How a Clock is the Missing Piece to Deep Space Travel
Imagine this scenario: You are tasked with shooting an arrow and hitting a target the size of a quarter. This in itself is a pretty difficult task that requires immense accuracy. Now, the quarter-sized target that you are tasked with hitting is sitting in Times Square, New York, and you are standing in Los Angeles. This is the example that Jill Seubert uses to contextualize her job as a deep space navigator. Seubert is in charge of steering spacecraft from the moment they separate away from the launch vehicle until they reach their final destination. She has the opportunity to adjust the course of the spacecraft (the arrow in the analogy) only a couple of times along its trajectory. But in order to make the necessary adjustments, she must know the exact location of the spacecraft at any given moment in time.
How does one track the position of a spacecraft that is billions of miles from Earth moving at tens of thousands of miles per hour? The concept is similar to an echo. A signal is sent from here on Earth to the spacecraft. Once the spacecraft receives the signal from Earth, it sends the signal directly back. That signal travels at the speed of light during its journey, so the amount of time it takes for the signal to return back to Earth can be used to calculate the distance between the Earth and the spacecraft. For example, Voyager 1 has been traveling through space since 1977 and is currently roughly 14 billion miles from Earth traveling at 38,000 mph. At this distance, the signal traveling between Earth and the spacecraft takes 40 hours to make a roundtrip. In that time, Voyager 1 has already travelled another 1.5 million miles farther from Earth. Measuring the amount of time it takes for the signal to travel between Earth and a space probe is absolutely critical for deep space exploration and can be the difference between a successful and a failed mission.
For this reason, incredibly precise clocks are paramount for deep space missions. The only time measuring tool that is equipped for the job is the atomic clock. The atomic clocks used here on Earth measure the time that it takes for the signals to return with accuracy better than one-billionth of a second. Not only are they incredibly exact, they are also extraordinarily reliable, meaning that there is very minimal “drift” as the clocks run for extended periods of time. “Drift” refers to how clocks tend to slowly mark longer and longer times as a “second” and this error could be catastrophic for spacecraft.
Now that it is understood why atomic clocks are necessary for deep space exploration, it would be beneficial to also understand how they work. These clocks track time based on the oscillation of atoms with an incredibly high degree of accuracy. The atoms of each element have distinct structures with a different number of protons in the nucleus, depending on the type of element. Atoms can have varying numbers of electrons that occupy distinct energy levels. With the correct amount of energy being passed to the atom, an electron can jump to a higher energy level around the nucleus. The amount of energy required for an electron to make this energy level jump is unique to each element, but consistent among all atoms of that element. Atomic clocks are made possible because the energy difference between the electron orbit rings have such a precise and reliable value. One second is the amount of time that it takes for a cesium atom to move 9,192,631,770 times between two particular energy levels. The number of times that the atom moves between the two energy levels is based on the natural resonance frequency of a cesium atom, which is 9,192,631,770 Hz. Natural resonance frequency is the frequency at which the most number of cesium atoms are altered or excited, thereby maximizing the amount of energy given off by the cesium. For reference, many modern clocks use a quartz crystal oscillator because quartz crystals vibrate at a very precise frequency (32768 Hz) when a voltage is applied to them. These clocks do not meet the standards of deep space navigation and can be off by a full millisecond in as few as six weeks. An error of just a millisecond would be a 185-mile difference when the signal is travellng at the speed of light.
In today’s “New Space Age,” we are sending more spacecraft to deep space than ever before. With manned missions to Mars on the horizon, the need for space crafts to navigate in real time is quickly increasing. A crew flying to Mars cannot wait extended periods of time for signals from Earth for instructions. By having an atomic clock onboard the spacecraft, the echo signal system used to track a distant spacecraft’s position is no longer necessary because the craft itself will be able to determine its position based on its own internal clock. However, one of the most prominent issues with current atomic clock technology is the sheer size of the equipment and the large amounts of energy that is consumed when operating. Deep space exploration requires compact tools, so the refrigerator size atomic clocks being used on Earth are unable to be sent into space aboard the spacecraft.
Fortunately, NASA developed and launched the Deep Space Atomic Clock (DSAC) in June 2019, ushering in the new age of atomic space clocks. No larger than a toaster oven, DSAC uses mercury ions instead of cesium atoms. Ions are just atoms that have an electric charge and can be better controlled while in the vacuum chamber that the clock stores them in. This prevents the mercury ions from interacting with the vacuum chamber walls which could cause changes to the atoms, and consequently frequency errors. Regarding reliability, it would take DSAC over 10 million years of constant running to be off by a single second.
With a clock as compact and precise as the Deep Space Atomic Clock, onboard autonomous navigation, or spacecraft that will be able to drive and navigate without relying on communication with Earth, is no longer a science fiction dream, but rather an upcoming reality. But the benefits of such a clock do not stop at just autonomous space travel. Seubert proclaims that this technology will allow for the building of GPS-like navigation systems on other planets and moons. Seubert looks even farther into the future and envisions “a network of communication satellites scattered throughout deep space broadcasting navigation signals” so that any spacecraft that picks up one of the signals would be able to travel from destination to destination with no direct tie to Earth at all.
Despite spacecraft historically being poor timekeepers, the miniaturization of the atomic clock will continue to transform the way that humanity explores deep space. Accurately measuring time from within deep space will fundamentally change how space navigation works, and with the introduction of onboard autonomous navigation, we will be able to send even more spacecraft to explore the vast universe.
Works Cited:
Brabaw, Kasandra. “Atomic Clocks Explained: NASA Set To Launch a Deep Space Timekeeper Monday.” Space.com, Space, 23 June 2019, www.space.com/atomic-clock-nasa-falcon-heavy-stp2.html.
“DARPA Making Progress on Miniaturized Atomic Clocks for Future PNT Applications.” Defense Advanced Research Projects Agency, Aug. 2019, www.darpa.mil/news-events/2019-08-20.
Greicius, Tony. “Five Things to Know about NASA's Deep Space Atomic Clock.” NASA, NASA, 3 June 2019, www.nasa.gov/feature/jpl/five-things-to-know-about-nasas-deep-space-atomic-clock.
Mann, Adam. “How the U.S. Built the World's Most Ridiculously Accurate Atomic Clock.” Wired, Conde Nast, 9 May 2018, www.wired.com/2014/04/nist-atomic-clock/.
Seubert, Jill. “How a Miniaturized Atomic Clock Could Revolutionize Space Exploration .” TED. May 2019, www.ted.com/talks/jill_seubert_how_a_miniaturized_atomic_clock_could_revolutionize_space_exploration?language=en.
