Einstein Goes to Space

The current landscape of space exploration is incredibly dynamic, with numerous developments taking place. I could have covered various topics, such as the Chinese Tianwen-2 mission aiming to return samples from an asteroid (another instance of China's Space Program operating at full capacity, as I discussed in my previous post here), SpaceX's ongoing challenges with the Starship program (which I had covered here), the discovery of a new Trans-Neptunian Object (TNO) dwarf planet that could have implications for Planet 9 (which I covered in my last post here), the proposed significant budget cuts for NASA, and much more.

Yet, an event that hasn't received much publicity but has captured my interest is the ACES mission, which could significantly influence our comprehension of fundamental physics, no less! Einstein's theory of general relativity suggests that gravity influences the flow of time. Experiments conducted on Earth have demonstrated that time moves more quickly in a weaker gravitational field, such as higher elevations, like mountain tops, compared to sea level. The Atomic Clock Ensemble in Space (ACES) mission plans to elevate this experiment by conducting precise measurements with two clocks set on the International Space Station, which orbits 400 km above Earth. These ultra-stable atomic clocks would create a "network of clocks" that links highly precise timepieces in space with the most accurate clocks on Earth, providing scientists with new insights into the connection between gravity and time.

The ACES payload (Figure 1) consists of two cutting-edge atomic clocks: the PHARAO (Projet d’Horloge Atomique à Refroidissement d’Atomes en Orbite) clock and the Space Hydrogen Maser (SHM). The PHARAO clock, developed by the French space agency CNES, uses lasers to cool caesium atoms to temperatures close to absolute zero, allowing for extremely precise measurements of time and frequency. The SHM, developed by Spectratime in Switzerland, provides short-term stability and complements the long-term stability of the PHARAO clock.

Figure 1: ACES's instruments from the ESA website

Launched aboard a SpaceX Falcon 9 from NASA's Kennedy Space Center on April 21, 2025, ACES travelled to the ISS as part of the 32nd SpaceX commercial resupply mission. Only four days later, the Station's Canadian-built robotic arm successfully installed the advanced timekeeping payload on the Earth-facing side of ESA's Columbus laboratory, where it is set to function for the next 30 months.

In addition to advancing our understanding of the fundamental laws of general relativity and opening up new possibilities for future research and applications in this field, the ACES mission will also contribute to time and frequency metrology, such as the next generation of atomic quantum sensors, very long baseline interferometry used for space navigation, and other areas of research that require a stable and accurate time base. The mission will serve as a test-bed for the space qualification of the active hydrogen maser and help optimize its performance for future space missions. 

In other words, the findings from the ACES mission will improve our capacity to expand and explore our Solar System. Not bad for two clocks. I'll keep on following the progress of the mission in the coming two years. Given the above, you might want to do the same. As always, onwards and upwards.