A groundbreaking mission spearheaded by the European Space Agency (ESA) is set to launch the most precise atomic clock ever sent into space, forming a network with the best terrestrial clocks. However, this innovative device has a limited operational lifespan, as it will ultimately disintegrate upon re-entry into the Earth's atmosphere at the close of the decade when the International Space Station (ISS) is decommissioned.

Dubbed the Atomic Clock Ensemble in Space (ACES), this ambitious project will generate a time signal with unparalleled accuracy, transmitting it via laser to nine ground stations as it travels at a staggering speed of 27,000 kilometers per hour. This synchronized network of clocks is poised to revolutionize timekeeping across the globe.

The significance of ACES extends far beyond mere timekeeping; it will enable scientists to test Einstein's theory of general relativity with remarkable precision. This theory posits that the flow of time is influenced by gravitational strength, and ACES aims to explore this concept in unprecedented detail. Furthermore, the mission will facilitate research into complex topics ranging from dark matter to string theory.

Scheduled for launch on April 21 aboard a SpaceX Falcon 9 rocket from the iconic Kennedy Space Center in Florida, ACES will be deployed to the ISS, where it will be mounted on the exterior of ESA's Columbus laboratory by the Canadian Space Agency's robotic arm, known as Canadarm2. This strategic placement will allow ACES to operate effectively in the vacuum of space.

The ACES package consists of two highly sophisticated clocks: one referred to as SHM, which maintains stability over short durations to assist in calibrating the other clock, named PHARAO. Together, these clocks are so precise that they will lose less than one second over 300 million years, boasting an accuracy that is tenfold greater than that of the clocks currently utilized in GPS satellites.

PHARAO is ingeniously designed based on a room-sized atomic clock located in Paris, showcasing remarkable advancements in miniaturization. Transforming this technology into a compact unit that occupies less than a cubic meter while enduring the challenges of a rocket launch and the harsh conditions of space was no small accomplishment.

To create a highly accurate clock signal, PHARAO employs a method known as atom fountain technology. It ejects a stream of caesium atoms, cooled to near absolute zero, and analyzes their interactions with microwave fields. While this process on Earth typically requires a device up to three meters tall, the microgravity environment allows for a more efficient operation, enabling the creation of a smaller and more manageable fountain of atoms.

Simon Weinberg, an ESA representative, highlights the extreme sensitivity of the clock, noting that even the proximity of a teaspoon could generate an electromagnetic field strong enough to disrupt its function. To give you some perspective, were trying to measure something better than a thousand million millionth of a second, explains Weinberg, emphasizing the complexity of this endeavor.

The ACES mission concept traces its roots back to the 1990s, initially envisioned for launch aboard the now-retired Space Shuttle, which concluded its operations in 2011. Once in orbit, it is expected to take approximately one and a half years before the first signals from ACES reach Earth-bound clocks. This timeline accounts for around six months dedicated to commissioning the device, followed by a year of data collection to isolate and mitigate noise interference affecting the clock signal.

After its anticipated operational period, ACES will continue to function until 2030, coinciding with the planned controlled deorbit of the ISS, leading to its eventual destruction upon atmospheric re-entry. By that time, it is likely that next-generation super-accurate timepieces, known as optical clocks, will have emerged, rendering atomic clocks significantly less relevant on Earth. However, the robustness and size of these new clocks may still pose challenges for use in space.

Weinberg mentions that ESA is considering the future of ACES, indicating a potential plan to launch a new generation of the ACES mission to replace the lost technology once the ISS concludes its operations. We are still a considerable distance from that point, and it will require gathering support and financing to ensure that we can turn this vision into reality, he adds, highlighting the ongoing commitment to advancing precision timekeeping in space.