In the paper the potential sources of energy of cometary outbursts have been reviewed. Considerations focus on four probable sources of the outbursts' energy. These are the polymerization of hydrogen cyanide HCN, impacts with the meteoroids, destruction of cometary grains in the field of strong solar wind and the transformation of amorphous water ice into the crystalline one. The values of released energy and jumps of cometary brightness caused by these mechanisms have been discussed. A modern approach to the problem of the thermodynamical evolution of the comet nucleus which includes amorphous water ice is considered as the starting point in the discussion presented in the paper. The main characteristics of an outburst of a hypothetical comet belonging to the Jupiter family comet are calculated. The obtained results are in a good agreement with the characteristics observed during the real outbursts of comets. The main conclusion of this paper confirms a general presumption that the cometary outbursts can have different causes. However, the hypothesis concerning the amorphous water ice transformation appears to be the most probable one.

The comet 29P/Schwassmann‐Wachmann 1 is an exceptional comet as far as cometary outbursts are concerned. Despite its large distance from the Sun (about 6 au), it shows quasi‐regular outburst activity, usually once or twice a year. Up to now there has not been a generally accepted model that explains this phenomenon. In the first part of this paper, the most well‐known hypotheses that attempt to explain the outburst activity of this comet are presented and critically analysed. The main aim of this paper is to present a model for the outburst activity of this comet. The model is based on the global analysis of the internal structure and physical and chemical processes that take place in the cometary nucleus. Numerical calculations were carried out for reasonable assumed values of a large range of cometary characteristics. The obtained results are consistent with observational data.

Abstract. The possibility of impacts and their results in relation to the cometary outbursts between comets and other small bodies in the solar system has been investigated. Taking into consideration certain physical features of cometary nuclei and impacting bodies, the probability of impacts of small bodies moving in the main asteroid belt with hypothetical comets which represent three types: Jupiter family comets, Halley family comets and long period comets has been computed. The probability of impacts between comets and meteoroids at large heliocentric distances has also been estimated. Potential consequences of these events in relation to outbursts of the cometary brightness have been discussed.

In this paper, a source of cometary outbursts is examined. As a potential cause of some outbursts, impacts of a comet with meteoroids are considered. The probabilities and consequences of such events have been investigated in relation to the case of the Comet 29P/Schwassmann–Wachmann 1 outbursts. This paper focuses on two problems. The first one concerns the changes in brightness of the comet after the impact of a meteoroid into its nucleus. It is shown that impacts of comets with even metre‐sized solid impactors give an immediate considerable jump in their brightness consistent with that observed during real outbursts. The second problem is to estimate how real such events are. It is concluded that the probability of impacts with metre‐sized projectiles in relation to the Comet 29P/Schwassmann–Wachmann 1 is very low – practically negligible. The possibility of self‐bombardment of this comet by pieces of cometary material ejected earlier from its nucleus was also discussed. It is shown that the probability of such events is also hardly probable. That is why the main conclusion of this paper is that the impact mechanism could not trigger the outburst activity of the considered comet.

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In this Letter the mechanism of destruction of ice grains as the cause of cometary outbursts and variations of brightness is reviewed. Such destruction can arise from the combination of two causes: inclusions of chemical compounds that are much more volatile than water‐ice placed inside cometary water‐ice grains; and the influence of the solar wind on such heterogeneous grains. Numerical calculations have been carried out for a large range of values of probable physical characteristics of cometary material. This approach gives a simulated increase in the cometary brightness that is consistent with the observed one. It is concluded that this mechanism is a probable and real explanation of the sudden activity of comets at a wide range of heliocentric distances.

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