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.
