Richard P. Binzel

Collisional evolution in the Eos and Koronis asteroid families - Observational and numerical results

Abstract Recent observational results ( R.P. Binzel 1987 , Icarus 22, 135–208) are analyzed to investigate the origin and evolution of the Eos and Koronis families. Rotation rates in the Eos family display a Maxwellian distribution implying a collisionally evolved population and are significantly faster than Koronis family and nonfamily asteroids. Observational and laboratory results suggest that the largest members in each family may still “remember” their parent-body rotation rates which were both in the range of 1–3 revolutions/ day. Results from a numerical model for the collisional evolution of rotational angular momentum are used to interpret the different rotational characteristics as possibly being due to different relative ages of the families. The age of the Eos family may be comparable with that of the Solar System while the Koronis family may be considerably younger. Koronis family members display a higher mean lightcurve amplitude and therefor may be more irregularly shaped. Alternatively, the higher amplitudes may be accounted for by a preferential low obliquity alignment of spin vectors, a testable hypothesis.

A photoelectric survey of 130 asteroids

A major observational survey was conducted to facilitate the investigation of asteroid collisional evolution processes. Photoelectric observations of 130 asteroids were obtained at the University of Texas McDonald Observatory and Cerro Tololo Inter-American Observatory on 150 nights between November 1982 and February 1985. The program objects (typically D ∼ 30 km) were primarily sampled from six distinct regions in the asteroid belt to illuminate two specific problems: the role of collisions in clearing the 31 resonance Kirkwood gap and the formation and evolution of the Eos and Koronis families. Over 400 individual lightcurves were obtained which when combined allowed rotational periods and lightcurve amplitudes to be derived for all but a few of the surveyed asteroids. These data have been calibrated to the Johnson B magnitude system. B−V and U−B colors were measured for a majority of the objects and absolute magnitudes, H, have been computed.

Improved Orbital and Physical Parameters for the Pluto-Charon System

Analysis of the observations of several Pluto-Charon occultation and transit events in 1985 and 1986 has provided a more detailed knowledge of the system. The sum of the radii of Pluto and Charon is 1786 � 19 kilometers, but the individual radii are somewhat more poorly determined; Pluto is 1145 � 46 kilometers in radius and Charon is 642 � 34 kilometers in radius. The mean density of the system is 1.84 � 0.19 grams per cubic centimeter, implying that more than half of the mass is due to rock. Charon appears to have hemispheres of two different colors, the Plutofacing side being neutral in color and the opposite hemisphere being a reddish color similar to Pluto.

Speckle interferometry of asteroids II. 532 Herculina

Abstract Speckle interferometry of 532 Herculina performed on January 17 and 18, 1982, yields triaxial ellipsoid dimensions of (263 ± 14) × (218 ± 12) × (215 ± 12) km, and a north pole for the asteroid within 7° of RA = 7 b 47 m and DEC = −39° (ecliptic coordinates γ = 132° β = −59°). In addition, a “spot” some 75% brighter than the rest of the asteroid is inferred from both speckle observations and Herculinas lightcurve history. This bright complex, centered at asterocentric latitude −35°, longitude 145–165°, extends over a diameter of 55° (115 km) of the asteroids surface. No evidence for a satellite is found from the speckle observations, which leads to an upper limit of 50 km for the diameter of any satellite with an albedo the same as or higher than Herculina.

Is 1220 Crocus a precessing, binary asteroid?

Abstract Photoelectric observations of asteroid 1220 Crocus (an Eos family member) show evidence for two distinct periods in its light variation: 30.7 days and 7.90 hr. The lightcurve amplitudes are 0.87 and 0.15 mag, respectively. The shorter period variation appears to be modulated over the longer period. Two periods are possible for a rigid body only if it is in a state of precession. The observations are shown to be compatible with a body in a forced precession state. This result leads us to hypothesize the existence of a satellite of Crocus as the source of the external torque. Calculations are presented which show that there are in fact dynamically possible “binary asteroid” solutions consistent with the observations. More photoelectric and perhaps direct (space telescope) observations are needed to resolve the true nature of this asteroid.

A photoelectric lightcurve survey of small main belt asteroids

Abstract A survey to obtain photoelectric lightcurves of small main-belt asteroids was conducted from November 1981 to April 1982 using the 0.91- and 2.1-m telescopes at the University of Texas McDonald Observatory. A total of 18 main-belt asteroids having estimated diameters under 30 km were observed with over half of these being smaller than 15 km. Rotational periods were determined or estimated from multiple nights of observation for nearly all of these yielding a sample of 17 small main-belt asteroids which is believed to be free of observational selection effects. All but two of these objects were investigated for very short periods in the range of 1 min to 2 hr using power spectrum analysis of a continuous set of integrations. No evidence for such short periods was seen in this sample. Rotationally averaged B (1,0) magnitudes were determined for most of the surveyed asteroids, allowing diameter estimates to be made. Imposing the suspected selection effects of photographic photometry on the results of this survey gives excellent agreement with the results from that technique. This shows that the inability of photographic photometry to obtain results for many asteroids is indeed due to the rotational parameters of those asteroids.

The rotation of small asteroids

The addition of the unbiased sample of R. P. Binzel and J. D. Mulholland (Icarus 56, 519–533) nearly triples the sample size of photoelectrically determined rotational parameters for main belt asteroids with estimated diameters (D) ≤30 km. Nonparametric stattistical tests which require no assumptions about the distributions or variances of the samples are used to examine rotational parameters for all D ≤ 30 km asteroids. A comparison of photoelectric and photographic results shows that the techniques have a highly significant difference in the range of detected frequencies. This difference does not allow photographic and photoelectric observations to be combined for meaningful statistical tests since a photographic bias toward smaller sample variances can induce statistical results that appear overly significant. Photographic observations also show a highly significant bias toward detecting asteroids with larger lightcurve amplitudes. The fit of a Maxwellian to the observed rotational frequency distribution can be rejected at a highly significant confidence level but the observed distribution can be acceptably fit by two Maxwellian distributions, which is consistent with the hypothesis that there are separate populations of slow and fast rotating asteroids. The frequency distributions of <15 km main belt asteroids and Earth and Mars crossers are not found to differ significantly. However, the larger mean lightcurve amplitude of the Earth and Mars crossing asteroids is found to be statistically significant. This latter result is interesting in view of the lack of any strong inverse amplitude versus diameter relation for small asteroids. No significant diameter dependence on rotational frequency is seen among only D ≤ 30 km asteroids. However, the inverse frequency versus diameter relation for D ≤ 120 km asteroids found by S. F. Dermott, A. W. Harris, and C. D. Murray (Icarus, in press) is found to be statistically significant using a linear least-squares analysis of photoelectric data only. No significant diameter dependence on rotational lightcurve amplitude is seen using linear least-squares analysis of photoelectric data for D≤30 and D≤90 km asteroids. However, a significant inverse amplitude versus diameter relation is found when this analysis is extended to D≤120 km asteroids. This finding may be consistent with the hypothesis of Dermott et al. that near 120 km there is a transition between primordial asteroids and their collisional fragments.

2 Pallas: 1982 and 3 lightcurves and a new pole solution

Abstract Photoelectric lightcurves of asteroid 2 Pallas obtained in March 1982 and May 1983 display amplitudes of 0.04 and 0.10 magnitude respectively. The latter lightcurve shows that Pallas was at a V (1,0) magnitude of 4.51 ± 0.02 when it occulted 1 Vulpeculae on May 29 1983. A least-squares best fit to an amplitude-aspect relation for all available lightcurve observations of Pallas between 1951 and 1983 yields two solutions for its pole position: λ = 200, β = 40 and λ = 220, β = 15, where the uncertainty regions corresponds to an overall estimate of ± degrees. Use of phase angle bisector coordinates (A. W. Harris, J. W. Young, F. Scaltriti, and V. Zappala (1984) Icarus 57 , 251–258) gave lower residuals than geocentric coordinates. The (220,15) pole position is favored since it is in very good agreement with an independent pole solution obtained by photometric astrometry (J. V. Lambert 1983 personal communication). This pole position implies that the latitude of the sub-Earth point at the time of the occultation was 22 degrees.

Physical parameters of near-earth asteroid 1982 DV

Abstract Visual and infrared observations were made of Amor asteroid 1982 DV during its discovery apparition. Broadband visual and near-infrared photometry shows that it is an S-class asteroid. Narrowband spectrophotometry shows an absorption feature due to olivine or pyroxene or both centered at 0.93 μm. Applying a nonrotating thermal model to 10-μm photometry, the geometric albedo is calculated to be approximately 0.27. The geometric albedo for a slowly rotating, rocky surface was calculated for 1 night to be 0.15, consistent with S-class asteroid albedos. Thus, 1982 DV is either one of the most reflective S-class asteroids known, or a significant amount of bare rock is exposed on the asteroids surface. For the nonrotating model, ellipsoidal dimensions for 1982 DV are 3.5 × 1.4 × 1.4 km.

The rotation, color, phase coefficient, and diameter of 1915 quetzalcoatl

Abstract Photoelectric observations of 1915 Quetzalcoatl on March 2, 1981 show that this asteroid has a rotational period of 4.9 ± 0.3 hr and a lightcurve amplitude of 0.26 magnitudes. B-V and U-B colors are found to be 0.83 ± 0.04 and 0.43 ± 0.03, respectively, consistent with Quetzalcoatl being an S -type asteroid. Additional observations from March 31, 1981 give a linear phase coefficient of 0.033 mag deg −1 and a mean B (1,0) magnitude of 20.10. The resulting estimated mean diameter for Quetzalcoatl is only 0.37 km, making it one of the smallest asteroids for which physical observations have yet been made.