Jeff Thornburg has always had an instinct for turning big, audacious ideas into hard reality. He once helped SpaceX transform a theoretical NASA lab experiment into the blazing heart of the world’s most powerful rocket engine. Now he’s betting he can do it again—this time under his own flag, leading Portal Space Systems—a young, hungry startup quietly plotting the future of spacecraft propulsion.
Portal, which Thornburg founded with Ian Vorbach and Prashaanth Ravindran in 2021, just closed a major funding round. Fifty million dollars, raised from an intriguing list of backers—Geodesic Capital, Mach33, ARK Invest, Booz Allen Ventures, AlleyCorp, FUSE—catapult the startup’s valuation to a quarter-billion. All for a radical idea: harnessing the focused heat of the sun as rocket fuel, not just as another source of electricity.
The technology at the center of this vision is called solar thermal propulsion. It’s simple in theory, ruthless in practice. Most satellites today either burn through chemical propellants or suck electrical juice from solar panels to power their cautious, low-speed thrusters. Portal aims to upend all that. Their engines are designed to concentrate sunlight, convert it into staggering temperatures, and use that sheer, raw thermal energy to fling spacecraft across empty black at speeds chemical thrusters only dream about.
That notion isn’t new, exactly. Since the 1960s, government labs have played with the idea—most recently as a pipe dream for probes leaping out of the solar system—but never managed to push it beyond paper or the vacuum chambers of test facilities. Thornburg and his team are out to seize what NASA shelved, and shove it into orbit.
Thornburg’s journey started in the United States Air Force, deep in the world of novel rocket engines. There, he grappled with the design of something called full-flow staged combustion—technical, efficient, powerful, and almost mythical among engineers. Elon Musk saw that hunger for pushing boundaries and lured Thornburg to SpaceX, where he turned vision into reality: the Raptor, now the muscle behind the towering Starship.
He wandered, gathering experience—briefly with Stratolaunch, briefly with Amazon’s Project Kuiper—before the urge to upend spacecraft propulsion reeled him back.
For Thornburg, solar thermal represents a leap as significant as the transition from steam to combustion on Earth. NASA understood its promise twenty-five years ago and said as much in a quietly sidelined report. But back then, space was sleepy. Occasional, cautious missions favored brute-force rockets over inventive onboard maneuvers. Who needed nimble satellites when you barely had any?
That changed fast. Thousands of satellites now coil around Earth. Military planners demand flexible, fast-moving hardware: surveillance, deterrence, positioning. Orbital drift and collision risk rise each year. Suddenly, slow is unacceptable. “China’s running circles around our spacecraft,” Thornburg remarked recently. “We need equivalent capability.”
Portal’s promise has snapped up both private dollars—$67.5 million—and $45 million from the U.S. military, drawn by implications for space warfare and rapid orbital response. Investors like Aaron Burnett, Mach33’s CEO, see a tidal shift coming: someday there may be millions of satellites aloft, all jostling for room, all in need of affordable ways to dodge, reposition, or keep out of trouble. “Portal could become the backbone for space mobility,” Burnett said, “a go-to for all kinds of users.”
Turning any of this into reality is an engineering gauntlet. Portal’s onboard electronics just survived their first orbital test last week. By autumn, a new prototype will take flight. The debut of their first working SuperNova spacecraft—Thornburg calls it a “fighter jet for orbit”—is slated for 2027, carrying the prospects of Portal’s engine into the realm of metal and vacuum.
Key to their advance is the synergy of cutting-edge materials and additive manufacturing—3D-printed, rugged, precision parts. Take their Hex thruster, which elegantly unites solar concentrator and rocket nozzle, squeezing every drop of power from the captured sunlight.
Rocket aficionados see nuclear propulsion as the “final boss” of in-space transportation, but the red tape and cost put that dream out of reach for upstarts. Portal, however, is tiptoeing toward that frontier; solar thermal’s logic isn’t too different from a reactor’s. When the U.S. is ready for nuclear rockets, Portal will already have flight-proven much of the machinery. “I’ll be able to mature this technology faster in space than we ever could building some $2 billion nuclear-safe ground test site,” Thornburg says. The subtext is clear: the finish line is orbit, not another dusty government lab. And Portal intends to cross it first.
