A meteoroid stream survey using the Canadian Meteor Orbit Radar: I. Methodology and radiant catalogue

Using a meteor orbit radar, a total of more than 2.5 million meteoroids with masses ∼10⁻⁷ kg have had orbits measured in the interval 2002-2006. From these data, a total of 45 meteoroid streams have been identified using a wavelet transform approach to isolate enhancements in radiant density in geocentric coordinates. Of the recorded streams, 12 are previously unreported or unrecognized. The survey finds >90% of all meteoroids at this size range are part of the sporadic meteoroid background. A large fraction of the radar detected streams have q<0.15 AU suggestive of a strong contribution from sungrazing comets to the meteoroid stream population currently intersecting the Earth. We find a remarkably long period of activity for the Taurid shower (almost half the year as a clearly definable radiant) and several streams notable for a high proportion of small meteoroids only, among these a strong new shower in January at the time of the Quadrantids (January Leonids). A new shower (Epsilon Perseids) has also been identified with orbital elements almost identical to Comet 96P/Machholz.     

CAMS: Cameras for Allsky Meteor Surveillance to establish minor meteor showers

First results are presented from a newly developed meteoroid orbit survey, called CAMS – Cameras for Allsky Meteor Surveillance, which combines meteor detection algorithms for low-light video observations with traditional video surveillance tools. Sixty video cameras at three stations monitor the sky above 31° elevation. Goal of CAMS is to verify meteor showers in search of their parent comets among newly discovered near-Earth objects.This paper outlines the concept of operations, the hardware, and software methods used during operation and in the data reduction pipeline, and accompanies the data release of the first batch of meteoroid orbits. During the month of November 2010, 2169 precisely reduced meteoroid trajectories from 17 nights have an error in the apparent radiant of the trajectory <2° and error in speed <10%. Median values of the error are 0.31° and 0.53 km/s, respectively, sufficient to resolve the intrinsic dispersion of annual meteor showers and resolve minor showers from the sporadic background. The limiting visual magnitude of the cameras is +5.4, recording meteors of +4 magnitude and brighter, bright enough to stand out from the mostly fainter sporadic meteors detected as under dense radar echoes.CAMS readily detected all established showers (6) active during the clear nights in November. Of the showers that needed confirmation, we confirm the theta Aurigids (THA, IAU#390), the chi Taurids (CTA, IAU#388), and the omicron Eridanids (OER, IAU#338). We conclude that the iota November Aurigids (IAR, IAU#248) are in fact the combined activity of the theta Aurigids and chi Taurids, and this shower should be dismissed from the list. Finally, there is also a clustering consistent with the zeta Cancrids (ZCN, IAU#243), but we cannot exclude that this is lower perihelion dust belonging to the Orionid shower.Data are submitted to the IAU Meteor Data Center on a semi-regular basis, and can be accessed also at http://cams.seti.org.     

The orbit of the Kappa Cygnid and related meteor streams

A study of the Kappa Cygnid and other minor streams of the August epoch is presented based on a computer search in a sample of 3518 photographic meteoroid orbits. Four different meteoroid streams with radiants in Cygnus, Draco and Lyra, were found. Three of these: the Kappa Cygnids, the Alpha Lyrids and the Zeta Draconids are identified with meteor showers reported by nineteenth century visual observers. The fourth stream, the August Lyrids, consists of six meteors with radiants in Lyra centered on = 277°.6, = 46°.2. No previous visual reports of this stream have been found. It is interesting to note that all four meteoroid streams are coincident in time; their orbits are all of short period and they all have very nearly the same orientation of semi-major axis.     

Using the youngest asteroid clusters to constrain the space weathering and gardening rate on S-complex asteroids

We have extended our earlier work on space weathering of the youngest S-complex asteroid families to include results from asteroid clusters with ages <10⁶ years and to newly identified asteroid pairs with ages <5 × 10⁵ years. We have identified three S-complex asteroid clusters amongst the set of clusters with ages in the range 10^5-6 years—(1270) Datura, (21509) Lucascavin and (16598) 1992 YC2. The average color of the objects in these clusters agrees with the color predicted by the space weathering model of Willman et al. (Willman, M., Jedicke, R., Nesvorný, D., Moskovitz, N., Ivezi?, Z., Fevig, R. [2008]. Icarus 195, 663-673). SDSS five-filter photometry of the members of the very young asteroid pairs with ages <10⁵ years was used to determine their taxonomic classification. Their types are consistent with the background population near each object. The average color of the S-complex pairs is PC₁ = 0.49 ± 0.03, over 5σ redder than predicted by Willman et al. (Willman, M., Jedicke, R., Nesvorný, D., Moskovitz, N., Ivezi?, Z., Fevig, R. [2008]. Icarus 195, 663-673). This may indicate that the most likely pair formation mechanism is a gentle separation due to YORP spin-up leaving much of the aged and reddened surface undisturbed. If this is the case then our color measurement allows us to set an upper limit of ∼64% on the amount of surface disturbed in the separation process. Using pre-existing color data and our new results for the youngest S-complex asteroid clusters we have extended our space weather model to explicitly include the effects of regolith gardening and fit separate weathering and gardening characteristic time scales of τ_w = 960 ± 160 Myr and τ_g = 2000 ± 290 Myr respectively. The first principal component color for fresh S-complex material is PC₁ = 0.37 ± 0.01 while the maximum amount of local reddening is ΔPC₁ = 0.33 ± 0.06. Our first-ever determination of the gardening time is in stark contrast to our calculated gardening time of τ_g ∼ 270 Myr based on main belt impact rates and reasonable assumptions about crater and ejecta blanket sizes. A possible resolution for the discrepancy is through a ‘honeycomb’ mechanism in which the surface regolith structure absorbs small impactors without producing significant ejecta. This mechanism could also account for the paucity of small craters on (433) Eros.     

Statistical model of meteor streams III. Stream search among 19303 radio meteors

Using a computerized technique of stream search, based on the statistical model of meteor streams, we have detected 72 additional streams in a sample of 19303 radio meteor orbits. The streams are found to have a tendency to cluster, partly along the ecliptic and partly in high-inclination orbits. Also noticed are specific relations among the detected streams, such as stream pairs, stream branches, and twin showers. A very probable association of a prominent stream, the a Capricornids, with the minor planet Adonis has been established, and possible associations of several streams with comets and minor planets of the Apollo and Albert types are also discussed. Identification of the detected radio streams with previously known streams is presented, and plans for future work are briefly outlined.     

Monte Carlo simulations of radio pulses in atmospheric showers using ZHAireS

We present predictions for the radio pulses emitted by extensive air showers using ZHAireS, an AIRES-based Monte Carlo code that takes into account the full complexity of ultra-high energy cosmic-ray induced shower development in the atmosphere, and allows the calculation of the electric field in both the time and frequency domains. We do not presuppose any emission mechanism and our results are compatible with a superposition of geomagnetic and charge excess radio emission effects. We investigate the polarization of the electric field as well as the effects of the refractive index n and shower geometry on the radio pulses. We show that geometry, coupled to the relativistic effects that appear when using a realistic refractive index n > 1, play a prominent role on the radio emission of air showers.     

Theoretical twin meteor showers

Attention is drawn to, and theoretical radiants calculated for, 27 hypothetical twin showers to Cook's (1973, in Evolutionary and Physical Properties of Meteoroids, NASA SP-319, U.S. Govt. Printing Office, Washington, D.C.) working list of meteor streams. Of these twin showers, 7 are previously known, 1 is a night-time twin to the ? Geminids, 9 are found to be contained among Sekanina's (1976, Icarus27, 265–321) radar streams, and the remaining 10 are undetected daytime showers. Minimum radii are computed for all of Cook's streams and are used to assess the possibility of detecting the twin showers.     

Is 2329 Orthos a dead comet?

Probably most meteor showers have a cometary origin. Investigation of Near-Earth asteroids' orbital evolution to determine whether they have related meteor showers is necessary to determine which asteroids evolved from comets. The results of calculations show that asteroid Orthos' orbit is an octuple Earth-crosser. Therefore, if Orthos has an old meteoroid stream it may produce eight meteor showers observable on the Earth. The existence of four Orthos' Northern meteor showers is confirmed by our search in the published catalogues of meteor radiants and orbits or in the archives of the IAU Meteor Data Center (Lund, Sweden).     

The Canadian Meteor Orbit Radar Meteor Stream Catalogue

The Canadian Meteor Orbit Radar is a multi-frequency backscatter radar which has been in routine operation since 1999, with an orbit measurement capability since 2002. In total, CMOR has measured over 2 million orbits of meteoroids with masses greater than 10 μg, while recording more than 18 million meteor echoes in total. We have applied a two stage comparative technique for identifying meteor streams in this dataset by making use of clustering in radiants and velocities without employing orbital element comparisons directly. From the large dataset of single station echoes, combined radiant activity maps have been constructed by binning and then stacking each years data per degree of solar longitude. Using the single-station mapping technique described in Jones and Jones (Mon Not R Astron Soc 367:1050–1056, 2006) we have identified probable streams from these single station observations. Additionally, using individual radiant and velocity data from the multi-station velocity determination routines, we have utilized a wavelet search algorithm in radiant and velocity space to construct a list of probable streams. These two lists were then compared and only streams detected by both techniques, on multiple frequencies and in multiple years were assigned stream status. From this analysis we have identified 45 annual minor and major streams with high reliability.     

Mostly Dormant Comets and their Disintegration into Meteoroid Streams: A Review

The history of associating meteor showers with asteroidal-looking objects is long, dating to before the 1983 discovery that 3200 Phaethon moves among the Geminids. Only since the more recent recognition that 2003 EH1 moves among the Quadrantids are we certain that dormant comets are associated with meteoroid streams. Since that time, many orphan streams have found parent bodies among the newly discovered Near Earth Objects. The seven established associations pertain mostly to showers in eccentric or highly inclined orbits. At least 35 other objects are tentatively linked to streams in less inclined orbits that are more difficult to distinguish from those of asteroids. There is mounting evidence that the streams originated from discrete breakup events, rather than long episodes of gradual water vapor outgassing. If all these associations can be confirmed, they represent a significant fraction of all dormant comets that are in near-Earth orbits, suggesting that dormant comets break at least as frequently as the lifetime of the streams. I find that most pertain to NEOs that have not yet fully decoupled from Jupiter. The picture that is emerging is one of rapid disintegration of comets after being captured by Jupiter, and consequently, that objects such as 3200 Phaethon most likely originated from among the most primitive asteroids in the main belt, instead. They too decay mostly by disintegration into comet fragments and meteoroid streams. The disintegration of dormant comets is likely the main source of our meteor showers and the main supply of dust to the zodiacal cloud. Editorial handling: Frans Rietmeijer.     

Triplicity and physical characteristics of Asteroid (216) Kleopatra

To take full advantage of the September 2008 opposition passage of the M-type Asteroid (216) Kleopatra, we have used near-infrared adaptive optics (AO) imaging with the W.M. Keck II telescope to capture unprecedented high resolution images of this unusual asteroid. Our AO observations with the W.M. Keck II telescope, combined with Spitzer/IRS spectroscopic observations and past stellar occultations, confirm the value of its IRAS radiometric radius of 67.5 km as well as its dog-bone shape suggested by earlier radar observations. Our Keck AO observations revealed the presence of two small satellites in orbit about Kleopatra (see Marchis, F. et al. [2008a]. (3749) Balam. In: Green, D.W.E. (Ed.), IAU Circ. 8928; Marchis, F., Descamps, P., Berthier, J., Emery, J.P. [2008b]. S/2008 ((216)) 1 and S/2008 ((216)) 2. In: Green, D.W.E. (Ed.), IAU Circ. 8980). Accurate measurements of the satellite orbits over a full month enabled us to determine the total mass of the system to be 4.64 ± 0.02 × 10¹⁸ kg. This translates into a bulk density of 3.6 ± 0.4 g/cm³, which implies a macroscopic porosity for Kleopatra of ∼30-50%, typical of a rubble-pile asteroid. From these physical characteristics we measured its specific angular momentum, very close to that of a spinning equilibrium dumbbell.     

Asteroid age distributions determined by space weathering and collisional evolution models

We provide evidence of consistency between the dynamical evolution of main belt asteroids and their color evolution due to space weathering. The dynamical age of an asteroid’s surface (Bottke, W.F., Durda, D.D., Nesvorný, D., Jedicke, R., Morbidelli, A., Vokrouhlický, D., Levison, H. [2005]. Icarus 175 (1), 111-140; Nesvorný, D., Jedicke, R., Whiteley, R.J., Ivezi?, ?. [2005]. Icarus 173, 132-152) is the time since its last catastrophic disruption event which is a function of the object’s diameter. The age of an S-complex asteroid’s surface may also be determined from its color using a space weathering model (e.g. Willman, M., Jedicke, R., Moskovitz, N., Nesvorný, D., Vokrouhlický, D., Mothé-Diniz, T. [2010]. Icarus 208, 758-772; Jedicke, R., Nesvorný, D., Whiteley, R.J., Ivezi?, ?., Juri?, M. [2004]. Nature 429, 275-277; Willman, M., Jedicke, R., Nesvorny, D., Moskovitz, N., Ivezi?, ?., Fevig, R. [2008]. Icarus 195, 663-673. We used a sample of 95 S-complex asteroids from SMASS and obtained their absolute magnitudes and u, g, r, i, z filter magnitudes from SDSS. The absolute magnitudes yield a size-derived age distribution. The u, g, r, i, z filter magnitudes lead to the principal component color which yields a color-derived age distribution by inverting our color-age relationship, an enhanced version of the ‘dual τ’ space weathering model of Willman et al. (2010).We fit the size-age distribution to the enhanced dual τ model and found characteristic weathering and gardening times of τ_w = 2050 ± 80 Myr and respectively. The fit also suggests an initial principal component color of −0.05 ± 0.01 for fresh asteroid surface with a maximum possible change of the probable color due to weathering of ΔPC = 1.34 ± 0.04. Our predicted color of fresh asteroid surface matches the color of fresh ordinary chondritic surface of PC₁ = 0.17 ± 0.39.     

Radar observations of Asteroids 64 Angelina and 69 Hesperia

We report new radar observations of E-class Asteroid 64 Angelina and M-class Asteroid 69 Hesperia obtained with the Arecibo Observatory S-band radar (2480 MHz, 12.6 cm). Our measurements of Angelina’s radar bandwidth are consistent with reported diameters and poles. We find Angelina’s circular polarization ratio to be 0.8 ± 0.1, tied with 434 Hungaria for the highest value observed for main-belt asteroids and consistent with the high values observed for all E-class asteroids (Benner, L.A.M., Ostro, S.J., Magri, C., Nolan, M.C., Howell, E.S., Giorgini, J.D., Jurgens, R.F., Margot, J.L., Taylor, P.A., Busch, M.W., Shepard, M.K. [2008]. Icarus 198, 294-304; Shepard, M.K., Kressler, K.M., Clark, B.E., Ockert-Bell, M.E., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J. [2008b]. Icarus 195, 220-225). Our radar observations of 69 Hesperia, combined with lightcurve-based shape models, lead to a diameter estimate, D_eff = 110 ± 15 km, approximately 20% smaller than the reported IRAS value. We estimate Hesperia to have a radar albedo of , consistent with a high-metal content. We therefore add 69 Hesperia to the Mm-class (high metal M) (Shepard, M.K., Clark, B.E., Ockert-Bell, M., Nolan, M.C., Howell, E.S., Magri, C., Giorgini, J.D., Benner, L.A.M., Ostro, S.J., Harris, A.W., Warner, B.D., Stephens, R.D., Mueller, M. [2010]. Icarus 208, 221-237), bringing the total number of Mm-class objects to eight; this is 40% of all M-class asteroids observed by radar to date.     

Orthos' meteor showers

Probably the majority of meteor showers has a cometary origin. Investigation of Near-Earth asteroids' orbital evolution to determine whether they have related meteor showers are necessary to determine which asteroids evolved from comets. The results of calculations show that asteroid Orthos' orbit is an octuple Earth-crosser. Therefore, if Orthos has an old meteoroid stream it may produce eight meteor showers observable on the Earth. The existence of four Orthos' Northern meteor showers is confirmed by our search in the published catalogues of meteor radiants and orbits or in the archives of the IAU Meteor Data Center (Lund, Sweden).     

Meteor streams associated with Jupiter family comets

Meteor showers have been observed for a considerable time, and the cause, meteoroids from a meteoroid stream ablating in the Earth's atmosphere, has also been understood for centuries. The connection between meteoroid streams and comets was also established 150 years ago. Since that time our ability both to understand the physics and to numerically model the situation has steadily increased. We will review the current state of knowledge. However, just as there are differences between the behaviour of long period comets, Halley family comets and Jupiter family comets, so also differences exist between the associated meteoroid streams. Streams associated with Jupiter family comets show much more variety in their behaviour, driven by the gravitational perturbations from Jupiter. The more interesting showers associated with Jupiter family comets will be discussed individually.     

Venus-intercepting meteoroid streams

This study is motivated by the possibility of determining the large-body meteoroid flux at the orbit of Venus. Towards this end, we attempt to estimate the times at which enhanced meteoric activity might be observed in the planet's atmosphere. While a number of meteoroid streams are identified as satisfying common Earth and Venus intercept conditions, it is not clear from the Earth-observed data if these streams contain large-body meteoroids. A subset of the Taurid Complex objects may produce fireball-rich meteor showers on Venus. A total of 11 short-period, periodic comets and 46 near-Earth asteroids approach the orbit of Venus to within 0.1 au, and these objects may have associated meteoroid streams. Comets 27P/Crommelin and 7P/Pons–Winnecke are identified as candidate parents to possible periodic meteor showers at the orbit of Venus.     

Is 2329 Orthos a dead comet?

Probably most meteor showers have a cometary origin. Investigation of Near-Earth asteroids' orbital evolution to determine whether they have related meteor showers is necessary to determine which asteroids evolved from comets. The results of calculations show that asteroid Orthos' orbit is an octuple Earth-crosser. Therefore, if Orthos has an old meteoroid stream it may produce eight meteor showers observable on the Earth. The existence of four Orthos' Northern meteor showers is confirmed by our search in the published catalogues of meteor radiants and orbits or in the archives of the IAU Meteor Data Center (Lund, Sweden).     

Polar wander of Mars: Evidence from giant impact basins

We investigate the polar wander of Mars in the last ∼4.2 Ga. We identify two sets of basins from the 20 giant impact basins reported by Frey [Frey, H., 2008. Geophys. Res. Lett. 35, L13203] which trace great circles on Mars, and propose that the great circles were the prevailing equators of Mars at the impact times. Monte Carlo tests are conducted to demonstrate that the two sets of basins are most likely not created by random impacts. Also, fitting 63,771 planes to randomly selected sets of 5, 6, or 7 basins indicated that the identified two sets are unique. We propose three different positions for the rotation pole of Mars, besides the present one. Accordingly, Tharsis bulge was initially formed at ∼50 N and moved toward the equator while rotating counterclockwise due to the influence of the two newly forming volcanic constructs, Alba Patera and Elysium Rise. The formation of the giant impact basins, subsequent mass concentrations (mascons) in Argyre, Isidis, and Utopia basins, and surface masses of volcanic mountains such as Ascraeus, Pavonis, Arsia and Olympus, caused further polar wander which rotated Tharsis bulge clockwise to arrive at its present location. The extensive polar motion of Mars during 4.2-3.9 Ga implies a weak lithosphere on a global scale, deduced from a total of 72,000 polar wander models driven by Tharsis bulge, Alba Patera and Elysium Rise as the major mass perturbations. Different compensation states, 0-100%, are examined for each of the surface loads, and nine different thicknesses are considered for an elastic lithosphere. The lithosphere must have been very weak, with an elastic thickness of less than 5 km, if the polar wander was driven by these mass perturbations.     

EURONEAR: First results

The European Near Earth Asteroid Research (EURONEAR) is a project which envisions to build a coordinated network which will follow-up and recover potentially hazardous asteroids (PHAs) and near earth asteroids (NEAs). We aim to include in EURONEAR two automated 1 m telescopes located in Chile and Europe, in addition to other non-permanent facilities. Astrometry will be the main aim of the project in order to secure and follow-up newly discovered NEAs, also to recover PHAs at their second or following oppositions, while photometry of bright PHAs will bring information on their physical properties. In this paper, first we review briefly the existent and past NEAs programs. Next, we include the results obtained in 2006 from three observing runs at Pic du Midi using the 1 m telescope, Haute-Provence employing the 1.2 m telescope, and Bucharest using a small 23 cm telescope. These add a total of 153 positions for 16 PHAs and NEAs, which were accepted by Minor Planet Center. Recently, a 1 m telescope was allocated by ESO in La Silla to be automated and used as the Southern dedicated facility by EURONEAR.