Free Astronomy Magazine May-June 2020

MAY-JUNE 2020 SMALL BODIES This bizarre possibility was recently discussed in a paper published in The Astrophysical Journal by Amir Siraj and Abraham Loeb (Department of As- tronomy, Harvard Uni- versity, Cambridge, MA). The two researchers de- veloped a hydrodynamic and radiative model to determine whether it is possible to detect sub- relativistic meteors gen- erated by solid particles 1 mm in size or larger. The assumptions in Siraj and Loeb’s study are reasonable based on the results of previous work. Back in the middle of the last century, Lyman Spitzer theoretically demonstrated that the radia- tion pressure of a supernova could acceler- ate dust grains to relativistic speeds. In the early 1970s, Satio Hayakawa had suggested that particles of such dust could be respon- sible for ultrahigh-energy cosmic rays. In the decades that have followed, other re- searchers further developed these topics, studying the processes that influence the origin and survival of relativistic dust, all the way to showing that, in our galaxy, there may exist dust grains that move at nearly the speed of light. We also have the cer- tainty that, in the past, the Earth has been targeted by matter fired by the explosion of relatively close supernovae. Evidence of this is found in the enrichment of iron-60, dis- covered at the end of the last century in rocks within the depths of the Pacific Ocean. This radioactive iron isotope is formed in na- ture by the explosion of supernovae, has a half-life of 2.6 million years, and decays into cobalt-60 (which in turn decays much more rapidly into nickel-60). The abundance of iron-60 found in the first 2 cm of the ocean floor, corresponding to the last 13.4 million years, suggests that a supernova could have exploded less than 100 light-years away from Earth in the last 5 million years. It is estimated that, in the geological period cov- ered by that thin layer of crust, twenty su- pernovae may have exploded within 1000 O n the right, many Geminids photographed above the Teide Ob- servatory, Tenerife, Canary Islands. [Flickr/StarryEarth, 2013] Below, a shower of Gemi- nids, the sum of five hours of many ex- posures. In the fore- ground, the two principal domes of the Las Campanas Observatory. [Yuri Beletsky, Carnegie, TWAN, 2015]