Robert L. Millis

Comet Bowell 1980b

Optical filter photometry, and optical and ultraviolet spectrophotometry data collected between November 1980 and June 1982 of Comet Bowell are presented. It was determined that Comet Bowell began producing significant amounts of OH, though not other species, before perihelion at a heliocentric distance near 4.6 AU. As the comet approached perihelion at 3.4 AU, OH production decreased and CN and C2 species were detected at normal concentrations. An outburst in April 1982 was dominated by OH. The grains in 1982 near perihelion showed increases in albedo between 3150 and 4500 A and 1.2 and 1.6 microns. The presence of two grains populations was suggested, together with the conclusion that Comet Bowell was physically a typical new comet entering the inner solar system from the Oort cloud for the first time.

Photoelectric observations of asteroids 3, 24, 60, 261, and 863

Abstract Observations of five asteroids, obtained from various observatories from 1978 to 1984, are presented. A Fourier analysis method was used to derive the composite lightcurves. This solution method yields a value for the rotation period, mean absolute magnitudes on each night of observation, and Fourier coefficients defining the shape of the composite lightcurve, to any degree specified. A major advantage of the method is that it yields formal error estimates for all the quantities computed. The Fourier coefficients derived can be used for studies of the shapes and pole orientations of asteroids and to define “rotation phase” in a more formal way, for connecting observations from one apparition to another. Periods, amplitudes, and phase relations are reported for four of the five asteroids. The presence of a substantial amplitude first harmonic in the lightcurves of 3 Juno, 24 Themis, and 60 Echo suggests that these asteroids may have significant surface albedo variegation. 60 Echo exhibits a difference of 0.2 magnitude in its mean absolute magnitude from different aspects, which suggests a somewhat polar-flattened figure. High-quality magnitude vs solar phase angle data were obtained for 24 Themis from 0°.3 to 21°. These data are perhaps the best available of a dark asteroid and provide a test for light-scattering theories.

Pluto's atmosphere

Abstract The stellar occultation by Pluto on June 9, 1988, was observed with a high-speed CCD photometer attached to the 0.9-m telescope aboard NASAs Kuiper Airborne Observatory (KAO). The occultation lightcurve, which probed two regions on the sunrise limb separated by about 200 km, reveals a clear upper atmosphere that overlies an extinction layer with an abrupt upper boundary. The observations demonstrate that the extinction layer extends along the portion of the sunrise limb bounded by the immersion and emersion regions, as well as long the corresponding portion of the sunset limb on the opposite side of the planet. In all, the total limb probed by the KAO data for extinction represents nearly half of Plutos circumference. Hence, the extinction layer may surround the entire planet. A model atmosphere is presented, from which is derived an occultation lightcurve that closely matches the data. In addition to the standard parameters describing the occultation curve by an isothermal atmosphere, our model includes the radius of the upper boundary of the extinction and the radius of unit observed optical depth as free parameters. Fits of this model to the immersion and emersion lightcurves show no significant differences in the derived atmospheric structure. A preliminary geometrical solution, based on three occultation chords, yields a half-light radius of 1214 ± 20 km. At this level, the mean scale height derived from the model fits to the KAO data is 59.7 ± 1.5 km. The corresponding ratio of temperature to mean molecular weight is 4.2 ± 0.4°K/amu, with the principal source of error arising from the uncertainty in the mass of Pluto. The extinction layer, whose upper boundary lies 25 km below the half-light level, has a minimum thickness of 46 km, a minimum vertical optical depth of 0.19, and a scale height of 33.4 ± 6.9 km. For a pure methane atmosphere, our results imply (for the clear atmosphere at the half-light level) a temperature of 67 ± 6°K, a number density of 8.3 × 10 13 cm −3 , and a pressure of 0.78 ωbar. Our occultation data are also consistent with a predominantly nitrogen atmosphere (such as that of Titan), in which case the temperature would be 117 ± 11°K. The substantially smaller scale height of the extinction layer may arise from properties of the “particles” causing the extinction or may indicate a lower temperature in this region. Since our analysis indicates that the extinction layer is optically thick at the limb of Pluto, determinations of Plutos radius by methods that use reflected light, such as speckle interferometry and observations of the mutual events, give results that refer to the “visible disk” of Pluto and not on the planets solid surface. Unit optical depth of the extinction layer (observed along the line of sight) lies at 1174 ± 20 km, a level consistent with the radius of Pluto derived from the mutual events (1142 ± 21 km). The mutual event radius is also consistent with the deepest level probed by the occultation: it lies at a radius of 1143 ± 20 km, which represents an upper limit on the surface radius. For a pure methane atmosphere, a surface pressure as low as 3 ωbar (the vapor pressure of methane at 50°K) would be consistent with the occultation data.

The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. II. Dynamical Classification, the Kuiper Belt Plane, and the Core Population

The Deep Ecliptic Survey (DES)?a search optimized for the discovery of Kuiper belt objects (KBOs) with the Blanco and Mayall 4 m telescopes at the Cerro Tololo Inter-American Observatory and Kitt Peak National Observatory?has covered 550 deg2 from its inception in 1998 through the end of 2003. This survey has a mean 50% sensitivity at VR magnitude 22.5. We report here the discoveries of 320 designated KBOs and Centaurs for the period 2000 March through 2003 December and describe improvements to our discovery and recovery procedures. Our data and the data products needed to reproduce our analyses in this paper are available through the NOAO survey database. Here we present a dynamical classification scheme, based on the behavior of orbital integrations over 10 Myr. The dynamical classes, in order of testing, are Resonant, Centaur, Scattered-Near, Scattered-Extended, and Classical. (These terms are capitalized when referring to our rigorous definitions.) Of the 382 total designated KBOs discovered by the DES, a subset of 196 objects have sufficiently accurate orbits for dynamical classification. Summary information is given for an additional 240 undesignated objects also discovered by the DES from its inception through the end of 2003. The number of classified DES objects (uncorrected for observational bias) are Classical, 96; Resonant, 54; Scattered-Near, 24; Scattered-Extended, 9; and Centaur, 13. We use subsets of the DES objects (which can have observational biases removed) and larger samples to perform dynamical analyses on the Kuiper belt. The first of these is a determination of the Kuiper belt plane (KBP), for which the Classical objects with inclinations less than 5? from the mean orbit pole yield a pole at R.A. = 27392 ? 062 and decl. = 6670 ? 020 (J2000), consistent with the invariable plane of the solar system. A general method for removing observational biases from the DES data set is presented and used to find a provisional magnitude distribution and the distribution of orbital inclinations relative to the KBP. A power-law model fit to the cumulative magnitude distribution of all KBOs discovered by the DES in the VR filter yields an index of 0.86 ? 0.10 (with the efficiency parameters for the DES fitted simultaneously with the population power law). With the DES sensitivity parameters fixed, we derive power-law indices of 0.74 ? 0.05, 0.52 ? 0.08, and 0.74 ? 0.15, respectively, for the Classical, Resonant, and Scattered classes. Plans for calibration of the DES detection efficiency function and DES magnitudes are discussed. The inclination distribution confirms the presence of hot and cold populations; when the geometric sin i factor is removed from the inclination distribution function, the cold population shows a concentrated core with a full width at half-maximum of approximately 46, while the hot population appears as a halo, extending beyond 30?. The inclination distribution is used to infer the KBO distribution in the sky, as a function of latitude relative to the KBP. This inferred latitude distribution is reasonably consistent with the latitude distribution derived from direct observation, but the agreement is not perfect. We find no clear boundary between the Classical and Scattered classes either in their orbital inclinations with respect to the KBP or in their power-law indices in their respective magnitude distributions. This leaves open the possibility that common processes have shaped the distribution of orbital parameters for the two classes.

Resonance Occupation in the Kuiper Belt: Case Examples of the 5:2 and Trojan Resonances

As part of our ongoing Deep Ecliptic Survey (DES) of the Kuiper belt, we report on the occupation of the 1 : 1 (Trojan), 4 : 3, 3 : 2, 7 : 4, 2 : 1, and 5 : 2 Neptunian mean motion resonances (MMRs). The previously unrecognized occupation of the 1 : 1 and 5 : 2 MMRs is not easily understood within the standard model of resonance sweeping by a migratory Neptune over an initially dynamically cold belt. Among all resonant Kuiper belt objects (KBOs), the three observed members of the 5 : 2 MMR discovered by DES possess the largest semimajor axes (a ≈ 55.4 AU), the highest eccentricities (e ≈ 0.4), and substantial orbital inclinations (i ≈ 10°). Objects (38084) 1999HB12 and possibly 2001KC77 can librate with modest amplitudes of ~90° within the 5 : 2 MMR for at least 1 Gyr. Their trajectories cannot be explained by close encounters with Neptune alone, given the latters current orbit. The dynamically hot orbits of such 5 : 2 resonant KBOs, unlike hot orbits of previously known resonant KBOs, may imply that these objects were preheated to large inclination and large eccentricity prior to resonance capture by a migratory Neptune. Our first discovered Neptunian Trojan, 2001QR322, may not owe its existence to Neptunes migration at all. The trajectory of 2001QR322 is remarkably stable; the object can undergo tadpole-type libration about Neptunes leading Lagrange (L4) point for at least 1 Gyr with a libration amplitude of 24°. Trojan capture probably occurred while Neptune accreted the bulk of its mass. For an assumed albedo of 12%–4%, our Trojan is ~130–230 km in diameter. Model-dependent estimates place the total number of Neptune Trojans resembling 2001QR322 at ~20–60. Their existence helps to rule out violent orbital histories for Neptune.

An investigation of the nucleus and coma of Comet P/Arend-Rigaux

Narrow-band photometry of Comet P/Arend-Rigaux in the visible and near-ultraviolet are presented for six nights during the 1884/1985 apparition, including three nights of simultaneous observations in the thermal infrared made with the NASA IRTF. The results indicate a period of rotation of 13.47 hr, dimensions for the nucleus of 13 x 8 x 8 km (under the assumption of prolate spheroid shape), and a steady decrease in the comets gas production from a month before perihelion to more that two months after perihelion. Results for the nucleus and the coma suggest that the nucleus is totally covered with a nonvolatile mantle. 51 references.

The size, shape, density, and Albedo of Ceres from its occultation of BD+8°471

The occultation of BD+8°471 by Ceres on 13 November 1984 was observed photoelectrically at 13 sites in Mexico, Florida, and the Caribbean. These observations indicate that Ceres is an oblate spheroid having an equatorial radius of 479.6±2.4 km and a polar radius of 453.4±4.5 km. The mean density of this minor planet is 2.7 g/cm3±5%, and its visual geometric albedo is 0.073. While the surface appears globally to be in hydrostatic equilibrium, firm evidence of real limb irregularities is seen in the data.

The Deep Ecliptic Survey: A Search for Kuiper Belt Objects and Centaurs. I. Description of Methods and Initial Results

We report here initial results of the Deep Ecliptic Survey, an ongoing new search for Kuiper belt objects (KBOs) and Centaurs using the 8K × 8K Mosaic CCD array on the 4 m Mayall Telescope at Kitt Peak National Observatory. Within the interval covered in this paper, useful observations were obtained during seven nights in 1998 October and November, 1999 April, and 2000 February. We used a novel technique to efficiently find and determine positions of moving objects. Sixty-nine KBOs and Centaurs with apparent magnitudes between 20.6 and approximately the 24th magnitude were discovered. Nine or 10 of the newly discovered KBOs appear to be in the 3 : 2 mean motion resonance with Neptune, and four appear to be scattered-disk objects. Three objects were found that may be in the 4 : 3 resonance. Sixty-two of the objects reported here have been observed on at least one additional night and have received designations. Our own follow-up astrometry was done primarily with the WIYN 3.5 m telescope in queue-scheduled mode and with the Steward Observatory 90 inch (2.3 m) telescope. Others, using a variety of telescopes, recovered a significant number of our objects. Although not a primary objective of the survey, positions of all main-belt asteroids, Trojan asteroids, and nearby fast-moving asteroids seen in our data also have been determined, and most have been reported to the Minor Planet Center. Through simulations and analysis of the existing KBO database, we have investigated the uncertainty to be expected in various KBO orbital parameters as a function of the extent of the astrometric coverage. The results indicate that the heliocentric distance of an object and the inclination of its orbit can be narrowly constrained with observations from a single apparition. Accurate determination of semimajor axis and eccentricity, on the other hand, requires astrometric data extending over additional apparitions. Based on the observed distribution of orbital inclinations in our sample, we have estimated the true distribution of orbital inclinations in the Kuiper belt and find it to be similar to that of the short-period comets. This result is consistent with the commonly held belief that the Kuiper belt is the source region of the short-period comets.

Narrowband photometry of Comet P/Stephan-Oterma and the backscattering properties of cometary grains

The results of narrowband filter photometry of Comet P/Stephan-Oterma are presented. For heliocentric distances between 1.95 and 1.58 AU prior to perihelion, OH, CN, and C2 production rates varied approximately as r to the -5th power. Following perihelion, an even steeper dependence of production rate on heliocentric distance was observed for these species. A pronounced brightening of the comets continuum near opposition is interpreted in terms of preferential backscattering by the cometary dust. While Comet P/Stephan-Oterma is one of the dustier comets observed, it shows no indication of compositional anomalies in either the gas or the dust.

Comet 19P/Borrelly at multiple apparitions: seasonal variations in gas production and dust morphology

We present analysis and results from both narrowband photometry and CCD imaging of Comet 19P/Borrelly from multiple apparitions. Production rates for Borrelly a few days prior to the Deep Space 1 spacecraft encounter were Q(OH) = 2.1×1028 molecule s−1, Q(CN) = 5.1×1025 molecule s−1, and A(θ)fρ = 400–500 cm. The equivalent Q(water; vectorial) = 2.5×1028 molecule s−1. We also find that the radial fall-off of the dust is significantly steeper than the canonical 1/ρ for aperture sizes larger than ρ = 2×104 km. In the near-UV, a strong trend in dust colors with aperture size is present. Imaging of Borrelly revealed a strong radial jet in the near-sunward direction that turns off late in the apparition. For the jet to appear radial, it must originate at or very close to the nucleus’ pole. Modeling the measured position angle of this jet as a function of time during the 1994 and 2001 apparitions yields a nucleus in a simple, rather than complex, rotational state with a pole orientation having an obliquity of 102.7° ± 0.5° and an orbital longitude of the pole of 146° ± 1°, corresponding to an RA of 214.1° and a Declination of −5.7° (J2000). There is also evidence for a small (∼8°) precession of the pole over the past century, based on our preferred model solution for jet measurements obtained during the 1911–1932 apparitions. Our solution for the orientation of the rotation axis implies a very strong seasonal effect as the source region for the jet moves from summer to winter. This change in solar illumination quantitatively explains both the nearly level water production measured in the seven weeks preceding perihelion and the extremely large decrease in water production (25×) as Borrelly moved from perihelion to 1.9 AU. A much smaller fall-off in apparent dust production after perihelion can be explained by a population of old, very slowly moving large grains released near peak water production, and therefore not indicative of the actual ongoing release of dust grains late in the apparition. Based on the water vaporization rate, the source region has an area of approximately 3.5 km2 or 4% of the total surface area of the nucleus, and water ice having an effective depth of 3–10 m is released each apparition from this source region.