Originally Posted by
DevilHorse
Here are my off-hand thoughts:
Just as a background, when a meteor/meteorite/bolide/bomb blows up on a trajectory to earth, the center of mass of all of the pieces land exactly where the object would have landed had it been intact. So if you put an explosive on an asteroid coming at earth, it blows the asteroid up, but the center of mass of the asteroid pieces still heads right through earth. Not that this applies in this case, but center of mass is important in a multi-body system.
While it appears that the Didymos' primary object should be unaffected, because the moonlet is the target and will be perturbed, this is not necessarily true. No matter where DART hits on the Didymos binary, the center of mass will be affected and perturbed. Since the goal is that the moonlet's course around the Primary will only be perturbed, it isn't likely that the DART satellite will have much of an impulse anyway. Not sure if DART will be hitting the moonlet head-on or just a glancing blow to give the moonlet an oompf in a direction. The 6.6 km/s impact velocity cited in the article could be while the moonlet is going 50 km/s around the Primary, so the impulse could be relatively small; we don't have the information. It also might not be perpendicular to the velocity of the moonlet, which takes the edge off the impulse and the effect. Lots of variables here.
Based on the numbers provided, the Primary object can be estimated to be about 115 times the mass of the moonlet. If the moonlet (or Primary) is hit in the direction of the path of the orbit, there will be little change in the direction of the Didymos system, just the speed (and therefore the ellipse/orbit); if the hit is perpendicular to the present orbit of Ditimos, then there won't be a change in the speed, but there will be a change in the path/ellipse/orbit. I presume that the NASA scientists are attempting to cause as much change to the moonlet trajectory with as little to the Didymos combo orbital trajectory as possible. They seem to be looking at a very small observable affect. It has to be small enough to see, but not big enough to do (unforseen) harm. We have major trouble solving the 3 body problem in physics and this is a 4 body problem (2 bodies, 1 satellite, and the earth). The 2 body problem is completely solvable with math. The 3 body problem is successively solvable with computers.
Anyway, looks to be too small to make a difference in the Didymos system to hurl it toward earth.
Larry
DevilHorse
Actually, I don't buy this. If you put a rocket on the side of a moonlet, and set it off, it would deflect the path of the moonlet in the direction opposite the rocket thrust. If instead of a rocket, you have an explosive, the same thing would happen -- the moonlet's path would be diverted. It would be much as what would happen if you placed an explosive charge on the side of an object sitting on the ground -- the object would be moved away from where the explosion occurred.
Sage Grouse
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