This paper contains analysis and selection procedures for target descent parameters of the probe for long-term contact interaction exploration of cold giant-planet atmospheres. Neptune is chosen as an example. Supposedly, such probe can be built with the use of a hot air balloon with the nuclear heat source. According to evaluation, a large descent module weighing 1500?kg of segmental-conical shape is needed. An important feature when solving this problem is the composition of the giant-planet atmosphere which consists mostly of hydrogen with high acoustic velocity and thermal conductivity. Multiversion aerodynamics equations integration was performed to find probe descent characteristics. Entry angle and velocity of entry into the atmosphere are determined according to the analysis of those characteristics. Computational modeling of gas flow over probe at entry into Neptune’s atmosphere with chosen trajectory was also performed. Results of the calculations were used to determine characteristics of the parachute system and to calculate the amount of heating of forebody shield at the time of descent. The obtained results may also be useful when designing research missions to Uranus which has atmospheric characteristics almost identical to the Neptune.