"Redefining the Second"... Korea to Compare ISS Atomic Clock and Space Time [Reading Science]

KRISS and KASI Begin Joint Research Comparing Atomic and Optical Clocks on the ISS
Korea Becomes Second in the World After Germany to Join ACES-ELT...
Expected to Contribute to the Redefinition of the "Second" in 2030

"How accurate does one second need to be?"

Korean researchers are officially joining an international project to directly compare atomic clocks aboard the International Space Station (ISS) with ultra-precise optical clocks developed in Korea. With the redefinition of the time unit "second" scheduled for the 2030s, Korea has entered the race for next-generation time standards.

An image showing the European Space Agency (ESA)'s Atomic Clock Ensemble in Space (ACES) attached outside the Columbus module of the International Space Station. Provided by KRISS

The Korea Research Institute of Standards and Science (KRISS) and the Korea Astronomy and Space Science Institute (KASI) announced on May 7 that they had signed a memorandum of understanding (MOU) for “Convergence Research Collaboration on Ultra-Precise Optical Clock Comparison Using SLR” at the KRISS headquarters in Daejeon.

Through this agreement, the two institutions will formally participate in the international joint project “ACES-ELT,” which will precisely compare the atomic clock installed on the ISS and the KRISS-Yb1, KRISS’s ytterbium optical clock, using a laser-based method.

Currently, the international standard for time is defined based on cesium atomic clocks. However, optical clocks, which are more than 100 times more precise, have recently emerged as candidates for next-generation time standards, prompting countries around the world to compete in verifying the performance of their optical clocks.

The challenge lies in comparing optical clocks from different countries at ultra-high precision. The traditional GPS-based comparison method has limitations in accuracy, and the fiber optic network method faces difficulties in connecting across continents.

To address these issues, the European Space Agency (ESA) has been promoting the “ACES (Atomic Clock Ensemble in Space)” project, which involves installing an ultra-precise atomic clock system on the ISS. In April, ESA completed the installation of the atomic clock ensemble on the space station.

ACES enables time comparison between space and ground at a precision over ten times greater than existing satellite methods. It is also used to test Einstein’s theory of relativity and fundamental physical laws.

ELT (European Laser Timing), in which Korea is participating, involves firing lasers at the ISS to compare time signals. Because the laser is aimed at the crewed ISS facility, strict safety approval from the European Space Agency is required. Korea is the second country in the world, after Germany, to receive this approval.

Overview of KASI-KRISS Cooperation Plan for Participation in the ACES-ELT Mission. Provided by KRISS

In particular, Korea and Germany benefit from having different observation times for the ISS, allowing them to collect continuous data needed to verify the stability of the ISS atomic clock.

Last year, the two institutions connected KRISS’s ytterbium optical clock and KASI’s Sejong Satellite Laser Ranging (SLR) system through a dedicated fiber optic network. This enables ultra-precise time signals from the optical clock to be transmitted via laser and directly compared with the ISS atomic clock.

The agreement includes the joint use of research equipment such as SLR and atomic clocks, joint research based on the ACES-ELT project, and the exchange and development of professional personnel.

KRISS President Lee Hosung said, “Through this international joint research, we can now verify the performance of optical clocks, which has been difficult due to technical limitations. We aim to take a leading role in laying the foundation for the redefinition of the ‘second’ scheduled for 2030.”

KASI President Park Janghyun commented, “It is highly significant that the two government-funded research institutes are joining forces for convergence research centered on the national strategic infrastructure, the Sejong SLR. We will continue to elevate the status of Korean science and technology in international space missions.”

This content was produced with the assistance of AI translation services.