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Space Science

New Horizons Probe's Images of Jupiter 86

SeaDour writes "The Pluto-bound New Horizons space probe, launched a little over a year ago, recently succeeded in passing through a narrow navigational keyhole by Jupiter. Using the gas giant's tremendous gravity, the craft now has a significant boost toward its final destination, shaving three years off the time it would otherwise spend en-route. As it passed through the Jovian system, the probe took some fantastic images of the neighborhood, including detailed observations of erupting volcanoes on Io, time-lapse photography of Jupiter's tumultuous atmosphere, and the faint ring system that was first discovered in Voyager photography. These new images prove the capabilities of the small probe, which is set to reach Pluto in 2015."
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New Horizons Probe's Images of Jupiter

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  • by Anonymous Coward on Monday March 12, 2007 @09:38PM (#18326199)
    You don't know what you are talking about, and the Wikipedia article is screwed up as well. The Wikipedia article claims that no momentum can be gained passing by a stationary massive object. I'll give a simple counterexample. Drill a bore straight through the diameter of the Earth. If you pull a vacuum in this theoretical bore and drop a spacecraft into it, it will exhibit simple harmonic motion. What is interesting is that the object will have the greatest absolute momentum at the center of the Earth. Now take a rocket and fire only towards the center of the planet when you pass outwards from the center--not inwards. If you do this long enough you will reach escape velocity and say bye bye to the planet. Do your math calculating the momentum and you will find that you have got more absolute momentum that just from the rocket alone.

    Why is this so? First, momentum is conserved, the extra momentum is in the Earth in the opposite direction. Second, you were able to amplify your momentum because you minimized the time that your spacecraft felt the strongest gravitational fields pulling it back towards the planet when you were heading outwards (by using your rocket). But you maximized the time that your spacecraft felt the strongest gravitational fields pulling you into the planet when you were heading inwards coasting (because your rockets only fire when you are heading outwards). Strictly speaking the Earth does not give a great example of how this would work since the highest gravity of the non-uniform density Earth is about 1000 miles under the surface (and is 0 at the core due to Gauss' Law). But it may be more obvious if you arbitrarily move the 'bore' or path of the spacecraft so that its closest approach to the Earth is 2000 km above the surface. In this case it is obvious that you would coast until you got to the closest approach to the Earth and then fire your rockets for a few minutes to minimize your time in the highest gravitational field.

    This is sort of what a flyby could do if it didn't use its rockets and the planet had a high orbital velocity. Due to the orbital velocity alone you could target your spacecraft so that the planet recedes minimizing your gravitational interaction on the flip side (which requires you to fly by close enough to change paths a little bit since no path change would not do anything even with a high speed massive object). Of course using rockets and this method together are better.
  • by radtea ( 464814 ) on Monday March 12, 2007 @10:39PM (#18326737)
    This is for the same reason as you give: it spends less time being slowed on the way out because its speed was higher than when it was falling inward.

    Nope. Think about it in terms of potentials and you'll see why this is not correct. The rocket's loss in gravitational potential energy coming out of the hole is exactly equal to the gain it got going in. It doesn't matter how fast it was moving at the start: the potential changes are determined solely by the source configuration because gravity is a velocity-independent force.

    Remember: Newtonian energy change is equal to the integral of force over distance, not time.

    The GP is correct in that mass discharged by a rocket deep in a gravity well has an added benefit. In terms of energetics you can think of this as being due to the gain in energy you get as the expended fuel falls into the well that you don't have to pay back when the spacecraft comes out of it.

    But there it is also the case that the orbital velocity of the planet generally gives a larger effect, although of course it would be misleading and silly to claim that this is not due to the planet's gravity, because what else would be causing the interaction between the planet and the spacecraft? It is true that if the planet had no orbital velocity nothing very interesting would happen, but the same would be true if it had no gravity. Not that either condition is likely to pertain to real planets.

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