Jack D. Drummond

Physical Properties of (2) Pallas

We acquired and analyzed adaptive-optics imaging observations of asteroid (2) Pallas from Keck II and the Very Large Telescope taken during four Pallas oppositions between 2003 and 2007, with spatial resolution spanning 32-88 km (image scales 13-20 km/pix). We improve our determination of the size, shape, and pole by a novel method that combines our AO data with 51 visual light-curves spanning 34 years of observations as well as occultation data. The shape model of Pallas derived here reproduces well both the projected shape of Pallas on the sky and light-curve behavior at all the epochs considered. We resolved the pole ambiguity and found the spin-vector coordinates to be within 5 deg. of [long, lat] = [30 deg., -16 deg.] in the ECJ2000.0 reference frame, indicating a high obliquity of ~84 deg., leading to high seasonal contrast. The best triaxial-ellipsoid fit returns radii of a=275 km, b= 258 km, and c= 238 km. From the mass of Pallas determined by gravitational perturbation on other minor bodies [(1.2 +/- 0.3) x 10-10 Solar Masses], we derive a density of 3.4 +/- 0.9 g.cm-3 significantly different from the density of C-type (1) Ceres of 2.2 +/- 0.1 g.cm-3. Considering the spectral similarities of Pallas and Ceres at visible and near-infrared wavelengths, this may point to fundamental differences in the interior composition or structure of these two bodies. We define a planetocentric longitude system for Pallas, following IAU guidelines. We also present the first albedo maps of Pallas covering ~80% of the surface in K-band. These maps reveal features with diameters in the 70-180 km range and an albedo contrast of about 6% with the mean surface albedo.

Earth-approaching asteroid streams

Abstract The osculating orbital elements of 139 Earth-approaching asteroids ( (through 1990KA) are compared with the D discriminants to identify the asteroids traveling in the most similar orbits. No Apollo associations are noted, but three Amor groups are identified whose members, if they were meteors, would be classified as comprising a shower or stream. Two of the streams have five members each, and are shown to be inconsistent with random groupings. One involves two V type asteroids, and the other five-member group has a secondary minimum in mutual orbital separation in the main belt, perhaps pointing to the location of a collision. The three meteorites with known orbits are also examined. Innisfree is most closely related to 1989DA, Pribram is questionably associated with 4486 (1987SB), and Lost City may be an outlier of a four-member asteroid association. Independent corroboration of these asteroid streams is provided by Halliday et al. (1990, Meteoritics 25, 93–99), who find four streams among 89 meteorite-producing fireballs, three of which are evidentally the meteor components of the asteroid streams. It is remarkable that in the face of disturbing perturbations asteroid streams could survive for any length of time, but if they are true nonrandom associations then the opportunity exists for studying an “exploded” asteroid in the near-Earth environment and through examination of pieces dropped by stream fireballs. The prediction is made that near-Earth asteroid search projects should find more members if they search the mean orbit of the streams.

Lidar observations of elevated temperatures in bright chemiluminescent meteor trails during the 1998 Leonid Shower

Seven persistent trails associated with bright reballs were probedwith asteerable Nawind/temperature lidar at Starre Optical Range, NM during the 17/18 Nov peak of the 1998 Leonid meteor shower. These chemilu- minescence trails were especially rich in Na. The average Na abundance within the trails was 52% of the background Na layer abundance, which suggests that the corresponding massesofthemeteorswerefrom1gupto1kg. CCDimages showthatthechemiluminescentemissions(includingNaand OH)areconnedtothewalls ofatube, whichexpandswith time by molecular diusion. Lidar proles within the trails show that the temperatures are highest at the edges of the tube where the airglow emissions are brightest. Approxi- mately 3 min after ablation, temperatures at the tube walls are 20{50 K warmer than the tube core and background at- mosphere. Neither chemical nor frictional heating provides a satisfactory explanation for the observations.

Observations of Persistent Leonid Meteor Trails 2. Photometry and Numerical Modeling

During the 1998 Leonid meteor shower, multi-instrument observations of persistent meteor trains were made from the Starfire Optical Range on Kirtland Air Force Base, New Mexico, and from a secondary site in nearby Placitas, New Mexico. The University of Illinois Na resonance lidar measured the Na density and temperature in the trains, while various cameras captured images and videos of the trains, some of which were observed to persist for more than 30 min. The Na density measurements allow the contribution of Na airglow to the observed train luminescence to be quantified for the first time. To do this, persistent train luminescence is numerically modeled. Cylindrical symmetry is assumed, and observed values of the Na density, temperature, and diffusivity are used. It is found that the expected Na luminosity is consistent with narrowband CCD all-sky camera observations, but that these emissions can contribute only a small fraction of the total light observed in a 0.5–1 μ bandwidth. Other potential luminosity sources are examined, in particular, light resulting from the possible excitation of monoxides of meteoric metals (particularly FeO) and O2(b1∑g+) during reactions between atmospheric oxygen species and meteoric metals. It is found that the total luminosity of these combined processes falls somewhat short of explaining the observed brightness, and thus additional luminosity sources still are needed. In addition, the brightness distribution, the so-called hollow cylinder effect, remains unexplained.

Meteor trail advection observed during the 1998 Leonid Shower

Sodium resonance lidar observations of meteor trails are reported from the 1998 Leonid shower experiment at the Starfire Optical Range, Kirtland Air Force Base, NM (35.0° N, 106.5° W). The lidar was operating in a spatially scanning mode that allowed tracking for up to one half-hour. Three trails are presented here whose motion allowed inference of radial as well as vector wind components and apparent diffusivities. The winds are derived independently using the narrow linewidth sodium (Na) resonance Doppler lidar technique and are compared with the tracking results.

Observations of persistent Leonid meteor trails: 1. Advection of the “Diamond Ring”

From a single image of a persistent trail left by a −1.5 magnitude Leonid meteor on November 17, 1998, the relative winds between 92.5 and 98 km altitude are derived, where the altitudes are determined by a sodium lidar. These are converted to true winds 82 sec after the appearance of the meteor by fixing the winds at 98 km to match the results of following the trail with the lidar for twelve minutes. The image and winds reveal a fine example of the effects of a gravity wave having a vertical wavelenth of 5.50 ± 0.02 km, a horizontal wavelength of 2650 ± 60 km, an intrinsic period of 19.5 ± 0.4 hours, and an observed period of 8.6 ± 0.1 hours. Effects of the gravity wave are still present in the wind field 70 min later.

Physical properties of the ESA Rosetta target asteroid (21) Lutetia I. The triaxial ellipsoid dimensions, rotational pole, and bulk density

We seek the best size estimates of the asteroid (21) Lutetia, the direction of its spin axis, and its bulk density, assuming its shape is well described by a smooth featureless triaxial ellipsoid, and to evaluate the deviations from this assumption. Methods. We derive these quantities from the outlines of the asteroid in 307 images of its resolved apparent disk obtained with adaptive optics (AO) at Keck II and VLT, and combine these with recent mass determinations to estimate a bulk density. Our best triaxial ellipsoid diameters for Lutetia, based on our AO images alone, are a x b x c = 132 x 101 x 93 km, with uncertainties of 4 x 3 x 13 km including estimated systematics, with a rotational pole within 5 deg. of ECJ2000 [long,lat] = [45, -7], or EQJ2000 [RA, DEC] = [44, +9]. The AO model fit itself has internal precisions of 1 x 1 x 8 km, but it is evident, both from this model derived from limited viewing aspects and the radius vector model given in a companion paper, that Lutetia has significant departures from an idealized ellipsoid. In particular, the long axis may be overestimated from the AO images alone by about 10 km. Therefore, we combine the best aspects of the radius vector and ellipsoid model into a hybrid ellipsoid model, as our final result, of 124 +/- 5 x 101 +/- 4 x 93 +/- 13 km that can be used to estimate volumes, sizes, and projected areas. The adopted pole position is within 5 deg. of [long, lat] = [52, -6] or[RA DEC] = [52, +12]. Using two separately determined masses and the volume of our hybrid model, we estimate a density of 3.5 +/- 1.1 or 4.3 +/- 0.8 g cm-3 . From the density evidence alone, we argue that this favors an enstatite-chondrite composition, although other compositions are formally allowed at the extremes (low-porosity CV/CO carbonaceous chondrite or high-porosity metallic). We discuss this in the context of other evidence.Context. Asteroid (21) Lutetia was the target of the ESA Rosetta mission flyby in 2010 July. Aims. We seek the best size estimates of the asteroid, the direction of its spin axis, and its bulk density, assuming its shape is well described by a smooth featureless triaxial ellipsoid. We also aim to evaluate the deviations from this assumption. Methods. We derive these quantities from the outlines of the asteroid in 307 images of its resolved apparent disk obtained with adaptive optics (AO) at Keck II and VLT, and combine these with recent mass determinations to estimate a bulk density. Results. Our best triaxial ellipsoid diameters for Lutetia, based on our AO images alone, are a × b × c = 132 × 101 × 93 km, with uncertainties of 4 × 3 × 13 km including estimated systematics, with a rotational pole within 5 ◦ of ECJ2000 [ λβ ] = [45 ◦ − 7 ◦ ], or EQJ2000 [RA Dec] = [44 ◦ + 9 ◦ ]. The AO model fit itself has internal precisions of 1 × 1 × 8 km, but it is evident both from this model derived from limited viewing aspects and the radius vector model given in a companion paper, that Lutetia significantly departs from an idealized ellipsoid. In particular, the long axis may be overestimated from the AO images alone by about 10 km. Therefore, we combine the best aspects of the radius vector and ellipsoid model into a hybrid ellipsoid model, as our final result, of diameters 124 ± 5 × 101 ± 4 × 93 ± 13 km that can be used to estimate volumes, sizes, and projected areas. The adopted pole position is within

Realization of a 50-watt facility-class sodium guidestar pump laser

A CW Na guidestar excitation source has been constructed and installed on the 3.5-m telescope at the Starfire Optical Range. This device is comprised of injection-locked Nd:YAG ring lasers operating at 1064 nm and 1319 nm and a doubly resonant cavity where sum-frequency generation of these wavelengths in LBO produces a diffraction-limited linearly-polarized 589-nm beam. Up to 50 W of 589-nm light for mesospheric guide-star generation has been produced. The injection-locked Nd:YAG lasers are capable of operating at up to 100 watts at 1064 nm and 60 watts at 1319 nm.

First observations of long‐lived meteor trains with resonance lidar and other optical instruments

In November 1998 the earth passed through a maximum in the cometary material responsible for the yearlyLeonids meteorshower. Themeteorstormeventpro- duced numerous examples of long-lived chemiluminescent trails|visible to the naked eye|over New Mexico, where a major observation campaign was centered. One trail was detected for over an hour with a CCD camera employing a narrow sodiumlter, and many others were observed for over ten minutes each. For the rst time, sodium densi- ties in such trails were measured while also being imaged in sodium light. Wehaveveried onesource of long-lived light emissions|a sodium-catalyzed reaction involving ozone| but it is far too weak to explain the visibility of such trails. Inaddition,wepresentanewexplanationforthecylindrical shell appearance long reported for chemiluminescent trails and show that ozone depletion by chemical processes is a possible explanation for this phenomenon.

Ultraviolet and visible photometry of asteroid (21) Lutetia using the Hubble Space Telescope

Context. The asteroid (21) Lutetia is the target of a planned close encounter by the Rosetta spacecraft in July 2010. To prepare for that flyby, Lutetia has been extensively observed by a variety of astronomical facilities. Aims. We used the Hubble Space Telescope (HST) to determine the albedo of Lutetia over a wide wavelength range, extending from ~1500 A° to ~7000 A°. Methods. Using data from a variety of HST filters and a ground-based visible light spectrum, we employed synthetic photometry techniques to derive absolute fluxes for Lutetia. New results from ground-based measurements of Lutetias size and shape were used to convert the absolute fluxes into albedos. Results. We present our best model for the spectral energy distribution of Lutetia over the wavelength range 1200-8000 A°. There appears to be a steep drop in the albedo (by a factor of ~2) for wavelengths shorter than ~3000 A°. Nevertheless, the far ultraviolet albedo of Lutetia (~10%) is considerably larger than that of typical C-chondrite material (~4%). The geometric albedo at 5500 A° is 16.5 ± 1%. Conclusions. Lutetias reflectivity is not consistent with a metal-dominated surface at infrared or radar wavelengths, and its albedo at all wavelengths (UV-visibile-IR-radar) is larger than observed for typical primitive, chondritic material. We derive a relatively high FUV albedo of ~10%, a result that will be tested by observations with the Alice spectrograph during the Rosetta flyby of Lutetia in July 2010.