An internet that transcends Earth is no longer science fiction but an engineering task with clear deadlines, demonstrations, and industrial partnerships. For the past two years, the European Space Agency (ESA) has been systematically connecting existing and new communication, navigation, and ground infrastructure programs into a coherent vision – the Solar System Internet (SSI). It is a "network of networks" for deep space that, similar to today's internet, would enable standardized and reliable data exchange between autonomous systems of different agencies and commercial operators, from lunar orbit to Mars and beyond.

From concept to operational demonstrations

SSI has grown out of two realities: the explosion in the number of planned missions to the Moon and Mars, and the maturity of technologies that were once experimental. Operational relay networks are active in space today, optical communications are reaching speeds that were recently unimaginable, and protocols adapted to disruptions and delays have become a subject of standardization and mission planning. In this landscape, ESA coordinates European actors through cross-directorate initiatives and preparatory studies, creating a common ground for interoperability and upcoming commercial services.

Why SSI is needed

The terrestrial internet assumes stable topologies, low-latency bidirectional paths, and low error rates. In the Solar System, the opposite applies: positions change, connections break, latencies range from minutes to tens of minutes, and energy and bandwidth are precious resources. Every single mission that "manually" solves these challenges is expensive, fragile, and hard to scale. SSI introduces layers of abstraction, standard protocols, contractually defined services, and common operational procedures, thereby lowering risk and cost while increasing performance. Along with communication, SSI includes Positioning, Navigation, and Timing (PNT) services for deep space, as well as autonomy elements needed when ground operators are too far away to "drive" a space vehicle in real time.

Three technological pillars

1) DTN – networks resilient to disruptions

Disruption/Delay Tolerant Networking (DTN) solves the basic problem: how to deliver data through an environment where routes emerge and disappear, and delays are immense. Instead of a permanently established session, DTN works on a "store-and-forward" basis, with packets (bundles) being reliably transferred node by node. ESA has already conducted multiple demonstrations in cooperation with international partners, including experiments linked to CubeSat missions and the ground segment. In practice, DTN reduces the need for every system element to "know everything about everything" – it is enough to know the next handover point and the rules that guarantee delivery when favorable geometry or a resource becomes available.

2) Optical communications – a leap in bandwidth

Laser links bring orders of magnitude higher speeds and energy efficiency compared to radio frequency (RF) systems, at the cost of stricter pointing requirements and atmospheric conditions upon reception. Demonstrations from the Moon confirmed downlink speeds in the order of hundreds of megabits per second a decade ago. In deep space, flying demonstrators on interplanetary probes have confirmed stable megabit speeds over hundreds of millions of kilometers, with occasional records rivaling terrestrial broadband connections when the distance is shorter. For SSI, this means that core "trunk" links – for example, Moon–Earth – can service entire constellations and fleets of users, with smart resource scheduling.

3) PNT for deep space – ODTS as a prerequisite for autonomy

The role of positioning, navigation, and time synchronization in deep space goes far beyond "GPS for Mars". Spacecraft must have a high level of autonomy in determining orbits and synchronizing time (Orbit Determination & Time Synchronisation – ODTS), and the network must provide long optical and RF ranges for precise distance and velocity measurements. SSI envisions PNT services being an integral part of the communication infrastructure, so that users – from orbiters and landers to mobile robots and temporary bases – can plan operations without "manually" tying back to Earth for every decision.

What Europe already has: ESTRACK and the Mars Relay Network

SSI does not start from scratch. The foundation is the existing ground network of deep space stations with 35-meter diameter antennas and a global distribution that enables coverage of the entire Solar System. These stations and the accompanying optical/KA infrastructure have been serving European and international missions for years. On the Red Planet, an inter-institutional relay architecture – operational today – connects surface elements (rovers and landers) with orbiters that forward data to Earth. In that network, European orbiters are not just "guests"; they often carry a critical part of the traffic and serve as key links when geometry and resources are favorable.

Operational coordination in Europe is centralized through a specialized office and associated information system that mediates between different control centers, standardizes formats, and automates the planning, execution, and evaluation of relays. This "hub" approach allows rovers and landers from different agencies to receive service via any compatible orbiter without needing to implement separate interfaces for each operator. Precisely this philosophy – single interface, multiple providers – is also mapped onto SSI as a future federation of networks.

Standards and interoperability

The success of SSI will depend on agreed protocols and service level agreements. At the network layer level, this means DTN and associated IETF/CCSDS specifications; at the resource management level, it means a service catalog, reservation methods, window allocation, priority schemes, and billing mechanisms for commercial users. at the security level, a combination of cyber protection, cryptography, and security zones in ground centers and transmission nodes is needed. As the number of participants grows, governance and technical management of standards become disciplines just as important as the technology itself.

Programs forming the SSI "network of networks"

Moonlight – the first European network beyond Earth

The European constellation around the Moon is conceived as a provider of communication and navigation services for hundreds of planned lunar missions in the next two decades. The constellation consists of one satellite with a focus on communications and four navigation satellites, arranged to prioritize the south pole. The link to Earth leads via dedicated stations, creating the core of a future lunar "internet" on which commercial services can later be offered to end users, from state expeditions to private robotics companies.

HydRON – "fiber in the sky"

HydRON is conceived as an optical transport network of multiple orbits with capacities in terabits per second. In the first phase, it includes demonstrators connecting low, medium, and geostationary orbits via optical links, and in later phases, it provides a "backbone" towards deep space as well. For SSI, this means the possibility that traffic from the Moon and – in the long term – Mars is aggregated and distributed through space optical nodes, without unnecessary reliance on expensive and time-limited RF windows at individual ground stations.

MARCONI – communications and navigation for Mars

The long-term plan envisions European "space tugs" delivering payloads to the Mars system and then remaining as elements of a constellation for communications and PNT. Thus, a permanent infrastructure is slowly built – resembling the first telecom network around another planet – which can provide services to future missions, including sample return, autonomous vehicles and, in the distant future, temporary human base camps. The concept envisions a multi-node constellation by the 2040s, whereby Mars becomes the first to get a "local internet" interoperable with SSI.

Nodes, trunk links, and operational reality

SSI is envisioned as a set of nodes (satellites, relay orbiters, lunar/mining landers with communication packages) connected by "trunk" links towards Earth and each other. In cis-lunar space, optical trunks ensure an order of magnitude higher throughput and lower energy cost per bit. DTN bundlers collecting data from surface and orbital users wait for the next window and hand over "upwards" without the need for constant end-to-end contact. On Mars, where windows and geometries are more complex, a good schedule of relays between the surface and orbit and further towards Earth is crucial; "hub" coordination helping to automate planning and reporting helps here too.

SSI Node-1 Pathfinder – the first step, near the Moon

A small, focused mission is designed to demonstrate a routine optical trunk between Earth and cis-lunar orbit and to use DTN operationally in a European mission for the first time. This moves from a "technology demonstrator" to regular operations that fit into a broader service regime. Node-1, in addition to the optical link, also includes experiments for precise synchronization and navigational positioning, laying the foundation for PNT services in an environment where heavy mission traffic is soon expected.

ASSIGN – the umbrella program connecting it all

Advancing Solar System Internet and GrouNd formalizes the approach: it anchors activities across multiple directorates, handles standardization, defines security frameworks and demonstration plans, and ensures that Europe has a defined role and interests in the global vision of SSI. Among the early tasks is the further elaboration and preparation of the Node-1 Pathfinder, but also connecting with existing programs like Moonlight, HydRON, and plans for Mars. ASSIGN thus does what internet development did on Earth: turning separate islands into a federated system with aligned rules and clear interfaces.

Lessons from optical link demonstrations

Optical links towards the Moon reached hundreds of megabits per second a decade ago, meeting criteria for high-throughput scientific missions and servicing numerous users. Deeper into the Solar System, a newer demonstration on an interplanetary spacecraft enabled bidirectional exchange of real telemetry data, with stable megabit speeds at hundreds of millions of kilometers and transfer records at shorter distances. For operational systems, this means two things: first, optical trunks are technically feasible and operable; second, an architecture is needed that intelligently mixes optical and RF links depending on geometry, weather, load, and user priorities – exactly what SSI envisions.

Security, reliability, and service maintenance

In a federated network with multiple owners and operators, security is not an add-on but a starting point. The level of trust and segmentation must be built into all layers: from authentication and encryption at the packet level to physical security of stations and optical terminals. Equally important, the network must survive the loss of nodes, performance degradation, and interruptions without a drastic drop in service quality; DTN is part of the answer, and the other part is agreed service levels and excess capacity on trunks so that priority traffic always has a path.

Economic model and market

As the number of users grows, the transition from "best effort" exchange to contracted services with catalogs, price lists, and support levels becomes inevitable. Moonlight is already charting the path towards the commercialization of lunar connectivity and navigation, while HydRON has the task of demonstrating that "fiber-like" capacities in space can serve as a transport backbone for deep space as well. In the Mars system, MARCONI introduces an environment where local services (relay, PNT, meteorology) can be offered in the long term to users arriving without their own "heavy" infrastructure. SSI ties all this into a coherent offer: a single interoperable network, multiple providers, multi-layered service.

The role of industry and consortia

European companies are in charge of systems engineering, strategy, roadmap mapping, protocol evaluation, and management. This ensures that mission requirements are translated into standards and references, and laboratory and field experiments spill over into operational rules and ground segment software tools. Particularly important are engineering efforts on DTN and interoperability between different control centers, where even "invisible" details like format conversions, reporting systems, and archiving make the difference between a successful relay and a lost opportunity in a timely manner.

Global context and the race towards standards

Europe is not alone in building infrastructure beyond Earth. Parallel to European plans, other space powers are developing new relay capacities around the Moon and preparing their own constellations, including test missions for future networks. On Mars, an international relay network has been serving daily for almost two decades to transfer commands and scientific data between the surface and Earth. Optical demonstrations in deep space further raise the bar of expectations – from transmitting high-definition video to transmitting telemetry from distances comparable to the maximum distance of Mars. In such an environment, standards, interoperability, and "peering" between networks are crucial so that users can independently choose providers, and missions have multiple routes and insurance against interruptions.

What follows by the end of the decade

On the timeline to the end of the 2020s, three parallel directions stand out. First, operational demonstrations: cis-lunar optical trunks, routine DTN in missions, and early PNT experiments. Second, networking of existing services: deep space stations, optical transport in higher orbits, and relays around the Moon and Mars. Third, standardization and governance: agreement on interfaces, security rules, priorities, and billing models. In addition, industrial contracts and tenders need to direct the development of terminals, lasers, PNT modules, and ground software so that the network is scalable. At the moment when the number of lunar spacecraft and users increases, SSI must be ready to accept traffic without improvisation.

What the user experience will look like

For mission operators, SSI brings familiar paradigms: Service Level Agreements, slot reservations, service catalogs, standard API interfaces, and telemetry instruments that allow performance monitoring. For scientific teams and industrial users, this means more stable windows and predictable costs – less time spent "hunting" for a communication opportunity, more on planning experiments and operations. For the wider European ecosphere, SSI means that engineering and software products developed for one mission can be scaled to many, fostering the emergence of new companies and services.

Open questions and risks

The key challenges are not just technical. Who decides on priorities when resources are tight? How is the balance between institutional and commercial users struck? What are the frameworks for sharing sensitive data and for responding to cyber incidents in a federated network? How much redundancy is enough and who funds it? On the technical side, optical trunks depend on weather conditions and pointing precision; RF links remain a necessary backup. DTN reduces complexity at the edges but requires robust routing, storage, and data expiration policies. PNT in deep space must simultaneously meet strict user needs and remain energy sustainable.

European advantage

Europe has two strategic advantages: experience in leading complex international missions and an industrial base that can deliver everything from laser terminals to ground planning software. With Moonlight as the first commercial offer beyond Earth, HydRON as a transport backbone, and MARCONI as a long-term plan for Mars, the contours of SSI are already visible. Additionally, operational networks and instruments like relay orbiters, deep space stations, and coordination offices are already delivering billions of bits of science and engineering data daily. The next steps – demonstration of cis-lunar optical trunks and routine DTN – are not a "leap into the unknown", but a logical upgrade of existing capabilities.

The bigger picture: towards a "Solar System Internet"

The idea of connecting networks of different owners and technological generations into a global architecture is not new – that is how today's internet was created. SSI applies this philosophy to space: interoperable standards, a minimal set of common rules, and clear connection points allow everyone to contribute and simultaneously retain autonomy. As new players and commercial networks appear around the Moon and, gradually, Mars in the coming years, precisely such an architecture will allow data to travel the shortest, cheapest, and most reliable path – regardless of who owns a particular link or node.

On the date of December 2, 2025, the roadmap is clear: preparatory studies have been done, industrial teams are working on demonstrators, and operational experiences and standards are maturing. The first nodes of the future "Solar System Internet" will be around the Moon; the next, gradually, around Mars. When these nodes start working routinely, not only the way data is exchanged will change, but also the way we design missions at all: from unique, fragile chains of links – towards networks that find the path themselves.