Themis
Themis is probably Europe’s most significant effort to develop a reusable rocket booster. The programme was officially adopted by ESA in late 2019, although preliminary work had been underway for several years prior, led by the French space agency CNES. Development is being spearheaded by ArianeGroup as the prime contractor, with contributions from companies across Europe. While primarily funded by the European Space Agency, Themis has also received €39 million in European Union support through the reuSable straTegic space Launcher Technology & Operations (SALTO) initiative.
The most recent tranche of funding was awarded in November 2024, with ESA allocating €230 million to ArianeGroup for continued development and demonstration. This funding covers ongoing work on the Themis T1H test article, construction of ground infrastructure at the Esrange Space Center in Sweden, and preparation for flight demonstrations. According to ESA, the aim is to mature Themis into an operational reusable stage, laying the foundation for Europe’s future access to space.
The Themis T1H (“1-Engine Hop”) test article will measure 29 metres tall and 3.5 metres in diameter. It is equipped with four landing legs for recovery and will use a single Prometheus engine during initial low-altitude flights. Prometheus is another major ESA initiative, focused on developing a reusable, low-cost, methane-fuelled engine capable of producing up to 120 tonnes of thrust. It supports multiple restarts and is designed for mass production. Development of Prometheus began under CNES in 2015 and was adopted by ESA in 2016.
Initial testing will take place at Esrange and will include a low-altitude “hop” test, during which the engine will be fired continuously, with ascent and descent controlled by varying thrust. This test is expected to be completed in 2025, though, given years of prior delays, even that timeline may prove optimistic.
In December 2024, ArianeGroup completed the first full mechanical integration of Themis in Les Mureaux, France. The fit-check assembled the tank, engine section, and structural components to verify proper alignment of all interfaces, an important validation milestone ahead of flight.
At Esrange, the core of the new infrastructure for Themis is Launch Complex 3 (LC3), which features a 40 x 40 metre launch and landing pad and a dedicated Launch Vehicle Integration Building (LVIB). The LVIB is divided into two symmetrical halls, Asgard and Midgard, allowing two customers to operate independently. ArianeGroup will use the Asgard section for final integration and pre-launch checks.
According to the Swedish Space Corporation (SSC), the commercial operator of Esrange, final adjustments are being made to support systems including the range warning system, low- and high-pressure nitrogen plants, dry air plant, firefighting robot, electric and fluidic ground support equipment, and propellant storage tanks. A dedicated Launch Control Center is being established at an ESA facility in Kiruna, around 40 kilometres from Esrange, from which all test flights will be monitored. In March 2025, SSC stated that delivery of the T1H article was expected “soon.” However, as mid-2025 approaches, any further delays could place pressure on the planned late-2025 hop test schedule.
An April update published on the official SALTO website outlines several key milestones that must be completed before the maiden hop test of the Themis demonstrator. These include the completion and validation of the launch and landing pads, final integration and control system tests of T1H inside the Launch Vehicle Integration Building, and the transfer of the vehicle to the pad for installation of its landing legs.
Following T1H testing, ESA and ArianeGroup plan to proceed with an upgraded version, Themis T1E (“1-Engine Evolution”), intended for higher-altitude flights. This new variant, part of the November 2024 funding package, will also be launched from Esrange. That funding round also included provisions to adapt the PF50 test stand in Vernon, formerly used for Vulcain engine tests, for long-duration Prometheus firings. The T1E campaign will expand the flight envelope, testing engine re-ignition, guidance and control, and propulsive landings under different conditions.
Further ahead, a multi-engine version of Themis is planned to test full-scale reusability. After completing the one-engine hops, a three-engine variant will launch from a new facility being built at the Guiana Space Centre in Kourou, on the site of the former Diamant pad. The test campaign will culminate in a high-altitude flight followed by a downrange landing on an ocean barge. A timeline has not been published, but this phase is expected in the latter half of the 2020s, following the Swedish test campaign. However, it would be difficult to suggest that initial testing of the multi-engine Themis variant could begin before 2028, given that the T1E version still needs to be built and tested at Esrange before operations can shift to French Guiana.
Callisto
Callisto is a smaller-scale technology demonstrator aimed at mastering the basics of reusable rocketry through an international partnership. The project was initially kicked off by DLR and CNES in 2015, with the Japanese space agency JAXA joining the partnership in mid-2017. At the time, the first test flight of CALLISTO was targeted to take place in late 2020. This gives one an indication of just how delayed this project has become.
Callisto will be a 13 metre-tall, single-stage demonstrator intended to validate vertical launch and landing techniques on a suborbital trajectory. The project’s goal is to reach an altitude of about 20 km and then safely return to the launch site for a propulsive landing, providing valuable data on flight dynamics, engine throttling, and reuse operations. The project will also focus on the time between recovery and relaunch and the extent to which the demonstrator will need to be refurbished between flights. The demonstrator is expected to conduct up to ten test flights over a six-month period.
After what appeared to be near stagnation, Callisto does appear to be moving forward with clear updates on hardware assembly. In early 2025, CNES officials confirmed that major components of the demonstrator were coming together, with the first integrated vehicle expected to be completed by late 2025.
In April 2025, the German Aerospace Center (DLR) completed the qualification campaign for the “Top Block” of the CALLISTO reusable rocket demonstrator. This component comprises the Vehicle Equipment Bay and the fairing, which house critical systems such as avionics, telemetry, communications, and flight control. The Top Block also integrates the rocket’s deployable control surfaces and reaction control system.
The inaugural test flight of Callisto is currently planned for 2026 from Europe’s Guiana Space Centre.
FROG
CNES’s FROG project (Fusée ROcket for GNC demonstration) is an experimental testbed aimed at perfecting landing algorithms and guidance systems for reusable launchers. Launched in 2017, the project employs a fast-paced development approach with multiple small teams working in parallel to rapidly design and iterate small, low-cost demonstrators. This approach was intended as a “break from traditional development methods,” allowing rapid learning and course correction as teams try different solutions.
The first prototype, FROG-T, was a tiny 2.5 metre-tall vehicle powered by a simple turbojet engine. In May 2019, FROG-T performed its maiden free flight. Over five test flights, it demonstrated stable takeoff, hover, and landing, reaching a maximum altitude of about 30 metres. These low-altitude hops, though modest in scale, allowed CNES to validate software and sensors for vertical landing. Each flight’s data helped fine-tune the guidance, navigation, and control (GNC) algorithms needed to autonomously land a rocket stage.
Now, CNES is preparing FROG-H, an upgraded version designed to push testing higher and closer to real rocket conditions. FROG-H stands 3.6 metres tall and, crucially, swaps the jet for a true rocket engine. The engine is a monopropellant rocket motor developed by Poland’s Łukasiewicz Institute of Aviation under an ESA contract. This international collaboration allows FROG-H to fly a more realistic trajectory with rapid thrust throttling, giving CNES GNC software a better analog to a full-scale booster landing. The detailed design of FROG-H was completed in late 2022, and fabrication followed throughout 2023.
The inaugural test flights of FROG-H were slated for early 2025. However, CNES has not indicated that these flights have taken place, and as we drift into June, it’s becoming increasingly difficult to argue that the “early 2025” launch target hasn’t slipped.
Reusable upper stages
While much of Europe’s reusability efforts have, until only recently, focused on first-stage booster recovery, Europe is now also exploring upper-stage reusability. In March 2024, ESA launched a Reusable Upper Stage Demonstrator project under its Future Launchers Preparatory Programme. Phase 1, which kicked off in late 2024, examined what technologies and maturation steps would be needed to build an upper stage that can survive re-entry and be flown again. This included technical trade-offs (materials, thermal protection, avionics) and an assessment of potential commercial applications for such a system.
By mid-2025, the project had moved into Phase 2, with ESA issuing a call for proposals to industry to advance the project to a critical design milestone. In practical terms, this means fleshing out designs and prototypes of key subsystems to the point of being ready for manufacturing and full-scale testing. Phase 2 will involve early hardware demos of high-risk items to remove uncertainties before committing to building the stage. ESA has, however, notably not yet publicly disclosed which companies are working on Phase 2, possibly reflecting the highly conceptual nature of the project thus far. It remains undecided whether the resulting reusable stage might be retrofitted to Ariane 6 or reserved for a next-generation launcher entirely.
In parallel to the ESA project, CNES has initiated its own programme for upper-stage reuse. In early 2025, the French space agency published a call for proposals under the name DEMESURE (a French acronym for Reusable Upper Stage Demonstration). CNES envisions developing technologies for an upper stage that could one day equip a two-stage launcher capable of lofting approximately 20 tonnes to Low Earth Orbit, which is essentially Ariane 6-class performance. The DEMESURE plan is to start with a subscale demonstrator, likely a fraction of the full size, to prove key aspects like re-entry heating resistance, autonomous guidance during return, and perhaps a powered landing or glide recovery.
PROTEIN
ESA kicked off its PROTEIN (Preparatory Activities for European Heavy Lift Launcher) project in June 2022 as part of its Future Launchers Preparatory Programme. The initiative aimed to assess the feasibility of developing a European super heavy-lift launch vehicle capable of delivering up to 100 tonnes to low Earth orbit (LEO), with a focus on reusability, cost-effectiveness, and environmental sustainability.
ESA awarded contracts to ArianeGroup and Rocket Factory Augsburg to conduct Phase 0/A studies. These studies explored various design concepts, including fully and semi-reusable launch systems, and evaluated the necessary technologies and infrastructure. Both companies proposed designs featuring high-thrust staged-combustion engines and considered the use of environmentally friendly propellants like liquid methane and hydrogen. In November 2024, ESA published executive summaries on both proposals. Since then, there have been no additional updates from the programme.
THRUST!
Launched in 2024, ESA’s THRUST! initiative is focused on advancing next-generation liquid rocket engines required for reusable rocket stages. In October 2024, ESA selected Rocket Factory Augsburg (RFA) and The Exploration Company to develop high-thrust, methane-fuelled engines under this initiative.
The Exploration Company, better known for its upcoming Nyx reusable cargo spacecraft, is developing a rocket engine capable of producing 200 tonnes of thrust. Called Typhoon, the engine is being co-funded by CNES and uses a staged-combustion cycle, making it comparable in power to SpaceX’s Raptor engine. It is not clear, however, whether the company’s work for THRUST! is directly tied to the Typhoon engine.
RFA, which has already developed the 10-tonne-thrust Helix engine for its small RFA One rocket, will design a larger engine for the THRUST! initiative. Unlike The Exploration Company, RFA has not yet released public details about this new engine. However, in an executive summary of a study conducted under ESA’s PROTEIN initiative, the company outlined a potential upper-stage engine that would be based on its Helix engine and would be capable of producing 60 tonnes of thrust. This approach may offer clues as to RFA’s direction under THRUST!.
Notably, THRUST! is not the only high-thrust reusable engine programme ESA is pursuing. In September 2024, ESA awarded Spain’s Pangea Aerospace a contract to design its Kronos engine. This full-flow staged-combustion engine is also designed to produce 200 tonnes of thrust and is intended for use on future heavy-lift rockets.
BEST!
ESA launched the Boosters for European Space Transportation (BEST!) initiative to foster innovation in reusable first stages and boosters. Following a call for proposals in 2024, ESA awarded initial development contracts to two companies: ArianeGroup and Isar Aerospace.
ArianeGroup is already heavily involved in reusability efforts, notably through its work on the Themis demonstrator and previous participation in the PROTEIN studies. However, it remains unclear what the company is specifically pursuing under the BEST! initiative, or whether the work overlaps with any of its existing projects. ArianeGroup is also developing the SkyHopper concept, which is partially funded by CNES and aims to enable recovery and reuse of the Maia rocket’s first stage. Maia is being developed by ArianeGroup subsidiary MaiaSpace.
Isar Aerospace, meanwhile, is working toward a second flight of its Spectrum rocket, a small, expendable launcher capable of delivering up to one tonne to low Earth orbit. The company’s first Spectrum launch in March 2025 ended in failure less than a minute after liftoff. In parallel with Spectrum, there have been reports that Isar is developing a larger vehicle, informally referred to as Spectrum 2. This new vehicle may be the focus of Isar’s BEST! activities, but neither ESA nor Isar has confirmed any specific details.
Conclusion
For now, Europe’s institutional reusability efforts remain cautious steps rather than bold leaps. Progress is slow, fragmented, and often weighed down by ambiguity, with no single programme yet charting a clear path to an operational reusable rocket. And yet, within these scattered initiatives lie the foundations of something more. With the right political will and full financial backing, ESA’s upcoming Ministerial Council meeting later this year could serve as a defining moment, not just for reusability, but for Europe’s broader ambitions in space. It is there, perhaps, that vision may finally give way to commitment, and the pieces could begin to align into a coherent trajectory forward.
