Applied Atomic Timekeeping in Space

In 1945, during the Richtmeyer Memorial Lecture at the annual meeting of the American Physical Society in New York City, Nobel Laureate Isidor Rabi made the first suggestion for a clock based on atoms. A device where the “tick-rate” of the clock is tied to the fundamental stability of atomic structure. Twelve years later, the first atomic clock was realized, and in the 1970s the first atomic clock was launched into space. In the nearly half-century since that first space flight, atomic clocks for space have become more precise, more resilient, and as a result have proliferated into diverse space systems. Here, we begin with a very brief overview of how space systems can employ atomic timekeeping, followed by a discussion of present-day and (near-term) next-generation atomic clocks for space systems.