RAS Discussion Meeting on “Comet-Asteroid Connections” The media have an interest in bombardment from space at the moment, but having watched “Asteroid” on TV recently it is fairly clear that they don't have much idea about what an asteroid is, or the links between asteroids and comets. This meeting was organised by Mark Bailey of Armagh Observatory and Iwan Williams of Queen Mary College, London and was held at Burlington House on March 14. I’m not sure that the media would have been much wiser after the meeting, and although one or two new ideas were presented, most talks were repeats of presentations at the ACM meeting last summer. David Hughes (Sheffield) kicked off proceedings with a review of the ‘zoo’ of small solar system bodies in a talk entitled “Distinguishing between Comets and Asteroids”. When the cumulative number of bodies larger than a certain size is plotted against the size, the slope of the line is different for asteroids (~-2) and comets (~-1.67). Several asteroidal families were formed by the collisional break-up of a parent; in the case of the Eos family this would be around 300 km in diameter. Most asteroids were solid, and consisted of several types, although a few might be re- aggregates after collisions. By contrast comets were formed by accretion and were low density irregular objects. He pointed out that there seemed to be no small comets. He didn’t think much of the idea of dormant comets as they would be embedded in a meteoroid stream and would soon be reactivated by collisions. Nathan Harris (Armagh) disagreed with this in his paper on “Inactive Comets Among the Near-Earth Asteroids”. Plotting the aphelion distance Q against the perihelion distance q, allows various groups of small bodies, such as main belt asteroids (MB), Trojan asteroids, Jupiter family comets (JFC) and near-earth objects (NEO) to be distinguished. NEOs have q less than 1.4 AU and most JFCs have Q greater than 4.2 AU. The NEOs can be split into class 1 with Q greater than 4.2 AU (and thus affected by Jupiter) and class 2 with Q less than 4.2 AU. Computer models suggest that it is possible to produce JFCs from both the Oort cloud (OC) and the Kuiper belt (KB), but Halley type comets (HTC) only from the OC. They also suggest it is impossible to populate the class 2 region, unless non-gravitational forces are included. Comets in this class include 2P/Encke, 111P/Helin-Roman-Crockett, 82P/Gehrels 3 and 107P/Wilson-Harrington, which are all in some sense ‘strange’. The current population of JFCs can be explained with an injection rate of about one per century and a dynamical lifetime for class 1 objects of around 10,000 years and 1500 years for class 2 objects. He would expect around 100 dead JFCs to be still hanging around, but many more dead HTC (between 3000 and 12,000). Fabio Migliorini (Armagh) presented results of his study on “Resonant Delivery of Asteroids and Meteorites from the Main Belt” which is part of a European Community funded GAPTEC project to investigate the NEO impact hazard. This project has to be multinational in order to win EC funding and will use ESO telescopes etc. He showed many viewgraphs (often in far too small print) of simulations of resonant scattering into earth crossing orbits. Most end up in the sun after a few million years. He suggested that the cosmic ray ages of meteorites didn’t reflect transport time since the parent collision, but possibly the time since the ‘grandfather’s’ collision. Neil McBride (Kent) spoke about “Meteoroids: the Debris of Comets and Asteroids”. Meteoroids can be observed from the ground as meteors and as impacts on spacecraft. We have a good idea of the mass distribution in the mean isotropic flux. However meteoroids also occur in streams. The flux from these can be calculated and it works out an order of magnitude less than the overall flux, which can be split into helion/anthelion. north/south toroidal and north/south apex components. The Taurids do contribute a significant amount to the anthelion flux. Meteor radars give a velocity distribution, but in the past have missed a lot of fast meteors (~60 kms-1). He suggests that the apex (fast) source is cometary as is the anthelion. The toroidal component probably is (but there isn’t much of it anyway). The helion source is slow and probably asteroidal in origin. Overall more than 60% of the flux could be cometary. The final talk before lunch was by Hans Rickman (Uppsala) who spoke about “Physical and Dynamical Interrelationships and Transition Objects”. He first looked at the formation region and growth mechanism of comets and asteroids. Comets formed far out in the solar system and are fragile, low density objects. The Trojan region could be a potential reservoir of objects. Icy crust formation on comets is a rapid, common process which may turn comets into apparent asteroids. Occasional collisions in the asteroid belt may temporarily turn asteroids into comets and produce dust bands. Because these events are rare there are likely to be few examples at any one time. There is a relative lack of old long period comets (LPC) with respect to new ones, implying that fading exists and a comet is mantled within about 10 revolutions. There is also a lack of old LPC with large perihelion distance implying inactivation. There are no HTC with q larger than 1.5 AU. There are a number of asteroidal objects with cometary orbits: Don Quixote (JFC), Damocles (HTC), 1996 PW (LPC, P ~5000 years, q 2.5). Its possible to decouple the JFC from Jupiter by a combination of close encounters with the Earth and Venus and secular resonances (which gives evolutionary links between MB and JFC). We don’t however know what fraction of comets, dead comets and asteroids form the transition NEOs. He also drew attention to the fact that there are no comets with H0 fainter than about 12. Fainter comets might be missed or mistaken for asteroids, alternatively small objects might erode very quickly once smaller than 1 km, especially if this is combined with fragmentation. He concluded by pointing out that comet 46P/Wirtanen is just such an object and is a target for the ESA Rosetta mission. Simon Green (Kent) resumed after lunch, on the topic of “Centaurs”. Only seven of these strange bodies are known: Chiron, Pholus and five others which orbit between Neptune and Jupiter. They might originate as Trojans, but more likely come from the KB. Their size is much larger than typical short period comets (SPC) at around 200 km. Chiron has a relatively high albedo of 14%, but the others are more typically 4%. There are probably ~5000 within detectable range with several hundred larger than 100 km. Chiron shows cometary activity and the rotational light curve shows features which point to either non sphericity or albedo variation across the surface. As the coma brightens the amplitude of the curve decreases. Only a small fraction of the surface is active. Surprisingly the absolute magnitude is brightest at aphelion. According to HST measurements still under debate the density is less than one. Pholus is very red and the reflectance spectrum (which is similar to that of KB objects) is a good match to tholins. They could be made by cosmic ray bombardment of cometary ices. The question of what makes Chiron active and Pholus inactive and if they are SPCs remains unresolved. Iwan Williams (Queen Mary) moved a bit further out to talk on “Edgeworth-Kuiper Belt Objects”. KBOs are near the limits of detection at a distance of 50 AU with a size of 100 km. There are not enough routine astrometric observations being made (because these are not exciting) and once discovered many are lost; even observations over 6 months are equivalent to only one point on the orbit. Observational selection is a problem because searches are only carried out near the ecliptic (because that is where they are expected to be seen) and away from the milky way. Unfortunately this currently excludes searches near the location of Neptune so the absence of objects here is not surprising. There may be two classes - Pluto types and the rest. Following the reported detection of many KBOs in an HST field, Anita Cochran has imaged another field with negative results. Vacheslav Emel’yanenko (Chelyabinsk) looked at the possibility of “Asteroids from Long-Period Comets”. New comets are often subject to strong fading post perihelion. Mathematical simulations suggest that there should be around 4000 HTC compared to the 20 observed if there lifetime is around 200 revolutions. Asteroid 5335 Damocles, which shows no cometary activity, has a perihelion distance of around 1 AU and has probably made around 600 revolutions. He suggested that large nuclei with radius greater than 5 km might be preferentially deactivated. Concluding the meeting Alan Fitzsimmons (Queen’s University, Belfast) summarised observations of “Comet Hale-Bopp”. The apparent standstill in the light curve last autumn could be explained if the large dust output gave rise to a strong phase effect. Careful selection of the observations gave a good straight line fit over the entire apparition to date, implying no outbursts or variations in behaviour. Similar molecules to those seen in 1996 B2 had been seen and new detections included SO, HCO+ and HCCCN. The isotope ratios of 12C/13C, 14N/15N and 32S/34S are very similar to those of the Earth suggesting that it formed in the same solar nebula that we did. Measurements of the rotation period gave results ranging from 12 hours to one year. The one year period was based on the fact that there was no apparent rotation of the jets over 13 days, however these images did show shells of material being emitted one the image from one day was subtracted from an image on the previous day. In a recent INT image 6 dust shells were visible and these implied a 12 hour rotation, however the lack of rotation of the jets couldn’t be explained. The latest telescope on La Palma is a 50mm camera lens attached to a cooled 2000 pixel CCD camera installed on the INT roof which had been specially made to follow the evolution of the gas and dust tails. Jonathan Shanklin