Yesterday the M5 crew encountered our first truly cloudy day. We’d had several rainy and foggy days in the past, but in each case the limited sunlight was enough to recharge our batteries through the solar panels. Yesterday, we were greeted with a grouping of fluffy grey clouds to the southeast — perfect for blocking sunlight from the solar panel array. As the Engineering Officer, I keep a series of spreadsheets on the habitat life support systems that help me project things like the crew’s power and water use. By studying how much energy our appliances and electronics use, and how well the solar panels recharge, I can predict our power supply and power drain. By the afternoon, it was clear that it would be impossible to get enough charge to sustain the crew overnight from sunlight alone.
The habitat has two backup power systems, a hydrogen fuel cell generator and a propane generator. The fuel cells will kick on automatically if the battery state of charge drops to 10% and acts as our primary power backup system. But the fuel cells can’t charge the habitat batteries; they can only power the habitat and provide direct power to our loads inside. Once the hydrogen is used, the habitat batteries will continue to drop until their critical stop at 3%. The propane generator can charge the batteries, and fairly quickly, but it requires an emergency EVA (extra vehicular activity) for the crew to exit the habitat and start the generator manually. We decided it would be better to achieve a small charge on the batteries to last us over night than to rely on the fuel cells, so we planned an emergency EVA.
James and I exited the hab about an hour before sunset to start the generator when our battery charge was just 42%. Our plan was to achieve a 60% state of charge and turn the generator off before it got dark. We could leave the generator running all night, but we would have to switch off the circuits in the hab to prevent over charging the batteries. It would mean the generator would run idle without load all night, and we wanted to prevent this. When we departed the airlock and walked around the habitat toward the generator, we were hit by the wind. It’s surreal when your EVA suit catches the wind and you can feel the air pushing you back and forcing your suit against you, but you can’t feel the breeze over your skin. The suits are bulky and awkward to work in, making it difficult to flip switches and push the buttons on the generator. Once started, we coordinated over the radio back to the crew in the habitat to switch the habitat from solar power to generator power on the breaker box.
We returned to the habitat to wait for the batteries to charge, and I ran a few calculations to see if we would charge the batteries fast enough for us to perform our second EVA before dark. It was going to be really close, and we would need to conserve power aggressively all night. The crew jumped into action to turn off all unneeded power drains. All computers, lights, power strips and appliances were shut off and unplugged. We’d be eating leftovers and ramen for dinner to reduce power needed for cooking.
When we felt we couldn’t wait any longer, James and I re-suited and exited the hab to turn off the generator. It was already dusk and the clouds were blocking much of the little sunlight left. It was cold, and I turned off the ventilation system in my suit to retain some body heat. Over the radios, we heard that the breakers had been flipped back to battery power and we were ready to shut off the generator. I switched on my headlamp and fumbled with the controls until we got the generator turned off. When the generator cut out, the silence was almost overwhelming. I’d never been out of the hab without my fans on and in the growing darkness I felt very small. The headlamp reflected its light onto my visor such that I couldn’t see through the light further than my own hands. I switched the headlamp off and by flashlight, James and I returned to the airlock. While we waited for the depressurization cycle to finish and allow us to enter, the sun fully set and we saw Venus rising over the hab. It was as if we were seeing Earth from Mars, and in the quiet darkness I felt truly alien.