What Is an Atomic Clock?
by Arielle Samuelson (excerpt)

In 2019, NASA introduced the Deep Space Atomic Clock. In this excerpt, writer Arielle Samuelson, who has written extensively about NASA's Jet Propulsion Laboratory, explains why this new "time machine" is important.

(1) 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.

(2) 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?

(3) 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).

(4) 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.

(5) 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.

(6) 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.

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

(8) 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?

(9) 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.

(10) 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.

What's unique about the Deep Space Atomic Clock?

(11) 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.

(12) 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.

(13) 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.

(14) 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.

1
Select the correct answer.
Read paragraphs 12 through 14 from the passage.

(12) 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.

(13) 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.

(14) 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.

How does the information in paragraph 12 connect the information provided in paragraphs 13 and 14?

 A. 
by explaining the similarities and differences between mercury ions and electrically neutral atoms
 B. 
by explaining the process navigators use to calculate a spacecraft's trajectory using communications to and from Earth
 C. 
by explaining how scientists determined that NASA's Deep Space Atomic Clock is 50 times more stable than atomic clocks on GPS satellites
 D. 
by explaining how the use of mercury ions in the Deep Space Atomic Clock makes autonomous navigation possible