Orbital mirrors, priced in photons: magnitude -16.7 in the beam, dusk out to 34 km

A paper accepted by Astrophysical Journal Letters and posted to arXiv has put modelled brightness figures on Reflect Orbital's plan to bounce sunlight onto the ground after dark ^[1][2]. That matters because the company's pilot satellite already cleared the US Federal Communications Commission in July, on the reasoning that light pollution and astronomical interference are not the FCC's problem to regulate ^[11][12].

The hardware first. The pilot, Earendil-1, carries an 18-by-18-metre reflector at roughly 600 kilometres altitude and is designed to drop a 2.5-kilometre patch of daylight on a customer-chosen spot on demand ^[3]. The commercial pitch is solar farms generating after sunset and disaster zones lit for rescue crews ^[4]. The pilot is a test article: the proposed constellation runs to about 50,000 satellites, with production units stretching to 54 by 54 metres ^[5]. That is nine times the reflector area of the pilot per spacecraft ^[6], and if the full fleet were built at that size it would amount to roughly 146 square kilometres of mirror in orbit ^[7].

The paper, by Miroslav Kocifaj and colleagues including Gaspar Bakos, models both the direct beam and the sky glow that sunlight throws off through Rayleigh and aerosol scattering on its way down ^[1][8][9]. According to that modelling, an observer standing inside the beam of a single 54-metre satellite would see a point source at magnitude -16.7, about four magnitudes brighter than a full moon ^[10] - a factor of roughly 40 in received light ^[13].

The scattered light is the part that travels. From 14 kilometres outside the beam, the paper finds the glow alone would outshine full moonlight across most of the sky, and it would still read as brighter than moonlight from 34 kilometres away ^[14]. The diffuse background at that point resembles dusk shortly after sunset, bright enough to wash out everything except the very brightest stars ^[15]. Reflect Orbital's long-term plans essentially call for running 400 mirrors on the same patch at once; the paper puts the resulting glow as obvious from 80 kilometres out ^[16]. Treated as a circle, that is a footprint on the order of 20,000 square kilometres per illuminated spot ^[17].

This is the useful thing the paper does. Until now the objection to orbital mirrors was aesthetic and procedural, and procedural objections lose. The FCC granted the licence over more than 1,800 public comments and formal objections from the American Astronomical Society and dark-sky groups ^[11][12]. A magnitude and a radius are different: they are the kind of quantity that can be written into a licence condition, a national park's lighting rules, or an observatory's siting analysis, and they can be measured against once Earendil-1 is operating.

What to watch. First, whether any agency claims jurisdiction, because as of this reporting none has ^[18] - the gap the FCC identified is still a gap. Second, whether the modelled numbers survive peer scrutiny and, later, photometry from the pilot itself; an 18-metre reflector is smaller than the 54-metre case the paper models, so the first in-orbit measurements will be a scaled test of the physics rather than a direct check of the worst case ^[3][5]. Third, whether the 400-mirror stacking case remains company policy, since that is where the modelled glow stops being local.