OK, maybe it's a long shot. But as long as we're passing through, we'll a look anyway.
That's the approach scientists are taking with a pair of sun-watching spacecraft called Stereo. The pair was launched in 2006. They've been trained on the sun, returning images scientists can combine into 3-D views of the star. Researchers now want the duo to hunt for debris that might have been left over from the construction of a hypothesized impactor that smacked Earth 4.5 billion years ago. You can read more about it here.
Scientists have dubbed the hypothesized Mars-sized planet Theia. And after the collision? Well, let's just say Theia was toast.
The glancing blow stripped the impactor of its outer layer, along with some of Earth's. Its iron core is thought to have melted and much if it merged with Earth's iron core. The outer material – mostly belonging to the impactor – gathered itself together and formed the moon.
That's the story scientists had pieced together up to 2005. And it explains a lot, astronomers say. Among other things, it explains why the moon has such a tiny iron core. It also explains why the the geochemistry of Earth and moon materials are so similar – they formed in the same region of the nebula of dust and gas that surrounded the early sun.
But where did the impactor actually come from?