Seeing 16 Sunrises in 24 Hours, Astronauts Need Different Ways to Tell Time in Space

A normal clock and routine won’t work while astronauts travel in space. Learn what helps them keep track of their days when traveling to the moon or orbiting Earth.

By Cody Cottier
Jul 9, 2024 6:30 PMJul 9, 2024 6:23 PM
Sunrise in space
(Credit: muratart/Shutterstock)


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Beyond our planet’s atmosphere, the system of timekeeping that gives structure to life falls apart. The words “day” and “night” mean something radically different when you’re completing an orbit of Earth every 90 minutes, as astronauts do aboard the International Space Station (that’s 16 sunrises and sunsets in each 24-hour period).

Since the human body and its circadian rhythms — patterns of sleep and wakefulness regulated by our internal clock — evolved here on Earth, we’re ill-suited for any other environment. In the topsy-turvy world of extraterrestrial time, where astronauts can’t rely on dawn and dusk to keep their usual schedule, they must follow a strict schedule of sleep and work. Any deviation from their natural cycle would quickly lead to physical and mental health problems.

Keeping a Routine in Space

A regimented routine keeps missions on track, so an astronaut’s life in space is almost entirely pre-planned. Activities on the ISS, from meals to exercise to maintenance, are slotted into five-minute increments. And it all must be in perfect alignment with the clocks back home.

“To do what you need to do,” says Todd Ely, a senior engineer at NASA’s Jet Propulsion Laboratory, “you need to be able to tell time wherever you’re at.”

Using precise atomic clocks, astronauts stay synchronized with Coordinated Universal Time (UTC), the global standard by which all clocks are ultimately set.

Of course, Einstein’s theory of relativity states that time is not universal: It passes differently under different conditions. If one person is moving faster than another, or is closer to a massive object, then time ticks slower for them. Though these relativistic effects aren’t enormous within our solar system (and at current spacecraft speeds), they still must be factored into calculations of time and trajectory anytime we venture beyond low-Earth orbit.

“If we did not account for relativity,” Ely says, “we would not get the right answer.”

Read More: How an Astronaut Sleeps in Space

Issues with Telling Time in Space

Close to Earth, the main difficulty lies in adapting to an environment that lacks familiar temporal cues. As you get farther away, however, communication with mission control becomes a greater problem.

Messages (in the form of radio waves) can only travel as fast as the speed of light, so it takes up to 14 minutes for news from Houston to reach a spacecraft near Mars, not to mention more remote regions of the solar system. That delay poses a serious challenge for tasks that require exact timing.

The clocks we use in day-to-day life — most of which employ quartz crystals — work well enough for our purposes, but they’re woefully inadequate for astronauts because they don’t measure time consistently. Even the best of them drift fairly rapidly.

According to NASA, after just six weeks, a quartz clock could be off by a millisecond. Though that may not sound like much, it adds up, potentially resulting in huge navigation errors. For space travel in general, and especially as astronauts start venturing farther from Earth, they need the next step up in precision: atomic clocks.

Atomic Clocks

Every clock relies on a mechanism to keep steady time — a “pendulum,” either literal or figurative. In quartz clocks, that mechanism is a crystal, which resonates at a specific frequency and generates electric current when stressed.

But due to manufacturing errors and environmental factors, crystal performance degrades over time. Atoms, on the other hand, are superbly stable. Those of the same element resonate at the same frequency whenever they absorb or release energy; they are, in effect, perfect “pendulums.”

Atomic clocks still involve a quartz crystal, but they check its oscillation against more consistent atoms. If the crystal’s frequency remains spot on, it will cause the atoms to transition to a higher energy state, like an opera singer breaking a wine glass with just the right pitch. If it’s off even a little, a jolt of electricity gets sent to the oscillator as a signal to adjust the frequency.

Read More: How NASA Is Prepping Mars Astronauts to Cope With Isolation and Other Extremes

We use atomic clocks for applications where accuracy matters most. The Global Positioning System (GPS), for example, depends on them to track our movements from one second to the next. Our phones receive signals from satellites, which come with timestamps obtained from atomic clocks, which then compute how long it took for the signals to reach us. With that information, our phones can pinpoint our position on Earth.

Similar principles apply to navigation in distant space. By measuring how long it takes for radio waves to travel back and forth between a spacecraft, scientists can calculate its distance from Earth. By measuring many of these two-way signal times in sequence, they can also tell its speed and trajectory. Putting all that data together, the location of a Mars orbiter can be determined within just a few meters.

Even after more than two decades at NASA, Ely says, “I’m always amazed by that.”

Read More: Why These 6 Items Are Not Allowed in Space

New Technology to Tell Time in Space

Since the early 2010s Ely has been engineering the next leap in navigation technology: The Deep Space Atomic Clock – an extremely stable device that uses mercury atoms and weighs a fraction of its ground-based counterparts.

The DSAC works so well that spacecrafts can calculate their position and velocity based solely on one-way signals from Earth, rather than waiting half an hour for the round-trip communication delay. This allows near real-time navigation, which helps with high-stakes maneuvers like landing on another planet or entering its orbit.

So far, the DSAC has only been tested experimentally. But during a year-long trial run from 2019 to 2020, the prototype’s performance was an order of magnitude better than current space clocks. It could soon become the default, especially as astronauts begin to journey beyond our moon, toward Mars and perhaps farther.

Without exceptionally accurate clocks, Ely says, “we would not be able to explore the solar system. We would not be able to get to these destinations reliably.”

Read More: Here Are 4 Reasons Why We Are Still Going to the Moon

Article Sources

Our writers at use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

Cody Cottier is a contributing writer at Discover who loves exploring big questions about the universe and our home planet, the nature of consciousness, the ethical implications of science and more. He holds a bachelor's degree in journalism and media production from Washington State University.

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