The dust coma of periodic comet Churyumov-Gerasimenko (1982 VIII)

The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ～2 × 10⁵ g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ～0.04 at 1.25 μm and ～0.05 at 2.2 μm.     

Infrared spectrophotometry of Comet IRAS-Araki-Alcock (1983d): a bare nucleus revealed?

Spectra of the central core and surrounding coma of Comet IRAS-Araki-Alcock (1983d) were obtained at 8–13 μm on 11 May and 2–4 μm on 12 May 1983. Spatially resolved measurements at 10 μm with a 4-arcsec beam showed that the central core was more than 100 times brighter than the inner coma only 8 arcsec away; for radially outflowing dust, the brightness ratio would be a factor of 8. The observations of the central core are consistent with direct detection of a nucleus having a radius of approximately 5 km. The temperature of the sunlit hemisphere was > 300 K. Spectra of the core are featureless, while spectra of the coma suggest weak silicate emission. The spectra show no evidence for icy grains. The dust producton rate on 11.4 May was ～ 10⁵ g/sec, assuming that the gas flux from the dust-producing areas on the nucleus was ～ 10^?5 g/cm²/sec.     

Radiometry of near-earth asteroids

We report 10 micrometers infrared photometry for 22 Aten, Apollo, and Amor asteroids. Thermal models are used to derive the corresponding radiometric albedos and diameters. Several of these asteroids appear to have surfaces of relatively high thermal inertia due to the exposure of bare rock or a coarse regolith. The Apollo asteroid 3103, 1982 BB, is recognized as class E. The Jupiter-crossing Amor asteroid 3552, 1983 SA, is confirmed as class D, but low albedos remain rare for near-Earth asteroids.     

On the albedo of the interplanetary dust

The zodiacal light brightness and measured spatial density of the interplanetary dust lead to a mean geometric albedo of 0.24 for the dust particles near 1 AU; whereas the composition of collected micrometeroids suggests a geometric albedo ?0.1. The data do not support the very low albedo (?0.01) proposed by A. F. Cook [Icarus33 (1978), 349–360]. The evidence is against a change in the mean particle albedo between 0.1 and 2 AU. Beyond 2 AU the data are unclear and a change in albedo is not ruled out.     

Infrared photometry of comets Bowell and P/Stephan-Oterma

We report broadband infrared photometry of comets P/Stephan-Oterma and Bowell between 1 and 20 μm. Their JHK colors are similar to P/Meier and P/Tuttle and are compatible with scattering of sunlight by micron-sized grains. The thermal emission from P/Stephan-Oterma showed an effective temperature significantly higher than that expected from a blackbody in equilibrium. The thermal emission can be models be fit by models of the dust coma consisting of micron-sized grains. Most of the flux at all observed wavelengths comes from the dust grains rather than form the nucleus.     

The Intermittently Embedded Herbig Ae/Be Stars ISO and Ground-Based IR Observations

We discuss silicate emission profiles observed with the ISO SWS and ground-based IR observations in a population of intermittently embedded Herbig Ae/Be stars which are viewed edge-on to their polarimetrically identified dust and gas disks. The ISO SWS observations confirm the lack of a simple correlation between system age and the profile shape. Comparison with laboratory silicates suggests that much of the observed variation is due to different annealing histories of the grains. This revised version was published online in August 2006 with corrections to the Cover Date.     

The dust distribution within the inner coma of comet P/Halley 1982i: encounter by Giotto's impact detectors

Analysis of the data from Giotto's Dust Impact Detection System experiment (DIDSY) is presented. These data represent measurement of the size of dust grains incident on the Giotto dust shield along its trajectory through the coma of comet P/Halley on 1986 March 13/14. First detection occurred at some 287000 km distance from the nucleus on the inbound leg; the majority of the DIDSY subsystems remained operational after closest approach (604 km) yielding the last detection at about 202000 km from the nucleus. In order to improve the data coverage (and especially for the smallest grains, to approximately 10(-19) kg particle mass), data from the PIA instrument has been combined with DIDSY data. Flux profiles are presented for the various mass channels showing, to a first approximation, a 1/R2 flux dependence, where R is the distance of the detection point from the cometary nucleus, although significant differences are noted. Deviations from this dependence are observed, particularly close to the nucleus. From the flux profiles, mass and geometrical area distributions for the dust grains are derived for the trajectory through the coma. Groundbased CCD imaging of the dust continuum in the inner coma at the time of encounter is also used to derive the area of grains intercepted by Giotto. The results are consistent with the area functions derived by Giotto data and the low albedo of the grains deduced from infrared emission. For the close encounter period (-5 min to +5 min), the cumulative mass distribution function has been investigated, initially in 20 second periods; there is strong evidence from the data for a steepening of the index of the mass distribution for masses greater than 10(-13) kg during passage through dust jets which is not within the error limits of statistical uncertainty. The fluences for dust grains along the entire trajectory is calculated; it is found that extrapolation of the spectrum determined at intermediate masses (cumulative mass index alpha = 0.85) is not able to account for the spacecraft deceleration as observed by the Giotto Radio Science Experiment and by ESOC tracking operations. Data at large masses (>10(-8) kg) recently analysed from the DIDSY data set show clear evidence of a decrease in the mass distribution index at these masses within the coma, and it is shown that such a value of the mass index can provide sufficient mass for consistency with the observed deceleration. The total particulate mass output from the nucleus of comet P/Halley at the time of encounter would be dependent on the maximum mass emitted if this change in slope observed in the coma were also applicable to the emission from the nucleus; this matter is discussed in the text. The flux time profiles have been converted through a simple approach to modeling of the particle trajectories to yield an indication of nucleus surface activity. There is indication of an enhancement in flux at t approximately -29 s corresponding to crossing of the dawn terminator, but the flux detected prior to crossing of the dawn terminator is shown to be higher than predicted by simple modelling. Further enhancements corresponding to jet activity are detected around +190 s and +270 s.     

Palomar and Table Mountain observations of 9P/Tempel 1 during the Deep Impact encounter: First results

We present the first results of the Palomar Adaptive Optics observations taken during the Deep Impact encounter with 9P/Tempel 1 in July 2005. We have combined the Palomar near-IR imaging data with our visual wavelength images obtained simultaneously at JPL's Table Mountain Observatory to cover the total wavelength range from 0.4 to 2.3 μm in the B, V, R, I, J, H, and K filter bands, spanning the dates from 2005 July 03-07. We also include in our overall analysis images taken on the pre-encounter dates of June 1 and June 15, 2005. The broad wavelength range of our observations, along with high temporal resolution, near-IR sensitivity, and spatial resolution of our imaging, have enabled us to place constraints on the temperature of the impact flash and incandescent plume of >700 K, and to provide mean dust velocities of order approximately 1.25 h after impact derived from our 1.64 μm observations. Our ejected dust mass estimates, as derived from our near-IR observations, are an order of magnitude less than those previously reported for visual wavelength observations.     

Infrared observations of Comet 81P/Wild 2 in 1997

Comet 81P/Wild 2 was observed in the thermal infrared over 6 months during its 1997 perihelion passage. The comet was most active in late February, about 3 months preperihelion; dust production declined by a factor of 3 between February and August. For the GIOTTO Halley dust size distribution, maximum dust production rate was ∼2 × 10⁶ g/s. The comet displayed a 10-μm silicate feature about 25% above the continuum, similar to several other Jupiter-family comets, but much lower than that seen in a number of Oort cloud comets.NASA’s STARDUST sample return mission will encounter P/Wild 2 98 days postperihelion in January 2004. Based on our observations at a similar point in the orbit and the Halley size distribution, we predict that the mass fluence on the spacecraft for a 150 km miss distance will be about 8 × 10⁻⁶ g/cm² for particles up to 1 cm in radius. The corresponding areal coverage will be about 10⁻⁴.     

Infrared emission from P/Halley's dust coma during March 1986

2 to 20 micrometers photometry of the inner dust coma of comet Halley was obtained at the NASA IRTF on Mar 6.85, 12.8, 13.75, 17.7, and 24.8. Positions offset 10" were measured as well as the central brightness. The strength of the 10 micrometers emission feature was observed to vary with location in the coma. The infrared emission is in general agreement with the dust size distribution measured from the Vega and Giotto spacecraft. Mar 6.8, 17.7, and 24.8 corresponded to strong dust jet activity. The strength of the 10 micrometers silicate emission is shown to be a sensitive indicator of grain size and thus of jet activity. Dust production rate on March 13.75, 6 h before Giotto encounter, was approximately 10(7) gm s-1.