# What Is an Atomic Clock?

The clock is ticking: A technology demonstration that could transform the way humans explore space is nearing its target launch date of June 24, 2019. Developed by NASA's Jet Propulsion Laboratory in Pasadena, California, the Deep Space Atomic Clock is a serious upgrade to the satellite-based atomic clocks that, for example, enable the GPS on your phone.

Ultimately, this new technology could make spacecraft navigation to distant locations like Mars more autonomous. But what is an atomic clock? How are they used in space navigation, and what makes the Deep Space Atomic Clock different? Read on to get all the answers

Why do we use clocks to navigate in space?

To determine a spacecraft's distance from Earth, navigators send a signal to the spacecraft, which then returns it to Earth. The time the signal requires to make that two-way journey reveals the spacecraft's distance from Earth, because the signal travels at a known speed (the speed of light).

While it may sound complicated, most of us use this concept every day. The grocery store might be a 30-minute walk from your house. If you know you can walk about a mile in 20 minutes, then you can calculate the distance to the store.

By sending multiple signals and taking many measurements over time, navigators can calculate a spacecraft's trajectory: where it is and where it's headed.

Most modern clocks, from wristwatches to those used on satellites, keep time using a quartz crystal oscillator. These devices take advantage of the fact that quartz crystals vibrate at a precise frequency when voltage is applied to them. The vibrations of the crystal act like the pendulum of a grandfather clock, ticking off how much time has passed.

To know the spacecraft's position within a meter, navigators need clocks with precision time resolution — clocks that can measure billionths of a second.

Navigators also need clocks that are extremely stable. "Stability" refers to how consistently a clock measures a unit of time; its measurement of the length of a second, for example, needs to be the same (to better than a billionth of a second) over days and weeks.

What do atoms have to do with clocks?

By space navigation standards, quartz crystal clocks aren't very stable. After only an hour, even the best-performing quartz oscillators can be off by a nanosecond (one billionth of a second). After six weeks, they may be off by a full millisecond (one thousandth of a second), or a distance error of 185 miles (300 kilometers). That would have a huge impact on measuring the position of a fast-moving spacecraft.

Atomic clocks combine a quartz crystal oscillator with an ensemble of atoms to achieve greater stability. NASA's Deep Space Atomic Clock will be off by less than a nanosecond after four days and less than a microsecond (one millionth of a second) after 10 years. This is equivalent to being off by only one second every 10 million years.

Atoms are composed of a nucleus (consisting of protons and neutrons) surrounded by electrons. Each element on the periodic table represents an atom with a certain number of protons in its nucleus. The number of electrons swarming around the nucleus can vary, but they must occupy discreet energy levels, or orbits.

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