Launching missions beyond our planet requires humanity’s most cutting-edge technologies and aspirational visions. But though these missions accomplish amazing feats, such as landing on other planets or hosting astronauts for months in orbit, there is one simple task they still cannot do all by themselves: tell time.
Current spacecraft rely on two-way communication from Earth to keep their clocks up to date, which is a necessary process for navigation, among other functions. Now, the time-keeping abilities of space missions are poised to be revolutionized by a new type of atomic clock that “will enable one-way navigation” and make “near-real-time navigation of deep space probes possible,” according to a study published on Wednesday in Nature.
In 2019, NASA’s Jet Propulsion Laboratory (JPL) launched an instrument known as the Deep Space Atomic Clock (DSAC) into orbit to conduct the first space-based demonstration of this next-generation technology, which has a dizzying range of future applications. Since the launch, DSAC has lived up to expectations by achieving a timekeeping stability that is an “order of magnitude better than existing space clocks,” despite the many challenges of the outer space environment, reports the study.
“Over the last few years, we've been analyzing data, listening to it, and watching it, so it's been steady, hard work,” said Eric Burt, an atomic clock physicist at JPL and lead author of the new study, in a call. “But now, culminating in this announcement, it's also very, very exciting.”
Atomic clocks use the excited states of atoms to count seconds, and they are by far the most precise timekeeping devices ever made. Many spacecraft already carry these advanced clocks to perform complex logistical calculations, such as the satellites of the Global Positioning System (GPS) constellation. Space missions that travel to other planets or voyage into the depths of the solar system rely on atomic clocks on Earth to provide them with updated time measurements, which is a value they need to calculate their position in space.
All of the atomic clocks that are currently operating in space use atomic beams or gas cells to trap the time-keeping atoms into a walled-in area. These clocks are extremely precise on short-term timescales, maintaining an error level of no more than one billionth of a second hour-to-hour, but the atoms bouncing off the walls cause this timekeeping stability to “drift.”
For this reason, the clocks on GPS satellites, and others, need to be frequently updated to account for drift, making them unstable timekeepers over longer periods. This need for long-term timekeeping stability has been a major limiting factor for clocks on deep space missions: it’s one thing to regularly correct clocks on GPS satellites close to Earth, where the communication time is low, but it would be inefficient to correct clocks onboard a probe traveling across the solar system, which is why these farflung spacecraft rely on Earth-based clocks to keep them on track instead.
Deep space missions have “to maintain a certain degree of precision for a longer period of time, which enables the spacecraft to be further away and for those spacecraft to be operating longer and have the clock still do its job,” Burt said.
DSAC is the first step toward this type of self-driving deep space probe that could tell time without having to contact Earth. The device is a toaster-sized “trapped-ion atomic clock,” which means it holds atoms in place with electromagnetic forces, rather than the traditional beams and gas cells used in other atomic clocks. This design eliminates drift caused by wall collisions and produces long-term stability, which has been demonstrated on Earth.
During its two-year run in space, DSAC showed that it could maintain ten times the long-term stability of existing space clocks, and that it was not substantially affected by outer space conditions such as variations in radiation, temperature, or magnetic fields.
“Each time you do a test, you learn something more, and you raise more questions,” Burt said. “We did a lot of work to refine our understanding of what we already knew on the ground.”
“Most of the sensitivities of the clock—how it reacts to temperature changes, magnetic changes, and that sort of thing—were well-known and it was just a matter of demonstrating that in the harsher environment of space,” he added. “There weren't too many surprises there, but nevertheless, the devil is in the details and we did investigate quite a bit.”
With its trailblazing mission now drawing to a close, the DSAC team has opened the door to new experiments that could revolutionize deep space travel and pioneer a host of other space applications near Earth. Deep space probes that carry future versions of DSAC could autonomously pilot themselves to locations around the solar system, and would not have to rely on Earth’s clocks to perform maneuvers such as orbital insertions or planetary landings. These clocks will also be essential for interplanetary missions that carry astronauts, as such high-stakes voyages will require real-time navigation on the spacecraft itself.
Beyond their navigational applications, trapped-ion clocks could work in tandem with other instruments to shed light on a host of different questions in planetary science and fundamental physics, from mapping out the subsurface seas of Europa to testing Einstein’s theory of general relativity. To that end, the next iteration of DSAC will fly on NASA’s VERITAS spacecraft, a newly approved voyage to Venus, to test out the clock’s capabilities on an interplanetary mission.
“The takeaway of all this is that the technology really has a tremendous amount of range, depending on what the application is,” Burt said.