Satellites Powered by Water? The Revolutionary Propellant Changing Spaceflight

Imagine a future where satellites are propelled not by toxic fuels or rare gases, but by plain old water. It might sound like science fiction, but water-powered satellite drives are rapidly becoming a reality. These novel propulsion systems use H₂O as a propellant – either blasting out superheated steam or breaking water into hydrogen and oxygen for combustion – to maneuver spacecraft in orbit. The appeal is clear: water is cheap, abundant, green, and far safer to handle than traditional rocket fuels esa.int, nasa.gov. As retired astronaut Chris Hadfield put it, being able to propel spacecraft with nothing more than solar energy and distilled water is “a great freedom,” especially since water is widely available in space (from lunar craters to comet ice) spaceref.com. In this report, we’ll dive into how water-powered propulsion works, its advantages and drawbacks, and the latest breakthroughs (up through 2025) that are pushing this technology from experimental demos to mainstream use.

How Do Water-Powered Satellite Thrusters Work?

Water alone doesn’t burn like a conventional fuel – it’s the reaction mass that gets energized and expelled to produce thrust. There are a few ingenious ways engineers have made water-fueled engines possible:

• Steam Propulsion (Electrothermal Thrusters): The simplest approach is to heat water into high-pressure steam and nozzle it out to produce thrust. These “steam rocket” or resistojet designs use electrical heaters or microwave energy to boil water. For example, Momentus Space’s Vigoride vehicle uses a Microwave Electrothermal Thruster (MET) that “microwaves water using solar power” until it boils into plasma, shooting out as a high-energy jet spaceref.com. It’s akin to putting a nozzle on a kettle or microwave oven – the expelled hot vapor pushes the satellite. Steam-based thrusters are low-thrust but very safe and mechanically simple. Japan’s startup Pale Blue proved such a system in orbit in 2023, using a water resistojet to adjust a small Sony satellite’s orbit by a few kilometers phys.org. Pale Blue’s design holds water at low pressure and vaporizes it at modest temperatures, an approach that validated two minutes of continuous firing in space phys.org.
• Electrolysis (Water Rocket Engines): A more energetic method is to split water into hydrogen and oxygen gases (via electrolysis) and then burn that mixture in a mini rocket thruster. In essence, the satellite carries unpressurized liquid water, then uses electric power from solar panels to produce combustible gases on-demand. NASA’s Hydros engine, developed with Tethers Unlimited, pioneered this approach spinoff.nasa.gov. Once in orbit, Hydros electrolyzes water into H₂ and O₂ stored in bladders, then ignites them in a chamber for bursts of thrust spinoff.nasa.gov. It’s “a hybrid of electric and chemical propulsion”, explains Tethers Unlimited CEO Robert Hoyt – solar power does the work of splitting water, but the resulting combustion gives a powerful push spinoff.nasa.gov. European engineers at ArianeGroup have a similar system in the works: a large water tank feeds an electrolyzer, with the hydrogen/oxygen gases ignited after about 90 minutes of generation, yielding roughly 30 seconds of thrust per cycle ariane.group. This cyclical charge-and-burn process can deliver thrust levels much higher than electric-ion thrusters (ArianeGroup estimates up to 14× more thrust per input power than hall-effect ion thrusters) esa.int. The trade-off is moderate specific impulse – i.e. fuel efficiency – that falls between conventional chemical and electric propulsion esa.int. Still, the performance is impressive: “Hydrazine has a specific impulse of 200 s as against 300 s for water,” notes ArianeGroup’s Jean-Marie Le Cocq, comparing their water engine favorably to the toxic fuel it could replace ariane.group.
• Ion and Plasma Thrusters Using Water: Water can also serve as the propellant in advanced electric propulsion systems. In these designs, water vapor is ionized or otherwise excited into plasma, then accelerated by electromagnetic fields to generate thrust (much like a xenon ion engine). For instance, Pale Blue is developing a Water Ion Thruster that uses a microwave plasma source to atomize water molecules and expel ions for thrust phys.org. Such systems can achieve much higher specific impulse (500+ seconds) because the propellant is expelled at extreme velocities reddit.com. Similarly, researchers have tested water-fed arcjet thrusters (~550 s Isp) and microwave plasma thrusters (up to 800 s Isp) reddit.com – performances on par with or exceeding many state-of-the-art electric thrusters. The challenge here is managing the plasma generation and preventing electrode corrosion from water’s by-products. But the potential is huge: high-Isp water thrusters could make water more mass-efficient than traditional fuels for certain missions reddit.com. These are still emerging technologies; Pale Blue’s first on-orbit demos of a water-ion engine are slated for 2025 via two missions with D-Orbit’s carrier spacecraft payloadspace.com. In the future, hybrid thrusters might even combine modes – e.g. a dual system that offers high-thrust steam burns when needed and efficient ion propulsion for long-duration cruising phys.org.

In all cases, the core idea is using electrical energy (from solar panels) to add kinetic energy to water mass and expel it for propulsion. Water itself is inert and non-toxic, which makes it uniquely convenient – it can be stored as a liquid (no high-pressure tanks needed at launch) and won’t blow up or poison handlers. The propulsion only “wakes up” once the satellite is safely in orbit and power is available to heat or electrolyze the water. This on-demand nature is exactly why NASA has been investing in water-based thrusters for small satellites: “PTD-1 will meet this need with the first demonstration of a water-based electrolysis spacecraft propulsion system in space,” said David Mayer, project manager for a 2021 test mission nasa.gov. The next sections will explore why this concept is so attractive – and what challenges still remain.

Benefits of Water Propulsion

Safety and Simplicity: Traditional satellite propellants like hydrazine or xenon are either highly toxic, corrosive, or require heavy pressurization. Water, by contrast, is “the safest rocket fuel I know of,” Mayer notes nasa.gov. It’s non-toxic, non-flammable, and stable at room temperature, making integration and launch far simpler and cheaper nasa.gov. No hazmat suits or complex fuel loading procedures are needed – “you can let undergrads play with it, and they’re not going to poison themselves,” jokes Tethers Unlimited’s CEO spinoff.nasa.gov. This safety factor is especially crucial for CubeSats ridesharing on rockets with expensive primary payloads, where stringent rules often forbid onboard explosives or high-pressure tanks nasa.gov. Water-powered systems stay benign until activated in orbit, easing range safety concerns. This has opened the door for even tiny CubeSats to have propulsion, which was previously off-limits due to fuel safety restrictions.

Low Cost and Ubiquity: Water is dirt cheap and universally available. There’s no supply chain bottleneck – any launch site in the world can obtain pure water easily (and spill some without incident). “Water is available everywhere on Earth and can be transported without risk,” emphasizes ArianeGroup’s Nicholas Harmansa, who is confident “water is the fuel of the future” ariane.group. Per liter, water costs pennies, whereas exotic electric propellants like xenon gas have seen price and supply fluctuations. The hardware for water thrusters can also be cheaper: no need for heavy-walled pressure vessels or toxic-material plumbing. Overall, using water can cut propulsion system costs by a factor of three compared to conventional systems, according to ArianeGroup’s estimates ariane.group. The European Space Agency found that a 1-ton satellite could save ~20 kg of mass by switching from hydrazine to a water-electrolysis engine, in addition to “largely reduced handling and fueling costs” esa.intesa.int. For commercial operators, these savings in mass and money translate to more payload and less risk.

In-Space Refueling and Sustainability: Perhaps the most exciting benefit is how water propulsion could enable a sustainable space infrastructure. Water is not only common on Earth – it’s abundant throughout the solar system. Ice deposits on the Moon, Mars, asteroids, and moons like Europa are essentially “space gas stations” waiting to be tapped mobilityengineeringtech.com. Unlike toxic fuels which would need complex chemical factories to reproduce off-Earth, water can be mined and used directly as propellant after minimal processing. This has huge implications for deep space exploration: a spacecraft could refill its tanks by harvesting ice at a destination and then journey onwards indefinitely. A pioneering demo of this concept came in 2019 when a team from UCF and Honeybee Robotics tested the WINE (World Is Not Enough) prototype, a small lander that mined simulated asteroid ice and used it to generate steam rocket thrust en.wikipedia.org. WINE successfully drilled icy regolith, extracted water, and hopped in a vacuum chamber on a jet of steam – proving a vehicle could “live off the land” and refuel itself for “eternal exploration” en.wikipedia.org. In the long term, water-fueled spacecraft could roam from asteroid to asteroid without ever needing a resupply from Earth en.wikipedia.org. Even in near-Earth operations, companies like Orbit Fab are eyeing water as a candidate for orbital refueling services, given how easy it is to handle. All of this makes water propulsion a cornerstone for the in-space economy that visionaries are trying to build: “we see water as a fundamental resource that’s key to that economy,” says Hoyt, who is designing next-gen Hydros thrusters with refueling ports for indefinite life spinoff.nasa.gov.

Environmental and Operational Cleanliness: As a green propellant, water produces no noxious exhaust – just water vapor or a trace of hydrogen/oxygen which quickly dissipates. This is great not only for Earth’s environment but also for sensitive spacecraft systems. Optical sensors or star-trackers won’t be fogged by residue, and there’s no risk of corrosive plume impingement on delicate surfaces mobilityengineeringtech.com. Chris Hadfield points out that water-based thrusters are ideal for servicing missions like boosting the aging Hubble Space Telescope, because they “can’t be spraying [Hubble] with any sort of residue from propellant” spaceref.com. The gentle, controlled thrust from a water plasma engine can raise or lower orbits without the intense jolts of chemical engines, reducing mechanical stress during delicate operations spaceref.com. In summary, water propulsion is not only friendlier to those launching and building satellites, but also to the satellites themselves and their celestial neighbors.

Illustration of a small satellite using a water-based thruster in orbit. Water-fueled propulsion can be achieved by electrically heating or electrolyzing water to produce thrust, offering a safer and “greener” alternative to traditional chemical rockets nasa.govnasa.gov.

Challenges and Limitations

If water propulsion is so great, why aren’t all satellites using it already? As with any new technology, there are trade-offs and hurdles to overcome:

Lower Thrust (in Some Modes): Pure water resistojet thrusters tend to have quite low thrust compared to chemical rockets. Boiling water can only expel it so fast (typically yielding specific impulse on the order of 50–100 seconds for simple steam thrusters reddit.com, blog.satsearch.co). This is fine for small CubeSats doing gentle tweaks, but it means maneuvers are slow. A 50 s Isp steam thruster provides “much worse bang for your buck” impulse-wise than a typical 300 s hydrazine thruster reddit.com. The industry is addressing this by moving to higher-energy approaches like plasma thrusters (500+ s Isp) and water bipropellant combustion (~300 s Isp) reddit.com, ariane.group. Still, the thrust-to-power ratio is a limiting factor – you need ample electrical power to get meaningful thrust from water. On small satellites, power is limited, so there is a ceiling on thrust unless they carry big solar panels or other power sources. This is why even the best water-ion engines will be suitable for slow orbit raising, not rapid orbital transfers (for now). Engineers must carefully weigh if a mission’s delta-V and timing requirements can be met with an electric water thruster or if a higher-thrust chemical system is needed.

Energy and Thermal Demands: Water may be easy to store, but turning it into hot gas or plasma requires a lot of energy. Electrolysis in particular is energy-hungry – splitting water is inherently inefficient, and then you still need to ignite the gases. The electrolyzers and heaters add complexity and can be points of failure. Managing the heat is another issue: boiling or plasma systems can run hot, which is tough in the vacuum of space where cooling is hard. Tethers Unlimited’s Hoyt noted the materials challenges in dealing with “hydrogen and oxygen and superheated steam” – corrosion and contamination can easily degrade a system spinoff.nasa.gov. Designers have to use special coatings and ultra-pure water to avoid electrode fouling and ensure long life spinoff.nasa.gov. These issues are being solved gradually (with better materials and by isolating the electrolyzer from the combustion chamber, for example), but it’s taken years of R&D to make a reliable engine. In fact, despite NASA theorizing about water rockets since the 1960s, only recently did a “practical water-electrolysis engine” emerge due to these technical hurdlesspinoff.nasa.gov.

Performance vs. Storage Trade-off: Water is bulky. It has decent density (1 g/mL, similar to many liquid fuels) but offers no chemical energy of its own. This means for high delta-V missions, a water propellant tank might need to be larger than a tank of more energetic propellants. Water’s saving grace is that advanced thrusters can pump in external energy to make up for this. For instance, a microwave electrothermal thruster feeding 5 kW into water can achieve ~800 s Isp reddit.com, effectively squeezing more performance out of each drop of water. But those power levels are only available on bigger spacecraft. Small satellites might be limited to lower Isp, making water less efficient by mass for them. There’s also the issue of water management in orbit: it can freeze if lines or tanks are not heated, or it can cause thrust instabilities if it flashes to vapor unpredictably. Engineers mitigate this with careful thermal control and pressure regulation (e.g. keeping water slightly pressurized so it stays liquid until intended to vaporize phys.org). Additionally, while water is non-pressurized at launch, some systems do require pressurizing it in space (or storing the electrolyzed gases in tanks at pressure). That reintroduces some complexity of pressurized systems, albeit after reaching orbit. Mission planners must also consider propellant boil-off – water in a heated tank could leak or evaporate over a long duration mission if not properly sealed and cooled.

Flight Heritage and Trust: As of 2025, water propulsion is still a relatively new player in operational fleets. Many satellite operators adopt a “wait and see” approach, wanting to be sure the tech is proven. Early adopters like HawkEye 360 (who flew water thrusters in 2018) and Sony’s Star Sphere program (2023) have helped build confidence geekwire.com, phys.org. But conservative customers might need more demonstrations, especially for critical missions, before ditching tried-and-true chemical thrusters. There have been minor hiccups too: for example, NASA’s Pathfinder Technology Demonstrator-1 (PTD-1) mission in 2021 aimed to prove Tethers’ Hydros thruster in orbit nasa.gov. While the mission was largely successful, any anomalies or underperformance (if any were encountered) are lessons that future iterations will improve upon. It’s worth noting that even successful tests have limited duration so far (minutes of firing). Long-term endurance of these systems (hundreds of firings over years) is being tested but isn’t fully validated in space yet. This is changing fast as companies like Momentus have now fired their water thrusters dozens of times on orbit nasdaq.com. Each new mission is expanding the envelope, bringing water propulsion closer to a mainstream option. In the meantime, engineers and regulators are carefully evaluating these thrusters to establish standards and best practices (for instance, making sure a “water-fueled” satellite can be safely deorbited at end-of-life by reserving a bit of water for a final deorbit burn – a requirement for space debris mitigation).

In short, the limitations of water propulsion – lower immediate thrust, energy needs, and early-stage development risk – mean it’s not a silver bullet for every scenario yet. But the rapid progress in the past few years suggests these challenges are being overcome one by one, as we explore next in the context of actual missions and players.

Early Innovations and Historical Milestones

The concept of using water as a space propellant has been floating around for decades. NASA researchers in the Apollo era recognized that water could be turned into hydrogen/oxygen – the same potent combo that powered the Space Shuttles – if one had energy available in space spinoff.nasa.gov. But through the 20th century, the idea remained on the drawing board; chemical rockets using storable toxic fuels were simply more mature and provided higher thrust for the technology of the time. It wasn’t until the miniaturization of satellites and advances in electric power that water propulsion gained new relevance. Here are some key early milestones leading up to the current state:

• 2011–2017: The rise of CubeSats (tiny satellites built from 10 cm cubes) created a need for equally tiny, safe thrusters. Research groups began revisiting water as an ideal CubeSat propellant since many launch providers banned chemical fuels on secondary payloads. In 2017, a Purdue University team led by Prof. Alina Alexeenko unveiled a microthruster called FEMTA (Film-Evaporation MEMS Tunable Array) that uses ultra-purified water mobilityengineeringtech.com. FEMTA employed 10-micron capillaries etched in silicon; surface tension keeps the water in place until a heater boils it, ejecting micro-jets of vapor. In vacuum chamber tests, a FEMTA thruster produced controllable thrust in the 6–68 µN range with specific impulse around 70 s futurity.org, sciencedirect.com. Four FEMTA thrusters (with about a teaspoon of water total) could rotate a 1U CubeSat in under a minute using only 0.25 W of power mobilityengineeringtech.com. This was a breakthrough in showing that even very low-power systems could impart meaningful attitude control using water. Alexeenko highlighted water’s appeal not just for Earth orbits but also for resource use in space – “Water is thought to be abundant on the Martian moon Phobos, making it potentially a huge gas station in space… [and] a very clean propellant” mobilityengineeringtech.com.
• 2018: The first operational use of water propulsion in orbit took place. A U.S. startup Deep Space Industries (DSI) had developed the Comet electrothermal thruster, a small device that boils water and shoots it out for maneuvering smallsats. In December 2018, DSI’s Comet thrusters flew on four commercial satellites: three were for the HawkEye 360 radio-frequency constellation and one for Capella Space’s radar imaging demo geekwire.com. These small satellites successfully used water propulsion to adjust their orbits, marking the debut of water-fueled engines working in space. Around the same time, a Japanese 3U CubeSat named AQT-D (Aqua Thruster-Demonstrator), developed at the University of Tokyo, was deployed from the ISS. AQT-D tested a water resistojet system in orbit in late 2019, demonstrating attitude and small orbit changes; this was an early in-space test by Japan that laid groundwork for the Pale Blue startup later blog.satsearch.co.
• 2019: NASA’s interest in water propulsion went from theory to practice. Tethers Unlimited, under NASA SBIR contracts and a “Tipping Point” partnership, delivered a flight-ready HYDROS-C thruster for CubeSatsspinoff.nasa.govspinoff.nasa.gov. NASA integrated this into the Pathfinder Technology Demonstrator 1 (PTD-1) mission, a 6U CubeSat. Though launch was delayed to 2021, this mission aimed to be the “first demonstration of a water-based electrolysis spacecraft propulsion system in space” nasa.gov. The mere approval of a water propulsion payload indicated NASA’s confidence in its safety and utility for small missions. In the private sector, DSI was acquired by Bradford Space in 2019 geekwire.com, shifting DSI’s focus fully to propulsion. Bradford continued marketing the Comet thruster as a non-toxic alternative for small satellites, and even large integrators took notice – LeoStella (the manufacturer for BlackSky’s Earth observation constellation) decided to adopt Comet water thrusters for its upcoming satellites geekwire.com. By the end of 2019, the momentum was clear: water propulsion had moved from lab prototypes to real spacecraft and was attracting serious investment.
• 2020–2021: Several significant events kept water thrusters in the headlines. A Washington-based startup Momentus Inc. emerged with bold plans for space tugs (orbital transfer vehicles) powered by water plasma engines. Co-founded by a Russian entrepreneur, Momentus garnered attention for its “water plasma propulsion” promises, although regulatory hurdles delayed its first launches to 2021. Meanwhile, in 2020, Japanese startup Pale Blue Inc. spun out of University of Tokyo labs, aiming to commercialize water propulsion in the Japanese and global market phys.org. Their roadmap included small resistojet units and more advanced ion and hall-effect thrusters using water. By early 2021, NASA finally launched PTD-1 (on SpaceX’s Transporter-1 rideshare) carrying the Hydros thruster nasa.gov. Over a 4-6 month mission, PTD-1 was slated to perform orbit changes using water fuel, proving out the performance and reliability needed for future use nasa.gov. This mission was a capstone of nearly a decade of work by Tethers and NASA, showing that even a shoebox-sized satellite could have a “low-cost, high-performance propulsion system” using water nasa.gov. 2021 also saw the European Space Agency complete a study on water propulsion viability, identifying it as a top choice for certain classes of missions (particularly 1-ton LEO satellites) and spurring companies like Germany’s OMNIDEA-RTG to begin development efforts in Europe esa.intesa.int.

This early history set the stage by proving the concept and early adoption. Next, we look at the current players who are scaling up water propulsion and the missions that are showcasing its capabilities.

Key Players Driving Water Propulsion Forward

By 2025, a vibrant ecosystem of companies and space agencies is pushing water-based propulsion from demonstration to deployment. Here are some of the notable organizations and their contributions:

• Tethers Unlimited (USA) & NASA: Tethers Unlimited (TUI) was a pioneer with its Hydros water-electrolysis thrusters, developed via NASA SBIR funding spinoff.nasa.gov. In partnership with NASA Ames and Glenn, TUI flew the Hydros-C on NASA’s PTD-1 mission, making it a trailblazer of water propulsion in CubeSats spinoff.nasa.gov. TUI also built larger Hydros-M units for 50–200 kg satellites under a NASA Tipping Point contract, delivering thrusters to Millennium Space Systems for testing spinoff.nasa.gov. NASA’s continued support (through programs like Small Spacecraft Technology and the upcoming On-orbit Servicing missions) indicates strong agency belief in water propellant for safe, refuelable spacecraft. TUI’s CEO Hoyt envisions water thrusters eventually equipped with refueling ports, able to top up from Orbit Fab depots or asteroid mining operations spinoff.nasa.gov.
• Momentus Inc. (USA): Momentus has developed a unique Microwave Electrothermal Thruster (MET) that uses water to create plasma jets, and integrated it into the Vigoride orbital transfer vehicle. Despite a bumpy road (including U.S. regulatory scrutiny and a delayed SPAC merger), Momentus successfully flew several Vigoride demos in 2022–2023. During its January 2023 Vigoride-5 mission, Momentus “tested its MET thruster in orbit with 35 firings”, validating the thruster’s performance in various use cases nasdaq.com. In one test, Vigoride-5 raised its orbit by ~3 km using water propulsion alone spaceref.com. Company board member Chris Hadfield has been a vocal cheerleader, highlighting that “we are finding far more water in our solar system” to use as propellant and that Momentus’ MET is basically “a nozzle on a microwave” that can even turn water into plasma for thrust spaceref.com. Momentus is now offering in-space shuttle services, leveraging the low cost of water to potentially compete on price. They have also proposed ambitious projects, like using a water-based tug to boost the Hubble Telescope’s orbit to extend its life spaceref.com. While Momentus is still proving its commercial viability, it has undeniably advanced the technology by demonstrating a scalable water propulsion system on orbit multiple times.
• Pale Blue (Japan): A startup born at the University of Tokyo, Pale Blue is the name to watch in water propulsion in Asia. In March 2023, Pale Blue’s water resistojet thruster propelled Sony’s EYE satellite (of the Star Sphere project) – the first on-orbit firing of a privately developed Japanese water engine phys.org. The thruster performed a two-minute burn that changed the CubeSat’s orbit as planned, a big milestone for the company phys.org. Pale Blue offers a range of thrusters: from the PBR- series (10, 20, 50) resistojet modules for small satellites, to the upcoming PBI water ion thruster and even a planned water Hall-effect thruster (PBH) by 2028 blog.satsearch.co. Their PBR-20 thruster (1 mN thrust, >70 s Isp) was tested in 2019 and 2023 flights, and a larger PBR-50 (10 mN thrust) launched in early 2024 for its first mission blog.satsearch.co. In 2025, Pale Blue is scheduled to demonstrate the world’s first 1U-sized water ion engine on two D-Orbit rideshare missions (June and October) payloadspace.com. The Japanese government is backing Pale Blue strongly – a 2024 program awarded the company up to $27 million to advance its water-based propulsion for commercial and defense applications (signaling national interest in non-toxic propulsion for satellites). With partnerships (like with Italian firm D-Orbit) and significant funding, Pale Blue aims to disrupt the smallsat propulsion market with safe, refillable water systems.
• Bradford Space (USA/Europe): After acquiring Deep Space Industries in 2019, Bradford Space inherited the Comet water thruster and has since supplied it to multiple satellite missions. The Comet is billed as “the world’s first operational water propulsion system” and has been implemented by several customers geekwire.com. Notably, HawkEye 360’s pathfinder satellites and Capella’s Whitney demo satellite in 2018 each used Comet thrusters for orbit maintenance geekwire.com. Seattle-based manufacturer LeoStella also chose Comet engines for the second-gen BlackSky imaging satellites that it builds, indicating confidence in Comet’s reliability geekwire.com. The Comet thruster provides about 17 mN thrust and 175 s Isp blog.satsearch.co, using an electrothermal heater to expel water vapor. Bradford markets it as a “launch-safe” replacement for hydrazine systems on small and medium satellites blog.satsearch.co. With offices in the U.S. and Europe, Bradford is also integrating Comet technology into future deep-space mission designs (e.g. their proposed Xplorer spacecraft bus for asteroid missions could use water propulsion to maneuver in deep space geekwire.com). As constellations proliferate, Bradford’s production of flight-proven water thrusters positions it as a key supplier for companies that want non-hazardous propulsion at scale.
• ArianeGroup & European Partners (EU): In Europe, the big aerospace prime ArianeGroup has taken the lead on water-based propulsion, aiming to equip next-generation LEO and MEO satellites. At their Lampoldshausen site in Germany, ArianeGroup’s team has built a hybrid electric-chemical water engine (very similar to Tethers’ Hydros concept) ariane.group. By late 2023 they revealed details: the system can electrolyze water in ~90 minutes and then fire a 30-second bipropellant burn, with an overall specific impulse around 300 seconds ariane.group. The design is modular and scalable – they can increase electrolyzer cells, tank size, or number of thruster chambers to meet different satellite requirements ariane.group. ArianeGroup claims the system could be “three times less costly” than current chemical propulsion for constellations ariane.group. With support from ESA and DLR (German space agency), ArianeGroup plans an in-orbit demonstration by autumn 2026 on the ESMS satellite, which will use the water engine for orbit adjustments and station-keeping ariane.group. This demo will validate the electrolyzer operation in microgravity and the performance of the dual-mode engine in space. Europe’s investment signals that they see water propulsion as a competitive and sustainable alternative for satellite networks, especially given upcoming regulations pushing for “green” propellants to reduce launch risks.
• Other Noteworthy Startups: Beyond the big names above, numerous startups across the globe are innovating in water propulsion. Aurora Propulsion Technologies (Finland) offers small ARM-series water thrusters for CubeSats, including modules for full 3-axis control of 1U–12U satellites using tiny water microjets blog.satsearch.co. SteamJet Space Systems (UK) has developed the aptly named Steam Thruster One and “TunaCan” thruster, which are compact electrothermal water engines that fit into the unused volume of CubeSat deployers blog.satsearch.co. These have been flight-proven on at least one CubeSat mission, showing that even nano-satellites can perform orbit maneuvers with a bit of heated water blog.satsearch.co. In France, ThrustMe (known for iodine electric thrusters) has explored water as a propellant in some concepts, and in Italy, ESA-funded startups are also considering water for small launcher upper stages or orbital tugs. Additionally, an intriguing entrant is URA Thrusters, which has outlined a suite of water-propelled systems – from a Hall-effect thruster that can use water vapor or oxygen blog.satsearch.co, to “ICE” electrolysis thrusters that combine MEMS-scale water splitting and combustion blog.satsearch.co, to a Hydra hybrid that pairs a Hall thruster with a chemical engine for flexible performance blog.satsearch.co. While some of these are still on the drawing board, the breadth of development underscores a point: water propulsion is not a one-trick novelty, but a broad technological movement attracting innovators worldwide.

A flight prototype of Tethers Unlimited’s HYDROS-C water propulsion system for CubeSats. This compact unit contains water tanks, an electrolyzer, gas bladders, and a rocket nozzle spinoff.nasa.gov. Such systems remain inert until reaching orbit, when solar power is used to split water into hydrogen/oxygen propellants for thrust.

Missions and Milestones: Water Propulsion in Action

Actual space missions in recent years have proven the feasibility of water-fueled drives and continue to push their capabilities. Below is a timeline of notable missions and demonstrations showcasing water propulsion:

• 2018 – First Orbit Use: HawkEye 360 Pathfinder satellites (3 in formation) and a Capella Space radar satellite each utilize DSI’s Comet water thrusters for orbit maintenance after launch in December 2018 geekwire.com. These became the first commercial satellites to operate on water propellant, completing maneuvers successfully and validating the thruster in space.
• 2019 – ISS Deployed Demo: The University of Tokyo’s AQT-D (Aquarius) 3U CubeSat, deployed from the International Space Station, fires its water resistojet thrusters in orbit. The system achieves attitude control and small orbital changes, marking Japan’s first in-space demo of water propulsion. This mission proved a multi-nozzle water thruster could work in microgravity and laid groundwork for Pale Blue’s later designs blog.satsearch.co.
• 2021 – NASA PTD-1: Pathfinder Technology Demonstrator-1, a NASA 6U CubeSat, conducts the first water-electrolysis propulsion test in orbit. Carrying ~0.5 liters of water, PTD-1’s Hydros engine performs programmed thrust maneuvers, demonstrating that splitting water into H₂/O₂ and burning it can propel a satellite as expected nasa.gov. This mission, lasting several months, verifies the performance, safety, and restart capability of the system, giving small satellites a new proven option for orbit control.
• 2022 – Vigoride Debut: Momentus launches Vigoride-3 (its first orbital service vehicle) in May 2022. Although initial thruster tests are limited (the vehicle experienced some anomalies in early operations spacenews.com), the mission sets the stage for incremental testing of the water-based MET. Momentus establishes contact and learns to operate the novel propulsion in the real space environment news.satnews.com, setting up improvements for subsequent flights.
• 2023 – Multiple Successes: This year is a turning point with several water propulsion victories:
  • Momentus Vigoride-5 (Jan 2023): Successfully executes 35 thruster firings of its water MET in orbit, raising its orbit and adjusting attitude using only water plasma jets nasdaq.com. This is a major proof that a larger vehicle (~250 kg) can use water propulsion for meaningful orbit changes.
  • Momentus Vigoride-6 (Apr 2023): Continues testing and even completes a customer orbit insertion (though a software timing issue led to a slight orbit inclination error) nasdaq.com. Vigoride-6 remains operational, further validating the propulsion system’s reliability.
  • Pale Blue EYE Demo (Mar 2023): Sony’s EYE CubeSat performs an orbit-raising maneuver using Pale Blue’s water thruster for ~120 seconds phys.org. The success of this demonstration – pushing the satellite closer to its target orbit for Earth photography – confirms the thruster’s orbital functionality and is widely reported as Japan’s entry into water propulsion phys.org.
  • EQUULEUS at the Moon (late 2022–2023): Although not widely publicized in mainstream media, it’s worth noting EQUULEUS, a JAXA-Univ. of Tokyo CubeSat launched to the Moon on Artemis I (Nov 2022), carried a water resistojet system for trajectory adjustments sciencedirect.com. It used water thrusters to successfully perform course corrections on its way to the Earth-Moon Lagrange point, demonstrating water propulsion in cislunar space – a first for beyond-LEO operations.
• 2024 – Scaling Up: Water propulsion begins appearing on more operational satellites:
  • Fleet Deployments: Hawkeye 360’s next batches of satellites and Capella’s newer SAR satellites continue to use water-based Comet thrusters in routine service, under Bradford’s support. Moreover, BlackSky’s Gen-2 satellites launched in 2024 incorporate the Comet water propulsion for orbit maintenance of the Earth-imaging constellation geekwire.com.
  • New Thruster Launches: Pale Blue’s larger PBR-50 thrusters see their first launch in early 2024 on a smallsat rideshare (exact mission undisclosed), aiming to provide ~10 mN thrust for a microsat in orbit blog.satsearch.co. This begins qualification of water propulsion for bigger smallsat classes.
  • Infrastructure: Companies like Orbit Fab announce plans to make water one of the fuel options for their proposed orbital propellant depots, and NASA’s TALOS project considers water-based “drop tanks” for deep space tugs – reflecting a broader acceptance that water will be part of the space logistics chain in coming years.
• 2025 – Upcoming and Ongoing: Exciting missions are on the docket:
  • Pale Blue D-Orbit Flights: The first water-ion thruster (PBI) will be flight-tested on D-Orbit’s Ion Satellite Carrier in mid and late 2025 payloadspace.com. These tests will measure the high-efficiency thrust and pave the way for commercial ion units that use water instead of xenon or krypton.
  • JAXA RAISE-4 Experiment: Japan’s space agency plans to launch the RAISE-4 tech demo satellite in 2025, which is slated to carry Pale Blue’s latest propulsion system (possibly the improved PBI) for testing in low Earth orbit blog.satsearch.co.
  • Momentus Commercialization: Momentus expects to transition from pure testing to operational missions, offering to ferry client payloads. By 2025, they aim to start providing orbit-raising services — for instance, taking small satellites from a rideshare drop-off orbit to a desired higher orbit — solely using water propulsion. This will be a litmus test of water thrusters’ economic viability in real missions.
  • ESA Water Engine Demo: In Europe, final preparations begin for the Spectrum Monitoring Satellite (ESMS) mission due in 2026, which by 2025 will have its water propulsion system integrated and undergoing ground testing ariane.group. If all goes well, this mission will become the first full-scale commercial satellite to rely on water for primary propulsion (not just as a demo unit).

This timeline shows a clear acceleration: from one-off experiments a few years ago to multiple spacecraft relying on water today, and many more in the pipeline. Each success builds confidence and heritage, which in turn attracts more users. By mid-2020s, water propulsion is moving out of the experimental phase and into the toolkit of mission designers.

Artist’s rendering of a small satellite (Sony’s EYE cubesat) which in 2023 used a Pale Blue water-based resistojet thruster to adjust its orbit phys.orgphys.org. The demonstration marked the first in-space use of water propulsion by a Japanese startup, and the satellite’s orbit change confirmed the thruster’s performance.

The Latest Breakthroughs (2024–2025) and What’s Next

The past two years have seen rapid advancements, and the trend is set to continue. Recent news and developments in 2024–2025 highlight how water propulsion is reaching new heights:

• Funding and Industry Support: Recognizing the strategic value of non-toxic propulsion, government agencies are investing in water thrusters. In 2024, Japan’s METI awarded Pale Blue a multi-billion yen grant (up to ~$27M) to scale up its water propulsion tech for commercial and defense satellites spacenews.com. This infusion will help Pale Blue increase thrust levels and develop larger systems suitable for bigger satellites. Europe’s Horizon programs are likewise funding green propellant solutions, with water-based designs front and center, as evidenced by ESA’s backing of ArianeGroup’s 2026 demo ariane.group. Even the U.S. DoD has shown interest in safe CubeSat propulsion for Space Force projects, where water’s safety is a selling point.
• Higher Power Thrusters: On the technology front, developers are pushing water engines to higher power and performance. One breakthrough on the horizon is water Hall-effect thrusters – combining the efficiency of Hall plasma engines with water propellant. Pale Blue’s planned PBH thruster for 2028 is one example blog.satsearch.co, and URA Thrusters’ conceptual Hydra system (dual Hall + chemical) is another blog.satsearch.co. If realized, these could handle missions that currently only chemical propulsion or large electric thrusters can do, like rapid orbit transfers or interplanetary trajectories, but with the benefit of easy refuel via water. Additionally, Momentus and others are studying how to increase the ISP of their METs further, possibly by using higher microwave frequencies or novel resonant cavities to superheat water more efficiently. Specific impulse of ~1000 s might be within reach in the next iterations, which would firmly put water thrusters in the league of traditional ion drives in terms of efficiency.
• Integration into Constellations: 2024 marked the first significant repeat deployments of water propulsion in satellite constellations. For instance, every new BlackSky imaging satellite now carries a Bradford Comet water thruster for orbit keeping, meaning dozens of identical spacecraft will operate on water propellant over their lifetimes geekwire.com. Hawkeye 360’s second-gen cluster (launched 2022–2023) also uses water-based propulsion for formation flying. This mainstream adoption is a breakthrough in itself – water propulsion is no longer just a one-off experiment, but a standard component in some fleets. Going forward, many proposed megaconstellations for IoT and Earth observation are considering green propulsion options, and water is high on that list due to its low system cost. As production of these thrusters scales up, unit costs will drop, further encouraging adoption.
• Novel Applications: Engineers are finding creative new ways to exploit water’s versatility. One idea in development is electrolysis-based attitude control – using tiny amounts of electrolyzed gas for precise attitude jets, then recombining the water, in a closed loop. Another is using water as the working mass in solar thermal propulsion: concentrate sunlight to directly heat water to steam for thrust (essentially a steam boiler in space powered by the Sun, which could be very efficient in inner solar system). Researchers are also testing water-based propellant for landers and hoppers for Moon/Mars. NASA’s lunar Flashlight mission (though ultimately it had issues) considered water as a candidate propellant early in its design. And looking further, water could be the propellant for nuclear thermal rockets or beamed-energy propulsion, where an external power source (like a ground-based laser) heats the water on a spacecraft to produce thrust reddit.com. Water’s benign nature allows for these outside-the-box concepts that would be unthinkable with toxic or rare propellants.
• Expert Endorsements: The water propulsion revolution has not gone unnoticed by space industry leaders. Chris Hadfield’s enthusiastic championing of Momentus’ water thrusters spaceref.com, and quotes like “I’m sure that water is the fuel of the future” from European project managers ariane.group, reflect a growing consensus that this technology is here to stay. In interviews and conferences (such as the Small Satellite Conference and the Space Propulsion Workshop in 2024), experts have lauded the balance of safety and performance that water systems offer. “Good propulsive performance needs to be balanced by safety – PTD-1 will meet this need,” said NASA’s David Mayer when introducing the first water-thruster demo nasa.gov. That statement neatly captures why water has gained traction: it hits the sweet spot between the high performance of chemical propulsion and the safety of electric propulsion. Space mission planners are increasingly echoing this sentiment in trade publications and panels.

As we stand in 2025, the trajectory for water-powered satellite drives is clearly pointing upward. The next big step is likely a flagship mission that truly relies on water propulsion for a critical objective – perhaps a lunar CubeSat that uses water to enter orbit around the Moon, or a servicing craft that autonomously refuels from a depot and tows a satellite. Each year, the envelope is being pushed. If current trends continue, by the late 2020s we could see water-based engines propelling spacecraft to asteroids and back, raising and lowering hundreds of satellites in orbit, and doing so with minimal environmental impact and full in-space refuelability. What started as an unconventional idea has grown into a practical technology that could make space operations more affordable, sustainable, and flexible than ever before.

Conclusion: A New Era Propelled by H₂O

Water-powered satellite propulsion is no longer a futuristic concept – it’s here, proving itself one mission at a time. In the span of a few years, we’ve gone from the first puffs of water vapor nudging a tiny CubeSat, to fully maneuverable spacecraft using water to change orbits and conduct complex operations. The allure of water as the ultimate space propellant lies in its elegant simplicity. As ESA’s technology report noted, water is “an underutilised resource – safe to handle and green”, yet containing “two very combustible propellants once electrolysed”, essentially packing the punch of rocket fuel in a benign form esa.int. This dual nature – easy storage as liquid, energetic use as gas – gives water a unique edge.

We are witnessing a convergence of factors making water drives practical: better small electric pumps and heaters, more efficient solar panels to power them, 3D-printed thrusters optimized for steam or plasma, and a booming demand for small satellites that need low-cost propulsion. The challenges (limited thrust, power needs) are being addressed with innovative engineering, and the successes are piling up. Importantly, water propulsion aligns with the broader push for sustainability in space – reducing toxic chemicals, enabling satellite longevity through refueling, and even utilizing extraterrestrial resources. It transforms water from just a life-support consumable into a versatile mobility enabler for space infrastructure.

In public imagination, “rocket fuel” has always been something exotic or dangerous. The idea that water – the same substance we drink and bathe in – could send satellites zooming around Earth or beyond is captivating. It lowers the barrier to entry for space endeavors (you don’t need specialized fuels, just ingenuity), and it sparks visions of spacecraft stopping by lunar ice mines or asteroid reservoirs to top off their tanks. The technology is still evolving, but its trajectory suggests that water-powered drives could become as commonplace in satellites as battery-powered motors are in cars. As one industry exec quipped, the old joke of “just add water” might well apply to the future of space travel.

In conclusion, water-powered satellite propulsion represents a paradigm shift towards safer, cleaner, and ultimately more expansive space operations. From small CubeSats to potential interplanetary probes, the humble molecule of H₂O is proving it has the right stuff to take us farther. As momentum (and no pun intended) continues to build, don’t be surprised when the next headline reads: “Spacecraft Fueled by Water Reach the Moon – and Keep Going.” The age of the water rocket has dawned, and it holds an ocean of possibilities for the next generation of space exploration spinoff.nasa.gov, spaceref.com.