Space Clock Redefines Time Measurement Itself

Time moves differently in space – and a revolutionary European timepiece launched this week will prove it with unprecedented precision.

The Atomic Clock Ensemble in Space (ACES) successfully reached orbit on April 21, launching aboard a SpaceX Falcon 9 rocket from NASA’s Kennedy Space Center. The mission puts the most accurate timekeeping system ever sent to space on the International Space Station, where it will test fundamental physics theories while losing just one second every 300 million years.

“The launch of ACES marks a major milestone for European science and international cooperation in space,” said Daniel Neuenschwander, Director of Human & Robotic Exploration at ESA. “With this mission, we are placing the most precise timepiece ever sent to orbit aboard the International Space Station — opening new frontiers in fundamental physics, time transfer, and global synchronization.”

At the heart of ACES are two remarkable clocks: PHARAO (Projet d’Horloge Atomique à Refroidissement d’Atomes en Orbite), developed by the French space agency CNES, and SHM (Space Hydrogen Maser), built by Safran Timing Technologies in Switzerland.

The PHARAO clock uses lasers to cool caesium atoms to nearly absolute zero (approximately -273 degrees Celsius), allowing for incredibly precise time measurements. Unlike Earth-bound caesium clocks that require significant height to function, PHARAO takes advantage of the Station’s microgravity environment to achieve the same precision in a much smaller package – roughly the size of a compact refrigerator.

Meanwhile, the SHM clock uses hydrogen atoms as its frequency reference, similar to technology used in Europe’s Galileo navigation satellites but with tenfold greater stability. Together, these instruments will maintain time with astonishing accuracy – equivalent to losing just one second in 300 million years.

The scientific implications extend far beyond simple timekeeping. ACES will directly test Einstein’s theory of general relativity, which predicts that gravity affects the passage of time. Previous Earth-based experiments have confirmed that clocks run slightly faster at higher altitudes where gravity is weaker. ACES takes this verification to new heights by comparing its space-based clocks with the world’s most advanced ground-based timepieces.

“We are thrilled by the opportunities that the clock network established by ACES will bring for fundamental physics research, geodesy applications and global timekeeping,” said Luigi Cacciapuoti, the ACES project scientist at ESA. “ACES is today responding to an urgent need in the scientific community and will surely play a key role in pushing towards the re-definition of the standard unit of time – the so-called SI second –, in terms of an optical frequency standard.”

The path to launch wasn’t straightforward. Engineering a device sensitive enough to measure time with such extraordinary precision – yet robust enough to survive the rigors of launch and space operations – presented enormous challenges over the project’s three-decade development.

“ACES is a highly sensitive apparatus made of intricate and interconnected subsystems that must work in harmony,” explained Thomas Peignier, ACES Principal Engineer. “The team faced many challenges and had to devise clever solutions. For example, to prevent the clocks from being damaged by exposure to magnetic fields, we conducted magnetic surveys before moving ACES anywhere, used special equipment to protect ACES during testing, and all tools, electronic devices and metallic pieces, down to the very nuts and bolts, are measured and demagnetised if necessary before they go near ACES.”

Later this week, a robotic arm will install ACES on the exterior of ESA’s Columbus module in the Earth-facing position. The facility will operate for 30 months, collecting data through at least ten measurement sessions, each lasting 25 days, as it orbits Earth 16 times daily.

The mission connects timepieces across continents through microwave and laser links that synchronize time between space and Earth with unprecedented accuracy. Ground stations throughout Europe, the UK, US, and Japan will communicate with ACES, compensating for atmospheric effects and the harsh conditions of space.

Beyond testing relativity, ACES will search for variations in fundamental physical constants and potentially detect evidence of dark matter. It will also enable scientists to measure differences in Earth’s gravitational field across continents and improve global time synchronization to levels impossible with current satellite systems.

As ACES orbits 400km above Earth, it pushes human understanding of time itself – potentially changing how we define our most fundamental unit of measurement while demonstrating Europe’s leadership in cutting-edge space science.