There’s really nothing easy about being a NASA astronaut on the International Space Station, and the same goes for spacewalking outside of the orbiting laboratory. It’s hard work, and when spacewalks stretch on for hours and hours, you have to be in excellent physical condition to make it through. The only upside — aside from being one of a select few to ever experience walking in space — is that space is usually pretty clean.

But there are always exceptions, and NASA astronaut Christina Koch, who was one half of the first all-female spacewalk that took place last Friday, managed the rare feat of getting dirty during her trip outside the space station.

In a NASA streaming broadcast on Monday, Koch explained that her work during the spacewalk resulted in a messy glove. A “contaminant” was found on the glove after Koch returned to the interior of the space station, and it wasn’t immediately clear where it came from.

After some discussion with engineers back on Earth, the best explanation thus far is that Koch’s glove came into contact with some grease on an external component. More specifically, NASA thinks that when Koch was riding the robotic arm, her adjustment of a foot restraint led to her glove brushing up against a section that had been lubricated, resulting in a messy glove.

The spacewalk was a huge success, with Koch and fellow NASA astronaut Jessica Meir performing some crucial maintenance to the space station’s power systems. The duo swapped out a malfunctioning battery charge-discharge unit with a new unit. The component allows the space station to manage its power reserves, making it an incredibly important piece of the system that ensures the space station remains powered up even when its solar panels aren’t receiving sunlight.

As for the “contaminated” glove, it’s unclear if NASA intends to use it again but in any case, it shouldn’t pose a problem for future spacewalks.