The Helix nebula (NGC 7293) is the closest planetary nebula with
a massive envelope of molecular gas (130 pc). It is far larger
in angular size than any other of this class, and offers the best
opportunity to explore the large and small scale structure of a
planetary nebula in great detail. In the open time program (COX_2),
we proposed a comprehensive study of the distribution of the
infrared emission in the Helix nebula by using ISOCAM, PHOT and
the LWS. The results have proven to be spectacular and provide
the first basis of a detailed study of the dust associated with
the small neutral globules in the nebula and even the previously
ejected, outer shell (Cox et al. in preparation). We propose to
follow-up these ISO results to investigate at the highest possible
angular resolution provided by ISO the small scale structure of
the infrared emission associated with the molecular envelope of
the Helix nebula. The field selected includes the two fields
recently observed with the Hubble Space Telescope. Furthermore,
we propose to investigate the spectral properties of the
mid-infrared emission in the neutral and the ionized regions of
the nebula using the ISOCAM CVF. This will enable to study the
excitation through the ionic lines and to probe the changes in
the dust populations across the ionized-neutral interface by
following the spectroscopic changes in the dust emission bands
and the behaviour of the continuum. Such observations are
fundamental for understanding the physical conditions in the
envelopes of planetary nebulae and will provide, at unmatched
linear resolution, detailed insights into basic processes which
are of general importance in the interstellar medium.