  We investigated three comets, which are active at large heliocentric
distances, using observations obtained at the 6-m BTA telescope (SAO RAS,
Russia) in the photometric mode of the focal reducer SCORPIO. The three comets,
29P/Schwassmann-Wachmann 1, C/2003 WT42 (LINEAR), and C/2002 VQ94 (LINEAR),
were observed after their perihelion passages at heliocentric distances between
5.5 and 7.08 AU. The dust production rates in terms of Afrho was measured for
these comets. Using the retrieved values, an average dust production rate was
derived under different model assumptions. A tentative calculation of the total
mass loss of the comet nucleus within a certain observation period was
executed. We calculated the corresponding thickness of the depleted uppermost
layer where high-volatile ices completely sublimated. The results obtained in
our study strongly support the idea that the observed activity of Comet SW1
requires a permanent demolition of the upper surface layers.
