Data Archiving: Where Does the Data Go?
What happens to the data for the Deep Impact mission? Stephanie McLaughlin is at work preparing data for release to the Planetary
Data System, a data archive where it will be available for use by others around the world. She explains:
"NASA's planetary missions are required to archive data into the Planetary Data System (PDS). PDS is a NASA-funded program that
archives and distributes planetary data, including ground-based data, to the science community. Data sets archived in the PDS are in the
public domain. The most popular PDS data sets are online and may be downloaded by anyone for free. Other data sets can be ordered
on CD or DVD for a nominal charge from the National Space Science Data Center."
What is involved in archiving?
Archiving data into the PDS involves describing each scientific data file so that a future researcher can easily work with it. The
description is an ASCII text file called a PDS data label. One label must be written for each data file in an archival data set. For
example, the PDS label for an image of comet 9P/Tempel 1 taken by the medium-resolution CCD camera (MRI) on the Deep Impact
flyby spacecraft will include information about the start and stop times of the exposure, the length of the exposure, what filter was
used, and much, much more. For an example of a PDS label for an MRI
image of comet Tempel is available
here. Of course, information to properly describe a data file can vary greatly across
instruments and missions, so PDS has a set of standards that must be followed when preparing an archival data set and writing the
data labels. The standards help maintain consistency across data sets archived in the PDS.
Archiving also involves writing and collecting documentation necessary for understanding a specific data set, such as all raw data
taken by the HRIV instrument during the encounter with comet 9P/Tempel 1. For example, PDS requires us to describe this data set in
an ASCII text file called a data set catalog file. Catalog files that describe the instrument and the spacecraft must also be written and
included with the data set. In addition to the required PDS catalog files the Deep Impact archive will include other supporting
documentation such as the data calibration report. This report is written by the mission's calibration team and provides a detailed
description of how a raw science image is processed into a calibrated science image. The science team uses the calibrated data for
analyses because these data have been corrected for instrument effects and are represented in physical units.
What are the challenges?
The biggest challenges have been the short duration of the mission and changes to the formats of the raw and calibrated data files
over the past few months. The mission was only seven months long, so we began designing the archive before launch to meet the
delivery date to PDS, scheduled for six months after the end of the mission. As the science team worked with raw and calibrated data
from flight, we realized the data files needed more information, so the files were reformatted which required revisions to the PDS
labels and the processes that automatically generate them. We expect all formats to be finalized by early November.
How are things going? Is the archive on schedule?
Deep Impact is still on schedule for delivering data to the PDS by the end of this year. We expect the data to be available online at
the PDS Small Bodies Node in January 2006.
Department of Astronomy, CSS 2337
University of Maryland
College Park, MD 20742-2421 USA