Portal Space Systems

Supernova masses around 500 kilograms and unfolds reflective sheets roughly seventeen metres across. They concentrate sunlight onto a 3D-printed component that is both heat exchanger and nozzle, heating ammonia and expanding it out the back. Portal claims that yields six kilometres per second of velocity change, enough to move between orbits in hours rather than the months an electric tug needs, and to keep doing it for five years.

Jeff Thornburg built full-flow staged combustion engines for the Air Force, then ran propulsion at SpaceX during Raptor, then went to Stratolaunch and Kuiper. His reason for starting Portal is that American space assets are strategically immobile: they fly predictable paths and cannot manoeuvre meaningfully without spending everything they carry. Solar thermal propulsion is the old idea he thinks fixes that, sitting between chemical thrust and electric efficiency, and needing no reactor because the Sun is already there.

Solar thermal propulsion has never operated in space, and Portal has not yet tested the part that makes it solar. Its headline ground firing in September 2025 heated the propellant with high-powered electric heaters standing in for concentrated sunlight, so the mirrors, the optics and the pointing remain undemonstrated end to end. No performance figures have been published at all: no thrust, no specific impulse, no operating temperature. Neither of the first two flights tests the idea either, since Mini-Nova, which reached orbit in March 2026, carries no propulsion, and Starburst-1 flies conventional thrusters. The solar thermal vehicle has moved from mid-2026 to 2027, and it will have to hold a seventeen-metre concentrator on the Sun with optical precision while thrusting. NASA and the Air Force both studied this decades ago and neither flew it.