Start your research:
NASA - https://www.nasa.gov
NASA Robotics Alliance Project - https://robotics.nasa.gov/edu/9-12.php
(Although this is directed to grades 9-12, 6-8 graders can understand most of the information)
ESA - https://www.esa.int/ESA
For building inspration, you can download building instructions from past LEGO models. Type in "Space" for the theme.
Q-Tip: On "almost" every challenge theme since 2000, there has been a mission that involves retrieving a loop model. On this past year's Hydro Dynamics it was the Pipe Removal. In Mars Mission (2003) it was an Ice Core like the model below:
In recent years another popular mission was to push something to make something rise & stay in position. I believe it first appeared in Body Forward (2010):
There are similar missions in the LEGO Education Space Challenge Set that you could practice.