Page 56 of The Mars House

Gale made a quiet sound low in their chest. “Is that all?”

“Afraid so.” Molotov looked like they were in pain. “It’s an emergency system, to tide us over if there’s a flaw in the sun fields or in the sand batteries. It was never designed to power Tharsis for any length of time.”

January was trying to look up what a gravity train was, but the internet was telling him too many different things for it to make sense.

“Ah—for those of us without an engineering degree, why’s a hundred and twenty megawatts bad?” Mx Francis said, focus coming back to the scientist after having stared into the middle distance, the way everyone did when they reading something online.

“Because Tharsis uses twenty megawatts per day,” Gale said.

There was a silence round the table.

“Hang on. What?” Mx Francis said at last. “You’re saying by the end of winter, we’re sometimes a week from total blackout?”

Gale stood up slowly and picked up a stylus to write on the screen on the wall. “I know not all of you know how the sun fields work, so here are the numbers. First: Tharsis needs—this is averaged for seasonal variation—fourteen gigawatts of power in one year—”

“What’s a gigawatt though, actually?” someone asked.

Gale glanced at the blank faces around the table. “Stop me if you don’t understand or if it’s too . . . heart-stoppingly boring.” They aimed the last at January. At first he thought they meant he was stupidest person in the room, but then realized that wasn’t like Gale, and that what they really meant was that he was the only one whose position here didn’t rely on pretending to know what was happening. He was the one who could ask.

“A gigawatt is a thousand megawatts; a megawatt is a million watts. A watt is a rate of energy use; one joule per second. A joule is about the energy it takes to lift an apple above your head on Earth. A ten-watt lightbulb uses ten joules per second. Ten apples.”

January had never heard it set out like that before. He made notes on his phone, feeling odd. He had assumed he would never understand things like this, but the way Gale talked made it sound straightforward.

“Second: the surface of Mars receives, at the very best, five hundred watts per metre squared from sunlight—so five hundred joules of energy every second. This is less than half of what Earth gets. Thirty per cent of that is reflected away by our atmosphere, sometimes more, so in the real world, it’s more like three hundred. Being stationed in the tropics at altitude is the best place to catch as much as possible, hence the Mariner Plains. A solar array like ours can get about fifty per cent of that energy, which is very good. At our best, in summer, we can therefore harvest about a hundred and fifty watts per metre squared. That means one square metre of Martian solar farm panels can power fifteen standard lightbulbs.”

January had to shuffle around his idea of how the world worked, because those numbers sounded shockingly low. All at once he could understand why power here was so expensive. If you had to build a whole square metre of solar panelling for even fifteen lightbulbs, then the amount you’d need to power a whole city, with power left to spare, was madness.

Gale was writing numbers as they went, in a steady stream, without having to glance away or pause to check anything. January sat forward, feeling bizarrely like he was at a performance. It was no small skill, to stand up unexpectedly in front of thirty people and reel off things like this by heart, in a logical order, without hesitating. Like any good teacher, Gale spoke in sentences so exact he could hear the punctuation.

“So: in order to generate twenty megawatts in a day—that’s twenty million watts—we need to cover about a hundred and forty square kilometres with heliostats. But, those are the best possible conditions, which we don’t usually get. In practise, we need more like three hundred square kilometres of solar mirrors. But that’s cutting it fine, that’s just enough, for a day, if we don’t have particularly bad weather; but, we are almost guaranteed bad weather for four hundred days of every year. What we need to do is produce excess in summer, when sunlight is plentiful, so we can add to the grid when demand is high but supply is low in winter. We have five hundred square kilometres to be on the safe side.” Gale nodded outside.

“The sunlight is concentrated by the mirror array on a tank of liquid salt in the height of a tower. That salt heats to a thousand degrees Celsius, and that thermal energy powers generators, which connect to the national grid. We have ten towers.”

Gale paused for a second. “It is not cost-effective to build many more fields than we realistically need. It’s fantastically expensive to build one. Even more to maintain it. Panels get smashed or torn, machinery has to be replaced, and liquid salt of a thousand degrees is not a forgiving substance. We can only grow at the pace the city does, or these arrays don’t pay for themselves. We can’t do what Earth does, we can’t generate a lot of power by putting solar fabric on clothes and roofs and cars, because most people live in the Valley, which gets even less sunlight than we do up here. They have to, because it’s the only place on Mars with enough air for first- and second-generation arrivals. That will improve as the terraforming process continues, but for now, that’s what we’ve got.”

January wondered whose bright idea it was to live on Mars in the first place. It seemed increasingly impractical.

“Therefore, we must run close to capacity at the very end of winter.” Gale looked around. “Does anyone have any questions? Communications and marketing staff need to understand this as much as scientists do or you’ll be trying to write press releases that make no sense to you.”

“We’re all going to starve and die, aren’t we?” Mx Francis said.

“Dehydration will get you ages before you can starve,” Mx Ren put in, looking quite cheerful about the idea of watching Mx Francis die of thirst.

“I think the only realistic option to tide us over will be something to do with the gravity trains,” Dr Molotov said.

January lifted his hand a little. “What is a gravity train?”

“Medieval,” Gale said, and the scientists sort of laughed. “It’s a basic way of storing energy. When you have excess power, you use it to raise something heavy to a height, so that the thing has plenty of potential energy—the energy of placement, yes? Apple has no energy,” they said, setting one on the floor. They lifted it up above their head. “Apple has potential energy. It could fall.”

January wondered how the apple knew.

“Martian gravity is very weak, so just rolling something up a hill doesn’t give it much energy, but fortunately we have a four-mile drop in our back yard, into the Mariner Valley. When we have excess solar energy, we use it to pull a chain of concrete blocks up the cliff. When we need that energy back, we let the blocks fall down their tracks again. The friction generated by applying the braking system gives us back most of the energy we put in by lifting them up in the first place.”

“Could we just crank them up again by hand or horsepower once they’ve been lowered? The trains,” someone from Mx Francis’s team said.

“Hell no,” one of the scientists snorted.

Gale fixed them with the serene stare they gave people when they were gauging exactly how much sausage meat it would be possible to make with the available human, and the scientist looked embarrassed.