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MS. STILSON: Palak from Chicago wants to know what experiments is Discovery carrying on board. MS. STILSON: Here's a good question from Kevin in Ottawa: How are payloads given priority, given the space limitations for each Shuttle flight? MR. HIGGINBOTHAM: That is a good question. It's a lot like packing for a camping trip. You only have limited amount of stuff that you can carry on your back and limited amount of weight that you carry, so you have to prioritize. First of all, you're going to carry the things that you need to keep you alive and keep you healthy. And then with whatever extra weight and volume you might have left over, you can carry other stuff that you might want to have. It's a lot like that for these missions.On the Space Station, we have two basic kinds of flight. We have assembly flights, where we're carrying up large pieces,building blocks to leave behind. And then we have resupply flights where we're carrying up supplies. On the assembly flights,the prioritization is relatively easy because that primary assembly element that we're carrying up takes up most of the payload bay. Now there will be a little bit of space leftover and a little bit of mass left over that we can use. And generally they will try to fly up spare parts that we might need for the Station. However, on a resupply mission, like this one,it's far more complicated and the process takes months and maybe even years to complete. And in that process, all the various disciplines that support the operation of the Station, both the flight crew office and the various technical disciplines that operate the various subsystems, get together and work with our Station program management to create a priority list of what really needs to get up there and they whittle that list down and try to optimize the maximum amount of stuff that we can carry up. Again, primarily focused on keeping the crew healthy, the Station operatingand if there's a little extra room left over for other stuff, we'll throw that on board, too.