Two things you can say about NASA; they sure do take pretty pictures, and they sure do like to share them. Not only does NASA place most of its images in the public domain -- freeing fans of any niggling little concerns about copyright -- it also places thousands of those images on the Web to make them available to as many people as possible. The Space Agency has literally dozens of these online photo albums scattered across the Web, and three of the most popular are GRIN , the Astronomy Picture of the Day , and Visible Earth .
GRIN (Great Images in NASA ) is a collection of more than one thousand images relevant to the history of NASA, with links to exceptionally large image files - so if you like a photo enough to print it, (and don't mind the time necessary for the download) your output won't be restricted to a medium-resolution postage stamp.
There are three methods of exploring the collection. For the goal oriented, a standard -- though unusually flexible -- keyword search engine can take the visitor quickly to specific images, while less precise surfers can browse the images via Center (Jet Propulsion Lab, Johnson Space Center, etc.) or Subject listings. In the latter case, 93 Subject categories are collected under such headings as Aeronautics, (with everything from SR-71 spyplanes to a world record breaking Paper Airplane) Space, (subdivided by program - Mercury, Apollo, etc.) and Historical Images.
The images themselves are displayed as a thumbnails, with links to background information and three additional file sizes - ranging from a few hundred Kilobytes (enough to fill a browser window) to several Megabytes (yielding an 8x10 inch, 300dpi image, 'suitable for framing'). While most files seem to top out at 3 or 4M, at least one shot (Buzz Aldrin looking back at Tranquility Base) reached more than 10M - so even broadband surfers will have to wait a few minutes for some downloads.
GRIN also has an extensive collection of astronomical photographs, but NASA's best known destination for this subject matter is probably the Astronomy Picture of the Day site. Operational since June 1995, this site features everything from the Moon, to close-up images of Saturn's rings, to Hubble's views of the far edges of the known universe. Each day's exhibit (the APOD's archive contains the largest collection of annotated astronomical images on the internet) includes a full-screen image accompanied by a text explanation, which is itself peppered with links to more detailed information. APOD images are available in two sizes, and while they don't approach the maximum output of GRIN, the larger pictures still permit closer inspection than most web images.
At the last site, NASA turns the camera back towards the point of origin. Visible Earth holds more than two thousand portraits of the home planet -- still and animated -- available through keyword search or Yahoo-style directory. Categories here include Atmosphere, (from hurricanes and lightning to aerosols and smog) Biosphere, Oceans, and Human Dimensions. This last category will probably be the first stop for most visitors, with images of major cities, nighttime illumination of the planet, and such 'cultural features' as the Great Wall of China, Giza, and Mount Vesuvius, but subjects that might only seem to be of academic interest, such as oceanic currents and atmospheric vorticity, can also yield striking images.
Directory listings also allow browsing by originating platform (aircraft, satellites, or manned spacecraft) or geographic location (so you can limit your search to your own back yard). Like the other two sites, images are accompanied by explanatory text, and like GRIN, large file sizes are offered in addition to thumbnails and full-screen renditions. (Occasionally, very large - a Space Shuttle capture of Boston measured in at over 15 megabytes.)
There is, of course, no need to view all three websites in a single sitting. These, and other NASA sites have enough material to cause near-fatal image exhaustion in overenthusiastic surfers - but if you pace yourself, the reward can be an virtually endless supply of beautiful, fascinating and historic photography.