THE TRUE EXTENT OF THE NODAL DISTRIBUTION OF THE PERSEID METEOROID STREAM

The observed semimajor axis distribution of the Perseid stream requires meteoroids to be ejected from the nucleus of P/Swift-Tuttle with velocities that have a Maxwellian distribution, the most probable velocity being about 0.6 km s(-1). The mass loss is reasonably symmetrical with respect to the time of perihelion passage. The cometary nucleus spins in a prograde fashion, maximum dust emission occurring at a lag angle of about - 5 degrees +/- 10 degrees. It has been found that the Earth only samples the 'inner edge' of the Perseid stream and that the core of the descending nodal distribution of the actual Perseids extends out to heliocentric distances of at least 1.2 au. In order to reach this conclusion the orbit of the Perseid parent comet, P/Swift-Tuttle, has been integrated backwards in time over a period of 270 000 years. Only over the last 160 000 years has the decay of this comet led to the production of any Perseid meteoroids that can be observed from Earth. Previously the mass of the Perseid stream was estimated to be about 3 x 10(17) g. This must now be taken to be a lower limit. In order to produce this mass, however, the mass lost per perihelion passage by P/Swift-Tuttle must have been much greater in the past than it has been over the last 20 or so orbits.