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Thread: Mars.

  1. #1 Mars. 
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    Sending spacecraft to the planets must involve a great deal of top technical work backed by scientific knowledge,
    Some craft are sent to orbit other solar system bodies, particularly the planets. Others are even able to land on planets such as Mars.
    I believe I am right in saying the craft appear to cover tremendous distances.
    At the moment Mars is around 35 million miles away and this its closest approach!
    Given the ability to send spacecraft with a great deal of accuracy, to reach an object, such as a planet, at great distances, why are the craft not sent to the rendezvous, with the planet, when it is at its closest point.
    This might be seen as simpler and also involve a shorter travel time span.
    This does not seem to be the approach used so clearly there must be sound reasons to use the normal method of covering much larger distances.
    Anyone give an answer?


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  3. #2  
    Moderator Moderator Janus's Avatar
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    It has to do with orbital mechanics and the energies/velocities needed. The trajectory that takes the lowest launch velocity from Earth orbit and reaches Mars orbit starts on one side of the Sun and end 180 degrees on the other side of the Sun. This is half of an elliptical orbit around the Sun with it closest point to the Sun at Earth orbit and its furthest point at Mars orbit.
    Like this:


    A smaller launch velocity will have the object fall short of Mars orbit, and a higher one would end up further from the Sun than Mars.
    An example of a higher initial velocity trajectory would look like this.


    An analogy would be like standing at the bottom of a building and trying to throw a rock onto it roof. There is a minimum speed at which it leaves your hand where it will reach the roof. You can throw it harder, but then it goes higher than the roof and you've spend more energy than you needed to.

    At this point you might be thinking " So why do we bother with all that elliptical orbit stuff? Why don't we just launch the craft straight up away from the Sun at just the right speed to just reach Mars orbit? The catch is that the Earth itself has an orbital velocity of some 30 km/sec. The elliptical orbit trajectory uses this to it advantage so that we just need to add some extra velocity to it to achieve are required velocity. If we tried to launch into a straight up way from the Sun trajectory, we would lose this advantage and would even have to expend fuel in order to cancel out the Earth's orbital velocity.

    The other consideration is the relative velocity of the craft when it get to the destination. If the object is to land or orbit the planet rather than just a fly-by. You will have to match speed with it when you get there, so you want the difference between them to be small if possible.

    So let's imagine that you were to try a short distance "straight shot" to Mars when it was at its closest. First, you would have to give it enough initial velocity for it to reach a point much further than Mars orbit (or as above, use fuel to kill off the velocity the Earth has given it to put it into a "straight up" trajectory.) In either case,when it gets to Mars orbit, it would be moving at a pretty good clip relative to Mars, ( with the straight up approach it would be at rest with respect to the Sun, Mars would be moving at 24 km/sec) and you would have to burn fuel to match velocities.

    Now we use rockets to make our velocity changes. And the greater the velocity change you need to make, the more fuel you need to carry. But this is not a direct proportion. For example, a doubling of the total velocity change needed increases the fuel requirement by over a factor of 6. A factor of 3 in velocity increase requires 19 times the fuel. even a 10% increase in velocity change means doubling the fuel.

    So you can see why we prefer to use the slower, longer routes over the short direct ones and why sometimes we use circuitous routes that pass by planets other than the target ones in order to use them to sling shot around and pick up some added velocity for "free".


    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  4. #3  
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    Thanks for the reply.
    I'm going to need some time to, hopefully, be able to get a decent grasp of your answer!
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  5. #4  
    Forum Freshman NoCoPilot's Avatar
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    Both the Earth and Mars are moving targets. When we send a spacecraft to Mars or anywhere else, it's not on a straight line trajectory (like the diagram Janus posted).

    We use the forward momentum of the Earth to push the satellite is a slow arc away from the sun until it intersects with our target. Launch windows are determined by when the target will cross paths with a least-effort path away from Earth -- not when the target is closest to Earth. THAT would require many times more fuel than is practical.
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