ACES: Atomic Clock Ensemble in Space
The Atomic Clock Ensemble in Space (ACES) is a European facility that will test fundamental physics from the outside of ESA’s Columbus module on the International Space Station. By creating a “network of clocks”, ACES will link its own highly precise timepieces with the most accurate clocks on Earth and compare them to measure the flow of time.
According to Einstein’s theory of general relativity, gravity affects the passing of time. Experiments on Earth have shown that time flies faster at higher altitudes, such as the tops of mountains, than at sea level. ACES will take this experiment to the next level, making precise measurements on the Space Station as it flies 400 km above Earth. The data gathered by ACES will offer scientists new insights into the relationship between gravity and time, advancing our understanding of fundamental laws of physics.
Two clocks
ACES includes two cutting-edge clocks: PHARAO (Projet d’Horloge Atomique à Refroidissement d’Atomes en Orbite) and SHM (Space Hydrogen Maser).
PHARAO is a caesium atomic clock developed by the French space agency CNES. The clock uses lasers to cool caesium atoms close to the absolute zero of temperature around –273 degrees Celsius; this allows extremely precise measurements of time and frequency.
On Earth, a caesium fountain clock is about two or three metres high to allow enough room for the caesium atoms to be launched upwards and interact with the microwave fields of the clock, before falling due to gravity. In free fall conditions on the International Space Station, the caesium atoms can be launched more slowly over a short distance and still have plenty of time for interaction; this allows a drastic reduction of PHARAO's size while retaining a high stability.
SHM is an active hydrogen maser, a device which uses hydrogen atoms to tell time, produced in Switzerland by Safran Time Technologies. This clock uses hydrogen as an atomic frequency reference and its operation resembles that of passive masers used onboard the Galileo satellites, but ten times more stable.
The ACES clock signal combines the excellent stability of SHM over a period of one hour with the long-term stability and accuracy of PHARAO. Together, these clocks provide timekeeping with a precision of one second over 300 million years.
In space
Once in space, a robotic arm will position ACES onto the Columbus module, where it will remain for 30 months to collect data. ACES aims to record continuous data over at least ten sessions of 25 days each. The experiment will be operated from Europe, through CADMOS in Toulouse, France, and the Columbus Control Centre near Munich, Germany.
The ACES clock signal is transmitted to a network of ground clocks by two time and frequency links: the microwave link (MWL) operating at microwave frequencies and the European Laser Timing (ELT) optical link. MWL ground terminals in Europe, the UK, the US and Japan will communicate with ACES to exchange time information whilst compensating for the effects of the atmosphere and the harsh conditions of space.
Satellite laser ranging stations in Europe, such as the observatory near Wettzell in Bavaria, connected to atomic clocks will also participate to the ACES experiments through ELT; their antennas will fire laser pulses towards the Space Station and receive the returning echoes that, after timestamping in the ACES and ground clock time, will be used to measure space-to-ground desynchronisation.
The science
The ACES science is truly interdisciplinary. By comparing clocks in space and on Earth, ACES will provide scientists with precise measurements to test Einstein’s gravitational time dilation effect, search for time variations of fundamental constants of physics and hunt for dark matter.
The global network of ground-based clocks connected through ACES will also enable scientists to measure geopotential differences across continents, perform clock synchronisation experiments and distribute global time scales.
Optical clocks have reached a fractional frequency stability and accuracy of one part in 1018, surpassing microwave clocks by two orders of magnitude. ACES will connect the best optical clocks across continents and compare them to one part in 1017, a level of precision that current satellite systems cannot achieve.
“We are thrilled by the opportunities that the clock network established by ACES will bring for fundamental physics research, geodesy applications and global timekeeping,” says 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,” he adds.
The journey
The ambitious project began over three decades ago, a testament to the highly complex nature of the technology in ACES. The design for PHARAO, which is now about the size of a small fridge, is based on an atomic clock which fills an entire room in the Paris Observatory. In those years, ACES has been a voyage of discovery as engineers have worked hard to produce an incredibly accurate measurement system.
“ACES is a highly sensitive apparatus made of intricate and interconnected subsystems that must work in harmony,” says 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. It is high-precision work for a high-precision facility,” explains Thomas.
Today, ACES is fully assembled at Airbus in Friedrichshafen, Germany, and is undergoing rigorous testing until the end of the year, after which it will be ready for its launch on a SpaceX Falcon 9 rocket, which is expected to take place in the first half of 2025.