Orbits of the two STEREO spacecraft

When the two STEREO spacecraft are launched, they are first placed in a highly eccentric orbit around Earth. In this Quicktime movie, the green dot represents Earth, the white dot stands for the Moon, and the letters "A" (red) and "B" (blue) stand for the STEREO "Ahead" and "Behind" spacecraft respectively. The yellow arrow represents the direction of the Sun, which changes as Earth moves along its orbit.

For the first few weeks, the two spacecraft are drifting slowly apart, but still stay fairly close to each other as they line up for their close encounter with the Moon one month into the mission. As they fly by, the Moon's gravity flings the two spacecraft out away from Earth. This is when the relatively small distance between the two spacecraft makes all the difference. The "Ahead" spacecraft is flung completely away from Earth, and becomes a satellite of the Sun, while the "Behind" spacecraft curves back to fly past the Moon a second time six weeks later, and gets flung out in the opposite direction.

This second Quicktime movie shows the two spacecraft once they've both settled into orbits around the Sun. As in the first movie, the green dot represents Earth, and the red "A" and blue "B" represent the STEREO "Ahead" and "Behind" spacecraft. The yellow dot represents the Sun. The orbits of Mercury, Venus, and Mars are also shown. The orbits of the two STEREO spacecraft are both more eccentric than Earth's, with the "Ahead" spacecraft orbiting slightly inside Earth's orbit, and the "Behind" spacecraft orbiting slightly outside. Because of these slight differences in the average distance from the Sun, the two spacecraft slowly drift away from Earth in opposite directions.

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