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A jolt of energy — in the form of microwaves — can cause an electron to rise to a higher orbit around the nucleus. The electron must receive exactly the right amount of energy — meaning the microwaves must have a very specific frequency — in order to make this jump.
The energy required to make electrons change orbits is unique in each element and consistent throughout the universe for all atoms of a given element. For instance, the frequency necessary to make electrons in a carbon atom change energy levels is the same for every carbon atom in the universe. The Deep Space Atomic Clock uses mercury atoms; a different frequency is necessary to make those electrons change levels, and that frequency will be consistent for all mercury atoms.
"The fact that the energy difference between these orbits is such a precise and stable value is really the key ingredient for atomic clocks," said Eric Burt, an atomic clock physicist at JPL. "It's the reason atomic clocks can reach a performance level beyond mechanical clocks."
Being able to measure this unchangeable frequency in a particular atom offers science a universal, standardized measurement of time. ("Frequency" refers to the number of waves that pass a particular point in space in a given unit of time. So, by counting waves, it's possible to measure time.) In fact, the official measurement of the length of a second is determined by the frequency needed to make electrons jump between two specific energy levels in a cesium atom.
In an atomic clock, the frequency of the quartz oscillator is transformed into a frequency that is applied to a collection of atoms. If the derived frequency is correct, it will cause many electrons in the atoms to change energy levels. If the frequency is incorrect, far fewer electrons will jump. This will determine if the quartz oscillator is off-frequency and by how much. A "correction" determined by the atoms can then be applied to the quartz oscillator to steer it back to the correct frequency. This type of correction is calculated and applied to the quartz oscillator every few seconds in the Deep Space Atomic Clock.
What's unique about the Deep Space Atomic Clock?
Atomic clocks are used onboard GPS satellites that orbit the Earth, but even they must be sent updates two times per day to correct the clocks' natural drift. Those updates come from more stable atomic clocks on the ground that are large (often the size of a refrigerator) and not designed to survive the physical demands of going to space.
Up to 50 times more stable than the atomic clocks on GPS satellites, NASA's Deep Space Atomic Clock is intended to be the most stable atomic clock ever flown in space. It achieves this stability by using mercury ions.
Ions are atoms that have a net electric charge, rather than being electrically neutral. In any atomic clock, the atoms are contained in a vacuum chamber, and in some of those clocks, atoms interact with the vacuum chamber walls. Environmental changes such as temperature will then cause similar changes in the atoms and lead to frequency errors. Many atomic clocks use neutral atoms, but because the mercury ions have an electric charge, they can be contained in an electromagnetic "trap" to prevent this interaction from happening, allowing the Deep Space Atomic Clock to achieve a new level of precision.
For missions going to distant destinations like Mars or other planets, such precision makes autonomous navigation possible with minimal communication to and from Earth — a huge improvement in how spacecraft are currently navigated.