Satellites encased in wood are in the works

LignoSat’s communications failed as it was launched from the International Space Station (ISS) on December 9th 2024 (see picture above). But it was a breakthrough all the same. For much of it was made of magnolia, a wood its builders at Kyoto University had picked for its strength, ease of working and (as demonstrated by previous experiments on the ISS) resilience to the hostile environment of space.

It proved a good choice. Though LignoSat’s temperature oscillated from -100 to +100°C as it passed in and out of Earth’s shadow, and it was also bombarded by the radiation of the solar wind, its wooden panels held firm, according to tracking by America’s Space Force, until its fiery atmospheric re-entry 116 days later.

Huld, a Finnish firm, prefers birch plywood to magnolia for WISA Woodsat, a test vehicle it helped design that is due for launch this summer. Such plywood is routinely cooled to -163°C when used to insulate tanks carrying liquefied natural gas.

Wood has several advantages over metal alloys as a satellite material. One is to reduce the amount of metal vaporising when satellites burn up on re-entry. In 2023 some 290 tonnes of space junk fell into the atmosphere. A study published that year found a tenth of the stratospheric sulphuric-acid particles it sampled contained such metal.

How much that matters, if at all, is unclear. But some people fear a build-up of metals at altitude will trigger chemical reactions which might, for instance, destroy ozone, a form of oxygen that absorbs harmful ultraviolet radiation. And build up they surely will. One forecast suggests that, by 2035, more than 2,800 tonnes of space junk a year will fall from orbit.

Swapping metal for wood, though, is not without hazard, observes Daniel Cziczo of Purdue University, who was one of the authors of the stratospheric-particle study. What might emerge from a reaction between the resulting soot and vaporised electronics remains unknown.

Such fears are unlikely, however, to rule wood out as a space material, for it has another advantage. Regulators are tightening the “design for demise” rules, intended to stop chunks of falling spacecraft reaching the ground. Satellites weighing more than about 300kg usually need special guidance systems to comply with these rules by ensuring controlled re-entry into a deserted part of the ocean. Dr Sakraker’s team think incorporating wood, which would burn up in the atmosphere, might permit spacecraft weighing up to a tonne to duck that additional cost and re-enter uncontrolled.

A further benefit is that radio signals are unperturbed by wood, so communications equipment need not be specially deployed once a satellite is in orbit. That will protect it from flecks of space debris, such as paint chips, and should reduce drag from errant atmospheric molecules, a big problem in low orbits. LignoSat2, intended for launch in 2028 into an orbit 400km up, will test this idea. Doi Takao, a former astronaut who is a member of the LignoSat team, reckons the reduced drag will extend the craft’s flight time by about 50%.

Wood is also cheaper than spacecraft alloys. And it absorbs vibrations, a plus for sensitive instruments. It insulates better than metal, too, meaning a craft’s heating coils will not have to be turned on so often.

It does have a downside. The vacuum of space can suck out moisture and organic compounds, weakening it—though not, experiments on the plywood used in the WISA Woodsat suggest, enough to matter. A protective coat of aluminium oxide may help. Overall, then, it looks possible that small spaceships may soon reverse the example of their maritime namesakes by rejecting man-made materials for their hulls and reverting to wood. ■