Graphene oxide sheets may be the ideal candidate for solar harvesting. Full radiation spectrum absorbance. High conductivity. Thermoelectric power generation that increases at very high (3000K+) temperatures. Lightweight. Inexpensive. 3D-printable. With cloth-like flexibility.
Might be time to fund a zero-gravity graphene factory in low Earth orbit.
Thermoelectric has terrible efficiency, tho. Also, graphene isn’t inexpensive, and actual macroscopic conductivity is much worse than aluminum or copper.
At the solar flux levels at the point of that probe (light intensity decays by distance squared), efficiency is probably mostly irrelevant. What's the challenge though is how you irradiate all the heat away to provide a large enough temperature delta.
I think that would mean only a dyson hemisphere, since Earth is illumnated by the entire hemisphere of the sun that faces us, due to the sun's sheer size.
To see the kinds of effects this would have on the gas giants would be fascinating. It would probably drastically alter the coriolis banding and spots on Jupiter and Saturn.
Venus would probably be the only other planet affected, and perhaps comet behavior might change. In fact, I wonder if planets would become so cold, that they might produce visible comas as they warm up, when they enter the sun's (now hemispherical) light cone. That's probably the most prominent effect I can see happening.
But does it have to totally block it out? What if we left space in between units so that it just dimmed the sun behind it. Hell, maybe it will help reduce global warming so we can just keep on keepin' on. No need to reduce output levels of anything other than the amount of sun we receive.
Plus, it would be fun to screw with societies in other parts of the galaxy that are using their Kepler satellite equivalents. Every time the Earth transits, the light would dip in a way that would lead the observers to think Earth was much larger, thereby much less massive (for the math to work). Great defensive strategy as they would not think our planet was worth pursuing.
I thought at first that an IMAX projector must be some sort of solar simulator, and the similar acronym to the ones in movie theaters was just a coincidence.
Turns out I was wrong and they just used old IMAX projectors they purchased off eBay.
This is a very naive question, but is there some distance from the sun at which it would heat an object in space to a temperature that we would consider comfortable?
I guess the obvious answer would be "the distance which the Earth is at," but what I really mean is could you be floating around in a spacesuit or some object with minimal shielding and you would neither overheat nor freeze to death.
A human in a spacesuit (or indeed anywhere) generates about 100W of heat. In the vacuum of space, the only way for this energy to escape is through radiation. The amount an object radiates depends on its temperature, its surface area, and its 'emissivity', which is simply 1-reflectance. Back of the envelope, a human sized object (2 m^2) with an emissivity of 0.12 (polished aluminum), generating 100W, will stabilize at almost exactly 20C - room temperature.
So, you don't need the sun at all! Make your spacesuit small and shiny and you can enjoy balmy temperatures deep in the inky blackness of the interstellar void, for as long as your food and oxygen lasts (food and oxygen ultimately being the fuel for your 100W).
Earth's average temperature without the atmosphere and the greenhouse effect would be quite cool.
Guess depends very much on the spacecraft design, how you absorb and radiate.
As a wild guess, probably further out then the Earth. The atmosphere here plays an important role in keeping us comfy. Even so, the tropical sun can be too much. Heat waves can be too much. Desert heat can be too much. Etc.
A brief google informs me that the sunny side of the Moon is around 127˚ Celsius, while Phobos goes up to about -4˚. So somewhere most of the way to Mars probably?
Air pressure on earth literally squeezes the water together, preventing it from turning into a gas until it's at a higher temperature.
More generally: practically all phase-changes depend on both temperature and pressure. Lower pressure almost behaves like higher temperature... within certain ranges (and it varies for every material).
E.g. for water, note that there are three major regions in its phase diagram[1]. At human-normal scales (the red horizontal line is 1 atmosphere) we see water boil/freeze at 100 and 0 celsius. If you lower the pressure though, you'll see there's a spot where all three phases meet, and below that there's no liquid phase separating solid and gas. When it's in near vacuum, you literally can't have water - ice evaporates (sublimates) directly into a gas, with no intermediate phase.
Given the diagram, this seems to happen somewhere around -60c in a serious vacuum. So in space or on the surface of the moon (without an atmosphere / something pressurizing it) you simply can't have liquid water - a portion of it will evaporate almost immediately, which steals energy from the remaining portion, causing it to freeze. After that, the solid portion just sublimates away (because the sunny side is 127c, well above the temperature needed to turn it into a gas).
What I am trying to figure out specifically -- to mental model -- is what impact this has on human comfort, per the original question.
So I have lived in very hot and humid places -- like Georgia, where it is often around 100F in summer -- and I have lived in even hotter places with a much dryer climate -- like the Mojave Desert, where it can get to 115F and I would wait until nightfall to go for walks for exercise at 99F and no sun. Humidity does make a big difference in how uncomfortable a temperature is. If it is hot and dry, you can stay reasonably comfortable if you get enough fluids and electrolytes into you and stay out of direct sunlight.
I'm trying to fit this into a mental model of that sort. Like is a boiling temperature going to burn you? Or, you know "It's a dry heat, man!" only "It's in the vacuum of space, man!"
Aaah, gotcha. Yeah, that's definitely fuzzier... like, what does boiling feel like if it's room temperature.
I imagine some of it depends on what our heat-sensors actually react to. At the least-interesting case, seems like being above boiling without heat would feel fizzy, just because it's physically doing that to the outer layers of your skin. Or something like laser hair removal, where it kinda feels like a hard slap.
A very cool mission (pun intended)! It makes me wonder if it would be possible to exploit that huge temperature differential behind the heatshield to generate power...
> Parker Solar Probe is powered by two solar arrays, totaling just under 17 square feet (1.55 square meters) in area. They are mounted to motorized arms that will retract almost all of their surface behind the Thermal Protection System – the heat shield – when the spacecraft is close to the Sun.
I guess that's exploiting the temperature differential for power systems but it's not exploiting the temperature differential for power generation.
Solar panels are just more reliable than any heat engine and actually using the temperature differential in a heat engine (stirling or whatever) would decrease the heat shields ability to reject heat. And it'd add mass in the form of radiators with space already at a premium.
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But more in general, I'm super excited about Solar Probe+ (Parker Probe) as well as the completion of the Advanced Technology Solar Telescope (Daniel K. Inouye Solar Telescope (DKIST)). They're both going to be exposing never before sampled, very exciting, territory.
The Solar Probe with actually in-situ measurements of the magnetic field of the corona (as opposed to inferences based on optical Hanle scattering or radio emission). It is going to put a lot of constraints on previously weakly constrained flare and CME models. Maybe in the 9 radius passes it might even detect (or more likely not!) the stupifyingly unphysical entire electron contents of the corona dumping down during flares most models call for.
And the ATST/DKIST with it's diffraction limited imagining and fast optical spectroscopy of the sun at 100s of km scales projected on the photosphere. There's so much going on in the photosphere and chromosphere that right now is just blurry blobs. When resolved I think there's going to be a lot to see.
Another way to think of it is that they already have too many solar panels because the probe has to power itself from Earth orbit to its final orbit near the Sun. So, anything that uses what you've already got (solar panels) with minimum additional mass and risk (the little motors to move the panels mostly behind the heat shield) is a winner.
There are ways, piezoelectric devices can generate power off temperature differential. And mechanically you could put a stirling engine on although the torque and rotational force likely would not be ideal.
WRT Stirling generators, there is already work ongoing for arrangement ssuitable to space applications, through new research on nuclear power sources (stirling radioisotope generators, and the Kilopower nuclear reactor). Generally the solution is to mount several Stirling generators such that their torques cancel each other out. Search for "managing the stroke of the pistons" at https://beyondnerva.wordpress.com/2017/11/19/krusty-first-of...
> They are mounted to motorized arms that will retract almost all of their surface behind the Thermal Protection System – the heat shield – when the spacecraft is close to the Sun.
Yes, there's a moving part. It only has to move twice (deploy, mostly retract) but it's still a moving part.
A motor that predictably moves an object from one point to another can be much more reliable than an array of heat-driven reciprocating engines that have to be in constant motion and rely on conducting heat toward the internals of the spacecraft in order to function.
"The chips that produce an electric field for the Solar Probe Cup are made from tungsten, a metal with the highest known melting point of 6,192 F (3,422 C). Normally lasers are used to etch the gridlines in these chips—however due to the high melting point acid had to be used instead."
I had no idea tungsten was used to make ICs. What kind of density is achieved by laser etching (on tungsten or otherwise)?
Or in more imaginable terms: 9 solar radii out at the closest passes in the far future. Here's an older sim clip showing the process, https://www.youtube.com/watch?v=IClNiZ2-ee4
I have no clue how large the sun is, for all I know it could be 0.000001AU or 0.01AU, but I do know how hot the sun can be at 1AU and that Mercury is scorched at 0.21AU so that's a lot more imaginable than a multiple of some unknown unit, particularly "in the far future" (sounds to me like 10k years, but given the topic of a single mission, maybe it's 30 years? Or 10? This is super vague, too.)
Yes, that's the kind of thinking that resonates with me as an astronomer, and when I talk to the general public and non-astronomer scientists, I find that a lot more people have a grasp of how hot it is at 1/5 of Mercury's orbit than 9X the radius of the Sun.
My understanding is that AU is measured from the center of the sun / object. Google tells me that the radius of the sun is ~0.0046AU (1/215th of an AU).
IMO light-seconds is actually a really nice unit for interplanetary distances. 1AU ~= 500ls. Not only is the scale nice for human arithmetic, but it serves a dual purpose of also telling you the light delay.
I'm pretty interested in this kind of thing but I couldn't tell you to within an order of magnitude Earth's distance from the Sun in metres or miles. I could work it out - it's about 6 light minutes and light travels 300,000 km/s - but I wouldn't expect everyone to do that. So expressing the distance in AU is relatable to me. Looking from the probe at perihelion, the sun will appear 25 times wider, or 625 times larger, than it does from Earth.
And does the fact that AU comes from the earth's distance to the sun make it self-centred? Sure, but everyone we need to communicate with would equally find it centred on them. Compared to other measures like "the distance from the king of England's nose to the tip of his outstretched finger" it's quite neutral.
If one AU is 1 meter, 0.04 AU is 4 cm. So it's, roughly, if 3 feet is between the Earth and the Sun, the probe would pass by the Sun at one and half inch.
(It is obviously easier to think in metric units.)
In Morgan's "Land Fit For Heroes" trilogy, Ringil does talk about the "pale sun" in the Grey Places, which people there call the "Muhn". But he'd just never heard of the Moon.
Because it had been destroyed, and became a ring, home of the Sky Dwellers. Or perhaps an orbital ring, as Stephenson has it in Seveneves. But there's nothing about a bombardment in Morgan's "Land Fit For Heroes". And that brings up the question about how a Moon breakup would really go.
If the Moon just broke up and did not blow up, the piece might not have been given much momentum towards Earth. In such a case they would slowly spread out in a ring. Their orbits would be stable and would decay outwards ever so slowly, as Earth seeps off their angular momentum.
funny... I knew a guy that had a penpal in another country, who asked him if it was the same moon that he saw when he looked up at night, as the one she saw...
Which is a totally legitimate question (albeit, one already answered long ago, but not everybody has gone to school, especially in developing countries).
>I guess that I'm more into cleverness and subtlety. Humor that directly or indirectly makes fun of people strikes me as sad, not funny.
The parent's joke (about the probe going to the sun at night, since during the day the sun is hot) is both clever (though not original, it's old), and in no conceivable way "directly or indirectly makes fun of people".
The sun does not appear round. This is a flat sun joke. We know it is round, this comment is not about that. It is just goofy humor.
My comment has the wrong parent too. On mobile, thought I was actually replying to the "go at night?" smart ass comment. That one was just so basic... good grief.
Normally, we are more serious here, and I read it, drew a blank, again, then chuckled.
I don't know. I mean, even if you went "at night" -- aka directly away from the Sun, always keeping the Earth in between -- you'd never get near the Sun. Unless you set out to circumnavigate the Universe. And in any case, once you got farther than the Moon's orbit, it wouldn't be night anymore, because you'd see the Sun.
So OK, approach the Sun edge-on. So is this a riff on flat-Earthers? I suppose that they might also have the Sun being flat. I can't quite imagine the geometry.
Graphene oxide sheets may be the ideal candidate for solar harvesting. Full radiation spectrum absorbance. High conductivity. Thermoelectric power generation that increases at very high (3000K+) temperatures. Lightweight. Inexpensive. 3D-printable. With cloth-like flexibility.
Might be time to fund a zero-gravity graphene factory in low Earth orbit.
https://phys.org/news/2017-07-gravity-graphene-space-applica...