www.lesswrong.com/posts/Y5cQYKYwAb2WwXXQQ/tech-i-m-skeptical-of-and-why
2 corrections found
It emits 174 petawatts into space
This number is too high. NASA’s energy-budget data imply Earth radiates about 240 W/m² to space on average, which is about 122 petawatts globally—not 174 petawatts, and that is for the whole Earth system rather than the atmosphere alone.
Full reasoning
NASA’s Earth energy-budget explanation says the Earth system absorbs about 240 watts per square meter of solar energy on average and that this is balanced by an equal amount of energy radiating back to space in thermal infrared. The same NASA page also explains that 340 W/m² is the incoming global-average solar power at the top of the atmosphere, before reflection.
So 174 petawatts corresponds to the wrong quantity: it is roughly the globally averaged incoming solar flux (340 W/m²) spread over Earth’s surface area, not the outgoing thermal radiation to space. Using NASA’s 240 W/m² figure gives about 122 petawatts for the whole Earth system. And because some thermal radiation escapes directly from the surface, the atmosphere alone emits less than that.
In short: 174 PW is not the atmosphere’s emission to space; it is closer to the gross incoming solar power before accounting for reflected sunlight.
2 sources
- Climate and Earth's Energy Budget - NASA Science
Globally, over the course of the year, the Earth system—land surfaces, oceans, and atmosphere—absorbs an average of about 240 watts of solar power per square meter... The energy that Earth receives from sunlight is balanced by an equal amount of energy radiating into space.
- Climate and Earth's Energy Budget - NASA Science
Averaged over the entire planet, the amount of sunlight arriving at the top of Earth's atmosphere is only one-fourth of the total solar irradiance, or approximately 340 watts per square meter.
You pretty much have to make it out of a carbon allotrope or some sort of glass.
That is too restrictive. There are several established high-performance fiber families for high-specific-strength applications that are neither carbon allotropes nor glass, including aramid, UHMWPE, Vectran/LCP, and PBO.
Full reasoning
This claim leaves out multiple well-known non-carbon, non-glass high-performance fibers.
A recent review of high-performance fibers lists major families including aramid fibers, ultra-high molecular weight polyethylene (UHMWPE) fibers, PBO fibers, polyimide fibers, and others—not just carbon and glass. In the specific context of space tether/cable materials, NASA has also documented testing of Kevlar, Nomex, Vectran, and Spectra in yarn, tether/cable, and fabric forms for the space environment.
So it is not correct that high-specific-strength fiber options are basically limited to carbon allotropes or glass. There are several important non-carbon, non-glass contenders already used or studied for demanding tether/cable applications.
2 sources
- The past, present and future of high-performance fibers - PMC
High-performance fibers mainly include carbon fibers, aramid fibers, ultra-high molecular weight polyethylene (UHMWPE) fibers, basalt fibers, polyphenylene sulfide fibers, polyimide fibers and poly(p-phenylene benzobisoxazole) (PBO) fibers, etc.
- Comparison of High-Performance Fiber Materials Properties in Simulated and Actual Space Environments - NASA Technical Reports Server
A variety of high-performance fibers, including Kevlar, Nomex, Vectran, and Spectra, have been tested for durability in the space environment, mostly the low Earth orbital environment. These materials have been tested in yarn, tether/cable, and fabric forms.