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I often hear the hypothesis that Jupiter decreases the chances of an asteroid strike on the Earth, but my question is, why does it only decrease the chances ?

While Jupiter can deflect some asteroids that were originally going to hit the Earth, it can also deflect some asteroids towards the Earth. How can we be sure that Jupiter only decreases the chances of an asteroid impact on Earth ?

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  • $\begingroup$ Do you have references stating this hypothesis of Jupiter decreasing the chances of impact. I'd like to read more, AFAIK Jupiter is the reason we have earth crossing asteroids. $\endgroup$ – Astroynamicist Dec 31 '15 at 14:27
  • $\begingroup$ Have you heard of the "cosmic vacuum cleaner" metaphor? Jupiter is massive enough to attract many comets and asteroids towards it, sort of (not really) like how a vacuum cleaner sucks in dust and other particles. $\endgroup$ – HDE 226868 Dec 31 '15 at 14:34
  • $\begingroup$ @HDE226868 I thought 'cosmic methphor ' was for blackholes. $\endgroup$ – Astroynamicist Jan 1 '16 at 2:41
  • $\begingroup$ @Astroynamicist The same thing has been used for black holes, too. $\endgroup$ – HDE 226868 Jan 1 '16 at 19:56
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Polyphant is correct, it can operate either way. It's hypothesized that Jupiter in 2:1 resonance with Saturn was the cause of the late heavy bombardment. Source

The 2 largest objects in the solar system operating in resonance tended to stir everything up. Many small objects including Neptune and Uranus were thrown around, some into the sun, some in towards the inner solar-system where some crashed into other planets and some out of the solar system or into the Kuiper belt, but all that ended 3.8 billion years ago.

The dummies answer is that most of the stuff in the solar system that had orbits that were likely to crash into a planet have already done so, and Jupiter sped that process up.

The slightly longer answer follows:

For the time being, the asteroid belt is mostly stable, as are Jupiter's two Trojan regions, L4 and L5 and Jupiter's hildas are also stable. So that's basically why we're in a low bombardment period now. The inner solar-system objects are mostly in stable orbits where they aren't in position to crash into planets. To collide, orbits need to cross and mostly the inner solar system objects don't cross orbits with planets.

See here:

Computer simulations suggest that the original asteroid belt may have contained mass equivalent to the Earth. Primarily because of gravitational perturbations, most of the material was ejected from the belt a million years after its formation, leaving behind less than 0.1% of the original mass. Since then, the size distribution of the asteroid belt is believed to have remained relatively stable.

Similarly, Jupiter Trojan objects, L4 and L5 and Jupiter Hildas are in mostly stable orbits due to the high mass of Jupiter, so it's a two fold answer. First is that late heavy bombardment removed most of the inner solar system objects that were bombardment candidates, and 2nd, the huge mass of Jupiter compared to the inner planets and the significant relative distance between Jupiter and Mars and the small mass of Mars), together creates a healthy amount of relatively stable orbital regions where asteroids can stay in stable orbit and not be in position to crash into the 4 inner planets or be subject to n-body instability of multiple body orbital systems. (I can add various articles on the stability of Jupiter's L4 and L5 and Hildas, but they're not hard to google).

For Kuiper belt objects, I don't believe Jupiter has much of an effect pro or con, for earth impacts, though it blocks a few of those, but it should only be a small percentage, but I suspect Kuiper belt object collisions with inner planets are less frequent. For inner solar system objects (asteroid belt, L4, L5 & Hildas), that's where Jupiter significantly reduces impacts with inner planets, for most of the last 3.8 billion years.

A curious sidebar is that Jupiter may, at some point in the future, stir things up in the solar-system again and create another much smaller heavy bombardment period. This would depend on the Jupiter-Mercury resonance and if Jupiter was able to pull Mercury away from the sun, as some models suggest it might.

This is low probability and if it happens it won't happen for billions of years, but if Jupiter does cause Mercury to migrate away from the sun, Mercury passing through the asteroid belt, hildas and L4 or L5 would likely cause a significant increase in bombardment.

This article doesn't mention the asteroid factor, but Mercury passing through any of the asteroid rich regions of the inner solar-system would stir things up quite a bit and significantly increase the chance of bombardment.

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With regard to collisions with earth, or inner planet collisions in general, you're right that Jupiter has both a positive and negative effect on asteroid and comet trajectories. On balance though, the effect is mainly positive.

From earthsky.org:

Some astronomers believe that one reason Earth is habitable is that the gravity of Jupiter does help protect us from some comets. Long-period comets, in particular, enter the solar system from its outer reaches. Jupiter’s gravity is thought to sling most of these fast-moving ice balls out of the solar system before they can get close to Earth. So long-period comets are thought to strike Earth only on very long timescales of millions or tens of millions of years. Without Jupiter nearby, long-period comets would collide with our planet much more frequently.

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But Jupiter creates both good and bad conditions for earthly life. Consider that its powerful gravity prevented space rocks orbiting near it from coalescing into a planet, and that’s why our solar system today has an asteroid belt, consisting of hundreds of thousands of small flying chunks of debris... today, Jupiter’s gravity continues to affect the asteroids – only now it nudges some asteroids toward the sun, where they have the possibility of colliding with Earth.

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