I understand that earth's orbit is not perfectly circular, alternating towards a slightly elliptical path every 100 thousand years or so. But I'm wondering if the other giant planets can alter our orbit through their immense gravitational fields...or are they too far away? Also, what factors would create the original shape of any planetary orbit. Why do some bodies have elliptical vs circular paths through space?
Planetary formation was probably chaotic. Over time those with stable orbits remained, while those without buggered off or went into the sun. In the past there were probably many planets all affecting each other, some being pulled into more elliptical orbits than others.
There are at least 3 eccentricity cycles, and probably more over longer periods of time. See the following link on the Milankovitch cycles. http://en.wikipedia.org/wiki/Milankovitch_cycles Earth's orbit will remain stable until a star or other large mass passes too close to the solar system. The odds of this happening are very low, but it will happen at some point in the future. It will probably happen long after the sun turns into a white dwarf.
How about gigantic asteroids striking the earth, like the doozy that wiped out the dinosaurs? Would that be enough of an impact to cause slight alterations in earth's orbit?
They are not really gigantic compared to the Earth. A six-mile wide asteroid, like the one may have caused the dinosaur extinction, has a mass of about 1 trillion tons or 1,000,000,000,000 tons. Coincidentally, this is about the same mass as Mt. Everest. By comparison, the earth has a mass of about 6,600,000,000,000,000,000,000 tons, or 6.6 million times more massive then the asteroid example. In the distant past, the Earth may have been hit by a planetoid with about 10 percent of Earth's original mass...see the following Wiki article... http://en.wikipedia.org/wiki/Giant_impact_hypothesis
All planetary and satellite orits are elliptical. It's just that some are more (or less) elliptical than others.
