Solid-Zinc Hall-Effect Thruster "Starlight" Achieves First In-Orbit Ignition — Muon Space Removes High-Pressure Gas From Satellite Propulsion

• What happened: U.S.-based Muon Space successfully achieved the first-ever in-orbit ignition of its "Starlight" Hall-effect thruster, which uses a completely solid metal (zinc) propellant, aboard its own dedicated test satellite, "SERT-III" — succeeding on the very first attempt.
• Why it matters: This marks the first-ever in-orbit demonstration of space propulsion using a cheap, non-toxic, room-temperature solid metal, eliminating the high-pressure gas tanks, plumbing, and valves essential to conventional electric propulsion, as well as toxic and corrosive propellants such as iodine.
• What to watch next: The delivery of the first units for customer missions by the end of 2026, and progress on the in-orbit demonstration of the larger-satellite-oriented "Gen2" thruster, planned for the first quarter of 2027.

First In-Orbit Ignition Success: A Room-Temperature Solid Metal Emits Plasma in Space

On September 10, 2026, U.S. space company Muon Space announced that its in-house-developed dedicated technology demonstration satellite, "SERT-III," had successfully achieved the first-ever in-orbit ignition of its "Starlight" Hall-effect thruster, which uses a completely solid metal (zinc) propellant. The actual in-orbit burn test was conducted on July 1, 2026, and the thruster operated normally on its maiden ignition attempt.

The telemetry data obtained showed a high degree of agreement with the predictive models derived from ground-based vacuum chamber testing. Ground radar and orbital tracking data also confirmed thrust generation and clear orbital transitions consistent with design values. Furthermore, no anomalous events were observed—such as electromagnetic interference (EMI) associated with the ignition process, sudden transient power fluctuations, or interference between the plasma and the satellite structure.

A Hall-effect thruster (HET) is an electric propulsion system that applies a radial magnetic field within an annular discharge channel to confine electrons, causing them to collisionally ionize the propellant gas; the resulting ions are then electrostatically accelerated and expelled. Because its specific impulse (a measure of fuel efficiency) reaches several to over a dozen times that of chemical thrusters, it has become widely adopted as the primary propulsion method for orbit transfer, station-keeping, and end-of-mission deorbiting in small to medium-sized commercial satellites. However, achieving an in-orbit demonstration in which a solid metal itself is vaporized and supplied as propellant in the space environment, forming a stable plasma discharge to generate thrust, represents a world-first achievement in the history of Hall-effect thrusters.

A Comparative Analysis of Propellants: Soaring Xenon Prices, Toxic Iodine, and Solid Zinc

From the earliest days of Hall-effect thrusters to the present, the noble gas xenon (Xe) has reigned as the standard propellant. Xenon has a large atomic mass, a low condensation temperature, is chemically inert, and is extremely easy to handle. In recent years, however, demand has tightened sharply amid the rapid expansion of commercial satellite constellations, and combined with geopolitical factors, prices have soared to several thousand dollars per kilogram. The resulting lengthened procurement lead times and cost inflation pose a serious risk for operators planning megaconstellations.

As an alternative, satellite fleets such as SpaceX's Starlink have moved to adopt krypton (Kr) and argon (Ar). While krypton is cheaper to procure than xenon, its lighter atomic mass and higher first ionization energy (approximately 14.00 eV) make it inferior to xenon in terms of propulsion efficiency and specific impulse density. More importantly, as long as krypton or xenon is used, spacecraft cannot avoid carrying carbon-fiber composite overwrapped pressure vessels (COPVs) rated for 150 to 300 atmospheres, along with high-precision multi-stage pressure-reducing valves, pyrovalves, and complex plumbing systems. High-pressure gas systems carry a constant risk of leakage, and have long been a major operational constraint at launch sites—requiring strict protective clothing and safety exclusion zones during pre-launch activities.

Among room-temperature solid propellants, electric thrusters using iodine have also been under demonstration in recent years. While iodine has high vapor pressure and vaporizes easily, it is extremely corrosive chemically and highly toxic biologically. Special corrosion-resistant coatings and rigorous sealing structures are required to protect satellite electronics, structural materials, and ground equipment from corrosion, and handling it involves specialized protective procedures.

Compared to these existing options, zinc (Zn) offers standout physical and operational characteristics. Zinc's first ionization energy is approximately 9.39 eV, lower than xenon's roughly 12.13 eV and krypton's roughly 14.00 eV. A lower ionization energy requirement facilitates more efficient plasma generation within the discharge channel. Additionally, zinc is abundant on Earth, and its price on the primary metals market is only a few dollars per kilogram—orders of magnitude cheaper than xenon.

Because it is a completely stable, non-toxic solid metal at room temperature and pressure, there is no need whatsoever for high-pressure tanks prone to rupture, pressurized plumbing, or complex valve assemblies. The propellant can be housed as a molded metal block or pellet within an ambient-pressure chamber, and only the necessary amount is locally heated to vaporize and supply it in a microgravity environment—an architecture that becomes feasible with this approach. This dramatically reduces the total part count and dry mass of the entire propulsion system.

Overcoming the Technical Hurdle: Thermoelectric Quartz Crystal Microbalances (TQCM) Prove Low Contamination

Although the concept of using metal as a propellant has existed for a long time, the primary reason spacecraft engineers have long hesitated to adopt it is the risk of "plume contamination." With noble gas propellants, neutral gas or residual particles ejected from the nozzle rapidly disperse into space, and even if they re-adhere to the structure, they evaporate rather than condensing.

With metal vapor, however, neutral metal atoms or low-energy scattered particles that stray from the ion beam can wrap around to the rear or sides of the spacecraft and re-condense on critical surfaces of the satellite. If conductive metal particulates accumulate on the cover glass of solar panels, the lenses of optical sensors, or the apertures of star trackers, this can reduce transmittance, leading to lower power generation or loss of observation capability. It could also alter the emissivity of thermal radiators, disrupting thermal control, or short-circuit the wiring between solar cells operating at high voltage—potentially leading to catastrophic functional failure.

To directly address this concern, Muon Space installed multiple thermoelectric quartz crystal microbalance (TQCM) sensors at various points on the SERT-III satellite structure. A TQCM is a high-precision sensor that exploits the piezoelectric property whereby the resonant frequency of a quartz crystal oscillator changes linearly in response to minute mass deposits on its electrode surface; it can measure deposition thickness in real time at the angstrom scale while controlling the temperature of the deposition surface using a Peltier element.

As a result of the in-orbit burn test, the TQCM data measured at positions closest to the thruster—the harshest locations—showed deposition levels far below the conservative predictions generated by numerical simulation models built before the mission. By optimizing the magnetic field topology at the discharge channel exit and the geometric shape of the vaporization-supply nozzle, the backstreaming of un-ionized neutral metal vapor was suppressed and controlled to a practically negligible level—a result now proven in the space environment.

The SERT-III Lineage and Development Structure: From Rapid Sub-One-Year Demonstration to Commercial Deployment

The name "SERT-III," which Muon Space bestowed on this demonstration spacecraft, reflects a tribute to NASA's historic experimental satellites that pioneered the early days of electric propulsion in space. In July 1964, NASA launched "SERT-I" (Space Electric Rocket Test I) on a ballistic flight, carrying a mercury electrostatic ion engine developed by Dr. Harold R. Kaufman and colleagues, and became the first in the world to prove that an ion thruster could generate thrust in space. Subsequently, "SERT-II," launched in February 1970, achieved several thousand hours of continuous long-duration operation in polar orbit, firmly establishing the practicality of ion engines.

Launched more than half a century after SERT-I and SERT-II, SERT-III symbolizes a mission marking the transition from the era of noble gases and liquid metals (mercury and cesium) to an era of non-toxic solid-metal propulsion.

Supporting this technological breakthrough is the propulsion startup Starlight Propulsion. The company was founded in 2022 by Todd Bailey (now Director of Propulsion at Muon Space) and Mark Hopkins, who had led propulsion development at U.S. small-rocket company Astra, and had been developing the foundational technology for solid-zinc propulsion. Muon Space acquired Starlight Propulsion in 2025, rapidly advancing the vertical integration of propulsion technology.

Of particular note is the speed of development. Typically, demonstrating a new propulsion architecture in orbit requires negotiating placement as a secondary payload on a customer satellite, or going through a satellite development cycle spanning several years. To avoid burdening a customer's commercial mission with the risk of unproven technology, Muon Space built SERT-III as its own dedicated demonstration spacecraft, based on the company's "Mission Foundry" concept. Muon Space designed, manufactured, and launched the SERT-III demonstration satellite in under one year, achieving successful first ignition on July 1, 2026, followed by deployment to customer missions within 2026 and the introduction of the Gen2 system for larger spacecraft in 2027 — an extremely rapid development and commercial deployment cycle. This speed was underpinned by the company's efforts to standardize spacecraft buses and by the strengthened development infrastructure resulting from significant funding rounds it has secured to date.

The commercial deployment roadmap has also taken concrete shape. The current-generation Starlight thruster, which has now completed its in-orbit demonstration, is expected to begin delivery and deployment to the first customer satellite constellation by the end of 2026. In addition, development is underway on a higher-thrust second-generation system, "Gen2," designed for larger spacecraft and higher-orbit transfers, with in-orbit testing planned for the first quarter of 2027 and the start of commercial service planned within 2027.

Redefining Constellation Mass Production and Spacecraft Architecture

The successful in-orbit demonstration of the solid-zinc Hall-effect thruster carries implications that extend well beyond improvements in individual thruster performance—it has the potential to bring structural change across the entire supply chain, from satellite production lines to launch-site operations.

In the operation of small satellite constellations comprising tens to hundreds of units for purposes such as Earth observation and communications, establishing a mass-production system for satellite structures is critical to business success. When high-pressure gas systems are onboard, considerable time and labor have historically been spent on activities such as waiting for delivery of certified pressure vessels, conducting high-pressure leak tests during satellite assembly, and performing non-destructive inspections of complex plumbing welds. Additionally, gas-filling operations at launch sites are classified as hazardous operations, requiring evacuation of work areas and strict adherence to safety procedures—leading to extended time on-site and higher operating costs.

Adopting a room-temperature solid metal propellant fundamentally eliminates these bottlenecks. Zinc pellets can be integrated directly into spacecraft in a standard factory environment, and no additional filling work is required at the launch site. Because there is no need to maintain high pressure, there is no concern about pressure loss from gas leakage even after extended ground storage, making the approach highly compatible with on-demand launch operations in which satellites are stockpiled for long periods before being launched immediately when needed.

Of course, technical caveats remain. The results from SERT-III to date confirm the soundness of initial ignition, short-duration burn characteristics, and early plume deposition data. It remains essential to continue verifying thermal cycling fatigue in the vaporization heater components, localized erosion of the thruster discharge chamber, and long-term plume deposition behavior over the thousands to tens of thousands of hours of cumulative operating time required for commercial use. Furthermore, for specialized observation missions carrying cryogenically cooled infrared sensors or ultra-high-precision optical mirrors, where the permissible deposition thickness threshold is extremely strict, optimal shielding design tailored to the satellite structure layout will continue to be required.

Even so, the benefit of being able to completely eliminate high-pressure gas—a potential hazard—from inside a satellite is enormous. If this design philosophy of bringing a common, room-temperature solid metal into space as an electric propulsion propellant takes hold, it will dramatically expand the design freedom of satellites and serve as a firm catalyst for further accelerating the pace of mass production in the small satellite industry.