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Rosetta Achieves Orbit Around Comet 54

Posted by timothy
from the what-hath-man-wrought dept.
schwit1 (797399) writes with an update on the European Space Agency's comet-exploring craft Rosetta: "Rosetta has successfully achieved orbit around Comet 67P/C-G and has transmitted its first close up images. More information here (1) and here (2) about the rendezvous and what science the mission scientists plan to do as they orbit the comet." As pointed out earlier by reader Taco Cowboy, this is the fruit of a 10-year mission. Reuters points out The mission performs several historical firsts, including the first time a spacecraft orbits a comet rather than just whizzing past to snap some fly-by pictures, and the first time a probe has landed on a comet. ... There is little flexibility in Rosetta's schedule this year. The comet is still hurtling toward the inner Solar System at almost 55,000 km per hour, and the closer it gets to the sun the more active it will become, emitting gases that can make it difficult to predict the trajectory of Rosetta and its probe.
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Rosetta Achieves Orbit Around Comet

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  • by Anonymous Coward

    Thank you!

  • by Vuojo (1547799) on Wednesday August 06, 2014 @02:58PM (#47616195)
    http://www.esa.int/spaceinimag... [esa.int] "This animation comprises 101 images acquired by the Navigation Camera on board ESA's Rosetta spacecraft as it approached comet 67P/C-G in August 2014. The first image was taken on 1 August at 11:07 UTC (12:07 CEST), at a distance of 832 km. The last image was taken 6 August at 06:07 UTC (08:07 CEST) at a distance of 110 km."
    • Ha, looks like a slightly more sophisticated versioin of Elite. Interesting to think that it's a glimpse into what future asteroid miners will witness as they approach ore bearing bodies.

  • by tulcod (1056476) on Wednesday August 06, 2014 @03:01PM (#47616223)

    Correct me if I'm wrong, but based on the little I learned from KSP, I don't think anything can reasonably get an orbit around a comet due to its lack of mass.

  • Sloppy reporting. (Score:5, Informative)

    by queazocotal (915608) on Wednesday August 06, 2014 @03:03PM (#47616243)

    It is not yet in orbit. (or rather - at the moment, propulsive manouvers are dominant - you can technically say you're in orbit if you jump off the ground, and not be wrong)
    Protip - orbits aren't triangular.
    https://www.youtube.com/watch?... [youtube.com] - is a two minute animation from ESA explaining the manoevers.

    10th sep - it begins its first orbit at 30km - and about 14 day period. After about half an orbit, on the 17th of Sep or so it is tilted 80 degrees and still remains in a 30km orbit.
    After a complete orbit, it then moves into 20km orbit, and around Oct 10, 10km.

  • "Rosetta and its probe"

    What in the world is that supposed to mean? Sounds dirty.

    • You are confusing this with 'probe the rosette' which is entirely different.
    • by mbone (558574)

      Rosetta carries within it a lander called Philae, which is intended to land on the comet in November. DLR is already picking out candidate landing sites - this video [youtube.com] shows the current set (each green dot is an error ellipse, which get stretched out over sloping terrain).

      Philae will harpon itself onto the surface, and one task will be to serve as one end of a low frequency radar system with Rosetta - it should be possible to image more-or-less the complete inside of the comet with this system.

      • It should be possible to image more-or-less the complete inside of the comet with this system.

        What is meant by "inside" in this context?

  • How close would the spacecraft have to be to the comet in order to achieve orbit? At just 2.5 miles long, I don't suspect its gravitational pull would be very significant.

    • Re:In Orbit? (Score:4, Informative)

      by Le Marteau (206396) on Wednesday August 06, 2014 @03:14PM (#47616341) Journal

      It will use thrusters to maintain orbit, because the gravity is indeed insufficient:

      "Rosetta will have to continue to fire its thrusters every few days to maintain a hyberbolic orbit at 100km above the rotating rock. "

      http://www.bbc.co.uk/news/scie... [bbc.co.uk]

      • by Anonymous Coward

        Hyperbolic orbit? I'm afraid the BBC got that wrong. The plan is to eventually get to as close to a stable orbit as you can about an irregularly shaped lump that's moving on its own, and that orbit won't look anything like a hyperbola. Yes, it will still have to correct often, but eventually the plan is to maintain an elliptical orbit for as long as possible. Try looking at http://www.esa.int/spaceinvideos/Videos/2014/01/Rosetta_s_orbit_around_the_comet for the pretty pictures.

        • by mbone (558574)

          Eventually, but not now.

          Right now the are spending a few 100 gm of propellant per day or so to maintain these triangular orbits, and each leg is hyperbolic (well, probably actually parabolic, but you get the idea).

      • by mbone (558574)

        This is something they are choosing to do. My understanding is this basically a safety thing. If the spacecraft went into a safe mode, in these slow flyby orbits it would keep moving slowly away from 67P, at maybe a km / hour or so, until the problem could be fixed. I heard that they didn't want to risk being in an unstable orbit, and maybe hitting the comet if they went into safe mode.

      • Re:In Orbit? (Score:5, Informative)

        by idji (984038) on Wednesday August 06, 2014 @05:33PM (#47617491)
        The comet 67P has a mass [wikipedia.org] of 3.14E12 kg
        Today the comet is 186,444,271 km from the Sun Where is Rosetta? [esa.int]
        Using F=GMm/R^2 [wikipedia.org], the Sun's gravity on Rosetta is equal to 67P's gravity on Rosetta at 700m from the center of Rosetta on 6 August 2014, which means that Rosetta will never really be completely within 67P's field. (At Perihelion on 13 Aug 2015, 67P's gravity field will be as strong as the Sun's only 250m from the centre) However, now that Rosetta is in the same orbit as 67P we can mostly disregard the Sun's gravity and the elliptical path that Rosetta and 67P now share as of today. (Earth's pull on Rosetta is at least a million times weaker than the Sun's pull - so forget any influence from the Earth's mass.)

        The "orbits" at 100km are called hyperbolic [wikipedia.org] because Rosetta is not trapped in 67P's gravity well since the gravity is so weak and because Rosetta is still moving FAST at 1 m/s. But this hyperbola is so weak it is effectively a straight line.
        Rosetta will turn 60 degrees after every 100 km of a hyperbolic path to make a triangular "orbit". This triangular path cannot be called an orbit because it is not a conic section [wikipedia.org], nor is the comet at a focal point of the conic section Kepler's First Law [wikipedia.org].

        These "straight"/"hyperbolic" paths of 100km and 50km are deliberately done for two reasons:
        -to calculate exactly the gravity field of the comet, because it is clearly not a uniform sphere. They will likely use radar&cameras to continuously measure the precise distance to the comet
        -to keep in front of the comet to avoid its coma and tail.
        After these maneuvers, Rosetta will go into a 30 km "orbit", so that the task of mapping 80% of the surface all happens from the same distance. This orbit is not natural and will be powered because a natural 30km orbit of 67P takes 26 days.

        Here's how to calculate the natural circular orbits for 67P (it won't be circular, because of the crazy shape, but close enough). Kepler's 3 Law [wikipedia.org] gives us
        T^2=4pi^2/GM*r^3. 4pi^2/GM=0.19 for this comet. G=6.67×1011 N(m/kg)2 [wikipedia.org]
        if r=30km=3e4m, the natural orbit would have a period of T=2.3e6 seconds=26.11 days
        If r=2.5km, the natural orbit would have a period of T=15 hours
        If r= 5km, the natural orbit would have a period of T=1.77 days
        If r= 100km, the natural orbit would have a period of 159 days So I could imagine that when Rosetta gets within 5km it is mostly using the natural orbit and hence saving fuel.
        • Glad to see that you jumped in on this: good description.

          Because the comet is so small, the gravity changes a lot with "altitude" from the surface. For a 2-km diameter sphere, say, then the difference in gravity between an altitude of 2-km and 6-km (i.e. between 4 and 8-km from the centre of the sphere) is a factor of 4. On the Earth, it barely changes at all between altitudes of 2 and 4-km, because this is a tiny change relative to the 6400-km radius of the Earth.

          So, yes, at 100-km and 50-km, we'll b

    • by mbone (558574)

      A 30 km orbit would take 8 - 25 days and have an orbital velocity of 0.1 - 0.3 m/sec, and would probably be stable.

  • Eleven years (Score:5, Informative)

    by mbone (558574) on Wednesday August 06, 2014 @04:15PM (#47616775)

    Rosetta was originally supposed to launch in January 2003, to comet 46P/Wirtanen, and was all ready to go when problems with the Arianne 5 forced a delay its launch. That meant finding a new target and a total redesign of the mission, leading to a launch in March, 2003.

    So, for the Rosetta team it has been the perils of Pauline since before the launch, and an 11 year mission to get to their comet.

  • by Andrio (2580551) on Wednesday August 06, 2014 @04:17PM (#47616795)

    They should rename come 67P to Rosetta Stone.

  • by RevWaldo (1186281) on Wednesday August 06, 2014 @05:00PM (#47617203)
    It was black as pitch on August the sixth
    Half a billion kliks from the sun
    I'd left Earth 'bout ten years ago
    And was ready to have some fun
    I'd buzzed past Mars and a coupla asteroids
    And saw this comet goin' roun' and roun'
    He says "Tin Can, this here's Rubber Duck
    And I'm about to put the hammer down"

    'Cause we got a little ol' convoy rockin' thru the night
    Yeah, we got a little ol' convoy, ain't she a beautiful sight?
    Come on and join our convoy, ain't nothin' gonna get in our way
    We gonna roll this truckin' convoy 'cross the invariable plane
    Convoyyyyy....


    .

Today's scientific question is: What in the world is electricity? And where does it go after it leaves the toaster? -- Dave Barry, "What is Electricity?"

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