People often picture the solar system as a cosmic clockwork. And why not? With few exceptions, the planets orbit the Sun in near-perfect circles, and the moons orbit their planets in the same manner, all moving with the famous regularity of the heavens.
Figure 1. Spaceflight typically requires the expenditure of considerable quantities of propellant. But after it blasted off from Earth, the Genesis probe was able to travel 1.5 million kilometers toward the Sun (green portion of the trajectory), which is some four times farther than the Moon’s orbit (gray circle). Genesis then orbited the Earth's L1 Lagrange point (white cross in foreground) collecting particles of the solar wind for two and a half years before traveling millions of kilometers along a circuitous path (blue) that looped by another Lagrange point, L2 (second white cross), before returning to Earth in September 2004. Amazingly, Genesis completed this vast trek using hardly any fuel. The probe did so by following one of the many possible low-energy paths through the solar system, routes that have long served as natural conduits between planets for asteroids and comets. Some of these conduits lead to collision with Earth, as the Genesis probe's path did by design.
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