What does a small, icy world roughly four billion miles from Earth have to do with the Nazis?

That’s the question NASA was wrestling with before it announced on Tuesday that a space object formerly known by the nickname Ultima Thule would now officially be named Arrokoth, a Native American word meaning “sky.”

The previous name was a Latin term that metaphorically means “a place beyond the known world.” When it was unveiled last year, NASA said the name was only temporary, but it still attracted criticism because the term has a historical association with the Third Reich.

But let’s start from the beginning.

What is Ultima Thule?

Until Tuesday, Ultima Thule was the informal name NASA used for a small, snowman-shaped object in the Kuiper belt, a desolate region of deep space that the agency said in a statement is home to “thousands of known small icy worlds.”

But that name was not registered with the International Astronomical Union and Minor Planet Center , which has international authority for the naming of objects in the Kuiper belt. Its official name was the impersonal-sounding “Kuiper belt object 2014 MU69.”