The two STEREO spacecraft orbit the Sun in orbits slightly different from the Earth. STEREO A orbits between the Earth and the Sun, while STEREO_B orbits beyond the Earth and the Sun. As a result, relative to the Earth, STEREO_A appears to move ahead of the Earth, while STEREO_B falls behind the Earth, in their motion around the Sun. In this configuration, the two spacecraft are now passing near the two stable Lagrange Points, L4 and L5, of the Earth_Sun system. The STEREO spacecraft are imaging these regions in the hopes of finding material that might have been left over from the original formation of the Solar System.