A group of scientists in a plane caught sight of a mysterious piece of space junk right as it burned up in Earth’s atmosphere above the Indian Ocean near Sri Lanka in November of 2016.

NASA and other space agencies around the world monitor a large percentage of the millions of bits of space debris that orbit the planet, and researchers had been expecting the object — appropriately named WT1190F — to re-enter Earth’s atmosphere when it was first spotted in October.

The International Astronomical Center (IAC) and the United Arab Emirates Space Agency hosted a team of veteran U.S. and German observers of spacecraft reentries to study the reentry of the object, which was approximately 1 meter (about 3 feet) in size. The object burned up on reentry and was not a threat to anyone on Earth due to its low density and small size.

But months, they still aren’t sure what the object actually was. They do have some ideas. The leading theory is that it’s the second stage of a rocket — though they have no idea which rocket.

Jonathan McDowell, an astrophysicist at the Harvard–Smithsonian Center for Astrophysics, told Scientific American that this could very likely be a lost piece of space history, returning to haunt us. According to the ESA, 8,500 objects larger than 10 cm orbit the Earth, along with 150,000 objects larger than 1 cm.

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