Time in space is measured using atomic clocks, just as it is on Earth, but with critical adjustments for the physics of moving at high speeds and operating in weaker gravity. A standard atomic clock on a GPS satellite drifts by about 38 microseconds per day compared to clocks on the ground, enough to cause positioning errors of roughly 10 kilometers if left uncorrected. Every space mission, from low Earth orbit to deep space probes, depends on solving this problem precisely.
Why Clocks Run Differently in Space
Two effects from Einstein’s theory of relativity pull time in opposite directions for any object in orbit. First, moving at high speed causes time to slow down relative to a stationary observer. The International Space Station travels at about 7,700 meters per second, and after six months in orbit, an astronaut has aged roughly 0.005 seconds less than someone on the ground. That’s special relativity at work.
Second, gravity itself warps time. A clock farther from Earth’s center, where gravity is weaker, ticks slightly faster than one on the surface. This is general relativity. For the ISS, the speed effect wins out, so time runs a tiny bit slower overall. But for GPS satellites orbiting much higher, the gravitational effect dominates, and their clocks tick faster than ground clocks. There’s actually a specific orbital altitude, about 1.5 times Earth’s radius, where the two effects perfectly cancel each other out. The ISS flies below that threshold; GPS and geostationary satellites fly above it.
GPS is the clearest everyday proof that these corrections matter. When the first experimental GPS satellite launched in the 1970s, engineers measured its clock running fast by 442.5 parts per trillion compared to ground clocks. General relativity had predicted 446.5 parts per trillion, a remarkably close match. Today, every GPS satellite has its onboard clock frequency pre-adjusted downward before launch so that it appears to tick at the correct rate when viewed from the ground. Even after that correction, residual relativistic wobbles of up to 23 nanoseconds per orbit cycle are calculated and transmitted to GPS receivers as part of the navigation signal.
Atomic Clocks Built for Spacecraft
Ground-based atomic clocks are extraordinarily precise, but they’re large, power-hungry, and not designed to survive rocket launches or the radiation environment of space. Shrinking that technology to fit on a spacecraft while keeping it accurate has been a major engineering challenge.
NASA’s Deep Space Atomic Clock (DSAC) represents a significant step forward. It uses trapped mercury ions held in place by electric fields, with magnetic shielding to protect them from environmental interference. The mercury ions vibrate at a specific frequency (40.50 GHz), and that vibration acts as a reference to keep a quartz oscillator locked to an extremely stable rate. The result is a clock that would accumulate no more than 1 microsecond of error over 10 years of operation. That’s more stable than the atomic clocks currently flying on GPS satellites, and small enough to ride on a spacecraft headed to Jupiter or beyond.
The practical advantage is enormous. Most deep space missions today rely on a two-way system: a ground station sends a signal to the spacecraft, the spacecraft bounces it back, and navigators on Earth measure the round-trip time to determine position. This works, but it means the spacecraft can never know its own location in real time. It has to wait for instructions from Earth, and the farther out it goes, the longer that wait becomes. With a sufficiently accurate onboard clock, a spacecraft could process one-way signals and navigate autonomously, cutting the communication delay in half and reducing dependence on Earth-based infrastructure.
How Time Transfer Works Between Spacecraft and Earth
Synchronizing a clock in space with one on the ground requires sending signals back and forth and carefully accounting for how long those signals take to travel. There are two main approaches.
In the simpler method, called half-duplex, a signal goes one way and then the other, like a conversation where only one person talks at a time. The system measures the total round-trip delay and assumes each leg took exactly half. This is how internet time synchronization protocols work, and it’s adequate when the signal path is stable. But if the delay fluctuates during the round trip, the estimate gets skewed.
The more robust method, full-duplex two-way transfer, sends signals in both directions simultaneously. Both stations record their measurements, exchange the data, and compute the difference. Because the signals travel through nearly identical conditions at the same moment, most sources of error cancel out. The international bureau responsible for coordinating world time (the BIPM) uses a satellite-based version of this technique for some of its most critical links between timekeeping labs, because it outperforms GPS-based methods. The tradeoff is complexity: both stations need full transmit-and-receive hardware.
Timekeeping on Mars
Mars has its own version of a day, called a sol, lasting 24 hours, 39 minutes, and 35.244 seconds. That’s only about 3% longer than an Earth day, close enough that mission controllers can work on a Mars-synced schedule, but different enough to cause gradual drift from Earth time that compounds quickly over weeks.
Since the Viking missions in 1976, Mars surface operations have used a 24-hour clock divided into Mars-hours, Mars-minutes, and Mars-seconds, each slightly longer than their Earth counterparts. Mission teams count sols from the day a lander touches down (designated Sol 0 or Sol 1, depending on the mission). There’s no single accepted Mars calendar with months and years, though many have been proposed. NASA does use a running count called the Mars Sol Date, which numbers sols sequentially from a reference point in December 1873, providing a universal way to timestamp events across different missions.
During the early weeks of rover missions like Spirit, Opportunity, and Curiosity, ground teams at the Jet Propulsion Laboratory actually shifted their own work schedules to align with Mars time. That meant showing up 40 minutes later each day, a disorienting experience that played havoc with sleep cycles but kept operations synchronized with the rover’s daylight hours.
A Time Standard for the Moon
As multiple countries and private companies plan lunar missions, the lack of a shared time standard on the Moon has become a real problem. In April 2024, the White House directed NASA to develop Coordinated Lunar Time (LTC), with a finalized strategy due by the end of 2026.
The policy requires LTC to meet four criteria: it must be traceable to Coordinated Universal Time (UTC) on Earth, accurate enough for precision navigation and scientific measurements, resilient enough to keep working if contact with Earth is temporarily lost, and scalable to environments beyond the Earth-Moon system. That last requirement is forward-looking, anticipating that whatever framework works for the Moon should eventually extend to Mars and beyond.
Why not just use UTC on the Moon? Because clocks on the lunar surface tick at a slightly different rate than clocks on Earth, thanks to the Moon’s weaker gravity. Over time, that drift adds up. LTC would track and distribute the offset between local lunar time and UTC, so that rovers, habitats, and satellites in lunar orbit can all coordinate precisely while still staying linked to Earth’s time system.
Using Pulsars as Natural Clocks
One of the more creative approaches to space timekeeping doesn’t involve building a better clock at all. It involves reading clocks that already exist in nature. Pulsars, the rapidly spinning remnants of dead stars, emit beams of X-rays with extraordinary regularity. Some pulse so predictably that they rival atomic clocks in stability over long periods.
X-ray pulsar navigation, known as XNAV, works by detecting these pulses with an onboard sensor and comparing their arrival times against a catalog of known pulsar profiles. By measuring the exact moment a pulse arrives and cross-correlating it with the expected pattern, a spacecraft can determine both its position and the time without any input from Earth. The technique uses cross-correlation, essentially sliding the incoming signal against the stored reference profile until the patterns line up, to pinpoint the pulse arrival with high precision.
XNAV is still a developing technology, and it requires an accurate onboard master clock to time-tag each detected photon. But it offers something no Earth-based system can: fully autonomous navigation anywhere in the solar system, using signals that are available everywhere and belong to no single nation’s infrastructure.
Next-Generation Optical Clocks
The frontier of space timekeeping is optical clocks, which use atoms that vibrate at the frequencies of visible light rather than microwaves. Because light oscillates roughly 100,000 times faster than microwaves, these clocks can slice time into far thinner increments, achieving precision at least ten times better than the best microwave atomic clocks.
The European Space Agency has been developing the Space Optical Clocks (SOC) project, which aims to operate a strontium-based optical lattice clock on the ISS. The target performance is a fractional inaccuracy below 5 parts in 10^17, meaning the clock would neither gain nor lose a second over roughly 600 million years. The payload would include an optical frequency comb to translate the clock’s light-frequency signal into countable pulses, plus microwave and optical links to compare the space clock with ground clocks across multiple continents.
Clocks this precise don’t just keep better time. They become scientific instruments. Tiny differences in how fast two clocks tick can reveal variations in gravitational fields, test predictions of general relativity with unprecedented sensitivity, and even detect gravitational waves. Placing one in orbit creates a tool for mapping Earth’s gravity field and probing fundamental physics in ways that aren’t possible with ground-based experiments alone.