An alternative theory, first suggested in 1882 by Ignatius L. Donnelly in "", is that the fire was caused by a meteor shower. This was described as a "fringe theory" concerning Biela's Comet. At a 2004 conference of the Aerospace Corporation and the American Institute of Aeronautics and Astronautics, engineer and physicist Robert Wood suggested that the fire began when a fragment of Biela's Comet impacted the Midwest. Biela's Comet had broken apart in 1845 and had not been observed since. Wood argued that four large fires took place, all on the same day, all on the shores of Lake Michigan (see Related Events), suggesting a common root cause. Eyewitnesses reported sighting spontaneous ignitions, lack of smoke, "balls of fire" falling from the sky, and blue flames. According to Wood, these accounts suggest that the fires were caused by the methane that is commonly found in comets. Meteorites are not known to start or spread fires and are cool to the touch after reaching the ground, so this theory has not found favor in the scientific community. Methane-air mixtures become flammable only when the methane concentration exceeds 5%, at which point the mixtures also become explosive, a situation unlikely to occur from meteorites. Methane gas is lighter than air and thus does not accumulate near the ground; any localized pockets of methane in the open air rapidly dissipate. Moreover, if a fragment of an icy comet were to strike the Earth, the most likely outcome, due to the low tensile strength of such bodies, would be for it to disintegrate in the upper atmosphere, leading to an air burst explosion analogous to that of the Tunguska event.