Keeping time in space

Autonomous deep-space navigation will require clocks that can work reliably in space without contact with Earth. But so far space clocks have been less stable than their terrestrial counterparts, in part because they confine atoms with atomic beams or gas cells, which are easier to implement, rather than the more stable trapped-atom techniques employed in Earth-based metrology (see the article by James Bergquist, Steven Jefferts, and David Wineland, Physics Today, March 2001, page 37 ). Trapping avoids some of the limiting factors to the stability of those techniques, such as the wall collisions in gas-cell clocks.

In 2011 NASA initiated the Deep Space Atomic Clock mission to develop a compact trapped-ion atomic clock for use in space. Now researchers at NASA’s Jet Propulsion Laboratory—including Eric Burt, John Prestage, Robert Tjoelker, and principal investigator Todd Ely—have demonstrated that such a clock can operate in space stably and enduringly.

The researchers installed their clock aboard General Atomics’ Orbital Test Bed spacecraft, shown in the image; it was launched into low-Earth orbit in June 2019. They switched on the clock two months later. The mercury-ion clock uses two RF traps: a quadrupole trap while the ions are loaded and prepared and a multipole trap while their hyperfine transition is probed.

Space limits the clock’s performance in ways not encountered on the ground. For example, over the course of an orbit around Earth, the magnetic field experienced by the clock varies more than in a typical laboratory setting by two orders of magnitude. But the Hg⁺ clock transition is less sensitive to magnetic variation than those of other atoms commonly used in space atomic clocks—the cesium atom clock transition, for example, is four times as sensitive as that of Hg⁺.

The spacecraft’s orbit passes through the South Atlantic Anomaly, which exposes the clock to heavy doses of radiation from high-energy protons. Anticipating that, the Jet Propulsion Laboratory researchers designed the control algorithm to filter out those effects.

The team tested the clock’s performance over a nine-month period. After 23 days of continuous operation, the frequency stability relative to a clock on the ground was 3 × 10⁻¹⁵, which corresponds to a time deviation of less than four nanoseconds. And the clock’s linear frequency drift of 3 × 10⁻¹⁶ per day is an order of magnitude better than those of previous space clocks and meets NASA’s goals for the mission.

The predicted lifetime of the current clock is three to five years, but the researchers are working to increase it to 10 years or more through improvements to the vacuum chamber and the clock’s deep-UV light source. (E. A. Burt et al., Nature 595, 43, 2021 .)