# An odd question on orbital dynamics

by parsec
Tags: dynamics, orbital
 P: 101 Is there any kind of celestial event that could occur that could completely lock the earth's rotation such that it is constantly facing the sun, in the same way the moon is tidally locked to the earth? What would it take? Could it be caused by the passage or entrance of a new gravitational body into the solar system?
 Emeritus Sci Advisor PF Gold P: 16,462 Yes. It's called time. Give it a few tens or hundreds of billion years, and it will happen on its own.
 Mentor P: 12,053 The earth will fall into the (expanding) sun long before that would happen. Impacts of asteroids, fly-bys of very massive objects or stuff launched into space can modify the rotation of earth. You need a lot of mass for any significant change, however.
 P: 101 An odd question on orbital dynamics Okay, let me rephrase. Is there any mass, position, linear and rotational velocity configuration of a foreign planet or body entering the solar system that could reduce that timescale for the earth's tidal locking to something in the order of days, weeks or months? Sorry if this is an absurd question, I don't know much about this sort of thing.
 P: 101 Is it possible without impact?
P: 101
 Quote by mfb fly-bys of very massive objects
Ah yes, this sort of thing. I wouldn't know where to begin calculating what kind of regime is plausible, but I'm guessing the nearby passage of a planet with mass greater than the earth could cause such a dramatic perturbation of the earth's rotation?

Would the more distant passage of a star be equally plausible?
Mentor
P: 12,053
 Quote by parsec Is there any mass, position, linear and rotational velocity configuration of a foreign planet or body entering the solar system that could reduce that timescale for the earth's tidal locking to something in the order of days, weeks or months?
No. Not without ripping earth apart. You would need many fly-bys or a very close orbit (which is tricky to achieve with a massive object) - in any case, I think the timescale would be at least millenia or more, but I did not check the numbers. It would produce serious, probably periodic floods everywhere apart from some mountains and increase volcano activity a lot.

More distance and more mass can increase the timescale of the influence a bit, but if the numbers get too large you ruin the orbit of earth as well (long before you see a slowed rotation).
 Emeritus Sci Advisor PF Gold P: 16,462 It can't be done with a flyby. You need a net torque with no net force.
 Mentor P: 12,053 A flyby would induce tides, which give a torque (similar to the earth/moon system). You get a net force and change the orbit, too, of course. As you need multiple flybys, those could cancel in the long run. Not very realistic, but it might be possible to do that in a planned way.
 Emeritus Sci Advisor PF Gold P: 16,462 Maybe I should have said "you can't do it with one flyby". The most realistic way to handle multiple flybys is an orbit.

 Related Discussions Astronomy & Astrophysics 7 Biology, Chemistry & Other Homework 2 Advanced Physics Homework 2 Introductory Physics Homework 1 Introductory Physics Homework 1