Thule, the orange moon nobody visits (yet)

Most travellers to Tessaly see Thule exactly once: a small orange coin sliding across the giant's pale bands, glimpsed from a lounge window on the way to Veyra. Fewer than thirty people a year go further, under permit, down through the haze to Amber Camp. I was one of six this season, and I suspect that in a few years the number will be a great deal larger.

A moon wrapped in fog

From orbit Thule has no features at all. It is a smooth ball of orange haze, a little darker towards the poles, with nothing beneath it showing. The haze is a kind of high smog, made when Ilyr's light breaks apart the gases at the top of Thule's thick atmosphere and the fragments recombine into fine, sticky particles. They drift down for years and gather in layers, and the result is a sky that hides everything.

Underneath it, the light is the first surprise. Noon on Thule is a dim, even amber, like the last half hour of an autumn afternoon on Calder, and it barely changes from one hour to the next. There are no shadows to speak of. There are no stars. And somewhere overhead, filling a great slice of the sky, hangs Tessaly, rings and all, which you will not see once from the surface. You are standing beneath a ringed giant, and you cannot see it. Some visitors find that maddening. Most, by the second day, find it oddly restful.

The cold, and Amber Camp

The second surprise is the cold, which is beyond anything on Calder: well below minus 170 degrees at the surface, day and night. The air is thick, much thicker than Calder's, so suits here are about heat and breathing rather than pressure. They are heavier and warmer than the suits you may know from Selka or Orrin, with heated gloves and boots, and an hour outside feels like a full day's work.

You are standing beneath a ringed giant, and you cannot see it.

Amber Camp is a research station first and a guest house a distant second. Its modules sit on a low rise above a frost plain, joined by insulated tunnels, and most of its people are there to study the haze: how it forms, how it falls, how it changes over Thule's long seasons. Visitors sleep in a single guest module of six bunks, eat in the shared mess with the researchers and take their turn at the washing-up. It is warm, cramped, sociable and a little chaotic, and nobody I met there wanted it any other way.

What visitors actually do

There are no sights in the usual sense, and the camp is honest about that. What there is, is the work, and guests are invited to share it.

• Balloon launches. Most mornings the camp sends a small instrument balloon up into the haze. Guests help to fill and release it, and in the thick, cold air it rises with an odd, unhurried grace.
• Walks on the frost plain. With a researcher, never alone, out along a line of marker poles. The ground crunches underfoot, and the haze swallows the camp behind you within a few hundred metres.
• Sampling. Collecting the fine orange dust that settles on trays set out around the camp. It is the haze itself, fallen.
• Listening. Sound carries well in thick air, and the plain is so quiet that you can hear the camp's fans from a long way off, and your own breathing above everything.

And every so often the haze thins. It happened once during our ten days, for about an hour: the amber lifted a shade, and a faint, banded paleness appeared high in the sky, where Tessaly was. Two of the researchers, who had been there for months, put down their work and came out to watch it with us.

How to get a permit

Permits are issued by the camp's research council, for stays of about ten days, six visitors at a time and only a few times a year. The application asks for a medical certificate, some evidence of suit experience (a walk on Selka or Orrin counts) and a short note on why you want to go. The council reads every one. Getting there means the standard nine-day transfer from Halcyon to Tessaly, and then a seat on one of the camp's supply landers, which run to the camp's schedule rather than yours: allow several days either side.

It is not cheap, it is not comfortable, and there is no lodge with a view at the end of it. That is precisely why it is worth going now.

The next Veyra?

It is worth remembering that Veyra, not so long ago, was much like this: an observatory with a few bunks and a handful of guests a year. Brightwater is still small, but its Lantern nights are now famous, and Veyra's Glass Shore has only just opened to small guided groups. Thule could follow the same path. The camp's researchers half expect it, and are divided about it. Some would welcome the visitors and the funding they bring. Others, quite reasonably, would like the plain to stay as quiet as it is now.

For the moment, Thule remains the moon that almost everyone has seen and almost nobody has visited. Go while that is still true.