I had a chance to speak with Chris Cassidy, one of the astronauts who flew on STS-127. On that mission, Chris took three space walks (totaling 18 hours), primarily having to do with unwrapping and activating the Exposed Facility of the KIBO lab, installing experiments on the platform, and replacing huge batteries that store solar energy for use when the space station is on the dark side of earth. (You can see an animated summary of STST-127 in this video – jump to minute 3:00 to see docking and operations)

Chris described the battery situation. It takes about 90 minutes for the space station to make one orbit. During that 90 minutes, the solar arrays spend half of it in sunlight and half in darkness. So the batteries charge and discharge every 90 minutes, and they had been doing that for about 10 years. In other words, they had endured over 50,000 charge/discharge cycles (a typical laptop can handle 500 to 1,000 cycles before the batteries are shot) and it was time to replace them. But changing space station batteries is not like popping new batteries in a flashlight. A battery pack for the ISS is as big as a small refrigerator. [More info on the batteries here and here]

One thing Chris mentioned is that, on a space walk, the astronauts go through those light and dark periods as well. In the sunlight, the temperature is 250 degrees F. In darkness the temperature is -250 degrees F. I asked him if he felt any of that, or did the spacesuit completely isolate him? He said the suit did a good job unless you were standing around in the dark. He also mentioned that, when you grab hold of a metal rail that has been sitting in the dark for a long period of time, or when you hook your boots into a cold metal restraining plate, you can feel that -250 degrees soaking the heat out of your hands and feet. Keep in mind that a chunk of dry ice is at -109 degrees F – cold railings are far colder than that.

He also mentioned that the transition from dark to light every 90 minutes is something you have to think about, because the sun in space is incredibly bright. The helmets they wear have a gold visor. It comes up when working on the dark side, but the instant the transition into sunlight occurs, the light is incredibly intense and painfully bright. So you have to think about it and get the visor down in time.

I asked Chris about the daily schedule on the space station. Astronauts get 8 hours to sleep. There are two hours of “pre-sleep” and “post-sleep” time to do things like eating, showering, relaxing, etc. And then 12 hours of work. Because time on the space station is so precious, those work hours are dominated by schedules, check lists and scripts. But there is also time for things like photography, as shown here:

Pictures from space

I asked him what it is like to be on a spacewalk. Chris mentioned several things. He had spent so much time training in the NASA’s Neutral Buoyancy Lab that he could have done his EVA activities with his eyes closed. He knew exactly what he was going to do, and everything looked and felt like it did in the pool. However, when working in the pool there is a little bit of damping provided by the drag of the water – you do not have that in the vacuum of space, so there is a slight tendency to overshoot. Also, in the pool you do not have the giant sphere of the earth spinning underneath you, and that was a little distracting the first time he stepped outside the airlock.

He also discussed the safety systems. When you are working outside on the space station, there are three things to keep you from flying away. First, there is a 85-foot-long tether that he described as being similar to a retracting dog leash. It uses a thin stainless steel cord on a spring-loaded spool that provides a constant 3 pounds of force. You clip this tether to a bracket just outside the airlock. That way, if you ever “fall off”, you get pulled back to the airlock automatically.

Once you arrive at the work site, you have a short tether that you clip in there. That keeps the reel in the first tether from pulling you back to the airlock as you are working.

Then, if both of those were to fail and you found yourself floating away from the station, there is a propulsion system called SAFER that uses a joystick and nitrogen jets to let the astronaut fly back to safety. You can see the SAFER unit and the safety tethers toward the bottom of this page.

A typical spacewalk:

Chris started his career as an astronaut in 2004. He was an astronaut candidate for two years (according to his bio, those two years are spent on things like, “scientific and technical briefings, intensive instruction in Shuttle and International Space Station systems, physiological training, T-38 flight training, and water and wilderness survival training.”) Once he was assigned to STS-127 he spent about a year training together with the crew for the mission.

[[[Jump to - The STS-130 Shuttle Mission #11 – The space shuttle has docked with the International Space Station]]]