排序方式： 共有6条查询结果，搜索用时 15 毫秒
1
1.
2.
Julie Ziffer Humberto Campins Javier Licandro Noemi Pinilla-Alonso Yanga Fernandez Schelte J. Bus 《Earth, Moon, and Planets》2005,97(3-4):203-212
We present near infrared reflectance spectra from 0.8 to 2.5 μm of two asteroids with low Tisserand invariant, 1373 Cincinnati and 2906 Caltech. We compare our spectra with cometary nuclei and other asteroids in their class. Asteroids Cincinnati and Caltech have Tisserand invariant values of 2.72 and 2.97, respectively, values less than 3 are considered suggestive of cometary origin. The observed spectral slopes in the near-infrared are consistent with both the spectra of cometary nuclei and of primitive asteroids. However, both asteroids have features in the near-infrared that are not seen in cometary nuclei, but are present in other X-type asteroids. 1373 Cincinnati has a sharp slope change between 0.75 and 1.0 μm and 2906 Caltech has a broad and shallow absorption between 1.35 and 2.2 μm. Our attempts to model the visible and near-infrared spectrum of these two objects, with the components successfully used by Emery and Brown (2004, Icarus
164, 104–121) to fit Trojan asteroids, did not yield acceptable fits.Visiting Astronomer at the Infrared Telescope Facility, which is operated by the University of Hawaii under contract to the National Aeronautics and Space Administration. 相似文献
3.
Yanga R. Fernández 《Planetary and Space Science》2009,57(10):1218-1227
Our current understanding of split, Jupiter-family comets is reviewed. The focus is on what recent studies of comets have told us about the nature of the splitting phenomenon. The goal is to not repeat the information given in recent reviews of split comets, but to build upon it. In particular, we discuss comets that have suffered splitting or fragmentation events in the past few years. These include comets (a) 57P/du Toit-Neujmin-Delporte, observed with a long train of fragments in 2002; (b) 73P/Schwassmann-Wachmann 3, which split in 1995 and was extensively studied during its relatively close passage to Earth in 2006, during which dozens of fragments were discovered and studied; and (c) 174P/Echeclus, a Centaur and potentially future JFC, which split in late 2005 and was the first such Centaur observed to do so. We also discuss recent observations by SOHO of split comets that are likely of short-period. The Spitzer Space Telescope has observed many JFCs and provided us with unprecedented detailed views of cometary debris trails, which may be thought of as a middle ground between “normal” ejection of micron-sized dust grains and the cleaving off of meter-to-kilometer sized fragments. We will also discuss potential breakthroughs in studying splitting JFCs that may come from future surveys. 相似文献
4.
Direct observations of the nuclear surfaces of comets have been difficult; however a growing number of studies are overcoming observational challenges and yielding new information on cometary surfaces. In this review, we focus on recent determinations of the albedos, reflectances, and thermal inertias of comet nuclei. There is not much diversity in the geometric albedo of the comet nuclei observed so far (a range of 0.025 to 0.06). There is a greater diversity of albedos among the Centaurs, and the sample of properly observed TNOs(2) is still too small. Based on their albedos and Tisser and invariants, Fernández et al. (2001) estimate that about 5% of the near-Earth asteroids have a cometary origin, and place an upper limit of 10%. The agreement between this estimate and two other independent methods provide the strongest constraint to date on the fraction of objects that comets contribute to the population of near-Earth asteroids. There is a diversity of visible colors among comets, extinct comet candidates, Centaurs and TNOs. Comet nuclei are clearly not as red as the reddest Centaurs and TNOs. What Jewitt (2002) calls ultra-red matter seems to be absent from the surfaces of comet nuclei. Rotationally resolved observations of both colors and albedos are needed to disentangle the effects of rotational variability from other intrinsic qualities. New constraints on thermal inertia of comets are consistent with previous independent estimates. The thermal inertia estimates for Centaurs 2060 Chiron and 8405 Asbolus are significantly lower than predicted by thermal models, and also lower than the few upper limits or constraints known for active, ordinary nuclei. 相似文献
5.
Humberto Campins Michael S. Kelley Yanga Fernández Javier Licandro Kelsey Hargrove 《Earth, Moon, and Planets》2009,105(2-4):159-165
We present initial results from a study of a sample of low-perihelion near-Earth asteroids (NEAs) using the Infrared Spectrograph (IRS) on NASA’s Spitzer Space Telescope. The 7–14 μm thermal emission spectra have been fitted with models of the thermal continuum to derive the asteroid’s effective diameter, geometric albedo and beaming parameter η. In this work, we concentrate on the thermal behavior and we find a trend of increasing η (lower thermal fluxes and cooler color temperatures) with increasing solar phase angle. The slope of this trend is somewhat different from that reported for other NEAs (e.g., Delbó 2004); if confirmed, this result would indicate that the thermal behavior of low-perihelion asteroids is different from that of other members of the NEA population. In addition, deviations of the observed continuum from the thermal model, which can be diagnostic of composition, are apparent in a few of our targets. A complete characterization of these intrinsically faint objects will benefit from the large ground based facilities described elsewhere in these proceedings. 相似文献
6.
A review of our current understanding of Comet Hale-Bopp’s nuclear size is presented. Currently the best constraints on the effective radiusare derived from late-1996 mid-IR data and near-perihelion radio data.Unfortunately the two regimes give differing answers for the radius. A possible reconciliation of the two datasets is presented that would place the radius at around 30 km. This is a large cometary radius compared to the others that are known, and this motivates a discussion of what makes a large comet different. From several possible large-comet properties, Hale-Bopp’s activity is analyzed, focusing on the production rates, coma jet features, dust optical depth, and relationship with the interplanetary dust environment. The optical depth is particularly important since an optically-thick inner coma could complicate attempted measurements of the “nucleus”. 相似文献
1