Right, that is true for NASA pictures. They can be used and shared without much concern.
But the ones that were featured in apod yesterday and today have private copyrights on them.
Apod being mirrored not only here on steemit, but also on twitter, facebook, instagram, reddit, etc are official mirrors of apod and therefore have allowance to use the same pictures as used on the official homepage.
Just please be aware of that.