Portal Space Systems Bets on Sun-Powered Spacecraft for the Future

A decades-old space propulsion concept is getting a modern comeback thanks to Portal Space Systems, a Washington-based startup led by aerospace veteran Jeff Thornburg. The company is developing spacecraft powered by concentrated sunlight, a technology it believes could transform how satellites move through space while supporting future commercial and national security missions.

Based in Bothell, Washington, Portal Space Systems is working to commercialize solar thermal propulsion, an approach that was explored years ago by NASA and the U.S. Air Force but was never brought into mainstream use. Instead of relying entirely on traditional chemical engines or electric propulsion systems, the technology uses concentrated sunlight to heat a propellant, creating thrust that can quickly move spacecraft between different orbits.

According to Thornburg, the idea was not abandoned because it failed technically. Instead, the economics at the time made it impractical. He believes today’s lower launch costs, combined with advances in additive manufacturing and modern engineering, have made solar thermal propulsion commercially viable.

Portal’s mission has already attracted strong interest from both investors and government organizations. Since launching in 2021, the company has raised more than $20 million in venture funding. In 2024, it also secured a $45 million public-private funding commitment from SpaceWERX, the innovation arm of the U.S. Space Force. The company expects to demonstrate its technology in orbit next year.

A Career Built Around Rocket Propulsion

Jeff Thornburg has spent decades working on advanced aerospace technologies. His career began after earning an aerospace engineering degree through an Air Force ROTC scholarship. While poor eyesight prevented him from becoming a military pilot, he instead pursued aircraft maintenance and later specialized in rocket propulsion.

After serving in the U.S. Air Force, Thornburg contributed to propulsion projects at organizations including Exquadrum, Aerojet, and NASA. In 2011, he joined SpaceX after a conversation with founder Elon Musk, where he helped develop the methane-powered Raptor rocket engine, one of the company’s most important technological achievements.

Following five years at SpaceX, Thornburg accepted an invitation from Microsoft co-founder Paul Allen to lead rocket propulsion development at Stratolaunch. There, he oversaw work on the PGA liquid rocket engine. However, Allen’s passing in 2018 and Stratolaunch’s strategic shift eventually brought that project to an end.

Thornburg later founded Interstellar Technologies, focusing on hydrogen propulsion systems. Although the company worked with organizations including NASA and Northrop Grumman, the COVID-19 pandemic disrupted investment plans and slowed its momentum.

His next stop was Amazon’s Project Kuiper, where he helped establish engineering and manufacturing processes for the satellite internet program before taking leadership roles at Agility Robotics and Commonwealth Fusion Systems.

Turning an Old Concept Into a New Business

The inspiration behind Portal Space Systems dates back to 2016, when Thornburg was invited to speak at a conference on the future of propulsion. While researching his presentation, he explored nuclear thermal propulsion, which uses heat from a nuclear reactor to generate thrust.

That sparked a different idea.

Instead of using a nuclear reactor, Thornburg wondered whether concentrated sunlight could produce similar results. The answer was yes. While solar heating cannot match the power of a nuclear reactor, it eliminates the need for expensive nuclear technology while delivering many of the same engineering advantages.

The concept remained in development for several years before Portal officially emerged. During that period, Thornburg balanced full-time engineering roles while building the startup during nights and weekends. Once Portal secured support from the U.S. Department of Defense, he shifted his full attention to the company.

Supernova and Starburst Lead the Company’s Roadmap

Portal’s flagship spacecraft, Supernova, is designed to rapidly transport satellites and payloads between different Earth orbits. Unlike existing electric propulsion systems that may require weeks or even months to complete orbital transfers, Supernova aims to perform those missions within hours or a single day.

The spacecraft will be roughly the size of a commercial refrigerator and will deploy reflective mirrors stretching about 55 feet wide to concentrate sunlight onto its propulsion system. Using ammonia as its propellant, the spacecraft’s 3D-printed Flare heat-exchanger thruster has already completed successful ground testing.

Before Supernova enters service, Portal plans to launch Mini-Nova, a compact demonstration payload roughly the size of a tissue box, aboard a future SpaceX rideshare mission. The test will validate the spacecraft’s overall system design in orbit.

Later in 2026, Portal expects to launch Starburst, a smaller free-flying spacecraft equipped with electrothermal propulsion. While not as powerful as Supernova, Starburst will provide customers with rapid orbital maneuvering capabilities ahead of Supernova’s commercial debut.

If development stays on schedule, Starburst could begin customer missions in 2027, the same year Supernova is expected to make its first operational flights.

Built for National Security and Commercial Space

Portal’s leadership believes fast-moving satellites will become increasingly important for national security and commercial space operations. The company has focused on developing technology that fills a growing need for rapid orbital mobility, particularly as space becomes more competitive.

Chief Operating Officer Ian Vorbach said the company carefully aligned its technology with real customer requirements rather than developing innovation without a clear market. Fellow co-founder Prashaanth Ravindran, who previously worked at Blue Origin, praised Thornburg’s engineering-first leadership style and ability to build strong technical teams.

Industry analysts have also taken notice. Stan Shull of Alliance Velocity described Portal as a company moving with exceptional momentum and credited Thornburg’s extensive propulsion expertise for its rapid progress.

Looking Beyond Earth’s Orbit

While Portal’s immediate focus remains on commercializing solar thermal propulsion, Thornburg is already thinking about the next generation of space travel. He believes nuclear thermal propulsion could eventually become part of future versions of the company’s spacecraft, enabling missions far beyond Earth orbit.

For now, Portal is concentrating on proving that sunlight alone can power a new class of highly maneuverable spacecraft. If successful, the startup could help reshape satellite transportation while opening new possibilities for future exploration across the solar system.

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