Engineers driving Mars rovers must live on 24.6-hour 'Mars time'
A Martian day, known as a sol, lasts 24 hours, 39 minutes, and 35 seconds—just slightly longer than a day on Earth. Because solar-powered and active robotic missions on Mars operate during daylight hours, mission controllers at NASA have had to adapt to "Mars time." Team members shift their daily work, eating, and sleeping schedules 40 minutes later each day, wearing custom watches to synchronize their lives with the Martian sun.
The Geometry of a Martian Day
A day on Mars is remarkably close in length to a day on Earth, but the slight difference creates profound challenges for planetary exploration. Planetary scientists refer to a Martian solar day as a 'sol.' While Earth completes a solar rotation in 24 hours, Mars takes approximately 24 hours, 39 minutes, and 35.244 seconds to complete one mean solar day. The term was formally adopted by NASA during the Viking lander missions in 1976 to provide a distinct, unambiguous unit of time for surface operations, separating the rhythm of the red planet from the terrestrial calendar.
The difference between a planet's sidereal day—the time it takes to rotate once relative to distant background stars—and its solar day—the time it takes for the Sun to return to the same meridian in the sky—stems from orbital motion. A Martian sidereal day lasts approximately 24 hours, 37 minutes, and 22.66 seconds. Because Mars moves along its elliptical orbit around the Sun while spinning on its axis, the planet must rotate slightly more than 360 degrees for the Sun to reach the same midday position overhead. This extra rotation adds roughly two minutes, producing the 24-hour, 39-minute solar day that governs sunlight on the Martian surface.
Operating by the Martian Sun
Planetary rovers and landers operate under the direct authority of the Martian sun. Solar-powered missions, such as the Mars Exploration Rovers Spirit and Opportunity, rely on solar panels to generate electrical energy for driving, heating vital components, and operating scientific instruments. Even missions powered by radioisotope thermoelectric generators, such as Curiosity and Perseverance, depend heavily on local daylight to capture visible-light imagery, assess geological targets, and navigate rocky terrain safely.
Beyond direct sunlight, robotic operations must align with communications windows. Rovers transmit data to and receive commands from Earth either through direct high-gain antennas or by relaying signals through orbiters circling overhead, such as the Mars Odyssey or Mars Reconnaissance Orbiter. These satellite passes occur at specific local solar times each Martian day. To make efficient use of every sol, ground control teams on Earth must analyze incoming data as soon as Martian evening arrives, plan the next sequence of commands overnight on Mars, and transmit instructions before the Martian morning begins.
Because the operational schedule is bound to the rover's local daylight, the flight control team at NASA's Jet Propulsion Laboratory cannot follow a standard Earth workday. A work shift that begins at rover sunrise will start roughly 40 minutes later each Earth day. Over the course of just three weeks, day shifts completely rotate into night shifts, forcing ground teams to abandon ordinary terrestrial routines.
Crafting Watches for Another Planet
When the Mars Exploration Rover team prepared for Spirit and Opportunity, engineers recognized that continually converting between Earth time and local Martian time was slow, confusing, and prone to error. Digital clocks could be programmed with custom code, but mission personnel needed an immediate, mechanical reference on their wrists while moving between planning rooms, mission control, and home.
Engineers turned to Garo Anserlian, a master watchmaker and jeweler based in Montrose, California. Adjusting a standard wristwatch to run on Mars time presented a delicate mechanical puzzle. Martian time runs roughly 2.7 percent slower than Earth time; 24 Mars hours equal 24 hours, 39 minutes, and 35 seconds of Earth time. To achieve this, Anserlian opened self-winding mechanical wristwatches and painstakingly adjusted the internal balance wheel and movement weights to slow the mechanism down by exactly 39 minutes and 35 seconds across a 24-hour period.
The result was a wristwatch calibrated to tick off 24 'Martian hours' in one sol. Because Spirit and Opportunity operated on opposite sides of the planet—in Gusev Crater and Meridiani Planum respectively—their local times differed dramatically. Some mission controllers working on cross-rover operations wore multiple wristwatches: one running on Earth time, one tuned to Spirit's local time, and another tracking Opportunity's sol.
The Human Toll of the 40-Minute Slip
Living on Mars time creates an unusual physiological strain. The human circadian pacemaker naturally runs slightly longer than 24 hours, but forcing the body to adjust to a 24.65-hour cycle continuously over weeks and months produces a state of perpetual, rolling jet lag. Every single day, a rover driver's alarm clock sounds roughly 40 minutes later than it did the day before.
Team members resorted to aggressive countermeasures to keep their sleep cycles in sync with their shifting shifts. Engineers taped heavy black plastic or aluminum foil over their bedroom windows to shut out bright southern California daylight when their schedules forced them to sleep at noon. Team members often wore dark sunglasses on their morning commutes home from the Jet Propulsion Laboratory to prevent terrestrial sunlight from resetting their circadian rhythms.
The social disorientation proved just as challenging as the physical fatigue. Within a few weeks, an engineer's Martian noon would coincide with Earth midnight, meaning they would eat dinner at eight in the morning and breakfast late at night. Maintaining relationships with family members living on an unyielding 24-hour cycle became a recurring logistical hurdle, as work, meals, and rest phases drifted entirely out of phase with the surrounding world.
Standardizing Martian Clocks and Coordinates
To manage operations across multiple landing sites, scientists established coordinate systems and timekeeping conventions modeled on Earth's systems. A mission tracks progress using sequential sol counts, typically beginning with Sol 0 or Sol 1 on the day of touchdown. Because each lander sits at a distinct longitude, each mission relies on its own Local Mean Solar Time (LMST) or Local True Solar Time (LTST), matching the sun's position at that exact location.
On a planetary scale, Mars possesses a global prime meridian defined by the small crater Airy-0, located within the larger crater Airy. Just as Earth uses Greenwich Mean Time and Coordinated Universal Time (UTC) to establish a global temporal reference, Mars has Coordinated Mars Time, often referred to as MTC. This baseline allows mission planners and planetary cartographers to synchronize orbital observations and astronomical phenomena across the entirety of the Martian globe.
Endurance and the Evolution of Mission Scheduling
Despite custom watches and darkened bedrooms, humans cannot sustain rigid Mars time indefinitely without severe fatigue and disruption to daily life. During early surface missions, such as the initial 90-sol primary phases for Spirit and Opportunity, full mission teams lived on Mars time to extract the absolute maximum scientific return during the guaranteed lifespan of the rovers.
As missions proved their durability and extended across years and decades, NASA developed modified scheduling strategies. Rather than forcing hundreds of operators to drift continuously around the clock, flight teams transitioned to hybrid planning schedules. In these models, data downlinks are processed during set terrestrial working hours, or shifts are staggered within manageable windows, eliminating the extreme physiological toll of endless 40-minute daily slips while continuing to drive rovers across the Martian surface.
Key takeaways
•A Martian solar day, or sol, lasts 24 hours, 39 minutes, and 35.244 seconds, which is roughly 2.7 percent longer than an Earth day.
•Rover operators must synchronize their work with the Martian daylight and communication passes, shifting their schedules roughly 40 minutes later every Earth day.
•During the Mars Exploration Rover missions, watchmaker Garo Anserlian mechanically modified wristwatches to run slower so engineers could directly read local Mars time.
•Long-term missions eventually adopted modified or hybrid schedules because living on continuous Mars time causes severe, rolling circadian disruption.