The Earth is approximately 150 million kilometers from the Sun.  Traversing this distance would be equivalent to circumnavigating the globe 3,750 times.  In this vast gulf, we sit in an area known as the "habitable zone" of a star system, sandwiched between our two closest neighbors:  Venus corewards, Mars outwards.  The difference in orbits between our blue marble home and these fellows amounts to 40 million kilometers on one side, and 80 million on the other.  In ga...