Some relevant links:
http://www.marssociety.org for manned Mars and other Mars research.
http://http://www.islandone.org/APC for advanced propulsion; the parent has other space related pages.
http://www.projectpluto.com/find_orb.htm and http://newton.dm.unipi.it/orbfit for orbit determination; the parent of the first has other astronomical software.
http://ssd.jpl.nasa.gov/horizons.html has an online ephemeris., based on JPL's ephemeris data. http://ssd.jpl.nasa.gov/iau-comm4/relateds.html has links to many useful documents.
https://www.willbell.com sells books and other reference material for astronomy.
http://caao.as.arizona.edu for telescopes and space telescopes.
http://hea-www.harvard.edu/QEDT/jcm/space/space.html has a comprehensive newsletter on current space missions.