The European Space Agency (ESA) made history on July 7, 2025, by establishing its first optical communication link with a deep space spacecraft. In collaboration with NASA, data was transmitted from the Deep Space Optical Communications (DSOC) experiment, located on the Psyche spacecraft. At the moment the link was established, the spacecraft was an incredible 265 million kilometers from Earth, which corresponds to approximately 1.8 astronomical units. This event is not only a technical marvel but also a milestone in the long-standing collaboration between space agencies, proving for the first time the possibility of interoperability between ESA and NASA in the field of optical communications, something that was previously reserved exclusively for radio-frequency based systems. This success is the first of four planned links during the summer of 2025.

This revolutionary step marks the beginning of a new era in space exploration, paving the way for a future where a high-speed "space internet" becomes a reality. "The first successful demonstration of deep space optical communication with a European ground segment represents a true quantum leap towards enabling terrestrial internet-like connectivity for our deep space spacecraft," said Rolf Densing, Director of Operations at ESA. His words confirm the importance of international collaboration, which, along with contributions from industry and academic partners, is crucial for such achievements.

Greek Observatories as the Key to Success

The laser link-up campaign began in Greece, where ESA transformed two existing observatories into high-precision optical ground stations. The Kryoneri observatory, located near Athens, played a central role in this endeavor. A powerful laser beam was aimed from it towards NASA's Psyche spacecraft. Although this initial signal, known as a "beacon," carried no data, its purpose was fundamental. It was designed with extraordinary precision to allow the DSOC instrument on the spacecraft to detect it, "lock on" to it, and send a return signal back to Earth.

That extremely faint return signal, after traveling hundreds of millions of kilometers, was captured at a second location – the Helmos observatory, situated on a neighboring mountain peak, 37 kilometers away. This separation of locations is crucial so that the powerful outgoing laser does not blind the extremely sensitive receiving equipment. "Enabling this two-way optical handshake meant overcoming two major technical challenges: developing a laser powerful enough to hit a distant spacecraft with pinpoint accuracy, and building a receiver sensitive enough to detect the faintest return signal, sometimes composed of just a few photons," explained Sinda Mejri, project manager for ESA's ground laser receiver.

Overcoming Cosmic Challenges

Establishing a stable link at such an extreme distance required solving a series of complex problems. Flight dynamics experts at ESA's Space Operations Centre (ESOC) had to compensate in real-time for numerous variables affecting the laser beam's path. This includes atmospheric density, temperature gradients, and the constant motion of the planets. The process is similar to those used in global navigation satellite systems, but with the added complexity brought by the vast distances of deep space and the need for ultra-precise pointing, measured in microradians.

Safety was also of paramount importance. To ensure the powerful laser beams posed no danger, parts of Greek airspace were temporarily closed for the duration of the transmissions. Every aspect of the operation was meticulously planned to minimize risk and maximize success.

Years of Preparation for a Historic Moment

Although the link-up itself was relatively short, it was preceded by years of dedicated work, research, and international cooperation. The construction of the ground stations for transmitting and receiving optical signals was a project in itself. The Ground Laser Transmitter integrates five high-power lasers with ultra-precise pointing controllers, housed in a special six-meter-long container with a lifting platform. This structure protects the sensitive equipment from sunlight during the day and raises it into the open after sunset.

On the other hand, the Ground Laser Receiver consists of a sophisticated optical bench so sensitive it can detect single photons. This receiver, which uses superconducting nanowire single-photon detector technology, is securely mounted on the back of the 2.3-meter Aristarchos telescope, located at an altitude of 2340 meters at the Helmos observatory. Back in April, the team conducted a dress rehearsal by transmitting a low-power signal to ESA's Alphasat satellite in geostationary orbit, at an altitude of 36,000 km, which serves as the primary testbed for optical communication technologies.

Clemens Heese, Head of the Optical Technologies Section at ESA, highlighted the team's incredible efficiency: "Despite the complexity of the task, the final installation of the laser, electrical wiring, and cooling systems was successfully completed shortly after their delivery on the same morning. Achieving 'laser installation and safe laser emission to the sky within a single day' is an outstanding testament to the team's precision, coordination, and dedication." The entire effort on the ground involved fewer than 20 people: seven at Kryoneri and twelve at Helmos, with assistance from two experts from NASA's Jet Propulsion Laboratory (JPL).

A Future Written in Beams of Light

This demonstration is much more than a technical feat; it is a window into the future of deep space communication. Optical links promise data transfer rates that are 10 to 100 times greater than current radio-frequency systems. "Combining this technology with those we have for radio-frequency communications is key to transmitting the ever-increasing amount of data generated by missions exploring space," said Andrea Di Mira, ESA's Ground Laser Transmitter project manager. Higher data throughput will allow for the transmission of high-resolution videos and vast amounts of scientific data from future missions to Mars and beyond, almost in real-time.

"We are proud that ESA is involved in the Deep Space Optical Communications (DSOC) experiment on our Psyche mission. It is a powerful example of what international collaboration can achieve and a glimpse into the future of deep space communications," added Abi Biswas, DSOC project technologist at NASA JPL.

The ASSIGN Programme and a Look Towards Mars

The success of this mission lays the foundation for ESA's proposed ASSIGN (Advancing Solar System Internet and GrouNd) programme, which will be presented at the ESA Council at Ministerial Level in November. "The goal of ASSIGN will be to unify existing and future radio-frequency and optical networks into a secure and resilient interoperable 'network of networks' for ESA missions, as well as for institutional and commercial partners," stated Mehran Sarkarati, Head of ESA's Ground Station Engineering Division and programme manager for ASSIGN.

Looking even further into the future, ESA is currently studying the concept of an electric propulsion tug for Mars, named 'LightShip', which would transport crewed spacecraft to the Red Planet. After delivering its payload, LightShip would move into a service orbit to provide communication and navigation services via the MARCONI (MARs COmmunication and Navigation Infrastructure) payload. Part of this payload will be an optical communications demonstrator, as a crucial step towards supporting future human missions.

The Power of International and Industrial Cooperation

ESA's participation in the DSOC demonstration was made possible by a consortium of leading European companies, including qtlabs (Austria), Single Quantum (Netherlands), GA Synopta (Switzerland), qssys (Germany), Safran Data Systems (France), and NKT Photonics Ltd (UK). Crucial support was also provided by the National Observatory of Athens, which enabled the transformation of its Helmos and Kryoneri observatories into deep space ground stations. The project was funded through ESA's General Support Technology Programme (GSTP) and Technology Development Element (TDE), confirming Europe's strategic commitment to developing advanced space technologies.

Source: European Space Agency