In 1998, it visited the moon, powering the flight of NASA’s Lunar Prospecter on its way to conduct a series of research missions through 19 months of lunar orbit. For the next 11 years it was just another anonymous piece of untraced space debris that humans have lost track of over the years. It popped up briefly in 2009 and 2013 but in October of last year astronomers picked up its trail once again, just in time for the whole world to watch it plummet back into the atmosphere, briefly eliciting fears of asteroid disaster and alien invasion.

The “it” in question is a mysterious space object that scientists called WT1190F the public knew simply as “WTF,” which has now been identified as the remains of a rocket motor that was part of a NASA research probe to the moon back in 1998. While it can be difficult to identify space debris, researchers picked up the object’s trail early enough, they were able to reconstruct its elliptical trajectory and conclude it bore the unmistakable “signature of something launched to the Moon,” Paul Chodas, an asteroid tracker at NASA’s Jet Propulsion Laboratory in Pasadena, California told Nature.*

The identification remains a hypothesis, but it’s a convincing one given the relatively few possibilities left other than a mysterious alien ship that happened to stop by the moon on its way over to us.

The scientists ruled out remnants of earlier lunar spacecrafts, like those used in the Apollo missions, on the grounds that any object from the moon in orbit around the Earth for more than 10 years would likely have crashed back into the atmosphere or slipped into a solar orbit. They also found evidence of titanium oxide in the spectra analysis of images of the debris, the same material as the Lunar Prospecter’s translunar injection module.

Scientists are currently monitoring about 20 other pieces of space debris, hoping that tracking them will make it easier to identify where and when it will fall back to Earth. The WTF debris may have been the first time astronomers were able to trace a piece of human-made space debris from its orbit back into the atmosphere, but given the amount of trash up there ready to fall at any moment, we better hope we can do more than just watch it all crash to Earth. Gunners, ready!