Today at 10:15 CEST, ESA’s Atomic Clock Ensemble in Space (ACES) began its journey to the International Space Station on a SpaceX Falcon 9 rocket launched from NASA’s Kennedy Space Center in Florida, United States.

ACES carries the most accurate clocks ever flown in space – PHARAO and the Space Hydrogen Maser – designed to keep time so precisely that they would lose just one second every 300 million years. Developed by the French space agency CNES and Safran Timing Technologies in Switzerland, these European-built clocks will work with a sophisticated time transfer time using microwave and laser links to synchronise the best clocks all over Earth.

Later this week, the Station’s Canadian robotic arm will install ACES on the exterior of ESA’s Columbus module. From its vantage point 400 km above Earth, ACES will link its ultra-precise clocks with the best timekeepers on the ground, enabling groundbreaking tests of fundamental physics, including Einstein’s theory of general relativity. Over its 30-month mission, it will carry out extended measurement sessions to investigate the very nature of time and enhance global time synchronisation.

ACES is a fully European mission led by ESA, with Airbus as a prime contractor, and will be operated from control centres in France and Germany (CADMOS and Col-CC).

“The launch of ACES marks a major milestone for European science and international cooperation in space. 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. ACES is a shining example of what Europe can achieve when we unite cutting-edge technology, scientific ambition, and strong partnerships”, says Daniel Neuenschwander, Director of Human & Robotic Exploration at ESA.

Would you like to find out more? Here’s seven things you probably didn’t know about ACES.