observed now. Oort selected 46 comets for his study that were best observed between 1850 and 1952. The distribution of the reciprocal of the semi-major axes showed a maximum frequency which suggested the existence of a reservoir of comets between away. This reservoir, located at the limits of the Sun's sphere of influence (astrodynamics), would be subject to stellar disturbances, likely to expel cloud comets outwards or impel them inwards. New model In the 1980s, astronomers realized that the main cloud could have an internal section that would start at about 3,000 AU from the Sun and continue up to the classic cloud at 20,000 AU. Most estimates place the population of the Hills cloud at about 20 trillion (about five to ten times that of the outer cloud), although the number could be ten times greater than that. The main model of an "inner cloud" was proposed in 1981 by the astronomer Jack G. Hills, from the Los Alamos Laboratory, who gave the region its name. He calculated that the passage of a star near the Solar System could have caused extinctions on Earth, triggering a "comet rain". His research suggested that the orbits of most cloud comets have a semi-major axis of 10,000 AU, much closer to the Sun than the proposed distance of the Oort cloud. Moreover, the influence of the surrounding stars and that of the galactic tide should have sent the Oort cloud comets either closer to the Sun or outside of the Solar System. To account for these issues, Hills proposed the presence of an inner cloud, which would have tens or hundreds of times as many comet nuclei as the outer halo. Thus, it would be a possible source of new comets to resupply the tenuous outer cloud. In the following years other astronomers searched for the Hills cloud and studied long-period comets. This was the case with Sidney van den Bergh and Mark E. Bailey, who each suggested the Hills cloud's structure in 1982 and 1983, respectively. In 1986, Bailey stated that the majority of comets in the Solar System were located not in the Oort cloud area, but closer and in an internal cloud, with an orbit with a semi-major axis of 5,000 AU. The research was further expanded upon by studies of Victor Clube and Bill Napier (1987), and by R. B. Stothers (1988). However, the Hills cloud gained major