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Comet Archives
I.INTRODUCTION
The International Halley Watch (IHW) grew out of a National Aeronau-
tics and Space Administration (NASA) sponsored study in 1979-80 led by Louis
Friedman of the Jet Propulsion Laboratory (JPL). The premise behind that study
was that some form of cooperation among the astronomers and space scientists
of the world would be necessary to make the most of the once per lifetime
tunity to study Comet Halley.
The original study at JPL suggested that the most important product
of the IHW should be an archive of all the data. The archive was not to
replace a normal interpretation and publication in technical journals. Rather
it was to complement the publications by establishing a comprehensive database
suitable for further studies requiring reduced but uninterpreted data from
observers and disciplines. The complete archive would be given to each
observer contributing to it and made available to institutes worldwide. Cost
of this publication would be borne by NASA.
II. DIGITAL DATA
A. BACKGROUND of the Comet Halley Armada Dataset
This dataset contains wide-field images of comet Halley obtained by
the Large-Scale Phenomena Network (LSPN) of the International Halley Watch
(IHW) during the interval of the Halley Armada spacecraft encounters: 1986
March 06-14. The images were archived, digitized, prepared for deposit on
CD-ROM by the Large-Scale Phenomena (L-SP) Discipline Specialist Team at the
NASA-Goddard Space Flight Center, Greenbelt, MD. This dataset contains 70
selected images of Halley from Armada Week, and 10 associated images of
calibration objects; the data have been subsampled to allow more data storage
on the disc. The image selection criteria were, in the main, quality of
individual images and contribution to the overall temporal coverage of Armada
Week. A secondary criterion was the desire to maximize the number of
represented observatories.
The comet images consist of a bright coma region in which is embedded a
small object, called the nucleus, that has a consistency of a dirty snowball.
The most dramatic feature of the comet is the extensive tail that is made up
of glowing ions or reflecting dust particles. In the former case, the ion
tail strecthes many miles from the coma and away from the sun. It has been
determined that the shape and structure of this tail is related to the
passage of a stream of charged particles emmanating from the sun as a solar
wind. Often times, the comet will appear to have disturbances that migrate
down the tail at accelerating speeds. Sometimes, the tail appears to
break away or disconnect from the main body of the comet due to this solar
wind interaction. As the comet sweeps around its orbit the ion tail acts as
a weather vane to indicate the state of interplanetary space.
B. DATA PRODUCTS for Comet Halley
Due to the modest data transfer rate inherent in the CD-ROM technology,
the process of reading an image is quite time-consuming, even though data
compression has been applied; for this reason, "browse images" are supplied.
These images are restricted to a maximum of 256 pixels on a side and preserve
the original sampling geometry. All browse images have been rescaled from
their original pixel values to 8-bits; for the vast majority of the images the
original values were 10-bits or 2-bytes. For a few images that were already
digitized by the observers and were not 2-bytes per pixel, the data were
rescaled from 4-bytes to 2-bytes before being compressed or subsampled.
The convention for naming files on the CD-ROM was adopted by the IHW
Lead Center to include a running number as a unique data qualifier for
chronological sorting. The name for the net (LSPN in this case) has been
abbreviated and concatenated with the number to yield the appropriate
filename (e.g., LSPNxxxx). For all calibration data, the filename is
LSPN4xxx to distinguish from the 3383 images archived for the Comet. The
full use of these calibrations is detailed in the FITS header file.
The file extensions follow suggestions by the Planetary Data System for
image (.IBG) data with detached label (.LBL) as illustrated in SPIDS v1.1.
.IBG - subsampled (browse) image
.LBL - detached (PDS) label
.HDR - FITS header with CR and LF
Note that in the ISO formatting process a version number (1 by default)
is attached to the filename. In MS-DOS this information is ignored but
for complex systems such as VMS, it is a necessary piece of information.
C. BACKGROUND to the Comet Giacobini-Zinner Dataset
The 1985 apparition of comet Giacobini-Zinner (GZ) became historical with
the success of the U.S. International Cometary Explorer (ICE), the space-
craft mission to a comet. On 11 September 1985, six days after perihelion,
ICE spacecraft passed through the coma, approaching to within 7800 km of the
comet's nucleus. Spectroscopic observations obtained while the ICE spacecraft
passed through the coma are included in the IHW Radio (RSN) and Spectroscopy
Network (SSN) archive together with good coverage of spectra obtained both
before and after perihelion.
It is important for cometary studies to determine the composition and
physical conditions of the object. A proven technique is the recording
of the emission spectrum as compared to a laboratory light source or standard
star absorption spectrum. Shifts recorded in the spectra can be caused by
motions as well as physical changes in temperature and pressure. Since
comets are unique solar system objects it is difficult to inter-compare,
but in general they evidence a typical "Jovian" composition such as water,
OH radical, methane, and ammonia. The radiation from the comet can be
recorded from the ultraviolet to radio part of the spectrum.
.
The detectors used to record the spectra in the archive include a
microchannel plate with output recorded on photographic film, an SEC Vidicon
International Ultraviolet Explorer (IUE) data, a photoelectric scanner,
an image dissector, and several different charge-coupled devices (CCDs).
The spectrographs were at foci of telescopes which ranged in size from 2 to
4 meters, situated in both the northern and southern hemispheres. Ten percent
of the archive data were obtained with CCDs and are in two-dimensional format.
In the case of the radio meassurements, a spectrometer sensitive to the
polarization of radio waves was used to record separate spectra.
C. DATA PRODUCTS for Comet GZ
The convention for naming files on the CD-ROM was proposed by the
IHW Lead Center to include the only unique data qualifier (the FILE-NUM
keyword value from the original FITS headers) for the submitted data. A
specific set of rules was established to identify the comet (G for
Giacobini-Zinner), discipline (given by numeric code), and subnet within
each discipline (given by a letter code) so that GNXXXXXL where XXXXX
is the FILE-NUM. A short list of this convention by discipline and
subnet (or experiment) is given below for the P/Giacobini-Zinner files.
Comet Discipline Number(N) Subnet (Exp) Code(L)
G Radio 6 OH Spec Line O
Spectroscopy 7 none X
The file extensions follow suggestions by the Planetary Data System
(SPIDS v1.1) for image data, including the .DAT extension for spectra; in
addition, following those proposed guidelines for dealing with the
SPECTRUM data structure, we consider the spectra as binary tabular data
(COLUMN_NAME, COLUMN_TYPE, COLUMN_BITS) which may be ordered. The
independent variable (WAVELENGTH) is described by the keywords
START_WAVELENGTH, WAVELENGTH_INTERVAL, and WAVELENGTH_UNIT. (A special
case for the Radio Studies data uses Doppler VELOCITY.)
C.REFERENCE
Martin, T., Martin, M., Davis, R.L., Mehlman, R., Braun, M., and Johnson,
M.: October 3, 1988, Standards for the Preparation and Interchange of Data
Sets, Version 1.1, JPL D-4683.
Wells, D.C., Greisen, E.W., and Harten, R.H. (1981). Astron. Astrophys.
Suppl. Ser. 44, 363.
III, Scientist: Edwin J. Grayzeck; NASA National Space Science Data Center,
Goddard Space Flight Center (under contract with U Maryland), Greenbelt, MD.