4 ms·

I assume that it isn't actually orbiting the comet, but instead is just adjusting to follow the same orbit as the comet, right?

by personZ 12y ago

FranOntanaya 12y ago
That's correct. And because the orbit forces it to slowly overtake the comet, it needs to slow down periodically to stay at 100km of it.
- exDM69 12y ago
  > That's correct. And because the orbit forces it to slowly overtake the comet, it needs to slow down periodically to stay at 100km of it. No, not quite. Remember, we're talking about a three body problem here so any intuition from two body dynamics doesn't really hold. The spacecraft travels 100km closer to the sun than the comet, but that's a very small fraction of the total distance. When you factor in the small acceleration from the comet body, we can no longer think of the orbit as being a slightly smaller orbit with a shorter period (and thus overtaking the comet). My best understanding is that the triangular "orbit" is to measure the mass and gravitational properties of the comet in order to orbit as well as scientific goals.
arethuza 12y ago
It's being described as a "triangular orbit" :-) "Rosetta will, follow, at least for now, a three-legged triangular orbit that requires a small thruster burn at each apex. The legs are about 100 km long and it will take Rosetta between three and four days to complete each one." http://blogs.esa.int/rosetta/2014/08/04/whats-happening-in-rosetta-mission-control-today/ http://blogs.esa.int/rosetta/2014/08/04/whats-happening-in-r...
- personZ 12y ago
  What a fascinating, seeming convoluted (but clearly intentional and purposeful), flight path. Fascinating stuff, and it's amazing it has that much residual thrust available.
  - Sharlin 12y ago
    AFAIK they're only doing it for a while to measure 67P's mass and density distribution, so they can calculate a "real" orbit for Rosetta later (but it will still require regular adjustments due to the highly irregular shape of the comet).
  - Gravityloss 12y ago
    This is a good illustration. https://twitter.com/rowhoop/status/496920940539052032/photo/1 https://twitter.com/rowhoop/status/496920940539052032/photo/...
  - exDM69 12y ago
    > and it's amazing it has that much residual thrust available. We're not talking about propellant residue, that was very well planned ahead. The spacecraft carried about 1.5 tons of propellant at launch. That gives the spacecraft a delta-v budget that is several kilometers per second. The next thruster burns (on the triangular "hyperbolic orbits") will be tiny, in the order of centimeters per second.
- 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.
        12y ago
Luc 12y ago
It's moving under power in a trajectory around it at the moment. I would call that orbiting, even if it's not due to the gravitational pull of the comet. http://www.newscientist.com/article/dn25995-cometmapping-rosetta-spacecraft-prepares-for-duty.html http://www.newscientist.com/article/dn25995-cometmapping-ros...

I assume that it isn't actually orbiting the comet, but instead is just adjusting to follow the same orbit as the comet, right?

by personZ 12y ago

I assume that it isn't actually orbiting the comet, but instead is just adjusting to follow the same orbit as the comet, right?

FranOntanaya 12y ago

That's correct. And because the orbit forces it to slowly overtake the comet, it needs to slow down periodically to stay at 100km of it.

exDM69 12y ago
> That's correct. And because the orbit forces it to slowly overtake the comet, it needs to slow down periodically to stay at 100km of it. No, not quite. Remember, we're talking about a three body problem here so any intuition from two body dynamics doesn't really hold. The spacecraft travels 100km closer to the sun than the comet, but that's a very small fraction of the total distance. When you factor in the small acceleration from the comet body, we can no longer think of the orbit as being a slightly smaller orbit with a shorter period (and thus overtaking the comet). My best understanding is that the triangular "orbit" is to measure the mass and gravitational properties of the comet in order to orbit as well as scientific goals.

arethuza 12y ago

It's being described as a "triangular orbit" :-) "Rosetta will, follow, at least for now, a three-legged triangular orbit that requires a small thruster burn at each apex. The legs are about 100 km long and it will take Rosetta between three and four days to complete each one." http://blogs.esa.int/rosetta/2014/08/04/whats-happening-in-rosetta-mission-control-today/ http://blogs.esa.int/rosetta/2014/08/04/whats-happening-in-r...

personZ 12y ago
What a fascinating, seeming convoluted (but clearly intentional and purposeful), flight path. Fascinating stuff, and it's amazing it has that much residual thrust available.
- Sharlin 12y ago
  AFAIK they're only doing it for a while to measure 67P's mass and density distribution, so they can calculate a "real" orbit for Rosetta later (but it will still require regular adjustments due to the highly irregular shape of the comet).
- Gravityloss 12y ago
  This is a good illustration. https://twitter.com/rowhoop/status/496920940539052032/photo/1 https://twitter.com/rowhoop/status/496920940539052032/photo/...
- exDM69 12y ago
  > and it's amazing it has that much residual thrust available. We're not talking about propellant residue, that was very well planned ahead. The spacecraft carried about 1.5 tons of propellant at launch. That gives the spacecraft a delta-v budget that is several kilometers per second. The next thruster burns (on the triangular "hyperbolic orbits") will be tiny, in the order of centimeters per second.
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.
        12y ago

Luc 12y ago

It's moving under power in a trajectory around it at the moment. I would call that orbiting, even if it's not due to the gravitational pull of the comet. http://www.newscientist.com/article/dn25995-cometmapping-rosetta-spacecraft-prepares-for-duty.html http://www.newscientist.com/article/dn25995-cometmapping-ros...