Astronomy & Celestial Mechanics
the sky, computed
Concept

Satellite constellations — where it appears

Large fleets of satellites in coordinated orbits, numbering thousands to tens of thousands, built for communication or observation. Their solar arrays hold most of the sunlit area in low orbit, and so most of the diffuse glow it adds to the night sky.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

Where in the sky satellites still shine. The whole sky above the horizon seen from below, zenith at the centre, north at the top and the Sun 20° below the western horizon. Shaded: directions in which a satellite at 550 km is sunlit — 71 per cent of the sky by solid angle. The lit region is a cap pressed against the western horizon and reaching up past the zenith, because satellites seen low in the west are far away and high above the shadow; the eastern sky is dark. Ignoring the penumbra moves its edge by under a degree.

The shadow that also switches off the satellites

After sunset the sky overhead darkens as the Earth's shadow climbs through the air, and the same shadow, climbing on past the top of the atmosphere, decides which satellites still catch sunlight. For a satellite five hundred and fifty kilometres up the zenith stays lit until the Sun is twenty-three degrees down — five degrees past the end of astronomical twilight — and at the latitudes of most large observatories' summers there is no hour of the night when it is not.

sky · Twilight The sunlit cross-section in orbit, by kind of object. Physical cross-section in square metres, for each class of object in orbit, from rounded population counts and typical sizes: active satellites 120,000; dead satellites 21,000; rocket bodies 57,500; tracked fragments, above 10 cm 665; fragments of 1–10 cm 407; fragments of 1 mm–1 cm 316. Of a total near 2.0 × 10⁵ m², the fragments below the catalogue's 10 cm hold 0.4% — the satellites and spent rocket stages, every one of them tracked, hold nearly all of it.

The sunlit area in orbit is almost all in things already tracked

The diffuse glow that objects in orbit add to the night sky depends only on how much sunlit reflecting area there is, and the debris too small to track was thought to be where most of that area might hide. It is not. Whether the smallest pieces hold the area depends on one number — whether the size distribution's slope is steeper or shallower than two — and for the fragments of break-ups it is shallower. The pieces below ten centimetres hold less than one per cent of the cross-section in orbit; the satellites and spent stages, every one of them catalogued, hold the rest.

sky · Twilight

Named alongside it

The objects these essays reach for when they reach for this one.

Sky brightnessAstronomical twilightAtmospheric dragBreakup modelCross-sectionEarths shadowLatitudeLow earth orbitObserving conditionsOrbital debrisSize distributionSolar depression

All concepts