Contents of: VI/111/./abstract/JDAVIES_JKDTRAIL.abs

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SCIENTIFIC ABSTRACT:
ISOCAM images of cometary dust trails are proposed to determine trail structure
at various positions along the trails of two periodic comets. Primary targets 
are P/Kopff and P/Churyumov-Gerasimenko for Autumn 95 and P/Kopff and  P/Gunn
for a spring 96 launch.  P/Encke and P/Pons-Winnecke are considered as 
alternates depending on launch date. 
Observations from IRAS (Davies et al, Nature 309 p315 1984) discovered an 
extended infrared tail (also referred to as a debris trail) associated with 
comet P/Tempel-2. Sykes et al (Science 232 p115 1986) independently discovered 
the Tempel-2 tail and subsequently identified a number of similar features in 
the IRAS skyflux plots.These trails may represent the main source of mass loss 
from comets yet they are too faint for ground based infrared observation. 
ISOCAM images at various positions close to the nucleus will probe the fine 
structure of the trail and, by determining its width, place constraints on 
the ejection velocity of the particles and provide inputs to theoretical 
models of dust ejection. 
OBSERVATION SUMMARY:
We have modelled various trails at the time of the mission and find the trail 
of P/Kopff to be about 1.5*10**-5 Jy/arc sec**2 at 12um. In a few min/frame 
ISOCAM would get s/n=10 in the LW10 filter (8-15 microns) using 6 arc sec pfov. 
Since the comet moves these must be done as fixed time observations.
The trails usually appear no more than 4 arc minutes wide 
so three frames at right angles to a trail taken with the 6 arcsec PFOV should 
be enough to cover one trail at one position around the comet's orbit and to 
get far enough away to reach the background. To obtain best sensitivity to 
these extended sources we will use the microscanning option in CAM AOT1 to 
scan across a trail at each position using the offsets to extend the 
coverage approx 2 ISO fields along the trail. We require similar scans through 
the trail at positions 0.25 degrees in front of the nucleus, close to the 
nucleus itself and 1 and 2 degrees behind. Observations are requested when 
the comet elongation is greater than 90 degrees in order to reduce the 
effects of the zodiacal background. JKD can supply positions on request.
These observations should allow sampling of the large (leading) particles, the
fresh trailing particles and a section of trail containing older particles.
These images will also show if there are regions of enhanced density within the
trails which would be prefered targets for spectrophotometric observations. If
the trail is found to be very clumpy it would be desirable to make
spectrophotometric observations of clumps and diffuse regions for comparison.
(For example a clump might be the result of a large particle falling to pieces
some time after ejection from the nucleus). Spectrophotometric observations with
ISOPHOT should take only a few seconds per filter, so these follow up studies
would be short compared to the CAM time required. The purpose of these
filter/spectophotometic studies is to define the spectrum of the trail particles
and compare the result to a black body since IRAS data indicates that the grains
are warmer than expected for black bodies at the appropriate solar distance
(Sykes et al ICARUS 86 236 1990). This may be due to the presence of very small
grains, as found in cometary comae, or to large grains able to support thermal
gradients and which behave, thermally at least, like small asteroids. 
Since the trail particles survive long after ejection from the nucleus they
must represent refractory grains from which all the ices have evaporated. The
grains are believed to be dark, consistent with the observations of the nucleus
of Halley, but modelling using the available IRAS data cannot be very precise.
ISO photometry will enable more detailed modeling to be done than is possible
at present. This will lead to a more detailed understanding of the relationship
between comets and the zodiacal dust complex, to  better understanding of
cometary dust production and cometary evolution. 
FIXED TIME OBSERVATIONS: (if any, otherwise drop this line)
Since the comets move these must be done as fixed time observations with
precise details to be agreed between the PI and the SOT near to the time of 
the observation.