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The Kuiper belt is a collection of small bodies (Kuiper belt objects, KBOs) that lie beyond the orbit of Neptune and which are believed to have formed contemporaneously with the planets. Their small size and great distance make them difficult to study. KBO 55636 (2002 [TX.sub.300]) is a member of the water-ice-rich Haumea KBO collisional family (1). The Haumea family are among the most highly reflective objects in the Solar System. Dynamical calculations indicate that the collision that created KBO 55636 occurred at least 1 Gyr ago (2,3). Here we report observations of a multi-chord stellar occultation by KBO 55636, which occurred on 9 October 2009 UT. We find that it has a mean radius of 143 ± 5 km (assuming a circular solution). Allowing for possible elliptical shapes, we find a geometric albedo of [0.88.sup.+0.15.sub.-0.06] in the V photometric band, which establishes that KBO 55636 is smaller than previously thought and that, like its parent body, it is highly reflective. The dynamical age implies either that KBO 55636 has an active resurfacing mechanism, or that fresh water-ice in the outer Solar System can persist for gigayear timescales., Multi-station observations of stellar occultations by the largest KBOs allow us to accurately measure their radii; also, we can probe for atmospheres and for small, nearby companions (4,5). Unfortunately, the [...]

Solar occultation in the infrared, part of the Spectoscopy for Investigation of Characteristics of the Atmosphere of Venus (SPICAV) instrument onboard Venus Express, combines an echelle grating spectrometer with an acousto-optic tunable filter (AOTF). It performs solar occultation measurements in the IR region at high spectral resolution. The wavelength range probed allows a detailed chemical inventory of Venus's atmosphere above the cloud layer, highlighting the vertical distribution of gases. A general description of the instrument and its in-flight performance is given. Different calibrations and data corrections are investigated, in particular the dark current and thermal background, the nonlinearity and pixel-to-pixel variability of the detector, the sensitivity of the instrument, the AOTF properties, and the spectral calibration and resolution. OCIS codes: 050.1950, 120.0280, 120.6200, 230.1040, 300.6340, 010.1280.

The radio occultation (RO) technique, which makes use of radio signals transmitted by the global positioning system (GPS) satellites, has emerged as a powerful and relatively inexpensive approach for sounding the global atmosphere with high precision, accuracy, and vertical resolution in all weather and over both land and ocean. On 15 April 2006, the joint Taiwan-U.S. Constellation Observing System for Meteorology, Ionosphere, and Climate (COSMIC)/Formosa Satellite Mission 3 (COSMIC/FORMOSAT-3, hereafter COSMIC) mission, a constellation of six microsatellites, was launched into a 512-km orbit. After launch the satellites were gradually deployed to their final orbits at 800 km, a process that took about 17 months. During the early weeks of the deployment, the satellites were spaced closely, offering a unique opportunity to verify the high precision of RO measurements. As of September 2007, COSMIC is providing about 2000 RO soundings per day to support the research and operational communities. COSMIC RO data are of better quality than those from the previous missions and penetrate much farther down into the troposphere; 70%-90% of the soundings reach to within 1 km of the surface on a global basis. The data are having a positive impact on operational global weather forecast models. With the ability to penetrate deep into the lower troposphere using an advanced open-loop tracking technique, the COSMIC RO instruments can observe the structure of the tropical atmospheric boundary layer. The value of RO for climate monitoring and research is demonstrated by the precise and consistent observations between different instruments, platforms, and missions. COSMIC observations are capable of intercalibrating microwave measurements from the Advanced Microwave Sounding Unit (AMSU) on different satellites. Finally, unique and useful observations of the ionosphere are being obtained using the RO receiver and two other instruments on the COSMIC satellites, the tiny ionosphere photometer (TIP) and the tri-band beacon.

The formation of C[O.sub.2] ice clouds in the upper atmosphere of Mars has been suggested in the past on the basis of a few temperature profiles exhibiting portions colder than C[O.sub.2] frost point. However, the corresponding clouds were never observed. In this paper, we discuss the detection of the highest clouds ever observed on Mars by the SPICAM ultraviolet spectrometer on board Mars Express spacecraft. Analyzing stellar occultations, we detected several mesospheric detached layers at about 100 km in the southern winter subtropical latitudes, and found that clouds formed where simultaneous temperature measurements indicated that C[O.sub.2] was highly supersaturated and probably condensing. Further analysis of the spectra reveals a cloud opacity in the subvisible range and ice crystals smaller than 100 nm in radius. These layers are therefore similar in nature as the noctilucent clouds which appear on Earth in the polar mesosphere. We interpret these phenomena as C[O.sub.2] ice clouds forming inside supersaturated pockets of air created by upward propagating thermal waves. This detection of clouds in such an ultrararefied and supercold atmosphere raises important questions about the martian middle-atmosphere dynamics and microphysics. In particular, the presence of condensates at such high altitudes begs the question of the origin of the condensation nuclei. Keywords: Mars, atmosphere; Occultations

On 1995 August 21, Saturn's moon Titan occulted the 13th magnitude star GSC5254-00997. The predawn event was observed at 2.3 [micro]m with the NASA 3.0-m Infrared Telescope Facility. We measured the average atmospheric scale height between latitudes 50 [degrees] S and 67 [degrees] S by fitting isothermal models to the immersion portion of the occultation lightcurve. Within the altitude range of 300 to 500 km, we find an isothermal scale height of Titan's sunset atmosphere of 55 [+ or -] 9 km. Our derived temperature of 180 [+ or -] 30 K is consistent with modeled temperature profiles derived from Voyager 1 IRIS observations (A. Coustenis and B. B6zard 1995, Icarus 115, 126-140) and the 28 Sagittarii occultation event (W. B. Hubbard et al. 1993, Astron. Astrophys. 269, 541-563). The atmospheric albedo asymmetry due to low level haze (Caldwell et al. 1992, Icarus 96,1-9) is inferred from the difference in Titan's center-of-light (determined from precise astrometric measurements of images of Titan and the star before and after the occultation) and the actual center of Titan (determined from the occultation analysis). We find the southern hemisphere to be brighter than the northern hemisphere, a result consistent with Combes et al. (1997, Icarus 129, 482-497), Gibbard et al. (1999, Icarus 139, 189-201), and Smith et al. (1996 Icarus, 119, 336-349). Key Words: Titan, atmospheres; structure, occultations.

We observed a stellar occultation of the star U149 by Uranus from Lowell Observatory and the IRTF on Mauna Kea on November 6, 1998. The temperatures derived from isothermal fits to the Lowell lightcurves are 116.7 [+ or -] 7.9 K for immersion, and 124.8 [+ or -] 15.5 K for emersion. The secular increase in temperature seen during the period 1977-1983 has reversed. Furthermore, the rate of decrease ([greater than or equal to] 1.2 K/yr) cannot be explained solely by radiative cooling. Although the temperature structure of Uranus' upper atmosphere may be related to seasonal effects (e.g., the subsolar latitude) or local conditions (e.g., diurnally averaged insolation), these observations suggest nonradiative influences on the temperature, such as adiabatic heating/cooling or thermal conduction. Key Words: Uranus, atmosphere; occultations; atmospheres, structure; atmospheres, dynamics.

A dozen lightcurves obtained during the ground-based observations of the occultation of 28 Sgr by Titan (3 July 1989) are reanalyzed. Profiles of density and temperature between altitude levels z of 290 and 500 km (pressures p from 110 to 1.4 (mu)bar) are derived. A mean number-density scale height of 50.5 +/- 1.4 km is found with no significant difference between immersion and emersion. Two inversion layers are observed at 425 and 450-455 km, respectively (p approximately 7 (mu)bar and p approximately 4 (mu)bar), with an increase in temperature of about 10 K in less than (delta)z = 10 km. These layers are visible both at immersion and at emersion, at latitudes ranging from 46 degrees S to 20 degrees N, and are thus global features of the stratosphere. The profiles of temperature gradients exhibit a clear cutoff at the adiabatic lapse rate, indicating that fluctuations lead to marginal convective instabilities. Although ray crossing can also cause an apparent cutoff of the temperature gradients, we estimate it probably does not play an important role in the observed cutoff, at least for the larger structures under study. The vertical power spectra of fluctuations show a general power law behavior, with an exponent close to -3, between vertical wavelengths of approximately 5 and 50 km. The finite stellar diameter and ray crossings can distort the real spectra, and we can only conclude that the original power spectra have slopes between -2 and -3. The horizontal structure of the atmosphere exhibits typical aspect (horizontal-to-vertical) ratios of 15-45, with a tail in the distribution with values as high as 100-200 for some structures. Finally, the horizontal spectrum of fluctuations is a power law with an exponent close to -4 (between horizontal wavelengths of approximately 25 and 250 km), if we assume it is separable from the vertical spectrum. Key Words: Titan; atmospheres (dynamics and structure); occultations.

We present the results of a comparison of the total extinction altitude profiles measured at the same time and at same location by the ORA (Occultation Radiometer) and Stratospheric Aerosol and Gas Experiment II solar occultation experiments at three different wavelengths. A series of 25 events for which the grazing points of both experiments lie within a 2 [degrees] window has been analyzed. The mean relative differences observed over the altitude range 15-45 km are -8.4%, 1.6%, and 3% for the three channels (0.385, 0.6, and 1.02 [[micro]meter]). Some systematic degradation occurs below 20 km (as the result of signal saturation and possible cloud interference) and above 40 km (low absorption). The fair general agreement between the extinction profiles obtained by two different instruments enhances our confidence in the results of the ORA experiment and of the recently developed vertical inversion algorithm applied to real data.

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 period [P.sub.s] = (0.109327 [+ or -] 0.000003) d and amplitude 0.09 mag, while the second, with period [P.sub.1] = (1.362 [+ or-] 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 ratio [d.sub.s]/[d.sub.p] = 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 ([P.sub.s]). The orbital period is [P.sub.1]. The mutual orbit's semimajor axis is estimated to be a = (2.7 [+ or-] 0.3) [d.sub.p]; the eccentricity is 0.07 [+ or -] 0.02. The similarity between the lightcurve of 1991 VH and those of 1994 A[W.sub.1] (Pravec and Hahn, Icarus 127, 431, 1997) and (3671) Dionysus (Mottola et al. 1997, IAU Circular 6680) 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, Nature 281, 51, 1996). A possible relationship between the population of binary asteroids and the belt of small near-Earth asteroids is discussed. Key Words: minor planets; multiple period lightcurves; binary asteroids.

We have observed a stellar occultation of GSC5249-01240 by Saturn's north polar region on November 20, 1995 from NASA's Infrared Telescope Facility (IRTF). This is the first recorded occultation by the polar region of a giant planet. The occulted region extends 88 km in vertical height and 660 km in horizontal length, over a region from 82.5 [degrees] to 85 [degrees] in planetocentric latitude and from 20 [degrees] to 30 [degrees] in planetocentric longitude. Based on isothermal model fits to the light curve, we find an equivalent isothermal temperature of 130 [+ or -] 10 K at a pressure level of 1.6 [+ or -] 0.1 [micro]bar, which corresponds to a half-light latitude of 83.2 [+ or -] 0.2 [degrees] and longitude of 24.1 [+ or -] 0.5 [degrees]. Using numerical inversion procedures, we have retrieved the temperature profile of the occulted region, which suggests an increase in temperature (with radius) of 14.5 K between 6 and 10 [Mu]bar. We also find temperature fluctuations of 1 to 5 K along the path probed by the occultation; if the observed temperature gradients of these fluctuations apply to the vertical direction only, then this region is super-adiabatic. More likely, these thermal gradients are due to a combination of diffractive scintillations and horizontal temperature variations. Given that isothermal model fits and numerical inversions cannot separate individual contributions to observed temperature gradients, such as from vertical variations, horizontal variations, and scintillations, this occultation requires further study. Key Words: atmosphere dynamics; atmosphere structure; occultations; Saturn atmosphere.

In this paper, we present results from analysis of GPS/MET occultation data obtained from the University Corporation for Atmospheric Research database. We have used high-rate occultation data to study the performance of an earlier 4D ionospheric electron content model - obtained tomographically with the use of the GPS constellation - with satisfactory results. In addition, we have processed UCAR level-one medium-rate occultation data by using the GIPSY package to obtain aligned phases. Under the assumption of local spherical symmetry, this phase information has been processed to yield ray-path bending angles through Doppler-shift analysis, which have then been used to yield profiles of electronic density via the Abel transform. These profiles give important information for preparing future tomographic work, although the limitations imposed by the working assumptions in this approach cannot be ignored. Index Terms - Inverse problem, ionosphere, occultations, tomography.

We observed selected mutual occultations of Jupiter's satellites in 1990-1991 at four wavelengths and we used the occultation profiles to constrain the areal distribution of selected spectrally active compounds on Io's surface. We performed high speed photometry of Io at four wavelengths while it was being occulted by another Galilean satellite. The wavelengths of the filters that we used for this work were 0.305, 0.345, 0.45, and 0.56 [[micro]meter]. These wavelengths were selected to constrain the areal distribution of sulfur and/or sulfur-bearing compounds and sulfur dioxide (S[O.sub.2]) frost, believed to be the two major surface constituents on Io. During a typical occultation, a Galilean satellite whose spectral properties are well defined passed in front of Io, gradually blocking the sunlight reflected by Io from reaching an Earth-based observer. In the observations reported here the occulting satellite was always Europa. Europa's contribution to the combined signal was removed and the remaining Io occultation brightness variations were compared to curves produced by synthesized occultations that would be expected assuming a two component surface, where one component is an unknown mixture of sulfur bearing materials and the other is S[O.sub.2] frost. We utilized the results of previous IUE derived ultraviolet longitudinal phase angle variation studies (Nelson, R. M., A. L. Lane, D. L. Matson, F. P. Fanale, D. B. Nash, and T. V. Johnson 1980. Science 210, 784-786) which localized the distribution of S[O.sub.2] on Io in longitude. Thus, we were able to vary the distribution of S[O.sub.2] in latitude and produce a synthetic lightcurve based on the reflectance properties of solid S[O.sub.2] and elemental sulfur. This was done by simulating different distributions of sulfur bearing materials and S[O.sub.2] frost on Io's surface and minimizing the difference between the synthetic lightcurve and the observed lightcurve. On Io's trailing hemisphere, we find that the region north of the equator between latitudes 0 [degrees] and 60 [degrees] and between 180 [degrees] and 270 [degrees] longitude to be highest in concentration of S[O.sub.2] frost. It also coincides with the highly reflective 'white areas' seen in the Voyager spacecraft television images. All of the observations reported are of Io's trailing hemisphere.

A 14th magnitude double star was occulted by 2060 Chiron on 1993 November 7. Observations of this event were obtained from five locations in California. An occultation by Chiron's nucleus was recorded at one of these sites, while a possible graze by the nucleus was seen at the next closest location. If this possible graze represents a true detection of the nucleus, Chiron's radius is determined to be 89.6 [+ or -] 6.8 km, assuming a circular outline for the shape. If the nucleus was not seen at the second location, Chiron's radius is only constrained as being greater than 90.2 [+ or -] 6.5 km. The presence of dust in Chiron's inner coma was also detected in four of the five occultation datasets. Lightcurve features were identified that may be explained by narrow, collimated jets of material, and by a much larger region of dust distributed asymmetrically about Chiron's nucleus. Periodic fluctuations in the dust may have been detected in one of the lightcurves. If this periodicity is real, and is induced as a result of Chiron's rotation, then a minimum expansion velocity for the dust is found to be [approximately]40 m [sec.sup.-1].

Observation of Saturn's central flash during the 28 Sgr occultation by the planet enables the development of a new model to study the stratospheric zonal winds in giant planets during central occultations. Observations performed in July 1989 with instruments at McDonald and Palomar Observatories revealed multiple stellar images near the limb of the planet in the infrared wavelengths. The flash of multiple images was strongly influenced by the midlatitude zonal winds.

The visibility of stars during lunar occultations was studied by comparing theoretical data with observations. The results showed that the theoretical model was capable of predicting lunar occultation visibility with a variation of only one-third of a magnitude. It was proposed that observers should use optimal magnification when the apparent dimension of the star's disk matches the eye's critical visual angle.

Parallel analysis of radio and polarimeter occultations of Saturn's Ring A density waves is attempted to form a standard model of wave structures, involving satellite mass, disk mass, and azimuthal phase shift as parameters. The results suggest that the polarimetric data better fits the theory than the radio observations, because the latter involves phase residuals. The latter may be due to the oblique incidence of the radio occultations. The findings indicate a young dynamical age for Saturn's rings based on standard theory torques.

Occultations of the few stars in Neptune's path during the 1990s may shed light on the uncertainties about the nature of the planet's rings. Conditions for Neptune's close approach to 68 stars between 1992 to 1999 are examined. Out of 40 ring plane occultations, 17 possible ring arc occultations are found. Photometric readings of a select few show that these have greater than 11 K magnitudes.

Martian aerosols in its equitorial region near the northern spring equinox were measured using solar occultations in order to explore their vertical structure and size distribution. The instrument used was specifically constructed for this type of measurement and consists of three dispersive systems. Five clouds were recorded in the experiment probably consisting of H2O ice particles.

The use of occultation transform imaging method facilitates the imaging of high-energy astrophysical sources. The Compton Gamma-Ray Observatory method uses large area, non-collimated detectors of the Burst and Transient Source Experiment. The earth becomes a stable occulting disk as the spacecraft travels in its orbit, and data from many occultations are changed into an image.

We explore the possibility of detecting small Kuiper belt objects (KBO) by serendipitous observation of stellar occultations: We show that such unpredictable occultations may allow us to detect a population of very small objects (typically of ~100 m radius at 40 AU), invisible by any other observational method, as long as (i) the assumed population fills up a sufficient area on the sky plane, (ii) the instrumental sensitivity and acquisition frequency are high enough, and (iii) the observed star has a small angular radius. This result is basically due to the diffractive broadening of the geometric shadow of small (assumed numerous) occulting objects. This diffractive broadening is more pronounced for smaller stellar disks and better photometric precision. Assuming there exist about [10.sup.11] objects of radius [Rho] [is greater than] 1 km, located between 30 and 50 AU near the Ecliptic, and that the differential size distribution varies as [[Rho].sup.-q] with the index q = 4 extending down to decameter-sized objects, we expect a number of valid occultations (i.e., a 4[Sigma] event) between a few to several tens per night, if we may obtain an rms signal fluctuation [Sigma] ?? 1% and observe a star in the ecliptic with an angular radius ?? 0.01 mas. Since this occultation rate is very sensitive to the index slope q and plummets when q ?? 3, a KBO occultation observation campaign could provide a decisive constraint on the actual slope of the KBO size distribution for subkilometer-sized objects. Blue O class stars are the best candidates for detecting KBOs since they have the smallest angular radius for a given visual magnitude. The occultation events are typically very brief (?? 1 s) and they are shorter but more numerous when observed in the antisolar direction, so rapid photometry ([is greater than] 1 Hz) is required and high-speed photometry (?? 20 Hz) is preferred. The French space mission Corot will provide an excellent opportunity to observe occultations by KBOs using high precision photometry. [C] 2000 Academic Press Key Words: Kuiper Belt objects; occultations; photometry.

We report the CCD observations of the Apollo-type asteroid 1996 FG3 obtained in April 1996 and December 1998. The lightcurves show a double periodicity which, following Pravec et al. (IAU Circ. 7074, 1998), is interpreted as the signature of mutual eclipses and occultations between a non-synchronous, slightly elongated central body and its satellite. We derive a period of rotation of 3.595 [+ or -] 0.002 h for the central body, while the orbital period of the satellite is determined to be 16.15 [+ or -] 0.02 h. A numerical model has been developed which describes the binary system as consisting of two coorbiting ellipsoids. The model incorporates the effects of shadowing and makes use of the Hapke photometric function. A fit of the model to the observations produces synthetic lightcurves which accurately reproduce the amplitudes, the timing, and subtler features of the eclipse events. Two possible orientations of the orbital plane normal vector (J2000) are found which can account for the observed events, one retrograde and one prograde-[[Lambda].sub.l] = 282 [degrees] [+ or -] 10 [degrees];/[[Beta].sub.1] = -87 [degrees] [+ or -]3 and [[Lambda].sub.2] = 262 [degrees] [+ or -] 10 [degrees]; [[Beta].sub.2] = +75 [degrees] [+ or -] 3 [degrees] --where the retrograde one provides a slightly better fit to the observations. We find a circular orbit normalized radius R = 2.9 [+ or -] 0.1 and primary body normalized semiaxes A = 1.05[+ or -] 0.02, B = 0.95 [+ or -] 0.02, C = 0.70 [+ or -] 0.10. For the secondary body we find normalized semiaxes a = 0.32 [+ or -] 0.03 and b = 0.23 [+ or -] 0.03. The satellite shortest semiaxis c is not well constrained by the observations and therefore we assumed c--b throughout the modeling procedure. We also find indications that the primary body might have albedo variations across its surface. The estimated parameters allow us to determine an average bulk density of 1.4 [+ or -] 4-0.3 g [cm.sup.-3] which is highly suggestive of a rubble pile structure. The primary body shape, together with its spin rate, are not compatible with a hydrostatic equilibrium figure. Key Words: asteroids; eclipses; occultations; photometry.

Sometimes, a blink is more informative than a steady look. This is certainly the case when it comes to stellar occultations, events that occur when a planetary body hides a [...]

Ultraviolet Spectrometer Observations of Neptune and Triton BEFORE THE VOYAGER ENCOUNTER, neither Nepture nor Triton had been detected at wavelengths shorter than 1700 [angstroms]. Measurements of the ultraviolet (UV) reflection [...]

Voyager Radio Science Observations of Neptune and Triton VOYAGER RADIO SCIENCE (RSS) observations of the Neptune system comprise coherent radio Doppler and ranging measurements for the study of Neptune's gravity [...]

We present observations of three Neptune central flash events: the 20 August 1985 occultation of n39 from the European Southern Observatory (ESO), the 12 September 1988 occultation of N51 from Pic du Midi, and the 8 July 1989 occultation of N55 from ESO. From simultaneous fits to the three central flash lightcurves, we determine the shape of Neptune's limb, and show that winds near the 0.38-mbar level have decayed to about [approximately equal to]0.6 [+ or -] 0.2 times their strength at 100 mbar, assuming that the latitude-dependence of the stratospheric winds follows the zonal wind profile of L. A. Sromovsky et al. (1993, Icarus 105, 110-141). From the measured decay of winds with height, we find that the average vertical shear in the zonal wind between 100 and 0.38 mbar is very close to the Voyager IRIS results for the 30- to 120-mbar region (B. J. Conrath et al., 1989, Science 246, 1454-1459), at the latitudes to which the central flashes are most sensitive. We also determine the shape of the limb at the 0.7-[micro]bar level from the 'half-light' points of the atmospheric immersion and emersion light curves of five stellar occultations by Neptune. The winds in this pressure regime have decayed to [approximately equal to]0.17 times their strength at the 100-mbar level, and the vertical shear in the zonal wind as determined from the thermal wind equation and Voyager IRIS measurements extends essentially unchanged all the way up to the microbar level of the stratosphere. Alternatively, a simple oblate model fit to the planetary limb gives an oblateness of [Epsilon] = 0.0180 [+ or -] 0.0010 and an equatorial radius of [r.sub.e] = 25262.7 [+ or -] 3.5 km. The corresponding rotation period is 16.59 [+ or -] 0.92 h, quite similar to that of Neptune's deep interior (16.11 h). This near corotation suggests that there is some coupling between Neptune's interior and the upper stratosphere, even though the intervening atmosphere between the cloud deck and the stratosphere has strong retrograde flow.