Marc V you are showing every one on this forum that you can't grasp the "mind buggling" problem of symetry between HC and GC and their being geometrically interchangeable. I flatter myself that I have some inkling of what you are referring to. I understand that in a system of moving bodies such as the solar system, it is possible, on the modelling board, to stick a pin in any one of the bodies in that system so as to immobilise it and to observe the motions of all the other bodies relative to the pinned object. The principle is well illustrated by the system of planetary gears (the name is no accident) used in automatic transmission gear boxes. Some time in the past -- on this forum if I remember correctly -- it was stated that an observer on any body in the system would, on pure observation and using the same reasoning as most here use to vindicate their beliefs, come to the conclusion that he was on a stationary body at the centre of the universe. They cannot all be right. I'll say it again -- they cannot all be correct and there is no physical reason which gives a bias to the Earth based observer. Now while "geometrically interchangeable" may have utility for explaining something, this is not the same thing as physical interchangeability. Put very simply, it takes more energy to accelerate a large mass to a given velocity than to accelerate a small mass to the same velocity. Again, it takes more energy to accelerate a small mass to a modest velocity than to accelerate the same mass to a great velocity. Space missions will not succeed if the assumptions about what is moving, in which direction and how fast are incorrect. A further example -- if you immobilise the tub in a spin dryer and turn it on, not only are you likely to wreck the laundry, but your clothes will not have the water removed even if you do manage to get the machine to spin around the tub. Now if you wish to convince me that I am wrong, you'll need to demonstrate that my example, including rough figures, is wrong. (I've reproduced it below). Paul D Here is my problem Marc. HC says that Earth revolves around the Sun at a distance of 149.6 * 10^6 km; at a velocity of 29.78 km/s; in a CCW direction (looking from the North); Mars revolves around the Sun at a distance of 227.92 * 10^6 km; at a velocity of 24.13 km/s; in a CCW direction (looking from the North); and has a solid thoretical framework to explain why this is so. The difference is thus -5.65 km/s in velocity and 78.32 * 10^6 km between concentric, and essentially circular, orbits. On the other hand, GS says the Earth is stationary; Mars revolves (effectively) around the Earth in a CW direction (looking from the North); at distances varying from 78.32 * 10^6 km and 377.52 * 10^6 km; at velocities varying from (very roughly) 5.71 * 10^3 km/s and 27.53 * 10^3 km/s; and has no explanation for these assumptions. In the GS scenario, assuming we take the closest approach for a rendezvous, Mars is travelling in the opposite direction from that which the planners of the mission relied upon, at a velocity difference of 5.72 * 10^3 km/s + 5.65 km/s = 5.73 * 10^3 km/s. This is roughly Earth escape velocity -- a large percentage of the maximum capability of our best launch vehicles -- about 1000 times the velocity difference expected and in the opposite direction! Should the GS position be the truth, the chances of a successful rendezvous, if the mission is based on HC data, are zilch, zippo, nil. ____________________________________________________________________________________ Feel safe with award winning spam protection on Yahoo!7 Mail. http://mail.yahoo.com.au