Free Astronomy Magazine May-June 2020

43 ation pressure of a supernova is only a working hypothesis or little more. A second doubt concerns the preservation of sub-relativistic speed by the grains with diameters on the order of 1 mm or larger. These grains would be mainly produced in the central regions of the galaxy, where the density of the interstellar medium exceeds by thousands of times the average density of one atom per cm 3 adopted to establish the potential frequency of sub-relativistic meteors. The doubt arises because a body of a certain mass, moving at very high speed in a medium, slows down significantly by coming into contact with a total mass of matter comparable to its own. Conse- quently, since only bodies of very small mass can be accelerated at relativistic or sub-rel- ativistic speeds, and since these bodies de- celerate rather quickly through a medium with relatively high density, the prediction of observing an average sub-relativistic me- teor per month appears quite optimistic. Another point of Siraj and Loeb’s hypothesis that does not seem convincing is the appar- ent brightness of the sub-relativistic mete- ors. Their calculations show that if the progenitor body had a diameter of 1 mm, it could generate about a billion photons of visible light in 0.1 milliseconds, a much higher flow than that coming from the brightest stars in the sky. In the case of a meteoroid with a diameter of 10 mm, the brightness of the meteor produced could exceed 10 million times that of Vega. Finally, a body placed at the maximum limit of the size scale considered by the researchers, 100 mm, would produce a Sun-like flash. To our knowledge, no one has ever re- ported observations of this kind – and one would imagine a record of such an event existing, considering that even the weakest flashes of light would still be much brighter than Venus. And what about the atmospheric traces left by the passage of hyper-fast meteors? The authors of the study don’t address this topic, but we can imagine that, for at least a few seconds or perhaps for a longer time, the section of the atmosphere af- fected by the phenomenon would be no- ticeably altered. !