[detritus]

Mein Gott, you're not just making this up ( https://www.datacenterdynamics.com/en/news/neevcloud-and-agn... ).

But.. why?

Musk bandwagon-hopping? Investor bamboozlery? (Same diff?)

Is my borderline childish understanding of basic physics THAT off because I cannot see the value or utility (beyond a small smear of niche edge cases) of space-based compute?

- ed, disclosure: You seem to have edited your response whilst I was typing mine, adding in valuable links. Thanks!

[rayiner]

> Is my borderline childish understanding of basic physics THAT off because I cannot see the value or utility (beyond a small smear of niche edge cases) of space-based compute?

In the real world, "physics" is not necessarily the gating factor. There is a major concern about the environmental footprint of terrestrial data centers, to the point where major U.S. states are enacting moratoriums: https://www.governor.ny.gov/news/first-statewide-moratorium-.... These legal and social roadblocks must be accounted for in analyzing the viability of orbital data centers.

[OneDonOne]

If the "physics" tells you that your satellite cannot radiate heat away from your nVidia GPU cluster because each H100 needs 1.1 meter square of radiator, then opinions do not matter. The same applies to power supply and bandwidth.

[rayiner]

The radiator is much more efficient than the solar panels (for obvious reasons), so that’s not the limiting factor.

As to power, Caltech is already at 2-3 pounds per square meter: https://magazine.caltech.edu/post/sspp-space-solar-power-pro...

> Another way to think about it: An SSPP spacecraft with a 60-meter-by-60-meter surface area made using today’s space PV-cell technology would cost $36 million and weigh nearly 9,000 pounds, or almost as much as a Ford F-450 truck. With the ultra-lightweight PV-cell technology Atwater envisions, it would cost just $450,000 and weigh about 300 pounds, or about as much as an IKEA three-seat sofa

That’s megawatt-level solar power under 5 tons using today’s leading edge technology. Starship super heavy can launch 100 tons into LEO.

As to bandwidth, Starlink V3 backhaul capacity is 1 terabit. Microwave radio frequencies have an insane amount of bandwidth.

[OneDonOne]

The Caltech Concept is just that — a concept. No prototype, no tests, no manufacturing, no results. When they achieve this order of magnitude improvement on a prototype scale, that's when we should take them seriously.

[rayiner]

I'm referring to the description of "today’s space PV-cell technology" (I believe the lightweight panels used to upgrade the ISS in 2021).

SSPP has already sent prototypes to space: https://www.caltech.edu/about/news/space-solar-power-project....

There's a ton of work being done on lightweight solar panels for space. E.g. https://ascentsolar.com/asti-technology-and-unique-advantage...

[selimthegrim]

I have a lot of respect for Atwater but I'm pretty sure a lot of people at Caltech think this is donor driven research.

[sfifs]

ISS today generates and radiates away about 120 KW of energy with its old tech 3250 sq m of panels panels and it's current radiators. That's what 3 H100 racks need

There may be an economic challenge - which seems to be the sort of problem mass manufacturing can solve very well.

There may be a compute model & latency problem, how do you organize model training when racks are much further apart than in traditional data centres (although speed of light is 50% faster in vacuum than glass fibre). But that's algorithms.

Relative to everything else in orbit, powering a rack of compute and some comms per satellite seems not really to be a physics problem.

[pfdietz]

What are the assumptions behind that 1.1 m^2 figure?

[alephnerd]

OP is quoting Mikhail Klassen at Planet Labs [0].

The cost of replacement is exorbitant for commercial usecases, but is acceptable for defense usecases.

The issue is too many people are looking at the commercial usecase while ignoring the defense usecase that is what is actually driving the conversation and dealflow in this segment.

[0] - https://www.mikhailklassen.com/posts/orbital-data-centers/or...

[pfdietz]

Ok, there are at least two bad assumptions there.

First, it assumes the radiator is at the same temperature as the GPU. But radiators become dramatically more effective as temperature increases, with radiated power increasing as the fourth power of absolute temperature. So a heat pump that drives the radiator at higher temperature could make the radiator far smaller. More power would be required (and the radiator would have to radiate this energy too) but the radiator could become much smaller.

The other problem is assuming the radiator is intercepting sunlight. But it can be shaded by reflective films or kept edge-on to the Sun.

[OneDonOne]

> Ok, there are at least two bad assumptions there.

> First, it assumes the radiator is at the same temperature as the GPU. But radiators become dramatically more effective as temperature increases, with radiated power increasing as the fourth power of absolute temperature. So a heat pump that drives the radiator at higher temperature could make the radiator far smaller.

Carnot's theorem and refridgeration cycles rear their head. Your heat pump still needs power to pump heat uphill., negating any savings from a smaller radiator. And what happens when you shrink a radiator? It becomes a high temperature radiator, meaning the GPU must operate outside its operational tmperature of about 70 degrees C. So small radiator = big pump + extra solar panels and batteries + dead GPU

Also, how do you deal with the added cost and complexity for the solar tracking mechanisms for the "edge-on-to-the-Sun" radiator?

[foobarian]

The other neat thing is that in space, the background temperatures are a lot lower than Earth due to lack of atmosphere, and available potentially more often than 1/2 the day cycle depending how high you're prepared to orbit.

[hahajk]

If I understand correctly, NY is concerned about the power and water consumption. 1. How do they get power in space? (solar) Do the same on earth at 1/100 the cost (or build 50x the number of solar to account for atmospheric loss and you will still come out ahead) 2. How do they get water in space (they don't). Whatever they do in space to not need water, do the same on earth! 3. People don't like living near datacenters - put them in remote areas hundreds of miles away from people, after all space datacenters won't have employees . Again, at 1/100 or 1/1000 the cost.

[xyzzyz]

The point is that while power and water is scarce in space, the NY state has no jurisdiction there. It might literally be easier to build data center in space than it is to overcome social and political pressure in NY.

[Dylan16807]

It's extremely unlikely that companies will find no locations that let them build zero power zero water data centers. And bulk AI stuff isn't latency sensitive either so the acceptable build radius is huge.

[mayama]

Most locations that could provide guaranteed power for western companies with stable governments are also full of NIMBY or NGO who could easily blockade those datacenters. Then you'll have to run to offgrid locations where you have to build 24hr battery for solar with closed loop cooling. Is 24hr battery+solar on earth cheaper than LEO satellites in constant sun without batteries? That's the question that would make or break satellite datacenters, as political climate against AI datacenters is increasingly getting bad with increasing electricity prices.

[Dylan16807]

> Is 24hr battery+solar on earth cheaper than LEO satellites in constant sun without batteries?

I'll be shocked if it isn't.

[lokar]

Exactly. This whole thing is not about being able to build whatever they can afford, it’s about being in a (stupid) race to build it as fast as possible.

[Dylan16807]

Even for ultimate speed I don't see how it works. Facilities to make and launch satellites add a lot of delays and need their own permits.

[TeMPOraL]

> Is my borderline childish understanding of basic physics THAT off because I cannot see the value or utility (beyond a small smear of niche edge cases) of space-based compute?

Don't worry, most of the people online didn't get the memo on this.

One of the major selling points of orbital compute is power supply - more specifically, it turns out that, compared to beaming power from space, it's projected to be cheaper to move compute upwell instead - atmosphere losses for beamed power are just too big. Of course this doesn't matter if you can get cheap, clean power from elsewhere (e.g. nuclear).

This is in general data center case. Here, GP says the motivation is reducing C2 RTT, which makes sense for military applications.

[notahacker]

The unit economic tradeoffs aren't quite that straightforward (beamed power is lossy enough to require more panels[1], but solar cells and satellite structures have longer useful lives than inference chips so over longer time periods you likely end up launching more stuff into space with disposable datacentres anyway[2], particularly given the datacentres also need bigger radiators. Other issues which favour power being beamed to the ground are not exposing those expensive chips to radiation, and being able to replace them on cycles dictated by inference chip innovation or end-of-life rather than fixed cycles depending on a satellite propellant budget...

The other comparison point is of course plain old terrestrial power sources, including energy storage in the mix if it relies heavily on solar power without a sufficiently global grid. Not having to launch into space buys a lot of power.

C2 RTT times and edge computation of large datasets collected in space make much more sense since unit economics aren't the driving factor, but are unlikely to need datacentres on the same scale as inference compute for the general population (another reason why this use case makes more sense)

[1]but I think the underrated problem with beamed power isn't just the low maturity of the technology, but that as soon as you start talking about sending power to earth via RF or laser you're going to encounter political opposition that makes objections to terrestrial datacentres seem tame.... [2]chips can in theory be replaced on orbit, but the "million satellite" filings are disposable. Replenishing propellant for a few large power stations, potentially on longer cycles, is a simpler task.

[pantalaimon]

> but solar cells and satellite structures have longer useful lives than inference chips

The limiting factor on the lifetime of any satellite is fuel. Especially for large structures that are influenced a lot by atmospheric drag and solar wind.

[notahacker]

Sure, but periodically refilling a propellant tank for a few large SBSP structure on-orbit involves less complexity than replacing racks of inference chips distributed across constellations of satellites as proposed by current filings. Which is why the latter structures are designed to be disposable and the former not.

SPSP proposals tend to operate in higher orbits than ODC proposals too, as they're less affected by latency and radiation, so their station-keeping requirements are less propellant-intensive.

[TeMPOraL]

> beamed power is lossy enough to require more panels[1]

What kills that is simply costs of land. You have to put these larger rectennas somewhere, and land ain't getting cheaper.

[notahacker]

Non-exclusive use of farmland or wasteland (or even - at least until maintenance complexity is considered - offshore) isn't a major barrier; we have many many miles of wires and pylons crossing farmland for regular terrestrial grids already. A rectenna looks more like a mesh than a set of panels: the idea is that the visible light spectrum passes through and the target microwave frequencies don't, so unlike a photovoltaic solar farm the land underneath is fine for farming. Nominally the land is safe to be occupied or passed through by humans too, although they might take some convincing. It becomes a problem when people start trying to get the rectennae banned...

[mr_toad]

> Non-exclusive use of farmland or wasteland (or even - at least until maintenance complexity is considered - offshore) isn't a major barrier

Not now, but with solar power output doubling every few years how long before we run out of land?

[notahacker]

Never. As the post you apparently quoted without reading said, we're talking about a rectenna composed of pylons and wires structured as a mesh that doesn't interrupt farming and can theoretically be offshore, not panels here

Space based solar power has much bigger challenges in gaining acceptance, proving the physics works in real world conditions and delivering on its promises than the theoretical possibility that there is absolutely no unoccupied region of land or sea

[rayiner]

Could we have giant nuclear reactors in space? What's the power loss from beaming down power?

[TeMPOraL]

> Could we have giant nuclear reactors in space?

We could. The usual stumbling block is how to ship fissile material upwell without the risk of a launch failure spreading highly radioactive material over several countries.

> What's the power loss from beaming down power?

I think it's about 10% from atmosphere alone, but you have to add losses from other components in the system, including light -> current -> RF and RF -> current legs, and I've seen estimates ranging from 15% to 40% efficiency end to end; this random article includes breakdown with estimates, that multiplies down to 37.5% efficiency end-to-end.

https://www.sciencedirect.com/topics/earth-and-planetary-sci...

EDIT:

This system design gives 7-14% end-to-end efficiency: https://arxiv.org/pdf/2206.08373

EDIT2:

Also to spell out another non-obvious aspect of beamed power, it turns out that it's not the efficiency that's the limiting factor per se, but land - you can improve efficiency by building larger rectennas, but it gets very expensive very quickly once you consider paying for land under them.

[pantalaimon]

We already have a giant fusion reactor in space

[TeMPOraL]

Yes, and "beamed power" is exactly about tapping into it some more downwell. As outlined in other comments, it's not currently the best option.

[OneDonOne]

See:

SNAP-10A

BES-5 (oops, sorry 'bout that Canada!)

TOPAZ-I

Kiwi, Phoebus, and NRX (Mars here we come!)

RD-0410 (Dossvidanya Solar System!)

SP-100

TOPAZ-II

Kilopower

[laughing_man]

Everybody keeps talking about cheap, clean power. But where I live the price of power has gone up dramatically in recent years. Free power (well, marginally free, anyway) is a major plus.

[wazoox]

In France, where we have relatively cheap and clean energy, they plan to build GW DCs with GWs setups of diesel backup generators. When these will run, the pollution will be staggering.

[detritus]

My concern is more in the realms of cooling. I know there's the potential for lots of 'free' energy up there, but how do you then ensure your space-based array of GPU farms bleed all of the resultant heat?

[TeMPOraL]

This is like "how do you ensure you can bring back home all the groceries you bought" kind of problem. Space 101. We know how much heat we can bleed off, how, and how fast, and this gives us bounds on how much power we can use, and that is the starting point - you design everything around that.

[alephnerd]

From a Defense perspective, it's acceptable if multiple ODC fails and you have to re-launch another one. This is why these are being treated as part of a mesh. These aren't supposed to be a commercial DC and are intended to be a mesh of multiple racks in orbit.

The fact that the US, China, Russia, and India have already deployed ASATs means a Kessler effect if a question of when and not if.

[laughing_man]

The Russians put the first ASats up in the '70s. Their existence doesn't require use.

[alephnerd]

The difference is India and China had an orbital near-miss in 2024 [0] and both India and China now field ASATs.

It's a matter of time before Japan, SK, Iran, Israel, UAE, KSA, etc build similar capabilities.

[0] - https://www.bloomberg.com/news/articles/2025-09-22/india-pla...

[Dylan16807]

> it turns out that, compared to beaming power from space, it's projected to be cheaper to move compute upwell instead

This is damning with the faintest of praise. Beaming power from space is a terrible idea.