Space Solar Power
vs Space Mirrors
Two very different technologies. Both involve sunlight and satellites. Both appear in the same headlines. The physics, economics, engineering, and players are almost entirely distinct — and conflating them leads to fundamental misunderstandings of both fields.
One Word That Changes Everything: Conversion
The critical difference is whether sunlight is converted or reflected.
Space-based solar power (SBSP) captures sunlight with photovoltaic panels in orbit, converts it to electricity, then transmits that electricity to Earth as microwave radiation or laser light, where a ground receiver converts it back to usable power. The chain is: sunlight → electricity → microwave/laser → electricity. Every conversion step loses energy. The value proposition is uninterrupted solar collection — no night, no weather, no atmosphere.
Space mirrors skip all conversion. A reflective surface in orbit catches sunlight and redirects it optically to a ground target — a solar farm, a city, a search zone. What arrives on the ground is sunlight. The ground's own solar panels then convert it to electricity, exactly as they would with direct sunlight. The chain is: sunlight → reflected sunlight → electricity (on the ground). No energy conversion happens in space at all.
These are not variations of the same idea. They share only the fact that they put something in orbit to interact with sunlight.
Side-by-Side
| ATTRIBUTE | SPACE-BASED SOLAR POWER | SPACE MIRRORS |
|---|---|---|
| CORE CONCEPT | Generate electricity in orbit via photovoltaic panels; transmit to Earth as microwave or laser | Reflect sunlight optically from orbit to a ground target; ground panels convert it normally |
| WHAT ARRIVES ON GROUND | Microwave radiation or laser light (electromagnetic, not visible sunlight) | Reflected sunlight (visible spectrum, same as direct sun) |
| IN-SPACE CONVERSION | Yes — sunlight to electricity, electricity to microwave/laser | None — passive reflection only |
| SATELLITE COMPLEXITY | Very high — photovoltaic array, power conditioning, phased-array transmitter, attitude control, thermal management | Low — thin-film reflective surface, attitude control, command/telemetry radio |
| SATELLITE MASS / COST | Hundreds to thousands of tonnes for GW-scale system; cost estimated at hundreds of billions | 10s to 100s of kg per satellite; constellation scale-up more affordable |
| OPERATIONAL ORBIT | Typically geostationary (GEO) at 35,786 km for continuous coverage | Low Earth orbit (LEO) at 400–600 km — only altitude where reflected beam intensity is useful |
| COVERAGE CONTINUITY | GEO provides 24/7 coverage of a fixed ground area | LEO: 3–5 min per pass per satellite; constellation required for near-continuous coverage |
| PROGRAMME MATURITY | Research phase since 1968; no commercial deployment; small demo missions underway (JAXA, ESA, CalTech) | Concept proven by Soviet Znamya experiments (1993, 1999); first commercial attempt underway (Reflect Orbital) |
| EFFICIENCY CHAIN | ~25% (PV) × ~85% (RF transmission) × ~85% (rectenna) ≈ 18% end-to-end | ~85–90% reflection × ground panel efficiency (~22%) ≈ 18–20% — similar result, far simpler path |
| BEAM SAFETY | Microwave beam must be designed to safe intensity; beam wandering a safety concern; significant regulatory complexity | Reflected sunlight at sub-direct-sun intensity; large footprint reduces intensity; no ionising radiation |
| MAIN ACTIVE PROGRAMMES | JAXA (Japan), ESA SOLARIS, CalTech SSPP demo (2023), UK SBSP programme | Reflect Orbital (Eärendil-1 mission, US) |
Why the Distinction Matters for Efficiency
SBSP — ENERGY CONVERSION CHAIN
Sunlight hits photovoltaic panels in orbit (approximately 25–30% efficient). The generated electricity feeds a transmitter — typically a phased-array microwave system — that converts electricity to microwave radiation (approximately 85% efficient). The beam travels to Earth and hits a rectenna (rectifying antenna), which converts microwave back to electricity (approximately 85% efficient). The compounded result is roughly 18–20% of the original solar energy delivered to the grid. This is not much better than ground solar, but the satellite is in constant sun — no night, no clouds. That continuity is the value proposition SBSP programmes are betting on.
SPACE MIRROR — ONE CONVERSION
Sunlight hits the mirror surface (approximately 85–92% reflectivity depending on material and age). The reflected beam travels to Earth and hits a solar farm. The solar panels convert it to electricity, exactly as they would with direct sunlight. There is no in-space conversion step. The total system efficiency is simply the reflectivity of the mirror multiplied by the efficiency of the ground panels — comparable numbers to SBSP, but achieved with vastly simpler hardware. The tradeoff is that a mirror's ground illumination intensity is always lower than direct sunlight, spread over a larger area, and available only during orbital passes. See Solar Economics for the energy analysis.
Why the Cost Structures Are Completely Different
SBSP is an enormously capital-intensive technology. A commercially viable SBSP system capable of producing gigawatts of power would require putting hundreds or thousands of tonnes of photovoltaic panels, power conditioning hardware, and microwave transmitters into geostationary orbit — at launch costs measured in billions of dollars per tonne until dramatically cheaper access to space exists. The UK government's 2021 assessment estimated a commercial SBSP system at approximately £16 billion in development cost before first watt is delivered. Japan's JAXA has spent decades and significant public funding on smaller demonstrators. As of 2026, no SBSP system has delivered commercial power.
Space mirrors start from a radically different cost base. A thin-film mirror satellite has very few components: the reflective film, a lightweight structure to deploy it, attitude control hardware, and a small radio for command and telemetry. Reflect Orbital's Eärendil-1 demonstrator is designed to launch as a small satellite. The company's argument is that the per-satellite cost is low enough that a multi-satellite constellation can be built incrementally, with each satellite generating revenue as it operates. The capital requirement is orders of magnitude lower than SBSP — which is precisely why a startup, rather than a national space agency, is attempting it first.
The economic weakness of space mirrors is intensity. The ground footprint receives sunlight at less than direct solar irradiance (approximately 1,361 W/m² in space, reduced by atmospheric passage and the mirror's non-unity reflectivity). Spread over a 5–8 km footprint, the per-square-metre addition to a solar farm's output is real but modest. The business case depends on capturing the premium electricity price during twilight hours — not on delivering large absolute power quantities. See Solar Economics for the full analysis.
Active Programmes by Category
SPACE-BASED SOLAR POWER PROGRAMMES
SPACE MIRROR PROGRAMMES
How the Confusion Happens
The conflation between SBSP and space mirrors is understandable: both put hardware in orbit to do something with sunlight, and both are described in shorthand as "orbital solar" or "space solar." The confusion is compounded by the fact that some SBSP proposals have included space mirrors as a component — large orbital reflectors designed to concentrate additional sunlight onto the SBSP satellite's panels, boosting their output. In those hybrid proposals, a mirror is a subsystem of an SBSP architecture, which muddies the conceptual separation.
Coverage of the Chengdu "artificial moon" proposal (2018) and Reflect Orbital's Eärendil-1 has repeatedly appeared alongside SBSP stories in technology journalism. The terms "space solar power," "solar power satellite," and "space mirror" are used inconsistently even within single articles. This disambiguation page exists because the two technologies have almost nothing in common beyond orbit and sunlight — and readers researching one should not end up misled about the other.
If you arrived here researching space-based solar power, the CalTech SSPP demonstration and JAXA's published roadmaps are the most current technical references. If you arrived researching space mirrors, our What Is a Space Mirror? page is the starting point, and Eärendil-1 is the most current active programme.