by Rex Parker, PhD director@princetonastronomers.org

2024 Mid-Summer Edition of Sidereal Times.  I hope you’re enjoying your summer and getting a chance to do some astronomy on the occasional clear night here in central Jersey. Our summer hiatus for meetings continues until Sept 10 when we’ll resume meetings at Peyton Hall, home of the Princeton University Astrophysics Dept.  

Despite plenty of clouds this summer, so far we have not had the problem of forest fire smoke in the northeast US atmosphere as we did last year.  So, I hope to see you for some deep sky observing at the AAAP Observatory at Washington Crossing State Park (see Observatory tab on the website for directions).  We have some incredibly fine telescopes and video astronomy equipment all tuned up for members use, and to bring the stars to the public on Friday nights.  For observatory opening status on those Friday nights (weather dependency) consult the AAAP website or social media sites on Friday afternoons.  Lately we’ve had quite a few members participating on Friday nights with many different telescopes on the field for comparison.  

The Cometary Origins of the Perseid Meteor Shower.  This year’s Perseids meteor shower might provide a good show if weather cooperates, as the first-quarter moon will set before midnight on the peak dates around Aug 10-12. Perseid counts usually increase late in the night before twilight, and as many as 50-70 per hour are predicted for darker sky locations this year. 

But what exactly are the Perseid meteors?

A few years ago, I returned from the NEAF conference in April with a small meteorite specimen.  Many of us have marveled at the US’s largest meteor (the Willamette meteorite, found near Portland OR) weighing in at 16 tons, on display at the Hayden Planetarium in NYC.  My 50 gram NEAF specimen is a fragment of the Canyon Diablo meteorite (Meteor Crater AZ), whose ~30 tons explosively dispersed over a ~10 km radius upon impact thousands of years ago.  Both of these examples are composed mostly of iron with ~5-10% nickel and 0.5% cobalt. The origin of the iron-nickel class of meteorites can be traced to the cores of large asteroids, similar in composition to earth’s core.  

In contrast, meteor showers like the Perseids originate from cometary orbital material which is quite unlike the iron nickel composition of the large meteorites.  Spectroscopic analysis suggests carbonaceous chondrites found on earth are similar to comets in composition, and some specimens may be from meteor showers.  Of course, only the rare larger meteors ever make it to the ground, and most are only a few grams in mass or even less.  Despite their low mass during entry into the atmosphere, their very high kinetic energies derive from velocities on the order of 100,000 mph, producing a brilliant transient incandescence if we are lucky enough to see them. Meteor showers result from earth crossing the orbital path of periodic comets, which over time have “leaked” small particles of dust and debris from the nucleus along the entire orbit.  The Perseid meteor shower of August is associated with the periodic comet 109P/Swift Tuttle.  This is a large comet with nucleus ~16 miles across, and a 133-year elliptical orbit which intersects our plane of the ecliptic at a sharp angle of ~113 degrees..  The comet crossed the earth’s orbital plane most recently most recently in 1992 (see Figure below, which I made using TheSkyX software).  According to information from the American Meteor Society, most of the Perseid particles have been part of the Perseid meteor cloud for at least a thousand years.

The Perseid meteors originate from the orbit of comet 109P/Swift Tuttle which crosses the plane of earth’s orbit closest to us on ~August 10-13.  Earth is moving to the right in this orbital diagram.  Image by RAP made using TheSkyX software.

Observing Challenge Update – T CrB Nova.  The challenge for AAAP members is to observe and record the light curve of the impending nova of the faint star T CrB (in the constellation Corona Borealis).  The star is now in pre-nova stage, and the outburst event could happen any day through September.  The star is recorded in some stellar databases and programs as  HIP 78322 (Hipparcos Catalog). This famous recurring nova has an 80-year cycle, and is one of only five recurring novae known in our galaxy.  Its magnitude, currently around ~10, will increase suddenly in brightness to magnitude ~2.   The goal is to observe the T CrB nova event in a telescope, and especially to record its light curve with an astro camera, being sure to get baseline data before the nova outburst.  Once it peaks, it should be visible to the unaided eye for several days and with binoculars for a week before dimming for another 80 years.