Imaging data is available on the Internet for most NASA planetary missions that have been flown since the early 1970s. Missions usually publish their image data six months to a year after they have been acquired, although the proprietary period varies. Some recent missions (including the Cassini mission to Saturn and the Mars Exploration Rover mission) make image data available on-line shortly after receipt through their own mission websites.
Data from older missions are often more difficult for the general user to access and retrieve, since the earlier mission data archives were developed using older technology that predate the Internet and the widespread use of powerful home computers. Images from earlier planetary missions present other challenges as well, since many of them were taken using camera systems, such as vidicons, that were quite different from today's digital imaging devices.
Of these, the Imaging Node, based at JPL, has the largest collection, as well as a good search tool. The Planetary Rings node, hosted by the SETI Institute, has a smaller collection, but the best search tool. Small Bodies, hosted by the University of Maryland, provides only limited search capability, but, uniquely, provides zipped archives (so that you can download an entire data set at once, if you wish).
The National Space Science Data Center (NSSDC) is another NASA archival storage center for data obtained from NASA space missions, located at the Goddard Space Flight Center in Greenbelt, Maryland. The NSSDC serves as NASA's central data repository for all NASA missions. The NSSDC archives contain data products provided to them by the PDS and data products from missions not supported by the PDS. The NSSDC does not support discipline nodes analogous to the PDS structure. Planetary image data is available for purchase on CD-ROM; it is mostly not available online. However, the NSSDC provides detailed background information on missions and instruments (including non-NASA missions) useful to interpreting the data. To find this background information, follow each mission's link to "Experiment Information" and "Data Set Information."
The European Space Agency has their own online archive of planetary imaging data, the Planetary Science Archive (PSA), which is similar to the PDS. Most ESA mission data is also available through the PDS.
Image Formats and Metadata
The images sent back from space missions are data -- the brightness or darkness of each pixel in an image represents a precise measurement of the intensity of light that is reflected or emitted by some object of interest to planetary scientists. Therefore, it is critically important to researchers that the value of every pixel in a planetary image be unaffected by the way it is stored. It is also important that every image be associated with metadata -- literally, "data about data." Image data from planetary missions would be nearly useless unless they were accompanied by such information as when it was taken, by what spacecraft, with what instrument, pointed at what target, and so forth.
Image data from the earliest planetary missions that returned imaging data (e.g., Mariner 9 to Mars, Mariner 10 to Venus and Mercury, Viking Orbiter and Lander missions to Mars, and the Voyager mission to the outer planets) was processed at the Jet Propulsion Laboratory (JPL) in the 1970s and 1980s. At that time, almost all of the data formats now in routine use did not exist. All of the images from these early missions were processed using a suite of image processing programs known as VICAR (for “Video image access and retrieval”). The final image data products were sent to the data archives in VICAR format. VICAR format includes a file header (usually ASCII text format) containing the metadata and image data stored line by line in individual records as binary data.
In the 1970s and early 1980s, the primary storage medium used to archive planetary image data was magnetic tape. After several years, physical deterioration of the tapes was observed. In addition, the computers and operating systems originally used to store the data onto the tapes were becoming obsolete. In the mid-1980s, NASA funded JPL to recover as much as possible of the image data from the original tapes and archive the data onto contemporary media. The data recovery task produced CDs containing as much data as could be recovered at that time. The data format was not changed during the preservation project, which means that data from missions archived during the data preservation task are still in VICAR format. Internet access to the CD-based data sets is provided by the PDS for those missions. Public domain software exists to read the data and convert it into data formats more commonly in use by today’s commercially available image processing software (click here for a list of software useful for opening vicar formatted data).
During the 1980s, the Planetary Data System established standards governing the format of image data delivered to the PDS. Later missions conformed to that format standard, and the PDS provides online software enabling users to download data from the PDS archives and convert it into other useful data formats. PDS formatted images usually have a .IMG file extension and have detached ASCII-formatted text labels containing the image's metadata.
FITS stands for "Flexible Image Transport System" and is the standard format endorsed by both NASA and the IAU for use with astronomical data sets. It is also increasingly being used for space image data, particularly for small bodies missions like Rosetta and Deep Impact.
The word "raw" is currently used in two contexts to describe spacecraft images. Recently, as the Internet has become widely accessible to the general public and standard image formats have evolved, several NASA planetary missions, starting with Mars Pathfinder, have made mission data available via the Internet within days or even hours of the receipt of data from the spacecraft. These early versions of the images are often called "raw" images. They are stored in lossy formats (such as JPEG), have typically not been calibrated by their instrument teams, and are usually not accompanied by much metadata, so they are not useable for scientific research. They are also usually "stretched" to enhance their contrast, which makes it impossible to generate "true color" images from them. They are released on the Web only to permit the public to watch the current status of a mission through a spacecraft's eyes.
"Raw" is also used to describe image data that is represented as it was returned from the spacecraft, before any calibrating or processing has been performed to correct for idiosyncrasies of the instrument, geometry, lighting conditions, etc. Vidicon cameras especially produced images with noticeable geometric distortions and brightness variations, but even images taken with modern CCD cameras must be calibrated before they are scientifically useful. Each instrument's science team processes, corrects, and verifies its own image products. The PDS receives both the initial releases of raw image products from current missions and the final scientifically corrected versions. ESA's Planetary Science Archive operates similarly.
Archival data records of planetary image data are available at a variety of "levels." There are standards that define the various levels of data processing that can be performed on planetary image data. The standards were developed by a National Research Council Committee on Data Management and Computation (CODMAC) in the 1970s. The standards are shown in the following table.
CODMAC Data Product Level
Raw Data. Data set corrected for telemetry errors and decommutated. Data are tagged with time and location of acquisition.
Edited data. Unresampled data that are still in units produced by the instrument, but have been corrected so that values are expressed in, or are proportional to, some physical unit. (For example, images that have been flat-fielded to remove blemishes.)
Resampled data. Data that have been resampled or reprocessed in such a way that the original edited data cannot be reconstructed. (For example, images that have had geometric distortion removed.)
Derived data products containing geophysical variables at the same resolution and location as the Level 1 source data. (For example, color images or anaglyphs produced by overlaying images without any geometric projection.)
Variables mapped on uniform space-time grid scales, usually with some completeness and consistency. (For example, images that have been map-projected and/or mosaicked.)
4 and above
Model output or results from analyses of lower level data (For example, maps of mineral abundance derived from spectral data.)
Free Software for Viewing Space Image Data
There is public domain software available that can open space image data and convert them to more familiar image formats:
NASAView (Win / Mac / Linux / Solaris)
Download from: http://pds.nasa.gov/tools/nasa-view.shtml NASAView is produced by the PDS and can open and display PDS format images and associated header information through a simple graphical user interface. It can also open some VICAR format images. Once opened, individual images can be saved in GIF or JPEG format.
Download from http://www.mmedia.is/bjj/utils/img2png/ IMG2PNG is produced by Björn Jónsson. It is a command-line program that can convert an entire folder's worth of PDS, VICAR, or FITS formatted files to PNG format, which can be displayed by most Web browsers and standard image software. Most space image data sets are supported, and when new ones appear, Jónsson is usually quick to update the software to handle them.
Download from http://rsb.info.nih.gov/ij/ ImageJ is an open-source Java-based image processing package. With help from the PDS Reader plugin, it can open most PDS formatted raw images. ImageJ can save images in a variety of formats, including TIFF, GIF, JPEG, AVI, and BMP. It can also be used for basic processing tasks such as creating color images, anaglyphs, and animations.
FITS Liberator (Win / Mac)
Download from http://www.spacetelescope.org/projects/fits_liberator/ Originally a plug-in for Photoshop, FITS liberator is now a stand-alone, open-source application that enables individual image planes of 32-bit FITS data to be stretched in a variety of ways before conversion down to 16- or 8-bit formats.
GIMP (Win / Mac / Linux / Solaris)
Download from http://www.gimp.org/ An open-source image manipulation software package that mimics much of the functionality of Photoshop. Currently it can handle only 8-bit image data, which is an obstacle to the performance of high-quality space image data processing, but they are slowly moving toward the ability to handle higher-bit-depth images.
Many thanks to Bill Green for his help in developing this page.
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