Expert: James Gort - 12/20/2011

Question
QUESTION: James,
I've heard many times about the high percentage of comets that have not hit the earth due to Jupiter's influence.  
Can you help me understand how Jupiter so influences them, when (I'm guessing) the majority of comets pass through Jupiter's orbit when Jupiter is not in the vicinity, say on the other side of the Sun?  
Thanks,
Paul

ANSWER: Hi Paul,

Very interesting question! But until recently, little research was done to back up the claim that Jupiter was a "protector" of earth. After all, Jupiter can also deflect comets to more closely approach earth!

Well, what you've heard many times may not be true. (Or at least there's a great deal of doubt). I'll refer you to one of the few studies on the subject. See http://arxiv.org/abs/0806.2795. Click on the "download pdf" button at the upper right side of the page.

Dr. Horner's research (as of 2009) concludes that the answer is not yet definitive, but Jupiter's role in protecting earth from asteroids (and comets) is highly suspect.

So, you see, the answer to your question is actually very complex. Jupiter sometimes helps by "sling-shoting" nearby comets to eject them from the solar system (if they happened to be close by, and not on the other side of the sun, as you point out!), but Jupiter can also disturb the cometary cloud and cause distant comets to head into the inner solar system. It's quite a complicated model that needs to be developed.

Until now, people have "guessed" at the answer. But the only answer you can trust is one arrived at through scientific study and (ideally) experiments.

Cheers,

Prof. James Gort



---------- FOLLOW-UP ----------

QUESTION: James,
Wow, thanks.  I have always intuitively questioned that assertion along the lines of your answer and the first few pages of the paper you cite.

The paper happens to mention the Late Heavy Bombardment, which reminds me of another question I'd like to ask.
My understanding is that the primary evidence of the bombardment is the frequency of craters on the moon dated to that time, and that the theory posits an abundance of incoming asteroids due to jostled orbits.  Is there other compelling evidence for the asteroid theory?  For instance, has it been considered that the bombardment might have been caused by a failed second proto-moon, perhaps having formed at the same time but opposite the moon and finally being broken apart?
Thanks,
Paul

Answer
Hi Paul,

The main compelling evidence for the asteroid theory seems to be the widespread bombardment at the same time (involving at least Mars, Mercury, and Vesta). But it seems the jury is still out, and the "broken up moon" theory (or even another planet) is still a possibility.

Harold Levison of the Southwest Research Institute in Boulder, Colorado has performed some simulations about possible mechanisms, including the failed moon scenario. See the first article from the New Scientist - http://abob.libs.uga.edu/bobk/ccc/cc040601.html. Other theories are advanced here - http://www.nature.com/nature/journal/v435/n7041/abs/nature03676.html and eo.ucar.edu/staff/dward/sao/dward617paper.pdf. And there are still other theories and models.

So you've asked another interesting (and still open) question. There's obviously still much we need to learn!

Cheers,

Prof. James Gort