How does Jupiter protect Earth from comets and such?

How does Jupiter "protect" Earth from comets and such?

I've commonly heard that Jupiter protects Earth by flinging comets out of an orbit that would otherwise leave them heading for Earth. I understand that Jupiter can throw comets and other objects out of their current orbit when they pass by and that Jupiter can add energy with a gravitational slingshot sending the comets far from the center of the solar system. However, why doesn't Jupiter fling comets into an orbit that will hit Earth as often as it flings them out of orbit? Can't it also gravitationally slow down the comets as well? Why does interaction with Jupiter give a net decrease in comets headed towards Earth rather than just evening out? Thanks!
 
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3 reasons I can see:
  • Some object hit Jupiter directly and vanish. The opposite process is not possible.
  • It can kick objects out of the solar system. The opposite process is very rare due to the low number of unbound asteroids entering the inner solar system.
  • For objects with orbits with small inclination (everything that was formed in the initial disk around our sun): an increase in inclination is more probable than a decrease, this reduced the impact risk.
 

Chronos

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Jupiter is extraordinarily massive compared to the inner planets. Imagine a binary star system. How many low mass objects will penetrate the Roche lobes between Jupiter and sol?
 
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Jupiter is a 300 times biggest than the Earth, so it's gravity is enormous, therefore it takes all the hits, it's our big sister :).
 

D H

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davenn

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Jupiter is a 300 times biggest than the Earth, so it's gravity is enormous, therefore it takes all the hits, it's our big sister :).
yup Jupiter is often referred to as a vacuum cleaner. collecting up much of the debris in the solar system

Dave
 

D H

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Except it doesn't. That's what a weak (by today's standard) simulation suggested almost two decades ago. Better simulations show that Jupiter is not such a nice guy after all. It might prevent the already rare impacts by long period comets, but it increases the impacts by asteroids and short period comets.
 
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why doesn't Jupiter fling comets into an orbit that will hit Earth as often as it flings them out of orbit?
Because there are more orbits that do not intersect earth than those that do.
 

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