Saturday, June 25th: Additional observations have made it possible to calculate 2011 MD's orbit quite accurately. Bill Gray, a well-known expert on orbital dynamics, has run the orbit backward in time, and is now quite sure that this asteroid could not have been close enough to Earth any time during the space age to have started off as a rocket booster. So it seems to be a genuine chunk of rock after all.
Original story Friday, June 24th: When I was working up the details for yesterday's story on asteroid 2011 MD, I gasped when I looked at the orbit diagram. Not only does this body's orbit intersect Earth's, but it's also in the same plane (within a few degrees), and moving at almost the same velocity. Somehow, it seemed unlikely to be an accident! Alan MacRobert, who sits across the hall from me, immediately suggested that it is indeed no accident — that this "asteroid" is actually the upper stage of a rocket. These upper stages are abandoned to become space junk after they've released their payloads. Most of them remain in Earth orbit, but it's perfectly possible for them to escape and start looping around the Sun all on their own. Since they start out with almost the same velocity as Earth, they tend to stay in similar orbits.
What significance does this have? Not much! For all practical purposes, 2011 MD is an asteroid now, whether its origin is natural or artificial. But somehow, it seems less glamorous to get up in the middle of the night to try to glimpse a rocket upper stage than a chunk of rock. After all, anybody can see a half-dozen rocket upper stages on any clear night just by looking in the right place at the right time. You don't even need optical aid! Just check out the predictions for your location from Heavens Above.