(meteorobs) early Persieds again

[wisetdf at Safe-mail.net]

I've been trying to have a read up on nodal pro/regression with respect to Perseid orbits.

One thing I found online was this chapter :-

http://meteor.uwo.ca/~pbrown/chap4.pdf

now, that's based on numerical integration from _assumed_ "reasonable" seed orbits, but I pumped some of the derived ancient orbital elements from Table 4.3 into the orbit plotter and they gave very similar results to the radiants clustered near xi Cas around the 15th of July and the smaller bunch near theta Cas around 24th to 25th.

Now, I'm assuming this graphic from the IMO 2010 meteor shower webpages :-

http://www.imo.net/galleries/calendar/2010/per08.png

is representing the expected radiant drift of the modern shower orbit(s)?

That is, there shouldn't particularly be any particular burst at those times from those positions if it's just the edge of modern orbits being sampled?  Just a smattering increase on the normal sporadic background, albeit with radiants in the right bit of sky.

Whatever, I can't find all that much on deep time Perseids, although there is a fair bit around on the past few hundred years up to 2000, which is understandable, as that spans the historical record, ie observed data and not just assumptions can be used to test.

The above chapter led me to this reference.

http://adsabs.harvard.edu/abs/1998Icar..133...36B

It seems that based on the 2000 year historical record that abstract gives a rate of nodal progression.

Now, based solely on the assumption that that nodal progression is the only factor involved (which is probably unlikely given comets do have nongravitational motion from outbursts from time to time) and that Brown and Jones feel this comet's orbit is somewhat stable, at least within the context of the inevitable pertubations from Jupiter etc (which cause the progression in the first place), that rate they quote would give the theta Cas smattering's orbit an age of very roughly about 80,000 years and the xi Cas mini-outburst roughly about 120,000 years.

Can't be as simple as that, surely?

I am noticing looking at new two station allsky stuff something of a suggestion though.  The newer stuff, like SonotaCo, is also all _time_, as much as possible that is.  Earlier projects tended to concentrate around the times of major showers, for simple logistical reasons.  For a start, they weren't as automated, or necessarily fixed site.

As the alltime allsky stuff seems to be coming along there looks to hint of a sparse 'background' consisting of a collection of fairly high inclination, and often retrograde, showers.  Some of this might just be a selection effect, as for a start they're consequently very fast in terms of velocity, and thus readily separated from any possible anthelion 'stream'.  I'm not entirely sure whether this also gives a selection effect of a larger distribution of meteors at the bright end too, as I only half know what I'm on about at the moment, and still have a lot of reading ahead of me.  For all I know fast streams have no necessary reason to have a brighter population index, as other factors must come into play, like meteor size and mass distribution for a start, but I'm suspiscious of the fact that a lot of the new discoveries in the IAU MDC working lists are streams with high velocity around the 60 km/s mark.

Of course, a selection effect has no need to mean artificial.  It may just be that highly inclined retrograde orbits last longer due to being less prone to pertubation, and thus that causes a positive bias towards their detection simply because that would lead to them being more common!  Brown and Jones cover this for the stability of the Perseid stream in the above papers.

Dunno.  Basically, the advent of not only allksy, but also alltime, showers, is promising stuff.

It also seems to dig out showers possibly lost in the past amongst a near contemporaneous ajdacent radiant shower.  How many past gamma Ursae Minorids have been though just very late Ursids I wonder?  And there's an interesting thing going on with those two too, the orbits of the GUMs are not at all like 8P/Tuttle, the parent comet of the Ursids, but they're not at all dislike it either, with geocentric velocity being very similar, time of year and radiant only displaced somewhat.  Weird stuff, I'll have to try and get my brain around it all somehow to see what is natural happenstance and what is hint of connection.

Cheers

John