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Here is a YouTube video from ESA demonstrating the orbit. It will be performing these small thruster (in the order of centimeters per second velocity change) bu

by exDM69 12y ago

Here is a YouTube video from ESA demonstrating the orbit. It will be performing these small thruster (in the order of centimeters per second velocity change) burns to stay ahead of the Comet while doing fine measurements of the acceleration caused by the gravity of the comet. When the mass and mass distribution has been established, the spacecraft will eventually settle in an orbit about 20 kilometers from the comet and finally down to 10 kilometers. http://youtu.be/Mf1zsACcXc4 http://youtu.be/Mf1zsACcXc4

3rd3 12y ago
It looks like it's only in a triangular orbit while approaching but it will eventually be in an elliptic orbit.
- exDM69 12y ago
  > It looks like it's only in a triangular orbit while approaching but it will eventually be in a elliptic orbit. Yes, exactly. But since the comet is highly irregular in shape and mass distribution (both are still unknown at this point!), the orbit won't be a perfect ellipse, not even close.
  - Throwaway823 12y ago
    Does the shape and mass distribution even matter? It's going to have a center of mass, so once that's located, shouldn't the orbit be close to a perfect ellipse?
    - exDM69 12y ago
      Yes, mass distribution and shape have a huge effect in the orbits. For example, the perturbations from Earth's equatorial bulge causes the perigee of an orbit to shift eastwards (see: Molniya orbit). The Moon's gravity field is very irregular and orbiting the Moon at low altitude is just black magic.
      - 3rd3 12y ago
        But it will be a stable orbit, right?
        dalke 12y ago
        The issues is that, technically, orbits aren't stable in the sense of being described perfectly as ellipse with a point mass at one focus. That approximation can describe an orbit for a while, but after time the position will differ from the predicted orbit, until at some point you have very little idea of where it will be. The time it takes for this to happen is called the Lyapunov time. For example, the Solar System is chaotic that way in the ~100 MY time scale. See http://en.wikipedia.org/wiki/Stability_of_the_Solar_System http://en.wikipedia.org/wiki/Stability_of_the_Solar_System . A Rosetta goal now is to figure out the comet's mass distribution, so that the orbital predictions can be better. The orbit won't be "stable" but it will be more "predictable." Even then, we expect Rosetta to release gas, dust, and water vapor which affects the orbit. I expect there will be orbital corrections over the next few years.
        exDM69 12y ago
        tl;dr: no, it won't be a very stable orbit for very long. One definition for the "stability" of an orbit is that if you give the orbiter a small nudge, it will not radically change its orbit. Lagrangian points L4 and L5 are not stable (with this definition) while L1, L2 and L3 are (only in theory, though, in practice perturbations matter a lot). Since the mass distribution is unknown, we can't quite predict if the orbit will be stable or not. The mass distribution may be very uneven and there may be spikes in the distribution (due to shape and porosity of the comet) that could cause the orbit to be unstable. The comet also rotates and makes things more difficult. Now the goal of the crew is to find an orbit that is reasonably stable and predictable given the knowledge of mass distribution they have. Nevertheless, the space craft will probably have to do thrust maneuvers to stay in orbit around the comet. And once the comet comes close enough to the sun and starts to lose mass to the vapor trail, all bets are off. That almost certainly implies some kind of unstability due to mass changes as well as the drag from the vapor trail. The comet may even break down to several pieces when it comes close to the sun.

by exDM69 12y ago

3rd3 12y ago

It looks like it's only in a triangular orbit while approaching but it will eventually be in an elliptic orbit.

exDM69 12y ago
> It looks like it's only in a triangular orbit while approaching but it will eventually be in a elliptic orbit. Yes, exactly. But since the comet is highly irregular in shape and mass distribution (both are still unknown at this point!), the orbit won't be a perfect ellipse, not even close.
- Throwaway823 12y ago
  Does the shape and mass distribution even matter? It's going to have a center of mass, so once that's located, shouldn't the orbit be close to a perfect ellipse?
  - exDM69 12y ago
    Yes, mass distribution and shape have a huge effect in the orbits. For example, the perturbations from Earth's equatorial bulge causes the perigee of an orbit to shift eastwards (see: Molniya orbit). The Moon's gravity field is very irregular and orbiting the Moon at low altitude is just black magic.
    - 3rd3 12y ago
      But it will be a stable orbit, right?
      - dalke 12y ago
        The issues is that, technically, orbits aren't stable in the sense of being described perfectly as ellipse with a point mass at one focus. That approximation can describe an orbit for a while, but after time the position will differ from the predicted orbit, until at some point you have very little idea of where it will be. The time it takes for this to happen is called the Lyapunov time. For example, the Solar System is chaotic that way in the ~100 MY time scale. See http://en.wikipedia.org/wiki/Stability_of_the_Solar_System http://en.wikipedia.org/wiki/Stability_of_the_Solar_System . A Rosetta goal now is to figure out the comet's mass distribution, so that the orbital predictions can be better. The orbit won't be "stable" but it will be more "predictable." Even then, we expect Rosetta to release gas, dust, and water vapor which affects the orbit. I expect there will be orbital corrections over the next few years.
      - exDM69 12y ago
        tl;dr: no, it won't be a very stable orbit for very long. One definition for the "stability" of an orbit is that if you give the orbiter a small nudge, it will not radically change its orbit. Lagrangian points L4 and L5 are not stable (with this definition) while L1, L2 and L3 are (only in theory, though, in practice perturbations matter a lot). Since the mass distribution is unknown, we can't quite predict if the orbit will be stable or not. The mass distribution may be very uneven and there may be spikes in the distribution (due to shape and porosity of the comet) that could cause the orbit to be unstable. The comet also rotates and makes things more difficult. Now the goal of the crew is to find an orbit that is reasonably stable and predictable given the knowledge of mass distribution they have. Nevertheless, the space craft will probably have to do thrust maneuvers to stay in orbit around the comet. And once the comet comes close enough to the sun and starts to lose mass to the vapor trail, all bets are off. That almost certainly implies some kind of unstability due to mass changes as well as the drag from the vapor trail. The comet may even break down to several pieces when it comes close to the sun.