On Earth, time is easy to measure—we use the rotation of the planet to define a day and the orbit around the Sun to define a year. But in the weightless, vast environment of space, keeping time is not so simple. With no sunrises or sunsets in orbit, and no fixed points of reference, astronauts and scientists must rely on technology and precise systems to measure time in space.
Why Time Measurement Matters in Space
Timekeeping in space is essential for:
- Coordinating communication with Earth
- Managing astronaut schedules (sleep, meals, experiments)
- Navigating spacecraft through deep space
- Timing precise maneuvers and orbital corrections
- Synchronizing onboard computers and international missions
Even a small timing error can lead to major mission problems—especially in space travel where distances and velocities are extreme.
How Astronauts Track Time on the ISS
Astronauts aboard the International Space Station (ISS) orbit Earth approximately every 90 minutes, experiencing 16 sunrises and sunsets each day. To maintain a stable schedule, the ISS follows Coordinated Universal Time (UTC)—the same global time standard used in air travel and satellite systems.
Crew members use Earth-based clocks and follow strict routines:
- 8-hour workdays with built-in exercise and rest
- Scheduled times for communication with mission control
- Lighting systems that simulate Earth’s natural day-night cycle
This helps regulate their circadian rhythm—the body’s internal clock—even when there’s no natural day or night.
What About Time on the Moon or Mars?
Future missions to the Moon or Mars raise new challenges for timekeeping:
- A lunar day lasts about 29.5 Earth days
- A Martian day (Sol) is about 24 hours and 39 minutes
NASA and other agencies are developing special clocks and planetary time zones to accommodate these cycles. For example, Mars missions use “Sol time” to track the days spent on the planet.
Deep Space Timekeeping
As spacecraft travel farther from Earth, delays in communication become significant:
- Light takes over 8 minutes to reach Mars
- Signals to distant probes like Voyager 1 take more than 20 hours
Spacecraft rely on atomic clocks and onboard computers to execute tasks autonomously. NASA’s Deep Space Atomic Clock experiment is paving the way for extremely accurate space-based navigation, like a GPS for space.
Can Time Pass Differently in Space?
Yes—according to Einstein’s theory of relativity, time actually moves more slowly in stronger gravitational fields and faster in weaker ones.
This means:
- Time passes slightly faster on the ISS (in orbit) than on Earth
- Deep space travelers would age slightly slower than people on Earth
This phenomenon, called time dilation, has been measured using atomic clocks and is a real consideration for long-term space travel.
Glossary
- UTC (Coordinated Universal Time) – The global standard for timekeeping.
- Circadian rhythm – The body’s internal 24-hour cycle that regulates sleep and activity.
- Atomic clock – A highly precise clock that uses vibrations of atoms to measure time.
- Sol – One full day on Mars, approximately 24 hours and 39 minutes.
- Time dilation – The stretching of time due to speed or gravity, predicted by Einstein’s relativity.
