From the Center for Archaeoastronomy. Data and Images NASA - Images, data, space missions, research, and news releases. The NASA page is the best starting place. Jet Propulsion Laboratory - Information on space missions, planets, and activities for teachers. National Space Science Data Center - More data from NASA missions.
Categories are astrophysics, space physics, planetary sciences, and earth sciences.
Dictionaries Dictionary of Science and Technology - More than 133,000 entries from all fields of science and technology, from Acoustical Engineering to Zoology. National Geographic Star Chart - Includes Hubble images. Sky and Telescope - Monthly sky highlights. History of Astronomy History of Astronomy - Links to biographies and source documents. Galileo Project - The life and times of Galileo, from Rice University.