SCIENTIFIC ABSTRACT

This proposal makes use of the opportunity given by ISO to study the
properties fo the different dust populations in interplanetary space
with the same instrument and identical observational procedures. For
technical reasons, the properties of dust in comets and of asteroid
surfacess are studied in separate central programme proposals, while
this central programme proposal deals with 

   - dust in the general interplanetary medium, 
   - dust in the asteroidal bands, 
   - dust in cometary trails. 

This procedure allows a thorough comparison of interplanetary dust with
the properties of dust released from its probable sources, comets and
asteroids, and thus contributes to the knowledge of the origin of
interplanetary dust. 

Distribution of the observing times:
 
 
      Observations        priority 1   priority 2   priority 3   sum
       ------------------------------------------------------------------
       general  medium      6478 s       15254 s      6478 s     28210 s
       asteroidal band         0 s       54369 s         0 s     54369 s
       cometary trail      24988 s           0 s      1859 s     26847 s
       ------------------------------------------------------------------
       total               31466 s       69623 s      8337 s    109426 s
 
                           (8.7 h)       (19.3 h)    (2.3 h)    (30.4 h)
 
                            29 %           64 %        8 %
 
 

OBSERVATION SUMMARY

  The properties of dust in the interplanetary medium, in asteroidal bands
and in comet trails will be studied with ISOPHOT by the same type of
measurements: photometry with ISOPHOT-P and ISOPHOT-C, either with full or
with reduced spectral coverage, and spectrophotometry with ISOPHOT-S.
In addition, scans perpendicular to the chosen asteroidal bands and comet
trail will be performed to detect structure and to determine the
position of maximum brightness for subsequent full photometry and
spectrophotometry. For all measurements the largest aperture (180 arcsec) 
will be used.

  We aim at relative photometric accuracies of at least a few percent or 
better, which is needed to separate bands or trails from the much brighter
background of zodiacal light. In spectrophotometry this accuracy will not 
be achieved for a single pixel. Here we take the longest available integration
time and will improve the signal-to-noise ratio by binning.
   The proposal consists of three parts:
       1. Dust in the general interplanetary medium
       2. Asteroidal bands
       3. Cometary trails
Object lists and breakdown of observations by AOT's will be given in the
three separate parts of this proposal. The common observing plan for photometry,
spectrophotometry or scans will be given here. The choice of filters
reflects the coordination with the proposals on "comets", "zodiacal
light" and "extragalactic background light".

For a detailed observing plan, special requirements and target positions,
see section scientific justification.