A detailed description of the Halley-type Comet C/2001 OG108 (LONEOS) has been derived from visible, near-infrared, and mid-infrared observations obtained in October and November 2001. These data ...represent the first high- quality ground-based observations of a bare Halley-type comet nucleus and provide the best characterization of a Halley-type comet other than 1P/Halley itself. Analysis of time series photometry suggests that the nucleus has a rotation period of 57.2+/-0.5 h with a minimum nuclear axial ratio of 1.3, a phase-darkening slope parameter G of -0.01+/-0.10, and an estimated H=13.05+/-0.10. The rotation period of C/2001 OG108 is one of the longest observed among comet nuclei. The V-R color index for this object is measured to be 0.46+/-0.02, which is virtually identical to that of other cometary nuclei and other possible extinct comet candidates. Measurements of the comet's thermal emission constrain the projected elliptical nuclear radii to be 9.6+/-1.0 km and 7.4+/-1.0 km, which makes C/2001 OG108 one of the larger cometary nuclei known. The derived geometric albedo in V-band of 0.040+/- 0.010 is typical for comet nuclei. Visible-wavelength spectrophotometry and near-infrared spectroscopy were combined to derive the nucleus's reflectance spectrum over a 0.4 to 2.5 D*mm wavelength range. These measurements represent one of the few nuclear spectra ever observed and the only known spectrum of a Halley-type comet. The spectrum of this comet nucleus is very nearly linear and shows no discernable absorption features at a 5% detection limit. The lack of any features, especially in the 0.8 to 1.0 D*mm range such as are seen in the spectra of carbonaceous chondrite meteorites and many low-albedo asteroids, is consistent with the presence of anhydrous rather than hydrous silicates on the surface of this comet. None of the currently recognized meteorites in the terrestrial collections have reflectance spectra that match C/2001 OG108. The near-infrared spectrum, the geometric albedo, and the visible spectrophotometry all indicate that C/2001 OG108 has spectral properties analogous to the D-type, and possibly P-type asteroids. Comparison of the measured albedo and diameter of C/2001 OG108 with those of Damocloid asteroids reveals similarities between these asteroids and this comet nucleus, a finding which supports previous dynamical arguments that Damocloid asteroids could be composed of cometary-like materials. These observations are also consistent with findings that two Jupiter-family comets may have spectral signatures indicative of D-type asteroids. C/2001 OG108 probably represents the transition from a typical active comet to an extinct cometary nucleus, and, as a Halley-type comet, suggests that some comets originating in the Oort cloud can become extinct without disintegrating. As a near-Earth object, C/2001 OG108 supports the suggestion that some fraction of the near-Earth asteroid population consists of extinct cometary nuclei.
Radar and Optical Observations of Asteroid 1998 KY26 Ostro, Steven J.; Pravec, Petr; Lance A. M. Benner ...
Science (American Association for the Advancement of Science),
07/1999, Letnik:
285, Številka:
5427
Journal Article
Recenzirano
Observations of near-Earth asteroid 1998 KY26 shortly after its discovery reveal a slightly elongated spheroid with a diameter of about 30 meters, a composition analogous to carbonaceous chondritic ...meteorites, and a rotation period of 10.7 minutes, which is an order of magnitude shorter than that measured for any other solar system object. The rotation is too rapid for 1998 KY26 to consist of multiple components bound together just by their mutual gravitational attraction. This monolithic object probably is a fragment derived from cratering or collisional destruction of a much larger asteroid.
Photometry and models of eight near-Earth asteroids Kaasalainen, Mikko; Pravec, Petr; Krugly, Yurij N. ...
Icarus (New York, N.Y. 1962),
2004, 2004-01-00, 20040101, Letnik:
167, Številka:
1
Journal Article
Recenzirano
We present new observations and models of the shapes and rotational states of the eight near-Earth Asteroids (1580) Betulia, (1627) Ivar, (1980) Tezcatlipoca, (2100) Ra-Shalom, (3199) Nefertiti, ...(3908) Nyx, (4957) Brucemurray, and (5587) 1990 SB. We also outline some of their solar phase curves, corrected to common reference geometry with the models. Some of the targets may feature sizable global nonconvexities, but the observable solar phase angles were not sufficiently high for confirming these. None is likely to have a very densely cratered surface. We discuss the role of the intermediate topographic scale range in photometry, and surmise that this scale range is less important than large or small scale lengths.
Lightcurves of 26 Near-Earth Asteroids Pravec, Petr; Wolf, Marek; Šarounová, Lenka
Icarus (New York, N.Y. 1962),
November 1998, 1998-11-00, Letnik:
136, Številka:
1
Journal Article
Recenzirano
We present the results of our photometric observations of 26 near-Earth asteroids (NEAs) in the range of absolute magnitudesH= 13.6–20.0 (diameters approximately 0.4–8 km). The synodic periods in the ...range 2.3–230 h were detected for 25 of them; 21 periods are new and in 4 cases we confirmed earlier determinations. In 20 cases the synodic periods are interpreted as being the rotation periods. Among the 5 exceptions, in two cases there remains an uncertainty whether the detected period is not half or twice that of the rotation period, and in another two cases—(3691) 1982 FT and 1997 BR—there were found large deviations of the lightcurve points from the mean curves that can be due to possible complex rotations of the small, slowly rotating asteroids. Overall, the short period end (2.3–3.3 h) of the spin rate distribution shows characteristics that are consistent with the hypothesis of their “rubble pile” structure, as noted by Harris (LunarPlanet. Sci.XXVII, 493–494); specifically, there is a “barrier” against spins faster than 2.3 h and the amplitudes of the fast rotating NEAs are smaller in comparison with the other, longer period NEAs. In the group of slow rotators (P> 12 h), the suggested presence of objects in excited rotation states must be confirmed by further observations using also different techniques. This slow rotators group may be actually more abundant than our results suggest (6 of 25 objects, i.e., 20–30%), since there is a bias against low-amplitude slow rotators in the groundbased photometric program.
Lightcurves of 7 Near-Earth Asteroids Pravec, Petr; Šarounová, Lenka; Wolf, Marek
Icarus (New York, N.Y. 1962),
12/1996, Letnik:
124, Številka:
2
Journal Article
Recenzirano
We present results of our CCD photometry of near-Earth asteroids (1627), (4957), (6569), 1993 UC, 1994 AH2, 1994 CB, and 1994 LX, that we made in 1994 and 1995. Synodic rotation periods were ...determined for 6 of them for the first time, and we provide constraints and show a consistency of our results with the earlier spin vector determination for (1627). Among the other objects, the most interesting observations were obtained for (4957), containing information useful for future spin vector determinations; 1993 UC, that is the second fastest rotator known among asteroids (period 2.340 hr); 1994 AH2, an object on the margin between asteroidal and cometary orbital types, with the probable rotation period of 23.95 hr; and 1994 CB, one of the smallest (est. 0.2 km) asteroids observed photometrically up to now.
We present the results of photometric observations of the Apollo asteroid 1991 VH. Its lightcurve consists of two components: the first is the rotational lightcurve with periodPs= (0.109327 ± ...0.000003) d and amplitude 0.09 mag, while the second, with periodPl= (1.362 ± 0.001) d, shows two minima with depth 0.16–0.19 mag, each with a duration of about 0.10 d, and little or no variation at phases between them. We present a model of the occulting/eclipsing binary asteroid with the secondary-to-primary diameter ratiods/dp= 0.40 that explains the observed lightcurve. In this model, the primary's rotation is not synchronized with the orbital motion and produces the short-period lightcurve component (Ps). The orbital period isPl. The mutual orbit's semimajor axis is estimated to bea= (2.7 ± 0.3)dp; the eccentricity is 0.07 ± 0.02. The similarity between the lightcurve of 1991 VH and those of 1994 AW1(Pravec and Hahn,Icarus127, 431, 1997) and (3671) Dionysus (Mottolaet al.1997,IAU Circular6680) suggests that binary asteroids may be common among near-Earth asteroids. Based on the three known cases, we tentatively derive some typical characteristics of this new class of asteroids. They are mostly consistent with the hypothesis that binary asteroids are generated by tidal disruptions of weak, gravitationally bound aggregates (so-called “rubble piles”) during encounters with the Earth (Bottke and Melosh,Nature281, 51, 1996). A possible relationship between the population of binary asteroids and the belt of small near-Earth asteroids is discussed.
A detailed description of the Halley-type Comet C/2001 OG
108 (LONEOS) has been derived from visible, near-infrared, and mid-infrared observations obtained in October and November 2001. These data ...represent the first high-quality ground-based observations of a bare Halley-type comet nucleus and provide the best characterization of a Halley-type comet other than 1P/Halley itself. Analysis of time series photometry suggests that the nucleus has a rotation period of
57.2
±
0.5
h
with a minimum nuclear axial ratio of 1.3, a phase-darkening slope parameter
G of
−
0.01
±
0.10
, and an estimated
H
=
13.05
±
0.10
. The rotation period of C/2001 OG
108 is one of the longest observed among comet nuclei. The
V-
R color index for this object is measured to be
0.46
±
0.02
, which is virtually identical to that of other cometary nuclei and other possible extinct comet candidates. Measurements of the comet's thermal emission constrain the projected elliptical nuclear radii to be
9.6
±
1.0
km
and
7.4
±
1.0
km
, which makes C/2001 OG
108 one of the larger cometary nuclei known. The derived geometric albedo in
V-band of
0.040
±
0.010
is typical for comet nuclei. Visible-wavelength spectrophotometry and near-infrared spectroscopy were combined to derive the nucleus's reflectance spectrum over a 0.4 to 2.5 μm wavelength range. These measurements represent one of the few nuclear spectra ever observed and the only known spectrum of a Halley-type comet. The spectrum of this comet nucleus is very nearly linear and shows no discernable absorption features at a 5% detection limit. The lack of any features, especially in the 0.8 to 1.0 μm range such as are seen in the spectra of carbonaceous chondrite meteorites and many low-albedo asteroids, is consistent with the presence of anhydrous rather than hydrous silicates on the surface of this comet. None of the currently recognized meteorites in the terrestrial collections have reflectance spectra that match C/2001 OG
108. The near-infrared spectrum, the geometric albedo, and the visible spectrophotometry all indicate that C/2001 OG
108 has spectral properties analogous to the D-type, and possibly P-type asteroids. Comparison of the measured albedo and diameter of C/2001 OG
108 with those of Damocloid asteroids reveals similarities between these asteroids and this comet nucleus, a finding which supports previous dynamical arguments that Damocloid asteroids could be composed of cometary-like materials. These observations are also consistent with findings that two Jupiter-family comets may have spectral signatures indicative of D-type asteroids. C/2001 OG
108 probably represents the transition from a typical active comet to an extinct cometary nucleus, and, as a Halley-type comet, suggests that some comets originating in the Oort cloud can become extinct without disintegrating. As a near-Earth object, C/2001 OG
108 supports the suggestion that some fraction of the near-Earth asteroid population consists of extinct cometary nuclei.
