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Post by twinstead on Aug 4, 2005 15:39:01 GMT -4
Basically because we're talking about orbits here, the Earth's velocity is added, but it's just not relevant. If a spacecraft stays very close to Earth -- say, in orbit around it -- then it is essentially drawn along with Earth in its path around the sun. Move that spacecraft far enough away from Earth so that Earth's gravity isn't a very strong force, then that spacecraft begins to pursue its own orbit around the sun. In order to do that, it has to basically move around the sun at the same speed as the Earth does. So just to stay at the same distance from the sun as the Earth, a spacecraft has to go at the same speed as the Earth. In order to advance outward to a higher orbit, such as Saturn's, the spacecraft must add energy to its trajectory. Because of the way orbital mechanics works, this actually results in a lower velocity (eventually), but in a higher altitude. This is how transfers between orbits are accomplished. Traveling between planets in the solar system is not like driving to Vegas. It's not a matter of pointing the vehicle in the direction of some stationary target and stepping on the gas, and the harder you step the faster you get there. There are gravitational forces at work in interplanetary trajectories that laymen almost always fail intuitively to understand. What is amazing to me is not that all this is done, but that it was figured out in the first place, much of it before space travel.
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Post by wildbill on Aug 4, 2005 15:39:30 GMT -4
> www2.jpl.nasa.gov/basics/bsf4-1.html > So the spacecraft lifts off the launch pad, rises above Earth's atmosphere, and uses its rocket to accelerate in the direction of Earth's revolution around the sun >>> This demonstrates that my statement is right: you must add velocity of the Earth to spacecraft's velocity.
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Post by wildbill on Aug 4, 2005 15:49:13 GMT -4
Professor JayUtah > So just to stay at the same distance from the sun as the Earth, a spacecraft has to go at the same speed as the Earth.
WRONG, TOTALLY WRONG The probe has a smallest mass. To stay at the same distance from the sun as the Earth the probe must go slower than the Earth.
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Post by blackpudding on Aug 4, 2005 15:55:38 GMT -4
Professor JayUtah > So just to stay at the same distance from the sun as the Earth, a spacecraft has to go at the same speed as the Earth. WRONG, TOTALLY WRONG The probe has a smallest mass. To stay at the same distance from the sun as the Earth the probe must go slower than the Earth. WRONG, TOTALLY WRONG the probe has smaller mass but it is in free fall so mass is irrelevant.
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Post by wildbill on Aug 4, 2005 16:04:59 GMT -4
> it is in free fall
>>> WRONG >>> There is no FREE FALL in space
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Post by LunarOrbit on Aug 4, 2005 16:05:44 GMT -4
Mass is not irrelevant. If something has mass it is affected by gravity. Even massless things like light are affected by gravity if it is strong enough.
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Post by LunarOrbit on Aug 4, 2005 16:06:42 GMT -4
There is no FREE FALL in space Huh? Do you even know what free fall is?
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Post by twinstead on Aug 4, 2005 16:14:39 GMT -4
Mass is not irrelevant. If something has mass it is affected by gravity. Even massless things like light are affected by gravity if it is strong enough. I assume black holes are an example of that?
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Post by LunarOrbit on Aug 4, 2005 16:18:22 GMT -4
I'm no Stephen Hawking, but I believe so.
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Post by wildbill on Aug 4, 2005 16:22:08 GMT -4
> I assume black holes are an example of that?
>>> Black holes at the moment are only in the science fiction
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Post by twinstead on Aug 4, 2005 16:25:30 GMT -4
> I assume black holes are an example of that? >>> Black holes at the moment are only in the science fiction LOL Dude, you're the last person whom I would trust concerning issues of astronomy; you are basically calling people who REALLY know astronomy idiots in this thread. The irony is not lost on the rest of us.
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Post by wildbill on Aug 4, 2005 16:30:21 GMT -4
> Dude, you're the last person whom I would trust concerning issues of astronomy; you are basically calling people who REALLY know astronomy idiots in this thread.
Who doesn't understand is not an idiot. Whose fault is it? It's a genetic problem.
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Post by sts60 on Aug 4, 2005 16:49:37 GMT -4
> So the spacecraft lifts off the launch pad, rises above Earth's atmosphere, and uses its rocket to accelerate in the direction of Earth's revolution around the sun That's part of it. To go into Earth orbit, it's easiest to launch in the same direction as the Earth is rotating. You're in orbit about the Sun as well, though. If you accelerate more or less in line with the Earth's instantaneous velocity about the Sun, enough to escape Earth's influence, you'll increase the "high" point of your orbit about the Sun. If you go the other way, you'll lower the decrease the "low" point of your orbit about the Sun.
Galileo and Cassini both did the latter in order to head in, towards Venus, where they borrowed a bit of that planets momentum, boosting a their own speed relative to the Sun and enabling them to get out to Jupiter and Saturn in reasonable times.
>>> This demonstrates that my statement is right: you must add velocity of the Earth to spacecraft's velocity.
Yes. Everyone familiar with space travel knows that, especially the interplanetary mission planners at NASA. Of course,if you're trying to get to Venus, the orbital speed of the Earth works against you.
Professor JayUtah > So just to stay at the same distance from the sun as the Earth, a spacecraft has to go at the same speed as the Earth.
WRONG, TOTALLY WRONG
No. If you're going faster than the Earth, you'll move further out. If you're moving slower than the Earth, you'll drop closer in to the Sun. It's basic physics. You can find this in any high-school physics textbook. Why don't you read the links I showed you?
The probe has a smallest mass. To stay at the same distance from the sun as the Earth the probe must go slower than the Earth.
No. That's completely wrong. It's the most basic physics. Pay attention, now:
The distance of an object orbiting the Sun does not depend on the objects mass if the object is considerably smaller than the Sun (the Earth to a very good approximation; certainly any spacecraft). For a circular orbit, the relationship is r = GM/v^2 where G = the gravitational constant = 6.67*10^11 N m^2 / kg^2 M = the mass of the Sun, 2 * 10^30 kg v = the objects orbital speed about the Sun
If a "coasting" space probe is moving in the same nearly-circular path as the Earth, it must be moving at the same speed as the Earth. If it slows down, it will enter an elliptical orbit whose "high point" is its original distance from the Sun and whose "low point" is closer to the Sun.
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Post by sts60 on Aug 4, 2005 16:53:54 GMT -4
Mass is not irrelevant. If something has mass it is affected by gravity. Even massless things like light are affected by gravity if it is strong enough. I assume black holes are an example of that? Our Sun is an example of that. The viewing of a star which was physically "behind" the Sun, during a total eclipse, provided direct confirmation of this (a consequence of general relativity).
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Post by JayUtah on Aug 4, 2005 17:00:24 GMT -4
WRONG, TOTALLY WRONG The probe has a smallest mass. To stay at the same distance from the sun as the Earth the probe must go slower than the Earth.
Thank you for proving that you have absolutely no understanding of orbital mechanics. This is the most common error made by laymen. Mass is not relevant to the relationship between orbital altitude and orbital velocity. For a given altitude above a given primary, all objects orbit at the same speed regardless of mass.
Why? Because an object of greater mass has greater momentum. That greater momentum translates to a greater desire to pursue a straight line path -- more resistance to change. However, that greater mass also produces a greater gravitational attraction and thus a greater force bending the object's path into a curved orbit.
Those two effects cancel each other out. That is how mass is rendered irrelevant.
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