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Zalucha A.M.,Massachusetts Institute of Technology | Zhu X.,Johns Hopkins University | Gulbis A.A.S.,Massachusetts Institute of Technology | Gulbis A.A.S.,Southern African Large Telescope and South African Astronomical Observatory | And 3 more authors.
Icarus | Year: 2011

We use a radiative-conductive-convective model to assess the height of Pluto's troposphere, as well as surface pressure and surface radius, from stellar occultation data from the years 1988, 2002, and 2006. The height of the troposphere, if it exists, is less than 1 km for all years analyzed. Pluto has at most a planetary boundary layer and not a troposphere. As in previous analyses of Pluto occultation light curves, we find that the surface pressure is increasing with time, assuming that latitude and longitude variations in Pluto's atmosphere are negligible. The surface pressure is found to be slightly higher (12.5-2.4+1.9μbar in 1988, 18.0-1.7+11μbar in 2002, and 18.5. ±. 4.7. μbar in 2006) than in our previous analyses with the troposphere excluded. The surface radius is determined to be 1173-10+20km. Comparison of the minimum reduced chi-squared values between the best-fit radiative-conductive-convective (i.e., troposphere-included) model and best-fit radiative-conductive (i.e., troposphere-excluded) shows that the troposphere-included model is only a slightly better fit to the data for all 3 years. Uncertainties in the small-scale physical processes of Pluto's lower atmosphere and consequently the functional form of the model troposphere lend more confidence to the troposphere-excluded results. © 2011 Elsevier Inc.

Zalucha A.M.,Massachusetts Institute of Technology | Gulbis A.A.S.,Massachusetts Institute of Technology | Gulbis A.A.S.,Southern African Large Telescope and South African Astronomical Observatory | Zhu X.,Johns Hopkins University | And 3 more authors.
Icarus | Year: 2011

We use a radiative-conductive model to least-squares fit Pluto stellar occultation light curve data. This model predicts atmospheric temperature based on surface temperature, surface pressure, surface radius, and CH 4 and CO mixing ratios, from which model light curves are to be calculated. The model improves upon previous techniques for deriving Pluto's atmospheric thermal structure from stellar occultation light curves by calculating temperature (as a function of height) caused by heating and cooling by species in Pluto's atmosphere, instead of a general assumption that temperature follows a power law with height or some other idealized function. We are able to fit for model surface radius, surface pressure, and CH 4 mixing ratio with one of the 2006 datasets and for surface pressure and CH 4 mixing ratio for other datasets from the years 1988, 2002, 2006, and 2008. It was not possible to fit for CO mixing ratio and surface temperature because the light curves are not sensitive to these parameters. We determine that the model surface radius, under the assumption of a stratosphere only (i.e. no troposphere) model in radiative-conductive balance, is 1180-10+20km. The CH 4 mixing ratio results are more scattered with time and are in the range of 1.8-9.4×10 -3. The surface pressure results show an increasing trend from 1988 to 2002, although it is not as dramatic as the factor of 2 from previous studies. © 2010 Elsevier Inc.

Christou A.A.,Armagh Observatory | Kwiatkowski T.,Adam Mickiewicz University | Butkiewicz M.,Adam Mickiewicz University | Gulbis A.,Southern African Large Telescope and South African Astronomical Observatory | And 3 more authors.
Astronomy and Astrophysics | Year: 2012

Aims. We investigated the physical properties and dynamical evolution of near-Earth asteroid (NEA) (190491) 2000 FJ10 in order to assess the suitability of this accessible NEA as a space mission target. Methods. Photometry and colour determination were carried out with the 1.54 m Kuiper Telescope (Mt Bigelow, USA) and the 10 m Southern African Large Telescope (SALT; Sutherland, South Africa) during the object's recent favourable apparition in 2011-12. During the earlier 2008 apparition, a spectrum of the object in the 6000-9000 Angstrom region was obtained with the 4.2 m William Herschel Telescope (WHT; Canary Islands, Spain). Interpretation of the observational results was aided by numerical simulations of 1000 dynamical clones of 2000 FJ10 up to 106 yr in the past and in the future. Results. The asteroid's spectrum and colours determined by our observations suggest a taxonomic classification within the S-complex although other classifications (V, D, E, M, P) cannot be ruled out. On this evidence, it is unlikely to be a primitive, relatively unaltered remnant from the early history of the solar system and thus a low priority target for robotic sample return. Our photometry placed a lower bound of 2 h to the asteroid's rotation period. Its absolute magnitude was estimated to be 21.54 ± 0.1 which, for a typical S-complex albedo, translates into a diameter of 130 ± 20 m. Our dynamical simulations show that it has likely been an Amor for the past 105 yr. Although currently not Earth-crossing, it will likely become so during the period 50-100 kyr in the future. It may have arrived from the inner or central main belt >1 Myr ago as a former member of a low-inclination S-class asteroid family. Its relatively slow rotation and large size make it a suitable destination for a human mission. We show that ballistic Earth-190491-Earth transfer trajectories with ΔV < 2 km s-1 at the asteroid exist between 2052 and 2061. © ESO 2012.

Adams E.R.,Planetary Science Institute | Gulbis A.A.S.,Southern African Large Telescope and South African Astronomical Observatory | Gulbis A.A.S.,Massachusetts Institute of Technology | Benecchi S.D.,Planetary Science Institute | And 4 more authors.
Astronomical Journal | Year: 2014

The Deep Ecliptic Survey (DES) was a survey project that discovered hundreds of Kuiper Belt objects from 1998 to 2005. Extensive follow-up observations of these bodies has yielded 304 objects with well-determined orbits and dynamical classifications into one of several categories: Classical, Scattered, Centaur, or 16 mean-motion resonances with Neptune. The DES search fields are well documented, enabling us to calculate the probability on each frame of detecting an object with its particular orbital parameters and absolute magnitude at a randomized point in its orbit. The detection probabilities range from a maximum of 0.32 for the 3:2 resonant object 2002 GF 32 to a minimum of 1.5 × 10-7 for the faint Scattered object 2001 FU 185. By grouping individual objects together by dynamical classes, we can estimate the distributions of four parameters that define each class: semimajor axis, eccentricity, inclination, and object size. The orbital element distributions (a, e, and i) were fit to the largest three classes (Classical, 3:2, and Scattered) using a maximum likelihood fit. Using the absolute magnitude (H magnitude) as a proxy for the object size, we fit a power law to the number of objects versus H magnitude for eight classes with at least five detected members (246 objects). The Classical objects are best fit with a power-law slope of α = 1.02 ± 0.01 (observed from 5 ≤ H ≤ 7.2). Six other dynamical classes (Scattered plus five resonances) have consistent magnitude distribution slopes with the Classicals, provided that the absolute number of objects is scaled. Scattered objects are somewhat more numerous than Classical objects, while there are only a quarter as many 3:2 objects as Classicals. The exception to the power law relation is the Centaurs, which are non-resonant objects with perihelia closer than Neptune and therefore brighter and detectable at smaller sizes. Centaurs were observed from 7.5 < H < 11, and that population is best fit by a power law with α = 0.42 ± 0.02. This is consistent with a knee in the H-distribution around H = 7.2 as reported elsewhere. Based on the Classical-derived magnitude distribution, the total number of objects (H ≤ 7) in each class is: Classical (2100 ± 300 objects), Scattered (2800 ± 400), 3:2 (570 ± 80), 2:1 (400 ± 50), 5:2 (270 ± 40), 7:4 (69 ± 9), 5:3 (60 ± 8). The independent estimate for the number of Centaurs in the same H range is 13 ± 5. If instead all objects are divided by inclination into "Hot" and "Cold" populations, following Fraser et al., we find that αHot = 0.90 ± 0.02, while α Cold = 1.32 ± 0.02, in good agreement with that work. © 2014. The American Astronomical Society. All rights reserved.

Gulbis A.A.S.,Southern African Large Telescope and South African Astronomical Observatory | Gulbis A.A.S.,Massachusetts Institute of Technology | Bus S.J.,University of Hawaii at Hilo | Elliot J.L.,Massachusetts Institute of Technology | And 9 more authors.
Publications of the Astronomical Society of the Pacific | Year: 2011

We present a high-speed, visible-wavelength imaging instrument: MORIS (the MIT Optical Rapid Imaging System). MORIS is mounted on the 3 m Infrared Telescope Facility (IRTF) on Mauna Kea, Hawaii. Its primary component is an Andor iXon camera, a nearly 60 square field of view with high quantum efficiency, low read noise, low dark current, and full-frame readout rates ranging from as slow as desired to a maximum of between 3.5 Hz and 35 Hz (depending on the mode; read noise of 6 e - pixel -1 and 49 e -pixel -1 with electron-multiplying gain 1/4 1, respectively). User-selectable binning and subframing can increase the cadence to a few hundred hertz. An electron-multiplying mode can be employed for photon counting, effectively reducing the read noise to subelectron levels at the expense of dynamic range. Data cubes, or individual frames, can be triggered to several-nanosecond accuracy using the Global Positioning System. MORIS is mounted on the side-facing exit window of SpeX, allowing simultaneous near-infrared and visible observations. Here, we describe the components, setup, and measured characteristics of MORIS. We also report results from the first science observations: the 2008 June 24 stellar occultation by Pluto and an extrasolar planetary transit by XO-2b. The Pluto occultation of a 15:8R magnitude star has a signal-to-noise ratio of 35 per atmospheric scale height and a midtime error of 0.32 s. The XO-2b transit reaches photometric precision of 0.5 mmag in 2 minutes and has a midtime timing precision of 23 s. © 2011. The Astronomical Society of the Pacific.

Zalucha A.M.,A. M. Zalucha Consulting | Gulbis A.A.S.,Massachusetts Institute of Technology | Gulbis A.A.S.,Southern African Large Telescope and South African Astronomical Observatory
Journal of Geophysical Research E: Planets | Year: 2012

We use a simple Pluto general circulation model (sPGCM) to predict for the first time the wind on Pluto and its global, large-scale structure, as well as the temperature and surface pressure. Wind is a fundamental atmospheric variable that has previously been neither measured nor explicitly modeled on Pluto. We ran the sPGCM in 2-D mode (latitude, height, and time varying) using the Massachusetts Institute of Technology general circulation model dynamical core, a simple radiative-convective scheme, and no frost cycle. We found that Pluto's atmosphere is dynamically active in the zonal direction with high-speed, high-latitude jets that encircle the poles in gradient wind balance and prograde with Pluto's rotation. The meridional and vertical winds do not show evidence for a Hadley cell (or other large-scale structure) due to the low-altitude temperature inversion. The horizontal variation in surface pressure is a small fraction of the previously derived interannual variation in surface pressure. The simple general circulation model output was validated with stellar occultation light curve data from the years 1988, 2002, 2006, and 2007. For 2006 and 2007, the best fit global mean surface pressure was 24 microbar, in 2002 it was 22 microbar, and in 1988 it was 12 microbar (1 microbar error bars). For all years the methane mixing ratio was 1% (0.2% error bars). This work is a first step for future Pluto, Triton, and Kuiper Belt object atmosphere general circulation models that will also include longitudinal variations and a volatile cycle. Copyright 2012 by the American Geophysical Union.

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