Louis A. Frank on Small Comets and Our Origins (4)

Figure 22. The recent findings of water ice or snow in the polar region of the Moon. Blue indicates a significant presence of this water.

For many years after the original publication of the small comet papers in 1986, a frequent objection to their presence in the vicinity of Earth was based upon reports during the Apollo missions that there was a remarkable absence of water or water snow on the surface of the Moon. The critics claimed that the frequent impacts of the small comets on the lunar surface should have left tell-tale signs of water. Of course, even though the Moon's gravity is relatively low, in the sense that it cannot prevent the high speed water outflow from the small comet impacts from escaping into space, there should be at least a little water ice or snow which is trapped in the surface crevasses and other shadowed areas of the lunar surface. The lunar surface is cold but any water ice or snow is quickly vaporized by direct sunlight. Last year a remarkable search for lunar water was reported by the Lunar Prospector spacecraft which is currently orbiting the Moon. A sensor on board this spacecraft was capable of remotely sensing the presence of hydrogen-bearing substances. The most likely substance with hydrogen is water. One of the first maps of this water on the lunar surface is shown in Figure 22. The view is the northern polar region of the Moon. The color blue indicates more water and red is lesser quantities. There are many tons of water in the polar region. This fascinating survey continues with the spacecraft orbit being adjusted to achieve lower altitudes and hence to obtain views with better resolution. The Moon is not dry.

Figure 23. The risks of challenging scientific dogma.

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 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. When the 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.

Figure 24. Seasonal variations during the months of November through January for atmospheric holes (Polar spacecraft, upper panel, and Dynamics Explorer-1, middle panel) and for radar meteors (lower panel).

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. 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 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. 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 an 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.

Figure 25. A diagram showing the inner disk and the Oort cloud of comets at great distances in the solar system, but still bound by the Sun's gravity.

The large numbers of the small comets in the vicinity of Earth provide stimulation of numerous questions on the horizon. Among these are:

The final frontier. In a desolate place not far from Tombstone, Arizona is a robot telescope from the University of Iowa which was constructed and is operated under the guidance of Robert Mutel, a professor at our university. The building which houses the telescope is shown in Figure 26. This building rolls to the left to allow the telescope to view the sky. This telescope, the Iowa Robotic Telescope (IRO), is seen in the photograph of Figure 27. It is a matter of great coincidence, and perhaps of irony, that this telescope can play a crucial role in the final confirmations of the small comets.


Figure 26. The building near Tombstone, Arizona which houses the Iowa Robotic Observatory.		Figure 27. The telescope with the protective roof of the building retracted so that the sky is viewed.

This will not be the first search of the sky for small comets with a ground-based telescope. About ten years ago Clayne Yeates, a scientist at the Jet Propulsion Laboratory in Pasadena, California designed a very clever way of detecting the small comets with the Spacewatch Telescope of the University of Arizona. His method relied upon passage of small comets by the Earth in an organized stream as inferred from the motions of atmospheric holes observed with Dynamics Explorer 1. Clayne, like so many other scientists in the 1980s, did not believe that the small comets existed. His technique to detect these small, dark, fast objects is shown in Figure 28. Telescopes are traditionally pointed so that they are staring at the stars. In order to see the small comets Clayne used the telescope in a "skeet shooting" manner. In other words, the telescope's pointing was moved in such a way as to keep the small comets in the sights of the telescope for a sufficiently long time that they would be recorded in the images.


Figure 28. Sketch of the way the telescope is moved in order to "skeet shoot" the small comets.		Figure 29. Expectations for the imprint of a small comet trailing across the sensor of the telescope.

To Clayne's surprise he in fact did find the small comets in his images and in numbers that were predicted from the observations of atmospheric holes. The small comets were clearly detected in the images. Astronomers insisted that the comets should be detected in two consecutive photographs. Clayne returned to the telescope and gained these pairs of images of each small comet. Astronomers returned with the ridiculous demand that they now needed three images. The small comets were there and the astronomers of the Spacewatch Telescope could only offer the now familiar "camera noise" as a defense. Because of his untimely death, Clayne was not able to continue his brilliant applications of ground-based telescopes in the pursuit of small comets.

Figure 30. The exciting detection of a small comet in a photograph taken with the Iowa Robotic Observatory.

The Iowa telescope is now being used to verify the existence of the small comets. The initial images already are being scrutinized. In order to prevent any further claims of "camera noise" the image of a small comet is taken with two exposures in the same picture. This simple method is shown in Figure 29. The small comet is moving in the telescope's view. A CCD sensor is placed at the focus of the telescope in order to record the image. These sensors are found in many of the present-day video cameras although the sensor in the Iowa telescope has some special design features. In order to distinguish a real comet signal from "camera noise" two exposures are taken with a mechanical shutter. One of the exposures is twice as long as the other and these exposures are separated by a shutter closed time equal to the short exposure time. The result for detection of a single small comet is two trails, one about twice as long as the other and separated by a gap with no trail. This is shown in the lower part of Figure 29. The ratios are not quite exact because of the small blurring due to the telescope's resolution. Also you will note that the trails are shown as dark, rather than bright trails. This inversion of dark and bright is done because the human eye can detect the events more easily in the images.

Figure 31. Scientific analysis of the event to show that the detection was not accidental.

An early candidate image of a small comet which was taken on October 19, 1998 is shown in Figure 30. The mottled appearance of the image is indeed due to camera noise. This picture is presented in order to provide the reader with an accurate accounting of the difficulties in detecting the presence of these small dark comets, and the reasons why these objects were not previously discovered by accident with ground-based telescopes. At the lower border of the photograph is the dark trail of a star. In the center image are the two trails of a single small comet. The analysis of the image proceeds by verifying that the trails conform to the very demanding restraints. The results of the analysis are shown in Figure 31. It is very exciting to find that the trails recorded by more than 200 individual photoelectric, or "picture elements," of the CCD conform to the expectations from special shutter operation. It would take billions of such pictures before a "noise event" of this type would occur. We are well on our way for the final confirmation of the existence of small comets. A large number of such detections is necessary to complete the confirmation.

Closing comments. I would like to note that it seems almost incredible that, two scientists, John Sigwarth and myself, from the University of Iowa have participated in the four major milestones in the discovery of small comets. Briefly these milestones are as follows:

1986	Initial detection of small comet impacts into the atmosphere with the Iowa camera on Dynamics Explorer 1
1989	Direct sightings of small comets with the Spacewatch Telescope of the University of Arizona
1997	Confirmation of atmospheric impacts and discovery of disrupting comets with the Iowa cameras on the Polar spacecraft
1999	Current search for small comets with the Iowa Robotic Observatory in Arizona