A data-driven look at orbital mirror brightness from every mission — past, present, and proposed. Magnitude comparisons, footprint calculations, tumbling flare risk, and what an observer on the ground would actually experience.
BRIGHTNESS SCALE
Magnitude Comparisons — All Mirrors
Astronomical apparent magnitude is a logarithmic scale. Each step of 1 magnitude represents a brightness factor of ~2.5×. Lower (and negative) numbers are brighter. The scale below places all orbital mirror missions in context.
VISUAL BRIGHTNESS SCALE — APPARENT MAGNITUDE
NOTE: Each magnitude step = ~2.5× brightness difference. The scale is logarithmic — the gap between the Sun and the Moon is enormous in real terms.
−26.7
The Sun
Reference point for solar illumination
−12.7
Full Moon
Bright enough to cast shadows; disrupts dark adaptation
−10 to −12
Tumbling Mirror (worst case)
Uncontrolled large-area reflector flare — brighter than full Moon, globally visible for seconds
−4.6
Venus (maximum)
Visible in daylight; easily seen at dusk/dawn; casts faint shadow
~−4 to −5
EARENDIL-1 (peak, overhead)
Comparable to Venus at its brightest — conspicuous at dusk, visible to naked eye as moving point
−1.5
Sirius (brightest star)
Reference for "very bright star"
~3–5
Znamya-2 (1993, actual)
Outside the beam, observers reported only a brief flash as the beam swept past — barely naked eye. The beam on the ground was full-moon equivalent but the satellite itself was faint due to imperfect reflector shape after deployment.
~3
Znamya-2.5 target (1999)
Intended magnitude before deployment failure — brighter than Znamya-2 due to larger 25m mirror. Comparable to a moderately bright star
3–5
Typical ISS / Starlink flare
Common point of comparison for existing satellite brightness
IMPORTANT NOTE ON EARENDIL-1 BRIGHTNESS
Reflect Orbital's stated target brightness for EARENDIL-1 is approximately Venus-equivalent (magnitude ~−4) when the mirror is directly overhead and optimally oriented. At lower elevations or non-optimal angles, apparent brightness drops significantly. The mirror is designed to be steerable — it can be tilted away from Earth between targeted passes, meaning it would not be continuously bright like a flat reflective surface.
FOOTPRINT DATA
How Large Is the Illuminated Area?
~5kmZNAMYA-2 · 1993Traced across European cities during demonstration pass
~5kmEARENDIL-1 · 2026Design target. Comparable to a city district. Moves at ~7.6km/s ground speed
The 5km footprint figure for EARENDIL-1 represents the region receiving meaningful supplemental illumination. The reflector is 18×18m, which from 625km altitude subtends a very small angle. The mirror is not focusing sunlight to a tight point (that would require a concentrating rather than flat reflector); instead it produces a relatively diffuse 5km beam of enhanced ambient light.
FLARE RISK
The Tumbling Mirror Scenario
RISK SCENARIO · WORST CASE
If an orbital mirror loses attitude control and begins tumbling, it becomes an uncontrolled specular reflector sweeping the sunlit side of Earth with a rapidly rotating beam. Depending on geometry, peak flares could reach magnitude −10 to −12 — comparable to or brighter than the full Moon — lasting several seconds per rotation cycle. A mirror at 625km altitude in a stable orbit without active deorbit capability could remain in orbit for months to years. Unlike a communications satellite failure, which is invisible, a tumbling mirror failure is visible to the entire hemisphere it is over.
This risk is not theoretical. In 2024, NASA's Advanced Composite Solar Sail System (ACS3) — a technology demonstration satellite deploying a large reflective sail — began rotating uncontrollably after deployment. ACS3's reflective area is much smaller than EARENDIL-1's proposed mirror and is not optimised for maximum reflectivity toward Earth. Despite this, the failure mode was real and public.
Reflect Orbital has described attitude control as a core technical challenge but has not published detailed failure mode analysis in the public domain. The FCC experimental licence does not appear to have required this analysis as a condition of approval.
OBSERVATION GUIDE
What Would You Actually See?
MIRROR
EXPECTED APPEARANCE
DURATION
FREQUENCY
NAKED EYE?
Znamya-2 (1993)
Brief flash outside beam, mag ~3–5. Beam on ground was full-moon equivalent but satellite faint due to imperfect deployment
~5 min per pass
Single demo
YES
EARENDIL-1 (planned)
Bright moving point, mag ~−4 to −5 near zenith
~3.5 min per pass
Multiple daily (targeted)
YES — bright
4,000-sat constellation
Near-continuous brightening of sky background; multiple simultaneous passes
Ongoing
Continuous over target regions
YES — pervasive
Tumbling mirror (worst case)
Multi-second flares, mag −10 to −12 — brighter than full Moon
Seconds per flare cycle
Until deorbit (months–years)
YES — alarming
// SEE IT FOR YOURSELF
These are the brightness predictions for a mirror that hasn't launched yet. In the meantime, you can see real satellites at real brightnesses right now — the ISS at mag −5.9 overhead, Starlink trains, Hubble, Tiangong, and thousands more, with live brightness predictions for your location.