BOTHELL, Wash. — Before he became the CEO of Portal Space Systems, Jeff Thornburg worked for two of the world’s most innovative space-minded billionaires. Now he’s working on an idea those billionaires never thought to pursue: building a spacecraft powered by the heat of focused sunlight.
Thornburg and his teammates are aiming to make Bothell-based Portal the first commercial venture to capitalize on solar thermal propulsion, a technology studied decades ago by NASA and the U.S. Air Force. The concept involves sending a propellant through a heat exchanger, where the heat gathered up from sunlight causes it to expand and produce thrust, like steam whistling out of a teakettle.
The technology is more fuel-efficient than traditional chemical propulsion — and faster-acting than solar electric propulsion, which uses solar arrays to turn sunlight into electricity to power an ion drive. Solar thermal propulsion nicely fills a niche between those two methods to move a spacecraft between orbits. But neither NASA nor the Air Force followed up on the concept.
“They didn’t abandon it for technical reasons,” Thornburg said. At the time, it just didn’t make economic or strategic sense to take the concept any further.
What’s changed?
“Lower launch costs, coupled with additive manufacturing, are the major unlocks to bring the tech to life, and make it affordable and in line with commercial development,” Thornburg said.
Thornburg argues that it’s the right time for Portal’s spacecraft to fill a gap in America’s national security posture on the high frontier. “There was no imperative for rapid movement on orbit in the 1990s,” he said. “Only recently have the threats from our adversaries highlighted the weaknesses in current electric propulsion systems, in that they have so little thrust and can’t enable rapid mobility.”
Portal’s vision has attracted interest — and financial support — from investors and potential customers. Since its founding in 2021, the startup has raised more than $20 million in venture capital. In 2024, Portal won a commitment for $45 million in public-private funding from SpaceWERX, the innovation arm of the U.S. Space Force. And next year, Portal is due to demonstrate its hardware for the first time in orbit.
So, how did Thornburg hit upon the idea of turning a decades-old idea into reality?
