This article on ScienceDaily.com tells of an asteroid whose orbit is so similar to Earth's orbit that the asteroid has been "following" Earth for hundreds of thousands of years. This confuses me: given that its mass is so much smaller than Earth's, I would expect that it would have to travel at a much lower speed around the sun, which suggests (to this novice) that we'd be overtaking it periodically, and at a rate far too high to allow it to sit behind us for thousands of years. Also, given its small mass again, if it were moving at the same speed around the sun as the Earth does, then its orbit should be much further out, shouldn't it? Can someone help me to understand this? I thought it might have something to do with Lagrange points, but I would have expected the article to mention that, as it seems like a strange coincidence.