&Bullet; physics 14.99

After a year in low Earth orbit, a NASA demonstration proves that an ultra-precise atomic clock maintains its stability in space and paves the way for real-time deep space navigation.

“Second star on the right and straight on until morning” worked for Peter Pan, but in real life traveling in the solar system is more complicated. Space navigation is based on the measurement of the transit time of radiated radio signals between a spaceship and the earth. Currently, all time measurements are made with earth-based atomic clocks, which means that a spaceship can wait up to several hours before its position information is sent back to it. Carrying out these timing measurements autonomously on board would significantly reduce processing time. Eric Burt and his colleagues from NASA’s Jet Propulsion Laboratory in California report the first successful test of a compact atomic clock with trapped ions in an orbiting spacecraft. The Deep Space Atomic Clock (DSAC) showed a long-term stability that exceeds current space clocks by an order of magnitude.

The most common clocks on board space probes are small and have a stable short-term performance at the expense of long-term stability. This is in contrast to the atomic clocks used on Earth and on GPS satellites where long term stability can be important. Atomic clocks keep time by coupling the frequency of an oscillator to the extremely stable frequencies associated with internal atomic states. GPS atomic clocks and conventional space clocks rely on trapping atoms in a box, with collisions with the box walls affecting performance and causing errors of around 1 ns after 1 day – a seemingly small amount that could add up to a navigational disaster on one years of space travel.

To optimize long-term stability, metrology researchers have developed atomic clocks with trapped ions that trap, cool and excite ions using electromagnetic fields. “A few decades ago it was recognized that trapping ions could revolutionize atomic physics in general and, shortly after, revolutionized atomic clocks,” says Burt. However, placing atomic clocks with trapped ions in space presents several challenges, from the violent conditions of launch to the extreme radiation environment of space.

Burt and his colleagues faced these challenges with the DSAC. At the heart of the watch design is a small cloud of mercury ions that is captured by electrical fields. Microwave pulses generated by the clock are directed into the cloud and some of the ions respond by changing their energy state. The number of ions that change state depends on how close the microwave pulse is to the correct frequency. By measuring this number, a frequency error can be calculated and used to correct the frequency of a crystal oscillator that is built into the ticking of the clock. This technology creates the almost perfect 40.5 GHz “tick” of the watch. The design avoids lasers, cryogenics or microwave cavities and thus enables a small and robust device with a power consumption of less than 50 W. While an earth-based atomic clock takes up the space of a refrigerator, the DSAC clock is the size of a toaster.

In laboratory tests, the researchers operated the DSAC technology continuously for nine months and observed a drift in the stability of the clock that corresponded to a 1 ns error over 20 days. This drift measurement resulted from tracking how much the clock frequency has changed compared to the specified value of 40.5 GHz. A change of 4 GHz, for example, would correspond to a drift of 10% or a deviation of 0.1; in the case of DSAC, the drift was only

$2.7th×1{0}^{–17th}$

Deviation per day.

For their space test, the team placed the DSAC on General Atomics’ Orbital Test Bed, which was launched into low earth orbit by a SpaceX Falcon Heavy rocket in June 2019. This space clock, which uses a slightly simpler capture mechanism than what was used in the lab test, has now delivered more than 12 months of data. By comparing terrestrial clocks and GPS satellite signals, Burt and his JPL colleagues found that the clock’s frequency was around exactly

$3rd×1{0}^{–16th}$

per day. This frequency shift corresponds to a 1 ns error over eight days and thus exceeds the stability of other space clocks by an order of magnitude. “In terms of performance, the long-term stability has been a huge success,” says Burt. “Another major achievement of the technology was its ability to work continuously from the ground without interactions.”

Nils Huntemann, research assistant at the German Institute for Metrology, points out that the mercury ion trap is the result of decades of work. “The big advantage is that the system itself is drift-free,” he says. “The DSAC flight is the first step towards bringing greater clock stability and accuracy into space, and there is still a lot of potential for advancement in technology.”

The technology has proven its suitability for applications that require autonomous operation. Future usage could allow spacecraft to focus on mission objectives and navigation in real time rather than constantly pointing their antennas earthward for position information and maneuvering instructions. Finally, sending multiple units into orbit around other moons or planets could provide a GPS-like navigation system for ground-based rovers or even human explorers.

–Rachel Berkowitz

Rachel Berkowitz is Corresponding Editor for physics based in Vancouver, Canada.

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