ESA’s reusable rocket demonstrator Themis has completed its first wet dress rehearsal on the launch pad at Esrange Space Center in Sweden, a milestone on the way to the vehicle’s first short test flight. The rehearsal on July 23 loaded the prototype with several metric tons of liquid nitrogen cooled to minus 200 degrees Celsius, simulating the full sequence of events before a liftoff and after a landing.
The test let ArianeGroup and SSC Space practice countdown and post-flight procedures and identify remaining anomalies under the extreme cold and high pressure the systems will face in flight. The simulated operations ran over a single day, according to European Spaceflight. ESA said the data gathered will play a key role in preparing Themis for its first flight, particularly complex operations such as getting the cryogenic engine ready for ignition. The agency noted the rehearsal approach was inspired by Ariane 6’s road to first flight, allowing operators to fine-tune procedures in safe conditions.
Themis will fly on liquid methane and liquid oxygen, but the rehearsal used liquid nitrogen for safety. Nitrogen has similar cryogenic temperature and density to the flight propellants but is neither flammable nor explosive.
Themis, which stands on four landing legs, is ESA’s flagship demonstrator for low-cost rocket recovery and reuse technologies and, at 28 meters tall, the first European full-scale reusable main stage. It is powered by the Prometheus engine, which has the same thrust as the main engine on Europe’s heavy-lift Ariane 6 rocket but burns liquid oxygen and methane instead of hydrogen. Liquid oxygen is cooled to as low as minus 183 degrees Celsius for fueling, and ESA notes methane offers advantages over other rocket fuels: it does not need the extreme cooling of liquid hydrogen, is less prone to leaks, and delivers higher performance than kerosene.
Themis was developed under ESA’s future space transportation preparation program, with ArianeGroup as prime contractor and multiple European industrial partners. The launch campaign is run through the Horizon Europe project SALTO, funded by the European Commission, managed by the EU executive agency HaDEA, and bringing together 25 consortium partners from 12 European Union countries.
The demonstrator arrived at Esrange in June 2025 and was erected on the pad in September 2025. The vehicle then spent the northern winter at Kiruna waiting out the snow before testing resumed, according to European Spaceflight. The wet dress rehearsal took place on July 23, with ESA announcing it a week later, on July 30. European Spaceflight noted the announcement came a full week after the event and that the rehearsal was the first substantive campaign update in more than ten months, the most significant milestone since the vehicle was erected. European Spaceflight describes the next step as a low-altitude hop test. ESA has not set a date for the first hop.
Sources
1. European Space Agency, “Major rehearsal takes Themis one step closer to flight”: https://www.esa.int/Enabling_Support/Space_Transportation/Future_space_transportation/Major_rehearsal_takes_Themis_one_step_closer_to_flight
2. European Spaceflight, “ArianeGroup Completes Themis Wet Dress Rehearsal”: https://europeanspaceflight.com/arianegroup-completes-themis-wet-dress-rehearsal/