to shift the locations of the maximums to higher solar longitudes. A discrepancy which is present for both the 1993 and Four major models of cometary meteoroid ejection are devel-1994 peak locations of 1-2 h between the observed and modeled oped and used to simulate plausible starting conditions for the flux profiles is most likely the result of emissions from 1862, formation of the Perseid stream. In addition to these physical which were observed to have a large component of their velocity variants, three different choices for initial meteoroid density out of the cometary orbital plane. (100, 800, and 4000 kg m Ψ3 ) are used to produce a total of 12
The cause of Perseid activity outbursts is found to be direct distinct initial models. The development and evolution of the planetary gravitational perturbations from Jupiter and Saturn stream are simulated for each model by ejecting 10 4 test meteorthat shift the nodes of stream meteoroids inward and allow oids at seven distinct mass categories over the full arc of 109P's them to collide with Earth. The last such perturbations was orbit inside 4 AU at each perihelion passage from 59 to 1862 due to Jupiter in 1991, and this effect combined with the return AD. All test meteoroids are followed to their descending nodes of 109P in 1992 produced the strong displays from 1991 to 1994. for times closest to the recent perihelion passage of 109P (1992).
On average, it is found that the Perseids observed each year In addition to these integrations, we have also performed longin the core portion of the stream left the parent comet (25 Ψ term integrations over the interval from 5000 to 10 5 years ago 10) Ψ 10 3 years ago. From the modeling, the total age of the using two plausible sets of starting orbits for 109P over this instream is estimated to be on the order of 10 5 years. From the terval.
simulations over the last 2000 years, the progression rate of We find that the choice of cone angle and precise cutoff the node of the stream is estimated at (2.2 Ψ 0.2) Ψ 10 Ψ4 distance for ejection make only minor modifications to the degrees/annum. overall structure of the stream as seen from Earth. The assumed The effect of terrestrial perturbations has been evaluated density for the meteoroids has a major influence on the present from the long-term integrations and found to play only a minor activity of the stream as radiation pressure moves nodal points role in the stream's development, producing a 5-10% increase further outside Earth's orbit and hence decreases the probability in the stream's nodal and radiant spread as compared to an of delivery for lower density meteoroids. The initial ejection identical simulation without the Earth. velocities strongly influence the final distributions observed The primary sinks for the stream are found to be hyperbolic from Earth for the first Θ5 revolutions after ejection, at which ejection due to Jupiter (and to a smaller degree Saturn) as point planetary perturbations and radiation effects become well as attainment of sungrazing states. Both the relative and more important to subsequent development. The minimum disabsolute contributions of these two loss mechanisms to the tance between the osculating orbit of 109P at the epoch of decay of the stream is found to be highly dependent on the ejection and the Earth's orbit is the principal determinant of assumed cometary starting orbits, with as much as 35% of subsequent delivery of meteoroids to the Earth.
initially released stream meteoroids removed by hyperbolic ejec-The best fit to the observed present flux location and peak tion after 10 5 years for the smallest Perseids on some starting strengths are found from models using Jones (1995) ejection orbits to less than 1% removed after the same time for larger velocity algorithm with an r Ψ0.5 dependence and densities bemeteoroids on other potential seed orbits. On average, it requires tween 0.1 and 0.8 g cm Ψ3 .
40-80 Ψ 10 3 years for a noticeable fraction of the initial population (ΟΎ0.1%) to be removed by these mechanisms, depending The recent activity outburst maxima observed for the Peron the chosen starting orbits.