In the week following the takedown of a Chinese spy balloon off the coast of South Carolina, three more unidentified objects have been shot down over Alaska, Lake Huron and Yukon, Canada.

A car-sized object in Alaska flew over “sensitive military sites” Feb. 10. It was much lower in altitude and smaller than the Chinese spy balloon, but it's the repeated headline, not the size, that’s raising questions about these four objects.

Here’s everything you need to know about how NASA and the U.S. government classify unidentified objects.

What does UFO mean?

UFO stands for “unidentified flying object,” a term for an aerial phenomenon whose cause or identity is unclear to the observer.

The United States Air Force coined the term in 1952, just five years after pilot Kenneth Arnold added “flying saucer” to the world’s lexicon. In 1947, Arnold flew past Mt. Rainier in Washington state where he saw “nine bright saucer-like objects.” Though he denied initially describing them as saucers, the name had already made its mark in popular culture.

According to the Air Force Declassification Office, UFOs were initially defined “as those objects that remain unidentified after scrutiny by expert investigators, though today the term UFO is colloquially used to refer to any unidentifiable sighting regardless of whether it has been investigated.”

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What does UAP mean?

The National Aeronautics and Space Administration uses the more precise term “unidentified anomalous phenomena” to describe “observations of the sky that cannot be identified as aircraft or as known natural phenomena.” Until December 2022, it was known as “unidentified aerial phenomena” rather than anomalous.

NASA began a study in October 2022 to further UAP data analysis, with promises of a mid-2023 report on its findings. The study is searching for the nature and origins of UAP, scientific analysis techniques, examining the risk to the National Air Space and ways to enhance air traffic management data acquisition systems.

In NASA’s 2021 UAP report, they classified five explanatory categories:

• Airborne clutter

• Natural atmospheric phenomena

• USG or U.S. industry development programs

• Foreign adversary systems

• Other

The report was largely inconclusive, but it stated that UAP “clearly pose” a risk to aviators and U.S. national security and NASA found “potential patterns” in shape, size and propulsion.

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This article originally appeared on USA TODAY: What does UFO mean? Definitions, NASA studies on UFOs and UAPs.