Meteor v elocities: a new l ook at an o ld p roblem

Meteoroids that orbit the Sun encounter the Earth with speeds between 11 and 74 km/sec. However, the distribution of the velocities of meteoroids between these limits is not weh known. The uncertainty is cansed by the difficulty in measuring the true flux of meteors at the extrema of the velocity distribution. Whilst the most comprehensive measurements of meteor flux are those obtained using radio techniques, meteors with speeds > 50 km/sec occur at heights where the effects of initial radius of the trail and diffusion significantly reduce the radio reflection from the trails; on the other hand the high dependence of the collisional ionization probability on velocity (to the power ~3.5) significantly inhibits the detection of meteors with speeds 74 km/sec) meteors. Both these radars exploit the phase information avaitable prior to the closest-approach (to) point.