NASA is an acronym (National Aeronautics and Space Administration) and, so it should be in all capital letters.
The first sentence assumes some previous knowledge of what the space platform is and where or why it might tumble. I almost want something more on the end: "tumbling to the the earth" or "tumbling out of orbit", but that doesn't mean it's wrong as is.
"Why do we need to build a space elevator when we already have space shuttles capable of sending man into space?"
Answer: Because the space shuttles are an inefficient use of resources, and a space elevator would increase our capacity to send people into space. It will allow us to send more people into space on a regular basis, whereas our use of space shuttles are limited due to the cost of fuel and other parts.
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