We all know the Earth has one moon, right?
Well, it sure seems like an obvious answer. Sure, there are a bazillion artificial satellites, and probably some dust and stuff, but the Moon is our only moon. A freakishly large moon for our size of planet, orbiting quite distant, but still, just one moon. Right?
Well, sort of. It turns out the Earth has relationships with other bodies that are . . . moon-like. And sometimes it captures new moons, which generally don’t stay long, because our huge Moon ejects them. There’s no way of knowing how many temporary moons the Earth has captured through the past few billion years, nor whether any of them stuck around for any significant length of time or were ever visible from the ground. But in 2002, an astronomer spotted what appeared to be one — the first ever formally identified. It was duly given the provisional designation J002E3, signifying the time and sequence of its discovery, and soon was confirmed to be orbiting the Earth.
This of course sparked a frenzy of astronomical observations. It was soon found to be very bright, suggesting it was perhaps 30 meters across, a size which caused some alarm, as orbital predictions suggested it would leave the Earth-Moon system and return around 2040. A 30 meter asteroid could cause severe damage, depending on where it hit. But then astronomers realized two other strange things. First, backtracing its orbit to try to find “precovery” images, they realized that it had likely last been in the Earth-Moon system in 1971 – during the Apollo program, when an object in such an orbit would surely be more conspicuous. And secondly, spectrographic analysis showed its surface was covered by a material never before seen in an asteroid: titanium dioxide. That’s never been seen on an asteroid, but we see it all the time on Earth; it’s very popular in white paint. Because that’s what the asteroid was covered with.
It wasn’t a 30 meter wide rock at all. Recalculating with the knowledge that it was covered in titanium dioxide and therefore bright white, they realized it was much smaller, less than half that size. In fact, once they put that fact together with the orbital data, they realized it wasn’t a rock at all. It was a rocket — the spent S-IVB upper stage from the Apollo 12 mission, which is painted white, and 17.8 meters long. The Apollo 12 mission was the last time a Saturn V was sent to the Moon without the upper stage being aimed at the Moon itself; all subsequent ones were deliberately crashed to generate moonquakes for the seismographs left by previous Apollo missions. Its predecessors expended the last of their propellants to boost themselves into heliocentric orbit, preventing them becoming navigational hazards, but this one had to burn its ullage motors a bit longer earlier in the flight, which left it with insufficient delta-vee to escape Earth. It wound up in a high and rather unstable orbit, and was soon lost to tracking, as attention was wrenched back to Earth with the sudden cancellation of Apollo.
Sometime in 1971, it wandered through a Lagrange point and slipped into heliocentric orbit; sometime near 2002, it slipped back in. It has left Earth orbit again, and has not been seen since 2003; it is too small to easily track away from Earth, but calculations indicate we’ll see it again in 2040, when Earth is likely to recapture it once again. It does present some hazard of impact with either Earth or Moon, but because we now know it to be a hollow rocket body rather than a chunk of nickel and iron, we know it will not be a major hazard after all. Rocket bodies fall to Earth all the time, usually landing fairly intact since they’re not going very fast, but this would be going very fast and would mostly burn up. Therefore, it is actually not listed as a Potentially Hazardous Asteroid.
And this isn’t the only time that’s happened. On August 28, 2006, astronomers working with the Catalina Sky Survey discovered an object provisionally designated 6Q0B44E. This object is in a very high orbit, higher than the Moon, and is much fainter than J002E3. It’s estimated to be just a few meters across, and it’s orbit is rather similar to J002E3. There’s also another, discovered in 2008 , which received the minor planet designation 2006 RH120. This object, like J002E3, appears to have a spectrum consistent with titanium dioxide, but it has been observed after escaping Earth, and observations of perturbations due to solar wind are not consistent with an artificial object, but rather with a more solid object such as a rocky asteroid or perhaps a fragment of the Moon. The jury, realistically speaking, remains out for both of these sometimes-moon objects; one is believed artificial, one is believed natural, but both have orbits reminiscent of J002E3 and the Apollo program. Are they space debris, or itinerant companions of the Earth?
And, of course, there is one more misidentification worth mentioning. Although it didn’t enter Earth orbit, an object was observed on an imminent near-collision course with the Earth. Its orbit was quickly confirmed by alert astronomers, allowing it to receive the minor planet designation 2007 VN84. But backtracing its orbit revealed it had made another startlingly close encounter with the Earth before — and then someone worked it out. It was the Rosetta spacecraft, performing a gravity assist maneuver.