Erran

What is Erran like?

Erran is a moon of Kharsis, the system’s arid mining planet. People call it la Errante, ‘the wanderer’.

• Size. 1,000 km in radius.
• Gravity. 0.867 m/s².
• Air. Thin: it is the only moon in the system with thin air. It has 0.654 kPa, under 1% of the pressure on Ithera, the temperate planet. It is 95.9% carbon dioxide, with a little argon and nitrogen.
• Living there. The air cannot be breathed. Work is done in pressure suits, and bases need an airlock.
• Day. It lasts five hours.
• Seasons. The most violent in the system, because its axis tilts thirty-one degrees. The noon sun climbs to eighty-one degrees in summer and gets no higher than nineteen in winter.

Where is it, and who holds it?

Erran orbits 199,991 km from Kharsis, once every 11.83 hours. The Nadir, one of the system’s two sides, keep their listening post there.

From the far face of Kharun, another moon of Kharsis, the planet is never seen. There, Erran is the only thing that breaks the field of stars. It rises wide and pale over the horizon, hangs there a while and is gone.

Why is its air ‘a season’?

Erran stores its CO₂ (carbon dioxide) in its polar caps and its regolith, the dust and loose rock of the ground. Every summer it lets it go. That is why the dossier on the moons of Aurin says its air “is a season and not an inheritance”. It is not air it has kept from the start. It comes from two places:

• The frost of the winter cap, at 148 K (125 degrees below zero), which gives up its gas.
• The rock, from which the tides draw more gas.

That thin air moves dust. Every summer brings granite dust storms that stain an almost black sky a pinkish grey. In the polar winter, cirrus clouds of CO₂ frost, thin and high, hang overhead.

Why does it not follow the other moons?

Kharsis’s other three companions are chained together: Kravane, Kharun and the asteroid Ashkar. Their periods keep a ratio so exact that they always rise and set at the same point on the horizon. The miners have used them as a clock for forty years without ever correcting it.

Erran does not. It orbits outside that chain, with its axis steeply tilted. So its rising point sweeps the whole horizon over the course of a season. No two of its dawns are alike.

It does not fit the Registro Aura, the first catalogue of the system, either, because it was never in it. Erran and the moons Pyra and Cinera were added later. Their names do not follow the suffixes the Registro used to classify what it saw from afar.

What is there to fear?

The miners of Kharsis do not trust Erran, and they have their reasons. Nothing that serves for navigation in the rest of the system works here. Not the chain, not the clock, not the fixed point on the horizon. It is the only moon where finding your way across the surface cannot be learnt by heart.

That is why the Nadir put their listening post right there. The dossier on the moons of Aurin gives the reason in a single sentence:

“An observatory you can never be sure how to approach is an observatory that is very hard to storm.”

Dossier on the moons of Aurin