Each body paragraph will have a topic sentence, which tells readers what your paragraph is going to be about and what you want to say about your topic. For example: 'One of the challenges that is unique to space is the fact that space is a vacuum, which is a risk for various reasons.' This topic sentence tells readers that the paragraph that follows it will be about one of the challenges that is unique to space: the fact that it is a vacuum and the reasons why that vacuum is a risk factor.
Each body paragraph will have supporting sentences that discuss the idea or ideas in your topic sentence, using examples and/or evidence to support that discussion. This evidence can be quotes from researched sources, personal experience and observation, statistics, stories or examples.