Atomic Time Heads to Space

A deep-space mission blasts off, but it’s not just going to the Moon. It's headed to Mars — farther than humans have ever traveled. How will the astronauts know where they are, and where they’re headed?

Astronauts who venture into deep space won’t be able to rely on Earth-based navigation systems like GPS. They will need to keep time and navigate on their own. That means they will need to carry their own clocks.

In 2019, NASA took an early step in this direction by launching the Deep Space Atomic Clock. The clock used ions (electrically charged atoms) of mercury as a frequency reference. Mindful of the unique constraints that space missions face, project leaders designed the clock to weigh less and draw less power than a typical atomic clock on Earth.

During the next year, agency scientists tested the clock’s performance. The device proved 10 times more stable than any previous space clock. This level of long-term stability would enable it, in theory, to support navigation independent of Earth for weeks to months at a time.

While atomic clocks have flown on GPS and other positioning and navigation satellites for decades, those clocks quickly drift away from true time. They must be regularly corrected by more accurate and stable clocks on Earth. This is possible because GPS satellites fly at an orbit around 20,200 km (12,550 miles), within range of ground stations with highly accurate and stable clocks.

NASA’s clock was stable enough to serve as a navigation aid without being corrected, project leaders say, although it will need to be refined further and made more robust before it’s ready for a mission to Mars or beyond.

Atomic clocks in space could have other benefits as well. Nations are preparing to return to the Moon as soon as 2026, not just to visit but to establish permanent facilities. As humans and their infrastructure populate the Moon, we will need a way to keep time there, just as we have here on Earth. Earlier this year, NIST scientists spearheaded a proposal for a lunar time zone based on a network of Moon-based atomic clocks.

Space clocks could also supercharge scientific research that already relies on atomic clocks on the ground. For example, integrating clocks into space telescopes could vastly extend the reach of a technique called very-long-baseline interferometry, which links distant telescopes to create extremely sharp images of faraway objects. Similarly, space clocks could enable physicists searching for dark matter or gravitational waves to cast a much larger net, making their searches much more sensitive.

When it comes to atomic clocks and science, the sky truly is the limit.

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