Astronautical definition of meteor storm †*

We define ZHR* as the number of meteoroids passing through 1000 km2 zenith area per hour which can form craters no less than 1 cm in diameter on an aluminum surface, and use it as a quantitative measure of the degree of hazard posed by a meteor shower to a spacecraft. The relationship between ZHR* and the conventional ZHR is deduced. We evaluate the ZHR* of large meteor showers since the 1990s and find that the Giacobinids (Draconids) of 1998 posed a much greater hazard to spacecraft security than the Leonids of 1999, 2001 or 2002.