From: yattering_nospam@web.de   
      
   Am 22.01.2017 um 15:11 schrieb Mike Dworetsky:   
   > For example, do you want the "enemy" to first stop the rotation (nearly all   
   asteroids spin with periods of the order 1 hr to 1 day) and then attach a   
   rocket motor to change the orbit so it collides?   
      
   If you can live with a longer time until impact, you do not need to stop its   
   rotation.  Just fire the engine periodically during 30-40% of the time   
   whenever it is heading into approximately the right direction.   
      
   > If you suppose that a main belt asteroid (mean orbital semimajor axis   
   2.7-2.8AU) has to change to an orbit with a 1AU perihelion, you could use a   
   transfer ellipse.  The duration of such a journey would probably be of the   
   order of a year or more.   
   > I'm not sure where you would find the right software.   
      
   In that case it is even doable without any software.  The asteroid's initial   
   kinetic energy (m=Mass, r_1=orbital radius) is   
   E_0 = m/2 Vo^2  (with Vo^2 = G Msun/r_1) and is to be decreased to   
   E_1 = m/2 (Vo-deltaV1)^2, with deltaV1 taken from [https://en.wi   
   ipedia.org/wiki/Hohmann_transfer_orbit#Calculation].   
   (This assumes instantaneous deceleration, so potential energy is unaffected.)    
   deltaV2 is not needed, since after impact, you're done. >:-)   
   Finding the best way to actually deliver this stupendous amount of energy   
   (E_1-E_0) to the asteroid is left as an exercise to the reader.   
      
   Jens.   
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