The passage of cometary nuclei near the surface of the Sun is considered analytically. It is found that within the solar chromosphere the ram pressure at the front of the nucleus significantly exceeds the material strength of cometary nuclei. This results in an aerodynamic fragmentation of the nucleus. At the same time the intensity of aerodynamic heating of the nucleus is more than the maximal intensity of the solar thermal radiation a thousand times. In such situation the rate of evaporation of the cometary nucleus will be significantly higher than the value traditionally accepted and used for interpretation of coronographic obsrervations of sungrazing comets. Observations of comets with extreme small perihelion distances in a wide range of spectrum, including soft X-ray range, are very reasonable for diagnostics of cometary nuclei and circumsolar region.