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Thread: Rosetta is There!

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    Rosetta is There!

    Quote Originally Posted by Superluminal View Post
    Perhaps, in August, we should start a new thread called, "Rosetta is There".
    As suggested. Going forward, please post updates here instead of in the Rosetta is on its way! thread.
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    Yay, Rosie! WE CAN DO IT!
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    I was just grooving on some of the pics. This is really exciting. Go, mankind! ...er, humankind!

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    Rosetta is THERE!

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    I was surprised to hear Rosetta is maneuvering to maintain an orbit around this relatively small primitive piece of the solar system. The attractive gravitational effect, being directly proportional to the mass of an object, could not be very large in this instance, I should think. Collecting all the data from the thruster usage required to keep Rosetta in orbit should give a pretty exact reading on what that mass is, which I'm sure they already have a very good estimate for. Unfortunately, that mass is not close to being uniformly distributed.
    Everyone is entitled to his own opinion, but not his own facts.

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    Well, while it's not much, that shape must make Earth's Moon look like a perfect sphere as far as gravity maps are concerned, and the moon is infamous for throwing off orbits.
    Last edited by ravens_cry; 2014-Aug-07 at 05:19 AM.

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    The comet's name is too long; they should just rename it Q-Bert.

    Actually, it looks more like a Peashooter from Plants Vs Zombies.
    Last edited by parallaxicality; 2014-Aug-07 at 10:19 AM.

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    The surface of this comet looks very much like the 81P/Wild comet. The same flat bottom pits ,sheer walls and generaly a very irregular surface.

    Where is the ice ? Because of course comets are full of ice . . .
    Last edited by galacsi; 2014-Aug-07 at 09:17 AM.

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    Quote Originally Posted by ravens_cry View Post
    Well, while it's not much, that shape must make Earth's Moon look like a perfect sphere as far as gravity maps are concerned, and the moon is infamous for throwing off orbits.
    I was trying to imagine standing on top of the "duck's head" and walking, or jumping, down the "neck" to the "body." Just can't get my head around it!

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    Quote Originally Posted by Hal37214 View Post
    I was trying to imagine standing on top of the "duck's head" and walking, or jumping, down the "neck" to the "body." Just can't get my head around it!
    Given that escape velocity is less than a typical human walking speed, I'd say it'd be very, very slow. It would be more like rock climbing, I'd think, with constant tethering, clip on, clip off

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    Quote Originally Posted by parallaxicality View Post
    The comet's name is too long; they should just rename it Q-Bert.

    Actually, it looks more like a Peashooter from Plants Vs Zombies.
    OMG, you're so right!

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    Quote Originally Posted by Cougar View Post
    I was surprised to hear Rosetta is maneuvering to maintain an orbit around this relatively small primitive piece of the solar system. The attractive gravitational effect, being directly proportional to the mass of an object, could not be very large in this instance, I should think. Collecting all the data from the thruster usage required to keep Rosetta in orbit should give a pretty exact reading on what that mass is, which I'm sure they already have a very good estimate for. Unfortunately, that mass is not close to being uniformly distributed.
    It will indeed be not easy to orbit this body . The escape velocity is only about 0.5 m/s !
    This means any object having a higher differential speed will move away .
    0.5 m/s in space is very very slow . Thats the reason , I guess , Rosetta has a lot of manoeuvering ( carefull using its thrusters ) to do . In a later stage the odd shape will not make things easier ...

    I looked up some data : Rosetta has 24 thrusters , each being able to give 10N of thrust . The spacecraft has a mass of 3000 kg ( at launch ) .
    If Rosetta would have to perform a 0.1 m/s manoeuver ( this alters its orbit relative to 67P significantly ! ) , it only has to trottle 1 thruster for only 30 seconds .
    Last edited by frankuitaalst; 2014-Aug-07 at 09:55 PM.

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    More information on Rosetta.

    http://m.space.com/26754-rosetta-com...40807_29286356

    " Europe's Rosetta probe made its historic arrival at Comet 67P/Churyumov–Gerasimenko yesterday (Aug. 6), making Rosetta the first human-built craft ever to rendezvous with a comet with the intention to enter orbit.
    Now flying about 62 miles (100 kilometers) from the comet, the Rosetta spacecraft is expected to gather data that will help scientists learn more about comets. Specifically, the findings will shed light on how the ancient, icy wanderers that roam through the solar system formed thousands of years ago.
    The European Space Agency-operated spacecraft may also help scientists find answers to some of the most pressing questions in space science today."

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    I didn't see that name mentioned in the link. What is a Q-Bert, and why would that be appropriate as a name?
    As above, so below

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    A foul mouthed little fellow who likes jumping on block pyramids.

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    Over the last few days I've spent too much time just staring at the thing... it's such a bizzare landscape. I've even started painting a picture of what strange landforms might be on the surface....

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    Quote Originally Posted by ravens_cry View Post
    Given that escape velocity is less than a typical human walking speed, I'd say it'd be very, very slow. It would be more like rock climbing, I'd think, with constant tethering, clip on, clip off
    No doubt! I'm thinking of how the gravity vectors must vary at different places - especially in the "neck" area.

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    67P Potential

    Quote Originally Posted by Hal37214 View Post
    No doubt! I'm thinking of how the gravity vectors must vary at different places - especially in the "neck" area.
    I had the same thinking about this and did some modelling , see result in attach .
    I modelled 67P as two spheres in contact with each other ; this is relative easy to calculate .
    Overall dimension : 4000 m .
    Mean density : 375 kg/m³ .
    The excel chart gives the overal shape ( blue line ) at a height of (in this case 100 m ) above the surface .
    The red line gives the potential along the lenght axis .

    In this model the potential is minimal in the "neck" (-0.2 J/kg ) . This is the most stable place to stay .
    The potential is maximal above the smaller part of the comet (-0.125 J/kg )
    This means that , if the surface were smooth , a particle being on the head of the duck , would accelerate towards the neck of the comet .
    Arriving at the neck , the particle might have a speed of about 0.3 m/s ( Kin Energy = delta potential enegry ) , and overshoot the neck , and even round the body !
    Of course , the above results are only a model .
    The actual shape is more complicated .
    Attached Images Attached Images

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    Quote Originally Posted by ravens_cry View Post
    Given that escape velocity is less than a typical human walking speed, I'd say it'd be very, very slow. It would be more like rock climbing, I'd think, with constant tethering, clip on, clip off
    I think it would be more like suborbital hopping. It'll take somewhere on the order of less than an hour to use suborbital hopping to go halfway around the asteroid. That's a bit slow compared to walking, but fast compared to rock climbing.

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    Quote Originally Posted by parallaxicality View Post
    The comet's name is too long; they should just rename it Q-Bert.

    Actually, it looks more like a Peashooter from Plants Vs Zombies.
    Nah, looks like a muddy boot.

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    Quote Originally Posted by IsaacKuo View Post
    I think it would be more like suborbital hopping. It'll take somewhere on the order of less than an hour to use suborbital hopping to go halfway around the asteroid. That's a bit slow compared to walking, but fast compared to rock climbing.
    Yeah, but if you're doing that, you might as well be in orbit anyway. The tether method gets you up close and personal and lets you investigate things and collect samples as you go.

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    Quote Originally Posted by ravens_cry View Post
    Yeah, but if you're doing that, you might as well be in orbit anyway. The tether method gets you up close and personal and lets you investigate things and collect samples as you go.
    Each suborbital hop can be short and you don't go far above the surface. The speed is a slow walk, so you get a great view.

    The key would be to keep a light and gentle touch with your feet on each hop. You don't want to either speed up or slow down, you just want to push upward with the minimum force necessary to remain as parallel with the surface as you can.

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    Given how rough that surface looks from here, I'd worry about running into things too. Sure, you're going pretty darn slow, but, on the other hand, you're in a spacesuit. You really don't want that to break.

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    With the comet's low density ~0.1 g/cc, I'm half expecting the landing harpoon's 90 m/sec velocity to be enough to sink all 4 meters or so of cable. Even w barbs, it might not hold well.

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    Quote Originally Posted by Squink View Post
    I'm half expecting the landing harpoon's 90 m/sec velocity to be enough to sink all 4 meters or so of cable.
    Any chance this will split the comet in two?
    Everyone is entitled to his own opinion, but not his own facts.

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    Quote Originally Posted by Cougar View Post
    Any chance this will split the comet in two?
    Possibly smithereens!

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    Quote Originally Posted by Cougar View Post
    Any chance this will split the comet in two?
    .
    I don't think the comet will be split by this small 438 gram harpoon , but I think Philae will surely sense the harpoon firing at this velocity of 90m/s .
    The lander has a mass of about 100 kg . The harpoon has a mass of 1/200 of the total mass .
    Conservation of momentum gives that the lander will get an upward boost of about 90 m/s *1/200 = ca. 0.45 m/sec.
    I don't know when the harpoon will be fired ( before or just after first contact ) . If after first contact the probe will sense a force which is enough to sent it in orbit again I think . So the winch system should get its work done and get the lander back to the surface in this case.

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    Where does that density value come from, Squink?

    The comet's gravity will almost certainly be very nonspherical, and that will actually be helpful in estimating its mass distribution. Does it follow the observed shape? Is it concentrated in some spots?

    This is even though gravity data is effectively 2-dimensional rather than 3-dimensional. Here's a multipole expansion of the gravitational potential:



    V is the potential, Vlm is the expansion coefficients, r is the distance from the center, Y is a spherical-harmonic function (it varies by direction), l is the multipole number and m is multipole-component number, which varies between -l and l. The coefficients are given by



    Essentially a moment integral. In the integrand, rl * (spherical harmonic) can be turned into sums of products of l of the rectangular coordinates, where one may repeat each coordinate.

    V00 is related to the total mass.
    The V1m are related to the offset of the centroid from the coordinate origin, and one can force them to zero with appropriate coordinates.
    The V2m are related to the moments of inertia.

    From the looks of it, the comet looks like it is rotating around the axis with the highest moment of inertia. That is the most stable configuration. But observing its gravity will make it possible to test that hypothesis. It will be interesting to see how large a multipole number they can get gravity data for.

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