The remaining critics of the small comet theory eagerly awaited the results of the survey of the seasonal variations of the frequency of small comet impacts into our atmosphere with the Polar spacecraft. Years before, the atmospheric hole rates as determined by the camera on board Dynamics Explorer 1 had been shown to be correlated with radar meteor rates observed with a ground-based station in Canada. The relevant Dynamics Explorer-1 observations were taken during November 1981 through January 1982 and those with the radar during November 1955 through January 1956, some 26 years earlier. The remarkable correlation of the seasonal variations as observed with these entirely different instruments was dismissed as happenstance by the critics. Their responses to these measurements are characterized by the donated photograph in Figure 23 [left]. When the Polar seasonal results were reported, and they most certainly were expected to disagree with the previous findings, then we would have shot ourselves in the foot and the small comets would be past history.
The Polar results were recently reported. Much to the dismay of the critics the seasonal variations agreed with the previous measurements with Dynamics Explorer 1 and the Canadian ground-based radar station. The observations from all three of these instruments are shown in Figure 24 [right]. The agreement is truly remarkable and there is no chance it is due to any error. From top to bottom are shown the rates for Polar, Dynamics Explorer, and the Canadian radar. The features of these rates are a maximum intensity during early November, a brief plateau during mid-November, more or less constant rates during late November through mid-December, another plateau of lesser intensities during mid-December through early January, and a well-defined minimum in mid-January with subsequent recovery. There is a further important feature of the radar observations to be seen in Figure 24. That is the large increases of rates associated with the atmospheric impacts of stony and iron meteors during such well-known showers as the Leonids and the Geminids. As expected because these are very small objects relative to the dimensions of a cometary water cloud there is no sign of these meteor showers in the Polar or Dynamics Explorer-1 records. The Canadian radar records both the meteor showers due to stony objects and the background events which are due to the infalling small comets.
The question arises as to where is the location of the source of this small comet population which is seen in the vicinity of Earth. We know from the orbits of the small comets as determined from the speeds of the atmospheric holes and from ground-based telescope observations that these objects are moving more or less in the ecliptic plane. That is, their motions are in the same general planes as those of the planets. Their speeds at Earth are such that their orbits will carry them to Jupiter's orbit and beyond. Their origins most likely lie beyond the planets, in a disk of comets which was formed during the birth of our solar system. A diagram of this disk, sometimes called the Oort inner cloud after the famous astronomer Jan Oort, is depicted in Figure 25 [left]. This sketch is not to scale for, if it were, the planets would be drawn so close to the Sun as to be not discernible. This disk extends for tens of thousands of Astronomical Units (AU), with 1 AU being the distance of our planet from the Sun. The gravity of passing stars or a, as yet undetected, dark planet can disturb comets in the disk and scatter them into the inner solar system of planets. Evidence of such a disk of comets, presumably both large and small, comes from the existence of a large spherical cloud of comets, the Oort cloud, which is also indicated in Figure 25. Their original location is thought to be in the inner cometary disk until their orbits were changed by passing large bodies such as stars.
The large numbers of the small comets in the vicinity of Earth provide stimulation of numerous questions on the horizon. Among these are: