From: alain245@videotron.ca   
      
   On Oct/30/2020 at 07:43, Dean Markley wrote :   
   > On Friday, October 30, 2020 at 4:24:42 AM UTC-4, JF Mezei wrote:   
   >> On 2020-10-29 15:52, Snidely wrote:   
   >>> Two defunct orbital masses seem to have not collided.   
   >> If 2 satellites, devoid of any fuel, one in retrograde and the other in   
   >> normal orbit, both on same plane and roughly same mass, ended up   
   >> colliding face to face, what would happen?   
   >>   
   >> So we end up with a flat pankake that has 0 speed and drops straight down?   
   >>   
   >> Behave as two tennis balls that bounce off each other, with the   
   >> retrograde now in normal orbit and the normal orbit SV now in retrograde?   
   >>   
   >> Nuclear fission explosion that creates a black hole and sucks all of the   
   >> universe?   
   >>   
   >>   
   >> When a car hits a cement wall at speed, it doesn't tend to explode into   
   >> a billion bits flying out. Right? Just curious on what the actual   
   >> behavious of a satellte would be. Is the energy level such that his is   
   >> no longer a mechanical collision, and it behaves very differently?   
   >>   
   >> In the case of more likely collision (a 50° satellite hitting a nearly   
   >> equatorial orbit for instance) would the satellite really spread debris   
   >> all over the place or would they remain more or less whole (with big   
   >> deformation where collision happened) and just see their   
   >> trajectory/orbit changed?   
   >   
   >   The Indian ASAT test ought to be a clear answer to your speculation.   
      
   Yes, the Indian (but also the Chinese and US) ASAT test gives a good   
   practical answer. But there are some interesting interactions going on   
   that, even if their practical implications are small, are interesting.   
   If two satellites, one prograde the other retrograde collide with a   
   collision speed of twice orbital velocity, at the point of impact, you   
   don't have metal bending like in a car accident. You have metal   
   vaporising, the result is more like an explosion than like a car   
   accident. That explains why the two don't just end up with zero velocity   
   and fall down. But it gets a little more complicated and more interesting.   
      
   As the ASAT test showed, some of the debris of the collision will end up   
   in an orbit with higher apogee than either the satellites had, meaning   
   that they end up with higher velocity. But the front of the satellite   
   obviously collides before the back of the satellite and it will start   
   being scattered in all direction while the back of the satellite hasn't   
   really had time to realize that it is in a collision. The front of the   
   satellite will scatter with some pieces having higher velocity than the   
   original satellites. So the back of the of the satellite can hit some   
   pieces of the front of the satellite with a collision speed even higher   
   than twice orbital velocity. This can give an even more explosive   
   collision, albeit with smaller pieces. This process can even be   
   multi-staged. The front of the satellite hits the front of the other   
   satellite, the middle of the satellite hits debris from the front of the   
   satellite with higher velocities and the the back of the satellite hits   
   the debris from the middle of the satellite with a yet again a higher   
   impact speed. Of course at each stage the debris hitting farther behind   
   parts of the satellite get smaller, but you can get some very small   
   pieces (individual atoms) completely ejected from Earth's gravity well   
   that way.   
      
   But all the pieces will end up in an orbit that passes by the collision   
   point (except tiny pieces which could be ejected from Earth's gravity   
   well). Essentially all pieces will have a perigee that will be lower   
   than the collision point. So, if the collision point is close to the   
   atmosphere, all pieces should be de-orbited in relatively short order.   
   But if the collision point is higher, you can have pieces with perigee   
   near the height of the collision point and apogee much higher. That can   
   be debris that will stay in orbit for a very long time.   
      
      
   Alain Fournier   
      
   --- SoupGate-Win32 v1.05   
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