Once your data is in a usable format, you will want to store the
data in a place where astronomers from all over the world can access it.
Before data is made public, it may go through a "proprietary
phase." This is a period of time during which only the team of
scientists who requested the observation will have access to the data.
The proprietary phase typically lasts a year (though this varies by
satellite), after which the data is available to any interested
During both the proprietary and public phases, the data is stored in
one central location, enabling scientists from around the globe to
access the data at any time. Proprietary data is password-protected
until it becomes public. Goddard's High Energy Astrophysics Science
Archive Research Center (HEASARC) currently archives data from 20
satellites covering 30 years of observations.
Why create an archive?
The primary purpose for an archive is to make data easily available
to as many interested people as possible. An astronomical archive serves
much the same purpose as a family album or vacation scrapbook. During a
family vacation, you likely take many pictures and collect brochures and
postcards from the sites you visited. After the vacation you collect
these pictures, postcards and brochures in a scrapbook. The scrapbooks
make it easy for you to look back through the pictures and to share your
photos and vacation memories with friends and family.
In an astronomical archive, the vacation pictures, postcards, and
brochures are replaced by images, lightcurves,
spectra, and raw data of lots of different celestial objects. An
astronomer looking through the archive may be conducting an historical
study, performing theoretical follow-up, conducting a survey, or
assuring that the data has been looked at and is not duplicated. Let's
look at each of these uses for an archive in a bit more detail.
Perhaps the most obvious reasons to look through pictures taken
years ago are to remember good times spent with your friends, to see how
your family has changed over the years, and to see if you still have
that picture of your graduation party. In this sense, the
scrapbooks serve as a historic record of your life and the lives of
people around you.
If you were an astronomer, you'd use an archive to trace the history
of an astronomical object that you've recently observed. Such an
archive search would give you a picture of how the object has changed
over time, allowing you to determine if the source has displayed new
characteristics since the previous observations.
In your new observation, you might discover a new phenomena;
however, only a detailed search of archived observations of the same
area of the sky would reveal whether or not the phenomena was truly new.
You might also use your photo albums in a "theoretical follow-up"
study. This use of an archive may not be as obvious as historical
studies, but it is quite important for astronomy and other sciences.
Imagine that while you were thinking about your last family reunion, it
suddenly struck you that all of the men at the reunion from your
father's side of the family had curly brown hair. You might hypothesize
that all of the men from your father's family have brown curly hair. If
you didn't have a record of past family reunions, you would have to wait
until the next reunion to start to test your hypothesis. However, with a
family album, you could immediately look back through years of family
reunions to try to verify your hypothesis.
As an astronomer, if you develop a new theory, you can go to
archived data to test that theory against observations. Prior to such
easy access to historical data, a theorist had to either befriend
observers and wait for new observations to test their theories, or
eyeball data from a printed journal. Now, with the data covering years
of observations, theorists have ready access to many observations,
enabling them to test their theories quickly with more reliable data.
Suppose you wanted to collect photos of all of your cousins on your
mother's side to try to determine what characteristics they inherited
from your grandma or grandpa. Such a "survey" could easily be done by
searching through your family albums and making a big sample of cousin
As an astronomer, you might conduct such a survey of astronomical
objects. You would collect many observations made of a single type of
object, like supernova remnants, by a single instrument and telescope. A
survey like this is a lot easier to do if you have access to an archive
of all the observations from that telescope. Even before all of the
observations of supernova remnants became public, you could produce a
good sample of supernova remnants.
Imagine that you are planning a second visit to Mount Rushmore. You
know that you already have a slew of pictures from your first trip.
Before you go on your trip, you might look back through your scrapbook
to see what you have pictures of, and which pictures didn't turn out
very well. On your trip, you will certainly take more pictures of the
presidents' heads, but this time you might remember some of your
pictures from the last trip, and try something new. Maybe you will take
pictures at a different time of day to get different lighting effects,
or maybe you'll make sure that you are standing in a different place
when you take the pictures to exploit a different angle. Your search of
your earlier pictures will prevent you from exactly duplicating the
pictures you already have in your album. Such a use for your scrapbooks
is called "assurance".
An astronomical archive provides the community assurance that all
observations have been analyzed and that unjustified duplicated
observations are not made. Since observing time on satellites is
expensive, it is important that all observations are at least analyzed,
even if the results are not noteworthy enough to publish. If all the
data are at least looked at, this will prevent unwarranted duplicate
observations from being performed.
The HEASARC archive
The archive for high-energy astronomical observations is held at
Goddard's High Energy Astrophysics Science Archive Research Center
(HEASARC). The goal of the HEASARC archive has always been to provide
the most up-to-date data files from any particular mission. To achieve
this goal, the entire archive is available through FTP and the Web. In 2002, the
HEASARC archive held over 1200 gigabytes of data, but this represents
files that are compressed and would occupy about 3 times as much space if
they were expanded. The HEASARC archive grows 1 gigabyte of
compressed data every day. And every month, astronomers download
about 200 Gb of data from the HEASARC website - that's 200,000 floppy
disks worth of data!
Next: Tell me about Data Analysis