The U.S. Space Force (USSF) turns four today. The youngest branch of the U.S. military was established on December 20, 2019, with the passage of the John S. McCain National Defense Authorization Act. Here are a few things to know about the newest U.S. military service.

Space Force was created to address space’s growing importance to national security and everyday life. Just as the U.S. Marine Corps is part of the Department of the Navy, the Space Force is organized under the Department of the Air Force. Space Force’s ties to the Air Force are understandable. It was created by merging twenty-three different Air Force units, and Air Force General John W. “Jay” Raymond was named its first chief of space operations. Last year, another Air Force veteran, General B. Chance Saltzman, succeeded Raymond as space operations chief. The Air Force’s influence over the USSF will likely continue for some time—it handles more than 75 percent of the Space Force’s logistics work.

Space Force’s mission is to organize, train, and prepare its service members “to conduct global space operations that enhance the way our joint and coalition forces fight while also offering decision-makers military options to achieve national objectives.” Its specific responsibilities include operating missile detection networks and the Geographic Positioning System (GPS) constellation—the set of satellites that your smartphone, among other applications, uses to pinpoint your location. The USSF also monitors intentional and unintended threats (e.g., “space junk”) to the 6,718 satellites active in space—more than half of which U.S. owners operate. And it works to enhance U.S. space strategy and the international rules governing space.

Members of the Space Force are called “guardians.” (They do not take their name from Cleveland’s professional baseball team or Marvel’s band of galaxy saviors.) Space Force’s motto is Semper Supra, or “Always Above.” As Space Force hits its fourth birthday, it has 8,600 uniformed guardians. To put that number into perspective, the next smallest service, the Coast Guard, has nearly 43,000 active-duty members. Space Force is expected to grow next year with a requested 2024 budget of $30 billion as individuals currently serving in the Army, Navy, Air Force, and Marines transfer into the service. But Space Force was established with the expectation that it would remain a small (and relatively agile) organization. So don’t bank on it rivaling the size of the Air Force (329,000 active-duty personnel), let alone the Army (482,000 active-duty personnel).

Although Space Force is the first independent service of its kind in U.S. history, it isn’t the U.S. military’s first space-centered program. Shortly after World War II ended, the Army Air Forces (the predecessor of the U.S. Air Force) turned its attention and funding to satellite and rocket technology. In 1985, the Defense Department organized the U.S. Space Command, planning military operations in space. In 2002, Space Command was absorbed into U.S. Strategic Command. It was reactivated as a distinct combatant command in 2019 and now works closely with Space Force.