Satellites shine because they reflect sunlight. They don’t produce their own light. Instead, their solar panels, antennas, and metallic body surfaces act like mirrors hundreds of kilometers above the Earth, bouncing sunlight down to observers on the ground. The reason they’ve become noticeably brighter and more numerous in recent years comes down to a combination of orbital geometry, surface materials, and the sheer explosion in the number of satellites being launched.
How Sunlight Reaches a Satellite at Night
The key to understanding satellite brightness is altitude. Even after the sun has set for you on the ground, a satellite orbiting at 550 kilometers up is still bathed in direct sunlight. The Earth’s shadow hasn’t reached it yet. This is why satellites are most visible during the hour or two after sunset and before sunrise: you’re standing in darkness, but the satellite is still in daylight, reflecting light straight down to you.
Sunlight isn’t the only factor. Light reflecting off the Earth’s surface, sometimes called earthshine, also bounces up and illuminates satellites from below. Research published in The Astronomical Journal found that this indirect illumination can add roughly one full unit of astronomical brightness, particularly during twilight. In some parts of the sky, earthshine makes satellites visible that would otherwise be too dim to see at all.
Why Some Satellites Are Brighter Than Others
Two properties matter most: how close a satellite orbits to Earth, and how its surfaces reflect light.
Closer satellites appear brighter for the same reason a flashlight looks brighter when someone holds it near your face. Starlink satellites orbit at about 550 kilometers, which is low enough to appear brighter than 99% of the roughly 200 artificial objects that were previously visible to the naked eye. By comparison, OneWeb’s constellation orbits at 1,200 kilometers and appears dimmer simply because of the extra distance. Geostationary satellites, parked at 36,000 kilometers, are far too distant to see without a telescope.
Surface material plays an equally important role. Smooth, flat surfaces like polished antennas or solar panels act as mirrors, reflecting sunlight in a concentrated beam. This is called specular reflection, and it’s what creates the most dramatic brightness events. Rougher or coated surfaces scatter light in all directions (diffuse reflection), which spreads the energy out and makes the satellite dimmer from any single viewpoint.
Satellite Flares: Those Sudden Bright Flashes
If you’ve ever seen a point of light in the sky suddenly blaze to startling brightness and then fade within seconds, you witnessed a satellite flare. This happens when a large, flat, reflective surface catches the sun at exactly the right angle to bounce a concentrated beam of light toward your location on the ground.
The most famous examples were Iridium flares, produced by a constellation of 95 telecommunications satellites launched starting in 1997. Each Iridium satellite carried three polished, door-sized antennas angled at 40 degrees from the satellite body. When one of these antennas caught the sun just right, it created a moving spot of illumination on the Earth’s surface about 10 kilometers across. From the ground, this looked like a brilliant flash lasting a few seconds. At their peak, Iridium flares reached a magnitude of negative 9.5, bright enough to see in broad daylight. For context, the human eye can detect objects down to about magnitude 6.0 on a clear night, and lower numbers mean brighter objects. A negative 9.5 flare was roughly 50 times brighter than Venus.
Solar panels can produce flares too, though they’re typically dimmer (around negative 3.5 magnitude) and last about twice as long because the reflective geometry holds for a wider range of angles.
Why There Are So Many More Visible Satellites Now
The number of satellites in orbit has grown dramatically. The European Space Agency estimates that around 100,000 satellites will be in orbit by 2030. Most of this growth comes from mega-constellations designed to provide global internet coverage. SpaceX’s Starlink alone has launched thousands of satellites into low Earth orbit, with plans for many more.
Because these constellations operate at low altitudes for faster internet speeds, they sit in the orbital sweet spot for naked-eye visibility. They’re close enough to appear bright, and they pass through the sunlit zone visible during twilight in large numbers. On any clear evening shortly after sunset, it’s now common to spot multiple satellites crossing the sky within minutes.
Efforts to Make Satellites Dimmer
Astronomers raised alarms early. When the first batches of Starlink satellites launched, their brightness was immediately apparent, and the International Astronomical Union responded with specific recommendations: satellites in operational orbit should be invisible to the unaided eye, with a brightness target fainter than magnitude 7.0 for satellites at 550 kilometers altitude.
SpaceX has tested two approaches. The first, called DarkSat, used a dark coating on reflective surfaces and reduced brightness by about 0.8 magnitudes in certain wavelengths compared to standard Starlink satellites. The second, called VisorSat, added small sun-shade eaves to block light from hitting the brightest surfaces. VisorSat performed better, with the average visual magnitude landing around 5.9 at operating altitude. That’s roughly one full magnitude dimmer than a standard Starlink satellite, which typically measures around 5.1. While this is an improvement, magnitude 5.9 is still visible to the naked eye under dark skies, falling short of the IAU’s 7.0 threshold.
The Impact on Astronomy
For casual stargazers, a bright satellite crossing the sky can be a novelty. For professional astronomy, it’s a growing problem. Satellites leave bright streaks across long-exposure images, ruining data that may have taken hours to collect.
A NASA-led study examining four space telescopes found the scale of contamination is striking. Roughly 40% of images taken by the Hubble Space Telescope could be affected by satellite light over the next decade. For the SPHEREx observatory, that figure jumps to about 96%. China’s planned Xuntian telescope and the European Space Agency’s ARRAKIHS mission face similar contamination rates. These aren’t ground-based telescopes struggling with city light pollution. They’re space telescopes sharing low Earth orbit with the very satellites causing the problem.
Ground-based observatories face their own challenges. Wide-field survey telescopes, which scan large swaths of sky to track asteroids or map the universe, are especially vulnerable because their broad field of view increases the chance of catching a satellite streak in any given exposure.
What Determines Brightness on a Given Night
Several factors combine to determine how bright a satellite appears to you at any particular moment. The solar phase angle, which is the geometric relationship between the sun, the satellite, and your eyes, is the most important variable. When a satellite is positioned so that sunlight bounces almost directly toward you, it appears brightest. As it moves across the sky and the angle shifts, it dims.
Time of year matters too. During summer months at higher latitudes, the sun doesn’t dip far below the horizon, which means satellites remain illuminated for more of the night. In winter, the Earth’s shadow climbs higher and satellites enter darkness sooner after sunset. Atmospheric conditions like haze or light pollution also affect what you can see, though the brightest satellites punch through all but the worst conditions.
Your best chance of spotting satellites, and seeing them at their brightest, is within about 90 minutes of sunset or before sunrise, looking toward the portion of sky still faintly lit by twilight. That’s the geometry where the most satellites are sunlit while you stand in shadow.