What a proposed wooden satellite could (and could not) do.
A strange satellite proposal made by a Japanese company in late 2020 caused many space experts to scratch their heads by 2021.
The proposal came from Kyoto University in collaboration with Sumitomo Forestry in Japan, although most of the information on the project comes from a BBC post in which the Japanese Aerospace Exploration Agency (JAXA) astronaut and Kyoto University professor , Takao Doi, who flew aboard the U.S. space shuttle on missions STS-87 and STS-123 to the International Space Station. STS-123 supplied and installed JAXA’s Kib? Module in 2008.
The idea is to design a wooden satellite as an alternative to conventional building materials. Although this strange idea may seem like a step backwards in the modern space age, wood as a light but durable building material actually has many preferred properties. However, the idea of a ‘WoodSat’ in response to the growing problem of space debris in low-earth orbit came under fire as the story made its rounds last week.
What kind of wooden satellite would (and wouldn’t) address
Of course, a wooden satellite would not solve the problem of space debris in orbit … or at least not in the way you would imagine or as many news outlets are currently reporting. The US space operations center Space-Track currently catalogs over 47,000 objects in low-Earth orbit, from active satellites to discarded rocket boosters to tiny debris up to an inch in size. It is estimated that millions of tiny man-made micrometeoroids exist in LEO, and these objects regularly “sandblast” satellites and spacecraft, including the International Space Station. A 2020 study suggests that we know even less about the man-made debris population in orbit.
The problem is, all of that junk in LEO is still darting around at a speed of about 7.8 kilometers per second, be it metal, plastic, or wood … and that speed is still going to be quite a blow no matter what it’s made of raging particles.
A micrometeoroid hits a dome window on board the ISS. Photo credit: NASA
How about satellite re-entry and air pollution? At first glance, the idea of a wooden satellite that would simply disintegrate upon re-entry seems like a good idea. After all, satellites contain a lot of exotic compounds and rare earth metals, right? Well, unfortunately, making the frame out of wood (a bad electronic conductor and essentially a carbon compound) doesn’t really change anything, as the electronics inside still have to be made of conductive compounds. Plus, many of the problematic compounds returning to Earth are toxic (think hydrazine fuel aboard the failed Phobos-Grunt mission in 2011-2012) and are occasionally radioactive and nuclear power plants. Radioactive re-entrances are a rarity these days, but have occurred in the past, such as the Aquarius lifeboat on Apollo 13 and the re-entry of Cosmos 954 over the Canadian Arctic in 1978.
Hundreds of re-entries take place on an average of a year, but even if they are all made of wood, this is still a trickle compared to the 15,000 tons of natural cosmogenic material that comes to earth each year.
Is there any practical use for ‘WoodSat’? Here’s a possible plus for a wooden satellite: Wood is largely transparent to radio waves, which means you can keep most of your communication and research antennas internal: after reaching orbit, no more bulky instruments are deployed. In fact, failure to deploy once in orbit has made many satellites fail … and that wouldn’t be a problem with WoodSat.
Most wood also contains a variable amount of moisture that would quickly evaporate into the vacuum of space, leaving porous gaps. The selected wood would also have to have low thermal expansion, as it would be exposed to high temperature fluctuations. So far, the exact composition of the wood Sumitomo Forestry would use for such a satellite is stated by the company as proprietary, although Takao Doi cites a study that was done on the exposed part of the kib. Laboratory on board the International Space Station to study the elasticity of various types of wood in space.
One half of a Ranger balsa wood impactor limiter. Photo credit: NASA
Has there ever been a satellite that contains mostly wood? Yes, the early NASA Ranger missions to the moon in the 1960s actually included a balsa wood impactor limiter ball designed to survive a crash on the lunar surface. Unfortunately, most of these early Rangers either failed shortly after launch or completely missed the moon. A wooden impactor aboard Ranger 4 may have come to rest intact near the Ioffe crater on the far side of the moon, but if it did, it never called home.
Still on the surface of the moon? The Ranger 4 spaceship on Earth shortly before encapsulation, with the balsa wood impactor limiter visible (the sphere just below the nose cone). Photo credit: NASA
Another interesting idea is to build a re-entry heat shield out of wood. China actually incorporated this feature with wooden impregnated white oak heat shields on its Fanhui Shei Weixing reconnaissance satellite, which reportedly worked quite well.
When could WoodSat go into space? Well, for now, Takao Doi and his team plan to launch an initial design in the years to come, which may hit the market by 2023. The design we see in the model released by Sumitomo Forestry appears to be based on a standard 6U Cubesat.
For now, ‘Woodsat’ remains an interesting option, although a more practical solution to the space debris problem is to incorporate disposal functions such as tows as standard equipment so that satellites can re-enter orbit shortly after their useful life is over.
Lead Image Credit: A wooden satellite model. Sumitomo forestry.
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