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Home Forums General Discussion Rosetta Target Double Comet

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This topic contains 83 replies, has 4 voices, and was last updated by  Tej 11 months, 1 week ago.

Viewing 15 posts - 16 through 30 (of 84 total)
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  • #7207

    Brian Blake
    Participant
    Topics: 187
    Replies: 409

    I think the journey time was to allow for gravitational flipping to increase the speed to catch the comet so as to save having to use lots of fuel thereby increasing the payload weight. This is the technique they used to propel the Voyagers through the Solar System and the fuel is then used to manoeuvre the craft.

    #7208

    Brian Blake
    Participant
    Topics: 187
    Replies: 409

    The journey time was I believe to use gravity assist to sling shot the craft fast enough to catch up to the comet. The technique was used with the voyager crafts to get them to journey through the Solar system.  This was done to reduce the amount of fuel on board which would have increased the payload weight and taken up space.  The fuel on board is used to manoeuvre.http://physics.stackexchange.com/questions/61960/what-is-the-fastest-a-spacecraft-can-get-using-gravity-assist. sorry for repeat but had not completed post.

    • This reply was modified 3 years, 1 month ago by  Brian Blake. Reason: Mistake
    #7210

    Andy Sawers
    Moderator
    Topics: 114
    Replies: 588

    Thanks for that, Brian. Interesting post and link. I realised it was because of Voyager-style sling-shotting but it still seemed to be taking the ‘scenic’ route! In the simulation on the Rosetta website the probe seemed to come back almost to Earth which made me wonder if it wasn’t taking a route that could have been made a bit shorter. But what do I know. I’m amazed at how they managed to model the flight path so precisely, no matter how long it took.

    PS: I think there should be an ‘Edit’ button on your posts allowing you to go in and change/add stuff, but please let me know if not.

    #7211

    Mike Meynell
    Moderator
    Topics: 119
    Replies: 749

    I believe that Rosetta has used 4 gravity assists to get to the comet – 3 of the Earth and 1 of Mars. As Brian says, it was the only way of getting to the comet and, most importantly,  exactly matching its velocity at the rendezvous point without using masses of fuel.

    I still remember my text book on this from university – Danby’s “Fundamentals of Celestial Mechanics”.  I’m itching to write another equation 😉

    #7212

    Brian Blake
    Participant
    Topics: 187
    Replies: 409

    Andy there is but I think I left it to late.

    #7219

    Andy Sawers
    Moderator
    Topics: 114
    Replies: 588

    I’m itching to write another equation 😉

    Oh, go on, Mike. Treat yourself – and us!

    #7227

    Brian Blake
    Participant
    Topics: 187
    Replies: 409
    #7229

    Mike Meynell
    Moderator
    Topics: 119
    Replies: 749

    Great article from Stuart. I remember the live TV programme about the Giotto mission to Halley’s Comet very well. It certainly didn’t make for good TV… the build up was all about seeing these fantastic images of a comet’s nucleus for the first time… and all we saw was a few coloured blobs on a screen, which looked like an image from a ZX Spectrum computer!

    Some expectation management was certainly required!

    Of course, when the data was finally analysed, they got some wonderful images. Such as…

    Giotto image of Halley's Comet

    HMC 68 Image Composite. Comet Halley. 14 March 1986.

    … but note the date on the image… over 4 years later!

    It takes time to analyse all of this data and it’s certainly not conducive to a live TV programme.

    #7230

    Mike Meynell
    Moderator
    Topics: 119
    Replies: 749

    Oh, go on, Mike. Treat yourself – and us!

    If you insist 🙂

    You can look at this as a three-body problem e.g. the Sun, the planet and the satellite. Of course, there is no useful solution to the three-body problem, despite hundreds of years of research… however, you can create solutions if you reduce the scope of the problem.

    There is a solution known as the Tisserand criterion, which works because the mass of the satellite (or comet, or other small body) is tiny in comparison to the Sun and a planet.

    The satellite’s orbital energy is determined entirely by its major radius a. The approximate conserved energy of the satellite before and after its close approach to the planet is:

    E = – 1 / (2a)

    If we let ae and l be the major radius, eccentricity and inclination angle of the satellites orbit before the close encounter with the planet and a’e’ and l’ be the corresponding parameters after the encounter, then the Tisserand criterion states that:

    (1 / (2a) ) + sqrt ( a ( 1 – e^2)) cos l = (1 / (2a’) ) + sqrt ( a’ ( 1 – e’^2)) cos l’

    This means that a spacecraft can make use of a close encounter with a moving planet to increase (or decrease) its orbital major radius a and, hence, to increase (or decrease) its total orbital energy.

    😉

    #7231

    Andy Sawers
    Moderator
    Topics: 114
    Replies: 588

    (1 / (2a) ) + sqrt ( a ( 1 – e^2)) cos l = (1 / (2a’) ) + sqrt ( a’ ( 1 – e’^2)) cos l’

    Mike, for Christmas we’ll get you the mathematical formula plug-in.

    I remember the live TV programme about the Giotto mission to Halley’s Comet very well. It certainly didn’t make for good TV

    Crumbs, I remember that, too – and the feeling of ‘Is that it???’ I remember thinking even Patrick Moore seemed to be pretending to be more excited than he really was. Though he would have realised the value and significance of what we were seeing, he would have had to admit it was underwhelming as television.

    #7232

    Mike Meynell
    Moderator
    Topics: 119
    Replies: 749

    Mike, for Christmas we’ll get you the mathematical formula plug-in.

    Difficult to follow, isn’t it?

    Try this…

    Formula

    #7259

    Tej
    Participant
    Topics: 36
    Replies: 588

    Mike on the formula rampage again 🙂

     

    Andy, I was trying to think up an analogy for sling shot acceleration going around the same mass more than once.  I wonder if the track and field event, Hammer Throw might help as one?

     

     

    #7260

    Tej
    Participant
    Topics: 36
    Replies: 588

    Only just catching up with the amazing Rosetta mission.  I’m a little bit shocked though.  Is that really all dirty ice?  It looks more like a piece of  Mars rock .   I am still yet to read up more on it, though.

     

    I couldnt help myself but upon seeing the close up near hi res image of of the comet, the first words that came out of my mouth was a famous Arnie line from Predator….I cant say the line here though, but if any of you saw that film, I think you know what I’m talking about…I am sure Brian does!

    Seriously, that is one bizarre object.  Any theories yet on how the hell it got that way?

    #7419

    Brian Blake
    Participant
    Topics: 187
    Replies: 409
    #7543

    Brian Blake
    Participant
    Topics: 187
    Replies: 409
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