re: Gravity Assist?

From: Geoffrey E Forden (forden@MIT.EDU)
Date: Fri Aug 26 2005 - 21:06:28 EDT

  • Next message: Ted Molczan: "TJM obs of 2005 Aug 27 UTC"

    Let me see if I can explain this.  The secret is thinking about it in the right
    frame.  In this case, that is the center of mass of the planet and spacecraft. 
    in this frame, the planet appears to be moving very slowly in from (say) the
    left and the spacecraft is coming in very fast from the right, accelerating as
    they approach each other (again, the planet hardly changes its speed but the
    satellite is accelerated quite a bit).  after they pass each other, they start
    to both decelerate until, when they are far away, they have the same speed as
    when they both started just as you expect.  Now, however, switch your view to
    the orginal frame.  The speed of planet has hardly changed by its encounter
    with the spacecraft but its moment has decreased.  The decrease in the planet
    moment's must be balanced by an equal increase in the spacecraft's momentum
    (which is mass times velocity).  Since the relatively light weight spacecraft
    has gained the momentum that the massive planet lost, its change in velocity
    must be huge.  Incidently, you can slow a spacecraft down by approaching it
    from the direction that the planet is moving in.  NASA used this reverse sling
    shot when it sent a space craft to venus (or was it mercury?) in a very long
    and complicated trajectory.  I hope that helps.
    Best, Geoff
    Quoting Jeff Umbarger <>:
    > Hey All,
    >     I know that this is somewhat off thread but can
    > someone explain how a planet can "slingshot" a
    > satellite that has come near it at a *higher* velocity
    > than before the satellite reached the planet? Here is
    > where I struggle with this idea: if satellite is
    > approaching a planet, then that planets gravity should
    > begin to accelerate that satellite towards it. But as
    > the satellite *departs* the planet after making
    > closest approach that same gravity should decelerate
    > the satellite by just as much - with no net positive
    > gain, right???
    >     Regards,
    >          Jeff Umbarger
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