver
post Nov 4 2014, 03:17 PM
Post #572


Member
***

Group: Members
Posts: 107
Joined: 1-August 14
Member No.: 7227



QUOTE (jgoldader @ Nov 3 2014, 07:07 PM) *
I imagine aeons of vacuum cementing, plus solar UV flux creating chains of hydrocarbons, resulting in a material that could be reasonably solid at low temperatures.


QUOTE
Would the ejection velocity be higher for lighter particles than heavier ones? If so, the 'snow' (assuming it is 'fallout') could be dominated by larger particles that are more likely to settle back to the surface. Perhaps the 'snow' is sand and gravel, with relatively little finer material. I think of it like cinders falling back around a volcanic vent (though we're dealing with escape velocities in a vacuum rather than convection).



What I imagine instead is a rock traveling around the sun followed by a cloud of debris particles, most of which are too light to be able to "go back and land" due to continuous gas emission, so they keep orbiting around the nucleus like a cloud of (damned) midges around your head. ;-)

Once eventually the comet "turns off", the cloud/coma requires months to come to rest and the particles to slowly fall back to the surface.
If Philae has retro-rockets for landing (didn't study its design yet) it could encounter serious target-visibility issues.... or it could just spread away all debris and uncover underlying solid terrain, considering there's no air to keep the debris flying around close to ground (thinking of apollo landing).

Hard to say,
nice to see. :-)
Go to the top of the page
 
+Quote Post
fredk
post Nov 4 2014, 03:45 PM
Post #573


Senior Member
****

Group: Members
Posts: 3778
Joined: 17-January 05
Member No.: 152



QUOTE (tanjent @ Nov 4 2014, 09:51 AM) *
The "three sigma" simulation ellipse shown in the ISSID.org reference mentioned by Doug above is quite small, less than 200 m in its longest dimension.

That reference isn't dated, and skimming through it I see no sign that the calculations were made for the actual chosen landing site. It looks just like early simulations.

For a body like C-G, I could imagine the actual landing ellipse size would be sensitive to the landing location, so the size of that simulated ellipse may be very different from that of the actual landing site.

As far as I know, we haven't seen actual landing ellipses for the actual landing site yet...
Go to the top of the page
 
+Quote Post
dvandorn
post Nov 4 2014, 06:13 PM
Post #574


Senior Member
****

Group: Members
Posts: 3408
Joined: 9-February 04
From: Minneapolis, MN, USA
Member No.: 15



QUOTE (mcgyver @ Nov 4 2014, 09:17 AM) *
If Philae has retro-rockets for landing (didn't study its design yet) it could encounter serious target-visibility issues....


No landing rockets. Philae is left in an orbit that will intersect the comet's surface at a relatively low speed. It has attitude control, but no retros. So, no target visibility issues due to rocket blasts.

-the other Doug (With my shield, not yet upon it)


--------------------
“The trouble ain't that there is too many fools, but that the lightning ain't distributed right.” -Mark Twain
Go to the top of the page
 
+Quote Post
Explorer1
post Nov 5 2014, 06:42 AM
Post #575


Senior Member
****

Group: Members
Posts: 1449
Joined: 13-February 10
From: Ontario
Member No.: 5221



Meanwhile, some new developments on the legal front:
http://blogs.esa.int/rosetta/2014/11/04/ro...ommons-licence/

So Wikipedia can finally illustrate them freely (along with everyone else).

T-8...
Go to the top of the page
 
+Quote Post
Dan Delany
post Nov 5 2014, 08:15 AM
Post #576


Junior Member
**

Group: Members
Posts: 23
Joined: 15-February 14
Member No.: 7141



QUOTE
Philae has been designed for compressive strengths between 2 kPa and 2 MPa. For a compressive strength less than 2kPa, Philae’s baseplate would touch the ground (but then effectively stopping further penetration) and the 360° rotation capability of the landing gear would be compromised. Still, all experiments could be performed. Only for compressive strengths < 100Pa (equivalent to tensile strengths of less than 5 . . . 10Pa) the mission objectives would be compromised.


Thanks Doug for these helpful figures. I was trying to visualize this in comparison to snow on Earth to get a feel for Philae's design tolerances, this is what I came up with. Please correct me if I'm making any bad assumptions here.

The minimum surface compressive strength Philae was designed for is 2kPa. This is comparable to snow/ice that can support the weight of a cube made of steel with 2.5cm (1") length sides (on Earth!). Qualitatively this is something like lightly compacted snow or snow with a icy crust, but definitely not fresh snow. The "mission objectives will be compromised" limit for surface strength is 100Pa. This is like snow that can support a sugar-cube-sized cube of ice with 1cm sides, or a cube of balsa wood with 6cm sides. These would still sink at least a bit in the freshest of powder snow, but that's really very little pressure - as a former Coloradan, I'd estimate this to be like wet snow, or fresh powder after sitting in the sun for an hour.

What this doesn't take into account is the fact that the compressive strength of the dusty layer could be very dynamic if it is compacted. Think of packing a snowball, it's easy at first but the more you compact it, the more pressure it takes. If compaction does play a part, it seems like it certainly won't hurt, as it can (presumably?) only increase compressive strength. However, compaction could introduce other effects like slippage if Philae lands on a (relative) slope. Anyway, thanks again for posting these numbers, finally helped me understand a bit more of what Philae was designed to handle.

-d

1 kgf/cm^2 = 98.0665 kilopascals.
density of (balsa wood, ice, steel) = (.0016, .00093, .008)
(((2.5^3) cm^2 * 0.008 kg/cm^3) / (2.5^2) cm^2) * 98 kPa/(kgf/cm^2) = 1.96 kPa
(((1^3) cm^2 * .000934 kg/cm^3) / (1^2) cm^2) * 98 kPa/(kgf/cm^2) = .091 kPa
(((6^3) cm^2 * .00016 kg/cm^2) / (6^2) cm^2) * 98 kPa/(kgf/cm^2) = .094 kPa
Go to the top of the page
 
+Quote Post
TheAnt
post Nov 5 2014, 04:59 PM
Post #577


Member
***

Group: Members
Posts: 484
Joined: 12-February 12
Member No.: 6336



A somewhat preposterous presentation, but it is about Rosetta. =)
Go to the top of the page
 
+Quote Post
JohnVV
post Nov 5 2014, 05:50 PM
Post #578


Member
***

Group: Members
Posts: 824
Joined: 18-November 08
Member No.: 4489



saw that the other day
it is up there with the IBM "linux" TV commercial from 11 years ago
https://www.youtube.com/watch?v=s7dTjpvakmA
Go to the top of the page
 
+Quote Post
foxfire
post Nov 5 2014, 11:58 PM
Post #579


Newbie
*

Group: Members
Posts: 7
Joined: 5-June 08
Member No.: 4184



QUOTE (jgoldader @ Nov 3 2014, 01:07 PM) *
Though smooth-looking, the material is very dark and so not just new-fallen snow. It must have dust/pebbles/organics in it at significant concentrations. It could have enough structural strength to hold together pretty tightly--I imagine aeons of vacuum cementing, plus solar UV flux creating chains of hydrocarbons, resulting in a material that could be reasonably solid at low temperatures.


My thought is that there may be layers of tar-like crust sandwiched between less consolidated material. Since C-G is periodic (~6.45 yrs) with a perihelion that has been reduced from 2.7 to 1.3 AU, it has had multiple episodes of "emissivity." At 1 AU, the moon's sunlit surface can reach temperatures exceeding 250 degrees Fahrenheit (123 Celsius) and its dark side has temperatures exceeding these values on the minus side of the scales. Assuming there are sufficiently complex molecules to cross-link--or "dusts" that can lithify--as components of the out-gased materials, these could be either directly sprayed onto the surface by the vents or fall back on it later. Near perihelion these materials could melted and annealed. Indeed, this could even happen during one rotation cycle for certain parts of the comet. It would seem possible that as the comet receded from the sun, the consolidated materials would be relatively immune from further "boiling" away, and yet, some of the residual gases and dusts could be captured by the comet. I agree that UV and cosmic radiation could continue to then operate to bind the "final coating." And, there is always the ongoing, fortuitously captured "micrometeor rain" adding to the regolith.
I would still kind of like to see snow or something analogously snow-like. There would still probably be layers like glacial firn (neve).
Go to the top of the page
 
+Quote Post
mcgyver
post Nov 6 2014, 09:08 AM
Post #580


Member
***

Group: Members
Posts: 107
Joined: 1-August 14
Member No.: 7227



Color images would be much better to understand surface features.
OSIRIS camera has even sixteen color filter! http://www.planetary.org/explore/resource-...tta-osiris.html
Why do we only see B/W pictures published?
Is there a full archive of raw images, like it happens for NASA Mars rovers? We could at least try building color images by ourselves by combining different filters; we all know it will be never be "true color", but at least it will not be just gray.
Go to the top of the page
 
+Quote Post
Malmer
post Nov 6 2014, 11:04 AM
Post #581


Member
***

Group: Members
Posts: 241
Joined: 22-August 05
From: Stockholm Sweden
Member No.: 468



QUOTE (mcgyver @ Nov 6 2014, 10:08 AM) *
Color images would be much better to understand surface features.
OSIRIS camera has even sixteen color filter! http://www.planetary.org/explore/resource-...tta-osiris.html
Why do we only see B/W pictures published?
Is there a full archive of raw images, like it happens for NASA Mars rovers? We could at least try building color images by ourselves by combining different filters; we all know it will be never be "true color", but at least it will not be just gray.


All the images from the mission will end up on the PDS when the proprietary period has ended. (And that's the end of that discussion)

The surface is spectrally very uniform. So the error between images taken at slightly different times and slightly different light is almost at the same level as the difference in spectral reflectivity between filters. It is NOT a trivial thing to do right. You need a very good shape-model and very good registration of images. You also have to compensate for the lighting differances. Otherwise you risk creating color that just isn't there. I believe that they will release images when they feel that they have something that is good.

I see no other problems in creating a true color image. The surface is dark but it is still very much visible. The sunlight out there is about 10% of what we have here on earth. The comet reflects about 4 percent of that so yes it is really dark now. but still very visible. (The exposure times they use for OSIRIS is less than a second.)

Remember that the moon reflects about 12% and it is very bright when you look at it at night. So if you normalize the light to sunlight at 1Au the comet would be just short of half as bright as the moon.

There is obviously the usual problem of creating sRGB from the spectra. But given the filter ranges and the spectral uniformity it should work well to create a spectra, convert to cie and from there to sRGB.

Go to the top of the page
 
+Quote Post
DDAVIS
post Nov 6 2014, 11:52 AM
Post #582


Member
***

Group: Members
Posts: 193
Joined: 8-February 04
Member No.: 10




'Color images would be much better to understand surface features.'

It may be very difficult to make a color image with the viewing angle and shadows changing between different filtered exposures, depending on the interval between them. They may have to do a more complicated version of what people do with Jupiter spacecraft photos and project the frames onto a 3D model rotated into place to register the three images. These would show color fringing in the shadows unless they decided to let the longest shadows 'fill in' the others. They may wait a rotation between exposures until the lighting is similar in all three to provide color information unaffected by lighting angle differences. But vantage point differences between such images could introduce further complications as color coverage gaps etc.

While anticipating a color product some time, is there Earth based spectral data one can use for overall average color information? I would guess the comet is a black-brown color.
Go to the top of the page
 
+Quote Post
mcgyver
post Nov 6 2014, 05:48 PM
Post #583


Member
***

Group: Members
Posts: 107
Joined: 1-August 14
Member No.: 7227



QUOTE (DDAVIS @ Nov 6 2014, 12:52 PM) *
'Color images would be much better to understand surface features.'

It may be very difficult to make a color image with the viewing angle and shadows changing between different filtered exposures, depending on the interval between them.

How long does it take to OSIRIS to change color filter?


QUOTE
The surface is spectrally very uniform

Thats' actually what would be interesting to know/see. There should be CO2 ice, water ice and carbon and many other stuff in different percentages down there; I guess color images could even highlight underground multylayering in zones where fracturing shows underground terrain.
Go to the top of the page
 
+Quote Post
aconnell
post Nov 6 2014, 06:15 PM
Post #584


Newbie
*

Group: Members
Posts: 9
Joined: 16-September 07
From: London UK
Member No.: 3893



http://www.esa.int/spaceinimages/Images/20..._October_2014_a

Has anyone seen the swear jar? laugh.gif
Go to the top of the page
 
+Quote Post
djellison
post Nov 6 2014, 06:27 PM
Post #585


Administrator
****

Group: Chairman
Posts: 14018
Joined: 8-February 04
Member No.: 1



QUOTE (mcgyver @ Nov 6 2014, 09:48 AM) *
How long does it take to OSIRIS to change color filter?



A google search for "osiris instrument rosetta pdf"

Reveals this paper http://pdssbn.astro.umd.edu/holdings/ro-a-.../osiris_ssr.pdf

Which states....

CODE
8.3 Positioning accuracy and filter encoder
Motor movement is achieved by sequential activation of
the 4 motor phases where always two adjacent phases are
simultaneously powered. Each activation step moves the
motors by one rotation step. A change to the next filter
position requires 27 motor steps in either direction. Filter
changes are completed in less than 1 s


It later states

CODE
9 Shutter mechanism
In each camera an electromechanical shutter in front of
the CCD controls the exposure. The shutter is designed to
support exposure times between 10 ms and > 100 s with a
maximum repetition rate of 1 s
-1
. Typical imaging might
use exposure times of 100 ms and repetition rates of one
image every 7 s. The shutter is able to expose the 28  28
mm2
active area of the detector with uniformity of better
than 1/500. A total of 50 000 shutter operations is
anticipated throughout the mission


Go to the top of the page
 
+Quote Post

41 Pages V  « < 37 38 39 40 41 >
Reply to this topicStart new topic

 



RSS Lo-Fi Version Time is now: 23rd October 2017 - 05:21 PM
RULES AND GUIDELINES
Please read the Forum Rules and Guidelines before posting.

IMAGE COPYRIGHT
Images posted on UnmannedSpaceflight.com may be copyrighted. Do not reproduce without permission. Read here for further information on space images and copyright.

OPINIONS AND MODERATION
Opinions expressed on UnmannedSpaceflight.com are those of the individual posters and do not necessarily reflect the opinions of UnmannedSpaceflight.com or The Planetary Society. The all-volunteer UnmannedSpaceflight.com moderation team is wholly independent of The Planetary Society. The Planetary Society has no influence over decisions made by the UnmannedSpaceflight.com moderators.
SUPPORT THE FORUM
Unmannedspaceflight.com is a project of the Planetary Society and is funded by donations from visitors and members. Help keep this forum up and running by contributing here.