'Hoods' are the semi-circular features and 'spikes' are the radial ones. Hale-Bopp currently has about 6 active vents producing prominent radial plumes near the head. For more on these, see Jerry Lodrigus's photographs which show them very clearly. The Hoods are more complex features which change on timescales of hours to days and are presumably produced by an interaction of the gas near the nucleus of the comet and solar radiation.
The above image is a composite of two Hubble Space Telescope, STIS CCD exposures taken at 04:05 UT (2 sec integration) and 04:06 UT (20 sec integration) on 27 Aug 1997. The heliocentric and geocentric distances of the comet were 2.476 AU and 2.989 AU, respectively.
The direction to the sun projected onto this image is at 53.6 deg clockwise from the +x-axis (where the +x-axis points to the right and the +y-axis points straight up), but the solar phase angle (the sun-comet-earth angle) is 18 degrees, which means that the direction to the sun is only 18 degrees from being perpendicular to the plane of this image.
A "long-pass" filter was used that transmits all light longward of approximately 5500 angstroms (10 angstroms = 1 nanometer) and rejects light shortward of that wavelength.
The image is 28 arcsec on a side, which subtends 60,680 km at the distance of the comet. Individual STIS CCD pixels are 0.0508 arcsec across, which projects to 110 km at the comet.
More details about this can be found at Hal Weaver's research page
Copyright 1997 Dr. Sten Odenwald
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