Future Tense

How Should Space Settlers Keep Track of Time?

Our earthly system of years, months, and days doesn’t make much sense on Mars or the moon.

Person in a spacesuit looking at a watch while surrounded by stars, inspired by Matisse's Icarus.
Lisa Larson-Walker

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We appear to be on the cusp of an era where humans live outside this pale blue dot. According to some newly leaked NASA documents, the agency hopes to build a moon base by 2028. That settlement will allow us to test the skills and tools we’ll need to try to make it to Mars, says NASA, and then who knows where we’ll go from there.

There are serious technical hurdles to establishing space settlements and enduring long-term flights: How will humans grow food? What happens to our waste? But there are also squishier, logistical ones that we take for granted here on Earth. One of the first things we’ll need to figure out is how to keep track of time.

The solution might seem straightforward: just bring a watch and a calendar, and mark off the days! And yes, this is how the only full-time space settlement—the International Space Station—handles things. The crew of the ISS operates on Greenwich Mean Time and, via their close contact with Earth, gets updates on the time.

But there are challenges to keeping space dwellers on an Earth-bound time-keeping system. One is the practicality of using a time-keeping system that ignores your local reality. Keeping astronauts on a 24-hour, GMT-based system makes things easier for ground control, but trying to keep Earth hours takes a toll on astronauts’ sleep, as their circadian rhythms are thrown off by the comparatively erratic light cycle: The ISS orbits Earth every 90 minutes, so over the course of a typical 24-hour Earth “day,” the crew sees 16 sunrises and sunsets.

While astronauts can be up in space for months at a time, there’s an impermanence implied in an ISS-tenure; we know they aren’t going to be up there forever. Space settlers establishing bases further afield might adopt Earth time and calendars while starting up a new society, but as time goes on, they might want a more permanent solution tailored to their lives.

Astronomers, science-fiction writers, and enthusiastic hobbyists have presented a range of proposals for new time-keeping systems for potential space settlements. Mars has particular intrigue to those dreaming of space settlements; it’s far enough that one imagines any settlers would be there for the long haul and would develop their own systems. Thomas Gangale, creator of the Darian calendar, came up with his design in 1985, and it’s since been used in several sci-fi books, including Hannu Rajaniemi’s The Quantum Thief and the Star Trek novel series Department of Temporal Investigations, which chronicles the adventures of an agency that investigates time travel. “If we’re going to send people to Mars and settle it, they’ll need to reckon time according to the natural rhythms of the planet,” Gangale says. His system adapts our traditional Earth time-keeping methods to the rotation and solar orbit of Mars. While Earth rotates once every 23 hours and 56 minutes, Mars’ rotation is slightly longer: A Martian day is just shy of 24 hours and 40 minutes, a unit astronomers call a sol. Mars’ solar orbit is about twice as long as Earth’s: It takes 687 Earth days to our 365, which comes to 668 sols.

We could just leave time measurement at the sol level, and in fact, some Mars calendar proposals suggest that. On Earth, the Julian calendar assigns each Earth day a number, counting up from Jan. 1, 4713 B.C., as “Day 0,” which makes it easier to calculate day-based milestones like use-by dates for food. (The Julian date as of July 2019 is in the 2,458,600s.) A Mars sol-based calendar would work similarly, counting up from a “Day 0” we’d need to decide on. Gangale says perhaps we could assume the start of any Martian calendar would be the date of Galileo’s first telescopic observation of Mars, in January of 1610. Or perhaps Day 0 could coincide with other important Mars milestones, like Mariner 4’s first successful flyby of the planet (July 14, 1965) or Mars 2’s first surface landing (Nov. 27, 1971).

But we humans are creatures of habit and will likely build societies that center around some of the same values and traditions we have here on Earth. “A lot of astronomers fail to consider societal aspects of keeping time; they’re all focused on astronomy,” says Gangale. “But what’s important is that it’s people and how they use time.”

We’ll likely want weeks and month measurements to keep track of social and socioeconomic milestones: holidays, birthdays, paydays, weekends, quarterly investor reports, vacation planning, farming. Gangale said he originally entertained the idea of synchronizing Mars months to the movement of its moons, the way Earth’s months are tied to the moon’s 27-day orbit, but it wasn’t practical: Phobos orbits Mars three times a day, and Deimos completes its orbit once every 30 hours—“much too short to use as a basis for dividing the year into usable fractions,” says Gangale. Instead, he broke up the year (the one that’s the equivalent of 668 Earth days) into 24 months of 27 or 28 sols each, and each month has four seven-day weeks. Days of the week are named after celestial bodies in our solar system; month names alternate between Latin and Sanskrit names for constellations of the zodiac.

The new day and month names are fun but also functional. By using distinct names, Mars settlers would avoid confusion with Earth time, “which is going to be important, since you’re going to have trade back and forth,” says Gangale. Tuesday on Mars wouldn’t always fall at the same time as Tuesday on Earth; January on one planet could be March on the other. Like a Martian day is a “sol,” future settlers could come up with different terms for “month” and “year” too, for full differentiation from Earth’s system.

Individual settlements might evolve their own conventions according to local constraints. Gangale has adapted the Darian calendar for Jupiter’s moons Io, Ganymede, Callisto, and Europa, and Saturn’s moon Titan, which could be humanity’s next outposts beyond Mars. For settlers staying closer to home, nonprofit group LunarClock.org advocates for Lunar Standard Time and the Lunar Calendar, where a year consists of 12 “days” (comparable to Earth months), each named for a person who has walked on the moon, broken up into 30 “cycles” (comparable to Earth days) of 24 moon-hours.

The other challenge settlers will face in developing alternatives to Earth time is actually keeping the time. Time on Earth is standardized by incredibly stable atomic clocks, which drift by just one second every 15 billion years. Unfortunately, they’re the size of refrigerators. Spacecraft keep time on board using ultrastable oscillators, which allow spacecraft to plan and execute maneuvers, but “they’re not [really] ultrastable—they drift over time,” says Jill Seubert, deep space navigator at NASA. (I dare you to find a better job title than Seubert’s.) The first ultrastable oscillator, or USO, was used on the Voyager probes in the 1970s, and they’re still in satellites today. Even over the course of an Earth day, those satellites must be recalibrated to ensure technologies like GPS are as accurate as possible. Being off by just a millionth of a second can mean a GPS discrepancy of hundreds of meters.

Currently, engineers take care of this issue by translating between precise Earth time and the spacecraft’s USO time readings. But as we get deeper into space or need to perform more instantaneous maneuvers, depending on Earth time would quickly become cumbersome. While the lag between Earth and the moon is only about a second, that balloons to between seven and 22 minutes between here and Mars, depending on the relative positions of the two planets, which varies over the course of its orbits. Seubert is the deputy principal investigator of a team of NASA engineers at the Jet Propulsion Laboratory building the first deep space atomic clock. “Right now, every spacecraft out there through deep space is being navigated by people on Earth; we compute the trajectory solution on Earth and upload it to the spacecraft,” says Seubert. Instead of relying on communications with Earth, spacecraft could carry their own atomic clocks, enabling them to keep more precise local time, which will in turn allow them to compute their own locations and execute maneuvers without help from back home. In late June, the deep space atomic clock hitched a ride into space aboard a SpaceX rocket; beginning in August, JPL scientists will evaluate the clock for a year to determine its stability.

Plus, it will be necessary for other technologies necessary for a space colony, both as a time keeper and for local GPS. Sci-fi movies often show astronauts driving around on the lunar or Mars surface with a map screen showing their progress. Currently, that’s not possible, but a deep space atomic clock could provide precise enough time and location to support that real-time tracking.

So far, most of the space time-keeping ideas floated have been modeled on what we do here on Earth. It may not be practical, but perhaps settlers will take the opportunity of leaving our planet to devise entirely new conventions. We could abolish weeks, months, and years altogether, relying instead on just local days, like the stardate system used in Star Trek or the Mars sol-based calendar. Or, more radically, perhaps space settlers will follow the Arctic town of Sommarøy in abolishing time altogether. With months of darkness and months of sunlight, these Norwegian citizens lack typical cues associated with a “day”—something space travelers on yearslong journeys or settlers in new lands are likely to experience as well. In an interview with Gizmodo, Sommarøy citizen Kjell Ove Hveding spoke of the tyranny of the clock. “You have to go to work, and even after work, the clock takes up your time,” he said. “I have to do this, I have to do that. My experience is that [people] have forgotten how to be impulsive, to decide that the weather is good, the Sun is shining, I can just live.” In space, the sun might not be shining, and the weather may not be good, but our exploration rides on the same home: that we will just live.

Future Tense is a partnership of Slate, New America, and Arizona State University that examines emerging technologies, public policy, and society.