Readit NewsWhen I came across this amazing project and wanted to share it to HN, I was debating whether to post the youtube link or the project page. I decided to post the project page and mention the youtube link in the description for those who prefer video, but somehow that description got posted as a comment instead (not sure how that happened?). Anyway as you said the video is embedded in the project page so it wasn't really necessary
Last year the presenter also wrote a blog post about the attack, which received some discussion at the time:
If I understand correctly, your two main benefits are broader spectrum and lack of PWM flicker. Did you measure the spectrum of the light from the prototype monitor? The light goes through several filters - first I assume the daylighting system has an IR filter to prevent overheating. Then it goes through the LCD itself, and the color filter array in front. Are you still left with a lot of IR (or the frequencies are considered beneficial) after all this?
Let's say you go to one of the illuminated areas that paid for reflectorbital-light and look up. What would you see? You would see a tiny bright spot flying past, with an angular size of about 10^-10 steradians [1].
This tiny spot has the same "brightness" (radiance) as the sun, because a mirror preserves radiance. However the mirror looks about 10 000 times smaller than the sun from your perspective (the sun has an angular size of about 10^-5 steradians). This means that the satellite would only give you 0.01% of the light compared to the real sun.
If you could somehow take 10k satellites and use them to illuminate the same spot, you could technically get it to resemble real sunlight. But imagine what this would look like: These satellite would need to be many enough / huge enough to cover a very significant portion of our sky, on the order of the apparent size our actual sun. They would be spread out in a sun-synchronous orbit, so they would be visible at dusk with this size, from all points on the earth. Would we really want that?
The founder has been thinking about using mirrors to collimate the sunlight to get around this problem, but it won't work. The collimator design he showed in a 2022 article [2] would decrease the focal spot from a 5km diameter to some smaller diameter as intended, but it would do so by throwing away light, not by increasing the brightness in this smaller spot. This is given by conservation of ètendue, one of the fundamental laws of nonimaging optics (where I do research).
[1] They are planning a 100sqm mirror at 600km altitude, which gives a solid angle of (100 m^2)/(600e3 m)^2
[2] https://www.vice.com/en/article/this-man-is-trying-to-put-mi...
Of course they don't reveal their neat trick now because from a strategic point of view it would be bad, because other people would copy them, and probably outpace them.
Once it's only a physics problem, it becomes quite easy. Here are two potential solutions to the physics problem : Liquid mirrors, and plasma mirrors. The easiest being the liquid mirrors so let's explain this one. In fact it's so easy, it's child play.
Have your kids ever played with soap bubble : It's the same thing but in space. You make a big ring (roughly of the same area of the spot you are trying to make on the ground) (thin circular wire that you fold so that it will unfold itself like these tents) and you flow liquid metal (like mercury) on top to form a thin layer, hold by surface tension (and eventually electric field to have finer control of the shape or the surface).
The mirror will be a few atoms thick, a design that must be fabricated in space directly. It's just that we are not used to thinking with low force zero gravity environment. You don't use motors to move your mirror, you use fields spawned from precisely positioned points (and you have to compensate for solar winds).
Of course there are still technological problems due to the harsh conditions of space, but it's not something unheard of : https://engr.ncsu.edu/news/2024/03/06/reflecting-the-future-...
Unfortunately, this is not how the size of the spot on the ground is decided. Sunlight, even if reflected by a perfectly shaped mirror, spreads by approx 1 meter every hundred meters. At the "edge of space" at 100km, your spot already has a 1km diameter, in reality with a higher orbit and imperfect mirror & tracking it will be much larger. The size of your (ideal) mirror decides the brightness of the spot, not the size of the spot.
Liquid mirrors in space seem like a cool concept though!
Crescent Dunes invented the algorithm for focus that prevents birdkills. And it had thermal energy storage, the first tower CSP at utility scale to do so, which removes the need to keep the heliostats always on standby focused together on one spot. Ivanpah was the last CSP built with no storage, and since then no bird kills. All 30 projects in China have been required to include storage.
Crescent Dunes (100 MW) had other problems as the first ever Tower CSP with storage at utility scale (Gemasolar had been built with storage a few years earlier in Spain, at 50 MW. Crescent Dunes had a major outage because a tank leaked, bankrupting the startup that developed it, but now its owned by a big Spanish outfit and has been back online for several years, supplying night solar to Las Vegas: https://www.solarpaces.org/what-happened-with-crescent-dunes...
By the way, I’m happy to find someone with CSP knowledge on HN. Are you working in the field?
You want to count particles per volume of air, so conventional sensors use a fan to have a constant volumetric flow and then count particles per second to infer particles per volume.
The way I interpret the above marketing language is that they use the optical sensor not only to count particles but also to measure the particle movement and infer airflow. So as long as there is some natural movement in the air, they can measure both particle count and volumetric flow, and thus infer particles per volume.