Astroblog

Wednesday, November 12, 2014

The Towering Cliffs of Comet 67P/Churyumov–Gerasimenko


Agilkia, the Philae landing site, looks pretty okay in this image (image credit ESA). Click to embiggen	But with a different lighting angle you can see it (indicated by box) is perched near a pretty worrying cliff. (image credit ESA). Click to embiggen

Lighting angle can make a huge difference to how a place looks. With the lighting above, Philae's landing environs look relatively smooth, with some scalloping. With the light at a different angle, the "scalloping" is revealed as terrifying cliffs.

Labels: 67P, comet, spacecraft

# posted by Ian Musgrave @ 11:32 pm 0 comments

Comet 67P/Churyumov–Gerasimenko in Celestia

Comet 67P//Churyumov–Gerasimenko and the Rosetta orbiter rendered in Celestia.

As usual, I have been trying to make a Celestia file for comet 67P.

And failing miserably. I tried to convert the ESA public shape model to celestia 3Ds format and failed miserably. So I ended up using Jack Selden's Rosetta files, with a surface map taken from this thread on the Celestia. The Celestia comet looks nothing like the rubber duck shape of 67P, but until some kind person points me to a pubic Celestia shape model that is what I am stuck with.

I utterly failed to make any of the XYZ orbital files work for me, so I just copied the orbit of Dactyl around Ida. Yes, that's cheating, but after days of frustration I wanted something that works. If you want a stunning Celestia simulation, go here.

As usual, copy the code below and save as it as a text file 67P.ssc in the Celestia extras folder.
==========================67P.ssc=======================================
# Taken from Jack Seldens 67P ssc file
# Created By Ian Shazell, Spacecraft Analyst for Rosetta
# On Behalf of the European space Agency, www.esa.int
# Comet 67P Churyumov-Gerasimenko, Celestia SSC file. www.shatters.net/celestia
#
#
"Churyumov-Gerasimenko" "Sol"
{
    Class "comet"
    Mesh "asteroid.cms"
        Texture "67p720x361.png" # texture from the Celestia forum from John Van Vliet
    Radius 1.98                             # http://forum.celestialmatters.org/viewtopic.php?f=4&t=690
    EllipticalOrbit
    {
        Period                   6.4451441700205338809034907597536
        PericenterDistance       1.2431213
        SemiMajorAxis         3.4632382
        Eccentricity            0.6410523343769
        Inclination            7.04073996
        AscendingNode          50.150595169
        ArgOfPericenter        12.77512076
                MeanAnomaly            -71.72623185
        Epoch                   2456778.5
    }
        Albedo            0.2
}

"Rosetta" "Sol/Churyumov-Gerasimenko"
{
        Class "spacecraft"
        Mesh "rosetta.3ds" # rosetta model http://homepage.eircom.net/~jackcelestia/files/rosetta.zip
        Radius 0.005

        EllipticalOrbit
        {
    Epoch         2449228.2028 # Using orbit of Dactyl around Ida
    Period             0.96534 #
    SemiMajorAxis     25.0     # Why? I can't make anything else work
    Eccentricity       0.13    #
    LongOfPericenter 310       #
    AscendingNode     90       #
                                   #
    Inclination        8       #
        }

        Albedo 0.5
}
======================end 67P.ssc==========================

Labels: 67P, celestia, comet

# posted by Ian Musgrave @ 10:38 pm 2 comments

BREAKING NEWS! Philae Lives! (Philae heads towards comet 67P)

UPDATE 21:57 ACDST: Telemetry received shows flywheel working and Landing gear deployed. Everything appears to be working.

XKCD's wonderful cartoon of Philae leaving Rosetta and heading for comet 67P, the cartoon updates so every time you visit there will be a different image of the journey.

Philae has separated from Rosetta and is heading towards the comet.

Until the lander and Rosetta are lined up properly we won't have any telemetry, the first telemetry should be at 12:00 UT (that's 11 pm AEDST).

BREAKING NEWS! Telemetry acquired from @ESA_Rosetta and @Philae2014 !! WOOT!

The ony problem has been the nitrogen thruster that was supposed to hold the lander down as the harpoons and ice screws deployed has apparently failed. Unless the surface is rally solid, this should not be a problem. Everything else seems to be working fine.

The lander is heading towards Comet 67P/Churyumov-Gerasimenko at a leisurely 2 Km per hour, it should arrive at the landing site in around 5 hours at 3:00 am AEDST (16:02 UT).

the Livestream is here http://new.livestream.com/ESA/cometlanding

Labels: 67P, comet, spacecraft

# posted by Ian Musgrave @ 9:47 pm 0 comments

Tomorrow we Land on a Comet


Small bodies landed (or about to be landed on) by spacecraft. Asteroid Itokawa size 540m x 270m x 210m (image Credit JAXA)	Comet 67P size 4.1×4.5 Km (image credit ESA)	Asteroid Eros size 34.4×11.2×11.2 Km (image credit JPL/NASA)

Tomorrow (Wednesday) at 04:03:20 UT, if all has gone well, the Philae lander will separate from the Rosetta spacecraft and begin its descent to Comet 67P/Churyumov-Gerasimenko, by 16:02:20 UT Philae will have either landed on the comet, or crashed, or bounced off and gone spinning into space (for Australians, that is 3 pm Wednesday and 3 am Thursday). Either way this represents a remarkable achievement.

Humans have done some extraordinary things in space, we have landed a laser wielding, atomic powered robot on Mars with a sky crane, parachuted a probe into the freezing methane clouds of Titan, sent a probe ballooning in the searing hell that is Venus's atmosphere, but nothing compares to this.

Mars, Venus and Titan are all challenges on their own, but compared to comets they are staid. They orbit predictably, they rotate predictably, gravity is predictably down and we had a reasonably good idea of what their surfaces are like (well Titan was a gamble, the probe could have landed in one of the methane seas).

But comets are a different kettle of fish, while their orbits are more or less predicable (given that outgassing can push them off course), they tumble rather than rotate and they vent gas and small particles that can .. inconvenience .. a spacecraft in a rather terminal way.

Comet 67P is even more of a nightmare, instead of a sort of battered potato shape most people expected, it is like a rubber duck from the dungeon dimensions. Flat plains are interspersed with boulder fields, craters and towering cliffs. The concepts of up and down on this weirdly spinning object are more like an Escher drawing than our traditional concept of a planet or asteroid.


Asteroid Itokawa from 4.1 Km (image Credit JAXA)	Agilkia, landing site of Philae from around 10 Km (image credit ESA). Even the safe site is a lot more bramatic than the two asteroid we have visited.	Eros surface from around 10 Km (image credit JPL/NASA)

While we have sent spacecraft scooting past comets before, even fired a copper slug into one, but we have never attempted to land on one. We have sort of landed on two asteroids. Itokawa, where the Hayabusa probe sort of bumped into it (and bounced off again with a precious dust sample) rather than landed per se, and Eros, where the Shoemaker orbiter wasn't actually designed to land but was rather crashed slowly on it. Itokawa is around 10 times smaller than 67P, and Eros 10 times larger, both giving entirely different challenges.

In contrast, Philae is designed to soft land and stay on the comet for months, taking measurements as the comet heats up. The descent will be slow, and it should touch down relatively softly, given the complicated calculations required to land the craft of the around 1 Km square patch of level surface Philae is aiming for, nestled in between boulders and scarps (and a rather dramatic cliff).

67P is a 60's science fiction cover art image of a comet. (Image credit ESA)

Unlike Itokawa and Eros, which had rather plain surfaces (even if scattered with alarming boulders), 67 P is a nightmare of chaos. Missing the landing site will be catastrophic.

If it succeeds in making it to the landing spot safely, Philae has a harpoon (a SPACE HARPOON!) to help anchor it to the surface as well as the equivalent of crampons on the lander feet.

The Agilkia site seems to be faily level, and covered with what looks like snow, but this is probably unconsolidated dust. What it is on top of nobody knows. While the comet has the overall density, and possibly the structural strength, of a snowbank, exactly how the material that makes up the comet, (rock, dust, ices, organic gunge) is organised is unclear.

Will there be a layer of rock cemented with ice, a layer of mostly ice, a messy honeycomb of rock and ice fragments with lots of space between? Will the dust layer be too deep and fragile for Philae to hold on to? Will the lander plummet into the surface like a rock thrown at a snowbank?

These are all questions that will be answered on the day. So until then we have to take a deep breath and wait. You can make a model of Rosetta while you wait.

You can watch the comet landing livestream here http://new.livestream.com/ESA/cometlanding
It will remain live until 19:00 UT (6 am Australian time).

In the meantime, you can watch this video of amateur images of 67P. http://vimeo.com/111177163

Labels: 67P, comet, spacecraft

# posted by Ian Musgrave @ 12:08 am 2 comments