Purge

The middle moon of Cronenberg, and the world the ladder measures the cheapest outer-system landing against.

The numbers

These are the figures the map's card shows when you hold your finger on Purge, plus the two speeds that decide what a flight there costs. Low orbit speed is how fast you must be going, sideways, to stay up just above the surface. Escape speed is how fast you must be going to leave altogether. Sphere of influence is how far Purge's pull reaches before something bigger takes over.

Stationary orbit is the height where one trip round takes exactly one of the world’s days, so a ship there hangs over the same spot on the ground. See Stationary orbits.

Its day and its orbit are the same length, so Purge keeps one face turned toward Cronenberg for ever. Land on the near side and Cronenberg hangs in the sky and never moves; land on the far side and you never see it at all.

What it is made of

What the world is built from. It is not something a ship measures, but it is why the ground looks and behaves as it does — an ice moon takes an impact differently from an iron one.

The air

None. Purge has no atmosphere whatsoever, and that decides most of what a flight here is like. There is nothing for a parachute to catch, so the whole descent is flown on the engine and the last metres are taken on the landing legs. There is nothing to heat a hull on the way in, so a heat shield is dead weight. And there is nothing to slow a ship on the way up, so the climb off the surface is as cheap as the gravity allows.

It also means there are no air bands to survey: leave the ground and you are straight into low space. See Science and readings for what that is worth.

The ground

Purge's surface is divided into biomes — bands running round the world, each with its own colour, its own typical height and its own roughness. Which one you come down in is recorded in your flight log, and the first reading taken in each one pays.

Where is the band's place round the world, in degrees from the world's own 0°, running out to ±180° on the far side. Typical height is where that biome's ground sits against the datum the map measures altitude from, and roughness is how far it wanders either side of that.

On the map Purge is drawn from a painted picture, and the three biomes are its three sectors, 120° each: the grey Cratered Plains round 0°, the dark, fractured Messina Chasma on one side, and the pale Bright Ejecta thrown out of its craters on the other. The colour under you as you come down is the colour of that sector.

Named landforms

Purge's ground is drawn from a table of named features rather than noise alone, as every world you can stand on is. Where you come to rest is recorded by name, which is why some achievements ask for a summit, a crater floor or a plateau rather than just a landing. All 7 of them are below, in order round the world, from the far side back to 0° and on round.

Where is the landform's place round the world, and height is how far it stands above (or below) the ground around it. Flat ground is the level run it promises — the width of the ledge, floor or summit a ship can set down on.

Getting there

Small and airless, and the drop from its orbit to its ground costs very little. The distance is the whole of the difficulty.

What a ship leaving Earth's pad needs, in total change of speed, to do each of these:

What it pays

Every place nobody has measured before pays discovery the first time an instrument reads it, and how much depends on how far from home the world is. Purge counts as 7.00 against Earth's 1.00, so every place on it pays that many times what the same place at home would.

A place is a world, a band of altitude and — on the ground — the biome you are standing in, and each instrument surveys each place separately. Science and readings explains the whole of it.