  Although the risk posed to spacecraft due to meteoroid impacts is dominated
by sporadic meteoroids, meteor showers can raise this risk for short periods of
time. NASA's Meteoroid Environment Office issues meteor shower forecasts that
describe these periods of elevated risk, primarily for the purpose of helping
plan extravehicular activities. These forecasts are constructed using a list of
meteor shower parameters that has evolved over time to include newly discovered
showers and incorporate improved measurements of their characteristics.
However, at this point in time, more than a thousand meteor showers have been
reported by researchers, many of which are extremely minor, are unconfirmed, or
lack measurements of critical parameters. Thus, a comprehensive approach is no
longer feasible. In this report we present a quantitative criterion for a
potentially hazardous meteor shower and apply this criterion to the list of
established meteor showers in order to determine which showers should be
included in our annual forecasts.
