It is always OK to make links to my pages. I don't understand why some people think it is good etiquette to ask permission to make links. To the contrary, I get so many such requests that it's quite a nuisance.
These pages are my intellectual property. You are free to copy and to redistribute unmodified copies for non-commercial purposes without restriction from me (but see below). Any other use should be consistent with normal copyright law and your conscience.
See my home page if you need to contact me.
The images in The Nine Planets are almost all from NASA. Here is a statement from NASA (applying to some but perhaps not all of its images):
No copyright is asserted for these images. If a recognizable person appears in an image, use for commercial purposes may infringe a right of privacy or publicity. These images may not be used to state or imply the endorsement by NASA or by any NASA employee of a commercial product, process or service, or used in any other manner that might mislead. Accordingly, it is requested that if these images are used in advertising and other commercial promotion, layout and copy be submitted to NASA prior to release.
National Aeronautics and Space Administration, Houston TX 77058
Here's another NASA statement:
All of the images presented on NSSDC's Photo Gallery are in the public domain. As such, they may be used for any purpose. NSSDC does ask, however, that you acknowledge NSSDC as the supplier of the data. In addition, where the source of the image (by project or as a specific person) is credited in the text, you should also acknowledge that, too.
Or see NASA's copyright guidelines page.
HST images are generally OK for non-commercial use. But see their copyright page.
It is always OK to use my text and NASA's images for classroom projects.
Those with some legal expertise may wish to examine the extensive Copyright Law Materials from the Legal Information Institute of the Cornell Law School.