General

1. Where can I find images of the comet P/Shoemaker-Levy 9 collision with Jupiter on-line?

A large (and growing) number of sites have these images. A set of over 200 images and links to many other sites is available on NSSDC's Comet Impact page.URL: http://nssdc.gsfc.nasa.gov/sl9/comet_images.html

2. Where can I find images of the Moon and planets on-line?

There are a number of places, both in and outside of NASA, where such images are available. Therefore, a comprehensive listing of such sites is neither possible or practical. However, there are some places worth pointing out.
The first such site is NSSDC's Photo Gallery, a service available from NSSDC for WWW users. The gallery shows a number of images (including non-planetary images) produced with data available from NSSDC's archives and other sources. The collection of photos is growing, so keep an eye out for new additions.URL: http://nssdc.gsfc.nasa.gov/photo_gallery/
Another site at NSSDC is the Catalog of Spaceborne Imaging. This service contains exclusively planetary images and will hopefully eventually contain all of NSSDC's plantary imaging in digital format.URL: http://nssdc.gsfc.nasa.gov/imgcat/
A list of other sites where one might find lunar and planetary images of interest is also available.URL: http://nssdc.gsfc.nasa.gov/photo_gallery/other_sites.html

3. What do the file names for the images and directories on the Voyager CD-ROMs mean?

On the Voyager imaging CD-ROMs, the top level directories indicate the object name, for example Jupiter, Saturn, Rings, Oberon. Under that lies directories with names like CnnnnXXX and, under each of these directories, lie images with names like Cnnnnmmm. These file names are derived from the actual exposure numbers, which are of the form xxxxx.yy. For the Voyager Imaging Science Subsystem (ISS), each exposure is tagged with the current spacecraft clock time. Because the spacecraft clock time is measured in seconds from launch, the number is quite large and gets larger as time goes by. Rather than having to deal with extraordinarily large numbers, the time is reduced in two ways to come up with the exposure number. First, the time is taken modulo 65,536 (16 bit) to come up with the first five digits of the exposure number (which, padded appropriately with zeros on the left becomes the xxxxx part). Second, the time is taken modulo 60 to come up with the last two digits of the number (the yy part). From the exposure number xxxxx.yy, one takes the first four digits and sets them equal to the nnnn portion of the directory. For the image names, one just elimates the decimal place. For example, the image 8966.31 can be found in the directory C0896XXX as file C0896631.IMG.

4. What do the file names for the images and directories on the Galileo CD-ROMs mean?

On the Galileo imaging CD-ROMs containing raw data, each of the images is placed into top level directories indicating the object name, for example Venus, Earth, Calibration, etc. Under that lies directories with names of the form Cnnnnnn and in each of those, images with names of the form xxxxR. These names are derived from the spacecraft clock time. Because the spacecraft clock time is measured in seconds (down to fractions of a second) from launch, the number is quite large and gets larger as time goes by. Because there are approximately 30 million seconds in a year and the mission is expected to last through 1997 at least, the file names could get quite big. Rather than do this, the last two digits of the spacecraft clock time and the first two places to the right of the decimal in the spacecraft clock time are used to make up the file name; the R on the end indicates it is "raw" (i.e., unprocessed) data. The directory names are then obtained by dividing the spacecraft clock time by 100 and eliminating the decimal places. If necessary, the number is padded at the beginning with zeros so that six numbers are present in the name. For example, an image taken at 00180626.00 spacecraft clock time becomes image number 2600R on directory C001806.

5. What is all this I hear about an upcoming planetary alignment? What effect will this have on the Earth and how frequently do these things occur?

The planets Mercury, Venus, Earth, Mars, Jupiter, and Saturn will be more or less lined up on 5 May 2000. There will be no observable physical effect on the Earth from this alignment. Depending on how strictly you want to define "alignment", the inner six planets line up every few hundred years or so. For more information, see the planetary alignment page.