Atomic clocks enter orbit to test relativity and redefine time standards

Time doesn’t pass uniformly across different gravitational environments. According to Einstein’s general theory of relativity, gravity affects how time progresses: the stronger the gravitational field, the more slowly time passes. This effect has been confirmed in ground-based experiments showing that clocks positioned at higher elevations tick slightly faster than those at lower.

To expand these findings beyond Earth’s surface, researchers have launched the Atomic Clock Ensemble in Space (ACES), a mission designed to push the limits of timekeeping in orbit. Operating aboard the International Space Station (ISS) at an altitude of about 400 km (250 miles), ACES will compare the behavior of atomic clocks in microgravity with those on the ground. This setup enables high-precision tests of gravitational time dilation in a new regime, helping scientists explore how gravity shapes the flow of time under conditions not replicable on Earth.

To achieve its objectives, ACES carries two complementary atomic clocks designed to function with extreme precision in the unique conditions of low Earth orbit.

Dual atomic clocks at the heart of ACES

ACES carries two advanced timekeeping instruments, the Projet d’Horloge Atomique à Refroidissement d’Atomes en Orbite (PHARAO) clock and Space Hydrogen Maser (SHM).

A product of the French space agency Centre National d’Etudes Spatiales (CNES), PHARAO is an atomic clock that relies on caesium atoms. It uses laser cooling techniques to lower the temperature of these atoms to approximately -273°C (-459.4°F). This ultra-cold state enables highly accurate time and frequency readings.

Terrestrial caesium clocks require significant height to let atoms travel upward and return under gravity. Aboard the ISS, where gravity is minimal, atoms drift slowly and don’t need much space. This makes it possible to build a more compact PHARAO clock while keeping its precision intact.

SHM is a hydrogen maser that relies on hydrogen atoms for precise timekeeping. Manufactured by Safran Time Technologies in Switzerland, it uses hydrogen as a reference for atomic frequency. Its design is similar to the passive masers aboard Galileo satellites, but with greater stability.

By integrating SHM’s stability with PHARAO’s precision, the ACES system delivers pinpoint timekeeping. This combination ensures that the clocks will differ by just one second after 300 million years.

Space deployment and global time transfer network

After launch, the ISS robotic arm installed ACES on the Columbus laboratory module, where it will operate for 30 months.

The mission is designed to record continuous measurements across ten sessions, each lasting 25 days. Operations will be managed from Europe, with Centre for Advanced Research and Development on Microgravity Science and Technology (CADMOS) in Toulouse and the Columbus Control Centre in Munich overseeing the process.

To support its physics experiments and precise timekeeping objectives, ACES shares its clock signal with a global network of ground stations. This is done using two separate systems:

1. Microwave Link (MWL): This radio-based system connects ACES with ground terminals across Europe, the UK, the US, and Japan. It allows for time data exchange and helps correct for delays caused by atmospheric interference and space conditions.
2. European Laser Timing (ELT): An optical system using satellite laser ranging stations in Europe, such as the one near Wettzell, Germany. These stations send laser pulses to the ISS and detect the return signal. By comparing the timestamps from both ends, scientists can assess any time differences between space and Earth.

ACES and the future of high-precision timekeeping

ACES will allow researchers to compare hyper-specific clocks on the Space Station with those on Earth. This will help test how gravity alters the passage of time, examine whether physical constants change, and search for possible evidence of dark matter.

Through its connection to a global network of ground-based clocks, ACES will also enable measurements of Earth’s gravitational variations. It will also support precise clock synchronisation and help maintain international time standards.

The precision of optical clocks has advanced to the point where they can measure time with an error margin as small as one part in 10¹⁸, making them more accurate than microwave clocks. ACES will make it possible to compare these clocks across continents with a resolution down to one part in 10¹⁷. This level of accuracy is something current satellite technology is not equipped to handle.

From prototype to spaceflight

The ACES project was initiated over 30 years ago. The PHARAO clock was originally occupying an entire room at the Paris Observatory. Now, it has been miniaturized to the size of a small fridge while maintaining its trademark precision.

After being assembled and rigorously tested at Airbus in Friedrichshafen, Germany, ACES was transported to NASA’s Kennedy Space Center in March 2025 for final preparations.

On April 21, 2025, it was launched aboard SpaceX Commercial Resupply Mission 32, docking with the ISS the following day. The payload was installed on the Columbus laboratory by the Station’s robotic arm, and by April 28, ACES was successfully powered on and established communication with ground control.

After completing a six-month commissioning phase, ACES will begin its two-year science mission of improving precision timekeeping.