Meteorites, meteors and comets

Meeting report A Although George Alcock and Albert Jones discovered comets, amateur visual comet discovery is now a thing of the past, thanks to asteroid search programmes such as LINEAR and spacecraft such as SOHO. Amateurs can still make significant contributions through visual magnitude studies. Observations of 153P/Ikeya-Zhang found variation across the course of its apparition, possibly reflecting the comet nucleus losing several metres from its surface as it rounded the Sun. Comet 2001 A2 (LINEAR) varied with a period of around a month, perhaps reflecting precession of the nucleus. Comet 46P/Wirtanen had a relatively normal light curve, but by contrast 67P/Churyumov-Gerasimenko, the new Rosetta target, was linear, peaking days after perihelion. David Hughes suggested that this might be due to a single active area becoming illuminated. The talk concluded with questions. Should observations be restricted to areas free of light pollution? No, as this would eliminate most observations from the UK. Do visual observers hallucinate? Probably, because the brain lets us see what we expect to see. Should light curves be compiled only from observations by experts? No, all observations are valuable. round 80 astronomers gathered on 10 May 2003 in the Open University Berrill Lecture Theatre in Milton Keynes, for a meeting organized by Jonathan Shanklin (British Astronomical Association), with help from Margaret Penston (RAS) and local organization by Barrie Jones (Open University). Displays available for inspection during the day included comet drawings by George Alcock, samples of Libyan desert glass, cuttings from old journals and information on the Journal of the International Meteor Organisation. The morning was devoted to meteorites and meteors. Monica Grady (Natural History Museum) set the scene with the formation of the solar system in a region similar to the Orion Nebula. Fragments of asteroids, which fall to Earth as meteorites, are the remnants of this time. Iron meteorites tell us about core formation. Stony-iron meteorites come from the boundary of core and mantle and are the most beautiful, with intermixed peridotite and nickeliron. The majority of meteorites are stony and contain chondrules and calcium-aluminium inclusions. Interstellar grains are also present as silicon carbide – from at least 35 stars – and diamonds. Meteorites may also come from comets, the Moon and Mars. On average one meteorite falls over the UK every 11 years. Falls are not predictable and the next one could be over Milton Keynes! BAA Meteor Section Neil Bone (BAA) then introduced the work of the BAA Meteor Section, which he directs. The BAA is the largest UK organization collecting amateur observations and had long associations with Harold Ridley and George Alcock. Today visual observers concentrate on rate information. Showers are listed in the BAA Handbook and a simple formula converts the observed rate into a zenithal hourly rate. The Perseids showed an unexpected spike of activity in the early 1990s, which was still present as late as 1997; such surprises show the value of visual observations. Photography can be scientifically October 2003 Vol 44 useful, especially trail photography and video. In addition, spectra taken by amateurs could be analysed professionally. Results are published in the Section newsletter and the BAA Journal. Our next speaker was Iwan Williams (Queen Mary University of London), talking about the formation, evolution and observation of meteor streams. Since the 1860s researchers have known that a lot of meteor showers are associated with comets. With the appropriate calculations, the orbits of comet debris and hence meteor storms can be predicted. It is a 3-D problem; the plane of the orbit can change, and with it the position of the nodes (where debris is expected). Recent TV results show evidence for hyperbolic orbits implying an interstellar origin (if real). Radar studies give us problems: they find very small meteors and lots of them – perhaps they are fragments of asteroids, not comets? A third problem is that the high density of meteors compared to that expected from comets: either there is evolution of meteoroids or some of the theory is wrong. Video advantages Andrew Elliott (BAA) concluded the morning session with a talk on video-recording meteors – straightforward with the right equipment – which could observe more than 100 meteors per night in good showers. As well as creating a permanent record, video can yield scientific-quality results. He used image intensifiers, which can be expensive, but the associated equipment – video camera, time inserter and video recorder – are within amateur budgets. Single-station work can yield meteor rates, magnitude distribution, radiant position and clustering information. Two-station work allows triangulation for accurate orbits, particularly with photography. Examination of video tapes takes about three times as long as the tape lasts, but software developed by German amateur Sirko Molau helps. He concluded with spectacular results from recent expeditions. After lunch, Jonathan Shanklin (BAA) gave a brief history of the BAA Comet Section. No shortage of short-period comets David Hughes (Sheffield University) then considered magnitude parameters from amateur visual observations, with the encouraging demonstration that there must be plenty of short period comets in the Jupiter family with larger q (1.5–3 AU) waiting to be discovered. The size of a comet can be found either directly with the HST or via a light curve. Different comets with different molecular emissions may have different slopes. There is a lot of scatter, which implies varied surface activity. The average Jupiter family comet starts with a radius of around 3 km and slowly shrinks. Short period comets are disappearing in front of our eyes and after 400 orbits (2500 years) half will have gone. If we are in a steady state, Jupiter must be throwing objects into the inner solar system. Alan Fitzsimmons (Queens University Belfast) 5.31 Meteorites, meteors and comets There are many areas in which professional and amateur cooperation brings benefits to both. Speakers at the RAS-BAA Pro-am discussion meeting on 10 May explored observational work on some of the debris in the solar system. The meeting concluded with a lecture given in memory of the noted amateur astronomer George Alcock. Jonathan Shanklin reports. Meeting report Amateur CCDs The session concluded with Nick James (BAA) describing CCD imaging by amateurs. Although automated searches such as LINEAR find practically everything, amateurs can carry out rapid follow-up, observe structure near the nucleus, do photometry and monitor faint objects. When LINEAR reports an object they usually have no idea if it is cometary or not. Astrometry is worthwhile for amateurs and particularly important for objects on the NEOCP. Photography doesn’t have enough dynamic range to show structure in the co (...truncated)