It was 2:30 in the morning when astronautical engineer Todd Ely watched as a little atomic clock—the size of a four-slice toaster—was launched into space on a satellite attached to one of the most powerful rockets in the world. He distinctly remembers a bright flash and a beating vibration that lasted long after the light went dim. “You feel it in your chest,” he recalls.
Also at the site was Ely’s colleague Eric Burt, a physicist who is an expert on atomic clocks. Despite all of the shake tests they had performed beforehand to ensure their delicate device could endure the journey into space, the violence of the launch left Burt in disbelief. “The whole Earth shakes,” he remembers. “I watched it from three miles away, thinking: How is our little clock going to ever survive?”
But it did. Ely and Burt are two leaders of the Deep Space Atomic Clock project at NASA’s Jet Propulsion Laboratory, and in September—more than two years after the clock’s deployment into low Earth orbit—the clock’s satellite was powered off, marking the end of its first mission. It’s the most precise clock to ever operate in space, and it’s paving the way for making real-time navigation of the cosmos a reality. “A robust onboard navigation system is going to be a fundamental component to human exploration beyond Earth,” says Ely, the project’s principal investigator. “And our clock can play a role in that.”
Atomic clocks, like every other kind, start with an oscillator: something that vibrates. “It could be as simple as a pendulum arm swinging, or it could be a quartz crystal like you have in your watch or iPhone,” Burt says. The frequency of that vibration, or how many oscillations occur in a second, is how clocks keep time, or tick.
But oscillators are fickle—the stability of their frequency degrades over time, a phenomenon known as drift. So, Burt says, atomic clocks pair an oscillator with a collection of atoms to help keep that frequency stable. (This clock uses mercury, but others have used cesium, rubidium, or strontium.) Atoms are made up of electrons circling a nucleus, and these electrons can exist only in specific, discrete orbits, based on how much energy they have. To jump into higher orbits, the electrons must be given energy of just the right frequency. That means scientists can monitor the stability of their clocks by observing the activity of the atoms it is paired with. “One way to envision it is that the atomic portion is just a steering wheel on the oscillator,” says Burt. “If it’s at the right frequency, then you get a lot of atoms jumping around. If it’s at the wrong frequency, nothing happens.”
In June, the team published a paper in Nature reporting that their clock has extremely low drift, corresponding to a deviation of less than four billionths of a second over the course of 23 days. “At this rate, the time over which this clock would lose a second is 1,000 years,” says Burt. This is much better than other clocks currently operating in space, which would be off by a second after about 90 years, although ground-based clocks are still ten to 100 times more accurate. “We would have been happy just to demonstrate operability,” he says. “Frankly, if we had turned it on and it worked, and then failed 10 minutes later, we’d be dancing in the streets.” But it accomplished a lot more than that.
James Camparo of the Aerospace Corporation thinks the drift of their clock is exceptionally low. “These on-orbit frequency stability results are very encouraging for the technology,” even though the clock did not operate in its optimal settings while in space, says Camparo, who holds a doctorate in chemical physics and was not involved in the study. He anticipates that during the next phase of the mission, the JPL team will achieve even lower frequency variations, further improving the clock’s performance.
This kind of precision timing will be needed for future deep space missions. Currently, navigation in space actually requires all of the decisions to be made on Earth. Ground navigators bounce radio signals to a spacecraft and back, and ultraprecise clocks can time how long the round trip takes. This measurement is used to calculate information about position, speed, and direction, and a final signal is sent back to the space vessel with commands on how to adjust course.
But the time it takes to send messages back and forth is a real limitation. For objects near the moon, the two-way trip only takes a couple of seconds, Ely says. But as you travel further out, the time required quickly becomes inefficient: near Mars, the round trip time is about 40 minutes, and near Jupiter, this increases to about an hour and a half. By the time you travel all the way out to the current location of the Voyager, a satellite exploring interstellar space, he says, it can take days. Far out into the cosmos, it would be impractical and unsafe to rely on this method, especially if the craft was carrying people. (Currently, uncrewed missions, like the Perseverance rover’s landing on Mars, rely on automated systems for navigation decisions that have to be made on short timescales.)
The solution, the JPL team says, is to equip the spacecraft with its own atomic clock and eliminate the need for ground-based calculations. The craft will always need to receive an initial signal from Earth, in order to measure its position and direction from a constant point of reference. But there would be no need to bounce a signal back, because the subsequent navigation calculations could be done in real time onboard.
Until now, this was impossible. Atomic clocks used to navigate from the ground are too big—the size of refrigerators—and current space clocks aren’t accurate enough to rely on. The JPL team’s version is the first one that’s both small enough to fit on a spacecraft and stable enough for one-way navigation to become a reality.
It may prove useful for ground travel too. On Earth, we use GPS, a network of satellites carrying atomic clocks that help us navigate on the surface. But according to Ely, these clocks aren’t nearly as stable—their drift needs to be corrected at least twice a day to ensure a constant stream of accurate information for everyone on Earth. “If you had a more stable clock that had less drift, you could decrease that kind of overhead,” says Ely. In the future, he also imagines that a large population of humans or robots on the moon or Mars will need to have their own tracking infrastructure; a GPS-like constellation of satellites, equipped with tiny atomic clocks, could accomplish this.
Camparo agrees, and says the device could even be configured to use on ground stations on Mars or the moon. “It’s worth noting that when we consider space-system timekeeping, we often focus on the atomic clocks carried by the spacecraft,” he says. “However, for any constellation of satellites, there has to be a better clock at the satellite system’s ground station,” since this is how scientists monitor the accuracy of clocks in space.
Ely and Burt are planning to send an even smaller version of their clock to hitch a ride on NASA’s VERITAS mission, which will head to Venus near the end of the decade. While the orbiter won’t depend on the clock to find its way to our twin planet—two-way navigation is still a more tried and true technique—the JPL team could demonstrate what VERITAS planetary scientist Erwan Mazarico calls a “shadow navigation,” by using the data collected by the main navigation team to verify how well one-way steering will work with their technology.
Mazarico is also interested in how the atomic clock might enhance the experiments that the VERITAS team plans to conduct once the orbiter reaches Venus. A primary goal is to fully characterize the planet, he says, and one way they can do that is by measuring radio frequencies. VERITAS will transmit radio waves, and the frequencies of those signals will change as they pass through Venus’ atmosphere and gravity field. Researchers can then extract information about the planet from the magnitude of those shifts. “Frequency is related to time,” Mazarico says, “and so timekeeping is quite critical to this whole field.”
The JPL team also wants to design a version of their clock that uses less power. Their first device runs on around 50 watts, just less than a lightbulb. “That’s not bad, actually,” Burt says, but that there are some ground-based clocks that operate using fewer than 10 watts. “So that’s the competition.”
In the meantime, the toaster-sized device from the original mission will continue to circle Earth, until the orbit of its host satellite eventually decays and the whole thing burns up in our atmosphere. Its flight has been a first, and critical, step towards a future in which humans can explore the depths of space and inhabit other worlds without relying on a communications tether to their home planet. “And at the heart of it,” says Ely, “will be an atomic clock.”