NASA launches the twin STEREO probes in search of remnants of the collision that produced the Moon.
Some scientists believe that the Moon was created when the Earth was hit by a Mars sized object. Theia was conceieved by Edward Belbruno and Richard Gott if Princeton. The Theia Hypothesis is a subset of the Great Impact Theory of lunar formation. Belbruno and Gott believe that the impactor that hit the Earth 4.5 billion years ago came from one the Earth-Sun Langrangian points. Langrangian points are the five places where the gravitational pull of two bodies cancels each other out.
Most people are familiar with Jupiter's Trojan points and the asteroids that reside there. Earth and the Moon also share Lagrangian points. NASA's STEREO probes are going to the two most stable Lagrangian points belonging the Earth-Sun system, L4 and L5. Part of STEREO's mission is to look for planetoids leftover form the formation of Theia. STEREO is primarily a solar observatory mission, similar to the NASA/ESA stalwart SOHO. SOHO has been in orbit around the Sun-Earth L1 Langrangian point since 1996. The STEREO will be passing through the Sun-Earth L4 and L5 points, which are 60 degrees ahead and behind of Earth in its orbit, and they will continue on their paths past both sides of the Sun.