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One thing I have learned over the years working at NASA is that while a rocket launch or a space walk may look like an amazing triumph of technology over environment, it is the people planning the activities that really make them work. For every hour of excitement that goes on in space, hours, if not days, of work and planning went into making it happen.

Case in point. When I started working the increment planning over a year ago, one of the tasks slated for the crew was to jettison the Early Ammonia Servicer (EAS) during an extravehicular activity, or EVA. This was definitely an intriguing activity and something we have never done before during the ISS’ history. It seemed pretty straight forward – an astronaut takes a piece of equipment about the size of a large fridge and throws it away in space.

But I knew better. Nothing is ever that easy in space.

The EAS was launched on the STS-105/7A.1 mission in August 2001. The ISS uses ammonia that flows through loops for cooling. Cold ammonia flows through tubes that interact with hot equipment, thereby picking up the heat. The warm ammonia then flows through large radiator panels jutting out from the ISS in to the darkness of space. The heat then radiates away making the ammonia again cold. This is very similar to how an air conditioner works; however, instead of Freon, the ISS uses ammonia. Ammonia is used since space gets extremely cold and hot depending on whether you are facing the sun or away; you need a material that won’t freeze or boil too easily. But just like an air conditioner, if the ammonia leaks out, the system will not work. Soon, critical equipment – computers, life support system, communication hardware – will overheat to the point of failing. In the event of a leak, the EAS was launched full of spare ammonia that could be added back into the loop.

But EAS was only to be used temporarily and then returned to Earth. Later this year, the P6 truss will be relocated from its current temporary early operation position on the top of the Z1 truss to the end of the port side of the truss (http://www.nasa.gov/mission_pages/station/structure/iss_assembly_4a.html). Before this delicate operation can be done, the 1,400 pound EAS has to be removed or it could cause damage. With delays in the shuttle program, the only way to remove it before the 10A mission (when the P6 is moved) is to jettison it.

So how hard can it be to just throw something away? Actually, pretty hard. First, you have to be very careful to make sure it will not hit any part of the space station. A 1,400 pound object, even at low speed, could do catastrophic damage to the delicate hull of the ISS. In addition, once it clears the ISS, you are not out of the woods. Due to the complexities of orbital mechanics, there is the potential that a few hours or days later, the EAS might come back to the vicinity of the ISS. Something with a lot of atmospheric drag, such as a towel, will experience a lot of orbital decay and quickly reenter the Earth’s atmosphere. A hard, dense object like a bullet will experience less drag and therefore change altitude little by the time it returns to the vicinity of the ISS. Early analysis indicated that unlike some other objects we have jettisoned in the past (towels or thermal blankets) the properties of the EAS are very nearly the same as the ISS (i.e., more like a bullet than a towel). As such, the EAS will not quickly decay and enter the atmosphere to burn up.

To avoid recontact, we figured out a plan that would call for a reboost of the whole space station, something we have to do periodically anyway to prevent its decay into the atmosphere. This reboost would occur right after the jettison. Thus, we could release the object -- which should stay in approximately the same orbit -- while the ISS reboosts itself into a higher orbit. So the next step to figure out is how quickly might it come back so we know how soon to plan the reboost.

This question has kept our Trajectory OPerations Officers (TOPO) very busy. The TOPOs have had to perform complex calculations to determine what will happen once the EAS is jettisoned. At what speed will it be released? What will be the atmospheric density, which changes as a function of the year and solar activity, which determines the drag on an object at the time of jettison? What angle to the ISS’ direction of motion should it be thrown? Will the EAS tumble after release? To understand these questions, hold your hand out of a car window while it is moving. When your hand is straight up, the drag from the wind is greatest; when the hand is horizontal, there is little resistance. Roll the hand and what you get is high drag, then low, then high (like a tumbling object). Eventually, it is likely the object will orient itself such that it has the minimum drag and stay that way. To perform the analysis, assumptions about all these variables have to be made. And since we want to make absolutely certain that nothing can go wrong, we assume all the worst cases for each parameter. One of the most critical parameters would be just how fast the astronaut could throw the EAS. So the next question to tackle – how fast could it be thrown?

To answer this we had to conduct some testing on what is called the Precision Air Bearing Floor (PABF). Imagine a giant air hockey floor where jets of air allow massive objects to move with no friction. Moving something along on the floor gives an astronaut a sense of how an object might move in space with no force of gravity acting on it. However, this only works for objects moving along the floor, so it is what we call a 2-dimensional model. So we put an astronaut in a space suit and then measured how fast he could throw the EAS while on the PABF. After some practice to determine the best way to release something in the very stiff space suit, we determined that the crew could push the object away at just about 1 MPH. With ISS traveling at 17,500 MPH, that may not seem like much, but it is just enough.

In the end, the TOPO estimated that in the worst case analysis, the EAS could return in as little as 36 hours, but more likely it will be in 4-5 days. Armed with this information we could begin planning the reboost after the EVA. Another factor to keep in mind when planning the reboost is that a few weeks later a Russian Progress resupply vehicle is arriving as well as the next space shuttle – 13A.1. You don’t want to reboost to too high of an orbit that then affects the launch and rendezvous of these vehicles. And to make things more fun, we have to plan so that these vehicles don’t hit the EAS on their way up.

At this point we knew how fast the astronaut could throw the object, but how fast would he actually release it? What if he thought it was released at 1 MPH and instead it was half that? This means it could come back faster then we were expecting! Ground radar can track objects but the object has to fly over them and it has to be far enough from the ISS itself to separate the two objects. This might take some time. So a team of engineers began looking at using the external cameras on the ISS to measure the speed. This is not as easy as it sounds. Think about seeing a plane fly overhead. Is it a big plane far away or a small plane up close? It can look the same. While we clearly know the size of the EAS, the point is that depth perception can be tricky. Using multiple cameras the team figured out how they could estimate the release speed with a few hours of calculations.

Next question – how do you best throw something away? Initially people thought the crew member would toss the object from its location on the P6. But to get the maximum push, you don’t want to be free floating when you toss it or some of the energy will go into the astronaut moving backwards in a great demonstration of Newton’s third law. So you put the crew member in a foot restraint attached in one of the sockets around the outside of the space station. However, it so happens that when a crew member is in one of these restraints large portions of the ISS will be in the astronaut’s field of view, and therefore a potential target if the object is not thrown perfectly.

So we decided to put the astronaut on the end of the Space Station Remote Manipulator System (SSRMS). The robotics arm can position the crew member in front of the ISS away from anything that could possibly be hit.

But that raises other questions. No one has ever jettisoned something this massive off the end of the arm. What would happen? While the arm is fairly strong when extended to its full 85 feet, throwing a big object off the end of it could cause it to recoil like a diving board after a swimmer has jumped off. Would the brakes that hold the arm rigid slip in response to the force? Could the flex cause damage to the arm itself? So the engineers who built the SSRMS for the Canadian Space Agency (CSA) had to perform a series of calculations to make sure the arm could handle the load. Since this was never in the original plan, new complicated computer models had to be developed. The actual speeds and forces measured in the PABF testing were used in their calculations. In the end, the CSA team decided the arm would flex some but no damage would result.

The final leg is to actually train the astronaut who will be doing this. As part of the training, the crew member would practice on the PABF floor as described above to get a feel of how hard to push. Recall that the astronaut will be on the end of the arm pointed into space away from the ISS holding a refrigerator-like object that blocks his field of view. There is little frame of reference to guide the release due to the limited visibility of the space helmet and the EAS obstruction. So the PABF is very important in providing a feeling for the person doing the jettison. But this only gives a 2-D feeling for handling an object. As with any space walk, a lot of time is spent in the Neutral Buoyancy Laboratory (NBL). This is basically a giant swimming pool where the crew can experience the floating sensation of an EVA. This can provide valuable training for all the tasks on an EVA but does not provide a good practice environment for jettisons due to the drag of the water. Additional training is provided by using the Virtual Reality (VR) lab. In there the crew can get a sense of how the jettisoned object will look as well what it will look like moving around on the P6 getting ready for the release.

So who do we train? The plan called for this EVA to occur in the stage after the 13A.1 mission during the summer of 2007. Therefore, we trained Clay Anderson as the person jettisoning the EAS off of the arm. Fyodor Yurchikhin will be the other astronaut on the EVA while Oleg Kotov will be the SSRMS operator. Due to a very busy increment, Expedition 16 will not be trained on this activity so we need to ensure it gets done by their trip in October even if the shuttle missions slip. But while Fyodor and Oleg will be up there for the 6 months from April to October, Clay is dependent on a shuttle flight which may move. To play it safe, Suni Williams was also given the training. This way, if the 13A.1 flight slipped too far, we could still do the jettison during the summer. In the end, the shuttle flights did slip. For reasons unrelated to the jettison, the space station program decided to move the replacement of Suni by Clay to the 13A mission. Either way, we were ready.

While primarily a U.S. activity, parts of the plan such as the reboost involve the Russian systems. Therefore, we have to work very closely with them on the plan. In addition, TOPO has to coordinate the Progress launch with their counterparts to make sure there are no issues there.

All this was analyzed and discussed in numerous meetings. Literally hundreds of people in 3 countries have been working this for well over a year. I think this will be an amazing event, but it all came about through careful planning.

- Bob Dempsey, Expedition 15 Lead Flight Director