Why Starlink Satellites Disappear Mid-Pass

This is the most common question from people who have just started watching satellites, and the answer is one of the more satisfying things you can observe without equipment.

The Earth casts a shadow, and it reaches into orbit

Sunlight travelling past the Earth leaves a cone of shadow stretching away on the night side, extending far beyond the Moon's distance. Everything inside that cone is in darkness. Low Earth orbit sits well within it for part of every lap.

A satellite at 550 km completes an orbit in about 92 minutes. On most orbits, part of that circuit is spent in sunlight and part inside Earth's shadow — the satellite experiences sunrise and sunset roughly sixteen times a day.

When you are watching a satellite in twilight, you are watching an object that is still in daylight while you are not. If its path takes it across the shadow boundary during your pass, it stops being lit, and since reflected sunlight is the only reason it was visible, it disappears.

You did not lose the satellite. You watched it fly into night, from the outside.

Why the fade is so abrupt

The transition is quick because the satellite is moving at about 7.6 km per second. The partially shadowed zone — the penumbra, where the Earth covers part of the Sun's disc from the satellite's viewpoint — is only a few hundred kilometres wide at that altitude.

At orbital speed, crossing a few hundred kilometres takes tens of seconds, and the visible dimming concentrates into the last few. From the ground it reads as a fade over roughly two to five seconds: noticeably gradual, but far faster than anything else in the sky behaves.

Some observers notice a brief reddening just before the satellite vanishes. That is real. It is sunlight refracted through the Earth's atmosphere at the shadow's edge — the same physics that makes a lunar eclipse red, seen from the other side.

Why entry is far more common than exit

Evening passes almost always end with the satellite fading out. Morning passes often start with one appearing out of nowhere partway up the sky.

The reason is which way the shadow boundary is moving relative to you. In the evening, Earth's shadow is sweeping eastward across the sky above you as the Sun sinks further below your horizon, so a satellite tracking eastward runs into it. Before dawn the geometry reverses, and satellites emerge into sunlight partway through a pass.

This is why pass predictions list a visible start and end that may be shorter than the satellite's actual horizon-to-horizon transit. The prediction is telling you the illuminated portion, not the geometric one.

The other reasons a pass ends early

Shadow entry accounts for most disappearances, but not all. If a satellite fades and the timing does not match a shadow crossing, consider these.

• Attitude change — the satellite rotated, and the surface that was reflecting sunlight toward you no longer is. Brightness can drop sharply and sometimes recover
• Increasing range — as a satellite descends toward the horizon it gets further away and dimmer, and eventually falls below naked-eye visibility before actually setting
• Atmospheric extinction — near the horizon you are looking through far more air, which absorbs and scatters light. A satellite can effectively vanish at 10° elevation while still being well above the horizon
• Thin cloud — high cirrus is close to invisible at night but will extinguish a faint satellite completely
• Local light — walking into a streetlight's glare, or glancing at a phone, destroys dark adaptation instantly and can 'lose' a satellite that is still perfectly visible

Flares: the opposite problem

Occasionally the reverse happens — a satellite briefly brightens dramatically, sometimes by several magnitudes, then returns to normal. This is a specular reflection: a flat surface catching the Sun at exactly the right angle to bounce it toward you.

Starlink satellites, being largely flat panels, are capable of this, and the effect is short because the geometry only holds for a few seconds at orbital speed. It is unpredictable in practice, since it depends on the satellite's precise orientation at that instant, which is not something published orbital data describes.

Using it to your advantage

Once you understand shadow geometry, you can read the pass list differently. Passes closest to sunset are the ones most likely to be cut short by shadow entry; passes deeper into twilight are shorter overall but more likely to run their full visible length.

The best passes are usually the ones that peak high while the satellite is still well clear of the shadow — which is exactly what a prediction showing a high maximum elevation and a full-length duration is telling you.