Useful strategies for answering "dumb" questions in a talk?
If somebody asks a dumb question, they're not going to feel good about the interaction whatever happens. If possible, get them back on the right track but avoid saying anything that could be interpreted as sarcasm and move on as quickly as possible.

The most likely thing is either that the questioner has missed something obvious or misunderstood something you said. So, in your first example, just point out that the rover is held there by the moon's gravity and move on. They're going to be embarrassed to have missed something so simple so it's important that you don't make them feel any worse about it; at least they'll probably understand the rest of your talk, now. In the far less common case of somebody making an assertion based on something that just isn't true, point out that the thing isn't true ("Well, the moon isn't made of cheese.") and move on. If they want to debate the point, offer to discuss it after the talk but don't let them derail you: everyone else in the room, you included, came for a talk about your thruster design, not an argument about whether the moon is a dairy product.