NASA's New Horizons spacecraft will conduct a close flyby of the cold-classical Kuiper Belt Object (KBO) designated (486958) 2014 MU69 on 2019 January 1. At a heliocentric distance of 44 au, "MU69" ...will be the most distant object ever visited by a spacecraft. To enable this flyby, we have developed an extremely high-precision orbit fitting and uncertainty processing pipeline, making maximal use of the Hubble Space Telescope's Wide Field Camera 3 (WFC3) and pre-release versions of the ESA Gaia Data Release 2 (DR2) catalog. This pipeline also enabled successful predictions of a stellar occultation by MU69 in 2017 July. We describe how we process the WFC3 images to match the Gaia DR2 catalog, extract positional uncertainties for this extremely faint target (typically 140 photons per WFC3 exposure), and translate those uncertainties into probability distribution functions for MU69 at any given time. We also describe how we use these uncertainties to guide New Horizons, plan stellar occultions of MU69, and derive MU69's orbital evolution and long-term stability.
Since insertion into orbit on December 7, 2015, the Akatsuki orbiter has returned global images of Venus from its four imaging cameras at eleven discrete wavelengths from ultraviolet (283 and 365 nm) ...and near infrared (0.9–2.3 µm), to the thermal infrared (8–12 µm) from a near-equatorial orbit. The Venus Express and Pioneer Venus Orbiter missions have also monitored the planet for long periods but from polar or near-polar orbits. The wavelength coverage and views of the planet also differ for all three missions. In reflected light, the images reveal features seen near the cloud tops (~ 70 km altitude), whereas in the near-infrared images of the nightside, features seen are at mid- to lower cloud levels (~ 48–60 km altitude). The dayside cloud cover imaged at the ultraviolet wavelengths shows morphologies similar to what was observed from Mariner 10, Pioneer Venus, Galileo, Venus Express and MESSENGER. The daytime images at 0.9 and 2.02 µm also reveal some interesting features which bear similarity to the ultraviolet images. The nighttime images at 1.74, 2.26 and 2.32 µm and at 8–12 µm reveal features not seen before and show new details of the nightside including narrow wavy ribbons, curved string-like features, long-scale waves, long dark streaks, isolated bright spots, sharp boundaries and even mesoscale vortices. Some features previously seen such as circum-equatorial belts (CEBs) and occasional areal brightenings at ultraviolet (seen in Venus Express observations) of the cloud cover at ultraviolet wavelengths have not been observed thus far. Evidence for the hemispheric vortex organization of the global circulation can be seen at all wavelengths on the day- and nightsides. Akatsuki images reveal new and puzzling morphology of the complex nightside cloud cover. The cloud morphologies provide some clues to the processes occurring in the atmosphere and are thus, a key diagnostic tool when quantitative dynamical analysis is not feasible due to insufficient information.
We present measurements of the wind speeds at the nightside lower clouds of Venus from observations by JAXA's mission Akatsuki during 2016, complemented by new wind measurements from ground-based ...observations acquired with the TNG/Near Infrared Camera Spectrometer (NICS) in 2012 and IRTF/SpeX in 2015 and 2017. The zonal and meridional components of the winds were measured via cloud tracking on a total of 466 Akatsuki images of Venus acquired by the camera IR2 using the 2.26 m filter, with spatial resolutions ranging from 10 to 80 km per pixel and covering 2016 March 22 to October 31. More than 149,000 wind vectors were obtained using an automatic template-matching technique, and 2947 wind vectors were inferred using a manual procedure. The meridional profiles for both components of the winds are found to be consistent with results from the Venus Express mission during 2006-2008, although stronger wind variability is found for the zonal component at equatorial latitudes where Akatsuki observations have better viewing geometry than Venus Express. The zonal winds at low latitudes also suggest a zonal variability that could be associated with solar tides or vertically propagating orographic waves. Finally, the combination of our wind measurements from TNG/NICS, IRTF/SpeX, and Akatsuki images with previously published data from 1978 to 2017 suggests variations of up to 30 m s−1 in the winds at the nightside lower clouds of Venus.
We present new imaging of the surface of Pluto and Charon obtained during 2002-2003 with the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) instrument. Using these data, we construct ...two-color albedo maps for the surfaces of both Pluto and Charon. Similar mapping techniques are used to re-process HST/Faint Object Camera (FOC) images taken in 1994. The FOC data provide information in the ultraviolet and blue wavelengths that show a marked trend of UV-bright material toward the sunlit pole. The ACS data are taken at two optical wavelengths and show widespread albedo and color variegation on the surface of Pluto and hint at a latitudinal albedo trend on Charon. The ACS data also provide evidence for a decreasing albedo for Pluto at blue (435 nm) wavelengths, while the green (555 nm) data are consistent with a static surface over the one-year period of data collection. We use the two maps to synthesize a true visual color map of Pluto's surface and investigate trends in color. The mid- to high-latitude region on the sunlit pole is, on average, more neutral in color and generally higher albedo than the rest of the surface. Brighter surfaces also tend to be more neutral in color and show minimal color variations. The darker regions show considerable color diversity arguing that there must be a range of compositional units in the dark regions. Color variations are weak when sorted by longitude. These data are also used to constrain astrometric corrections that enable more accurate orbit fitting, both for the heliocentric orbit of the barycenter and the orbit of Pluto and Charon about their barycenter.
We present a new computer program, SRTC++, to solve spatial problems associated with explorations of Saturn's moon Titan. The program implements a three-dimensional structure well-suited to ...addressing shortcomings arising from plane-parallel radiative transfer approaches. SRTC++'s design uses parallel processing in an object-oriented, compiled computer language (C++) leading to a flexible and fast architecture. We validate SRTC++ using analytical results, semianalytical radiative transfer expressions, and an existing Titan plane-parallel model. SRTC++ complements existing approaches, addressing spatial problems like near-limb and near-terminator geometries, non-Lambertian surface phase functions (including specular reflections), and surface albedo nonuniformity.
We present new light-curve measurements of Pluto and Charon taken with the Advanced Camera for Surveys High-resolution Camera on the Hubble Space Telescope. The observations were collected from 2002 ...June to 2003 June at 12 distinct sub-Earth longitudes over a range of solar phase angle 036-174--a larger range than previously measured. The new measurements of Pluto show that the light-curve amplitude has decreased since the mutual event season in the late 1980s. We also show that the average brightness has increased in the F555W (Johnson V equivalent) passband while the brightness has decreased in the F435W (Johnson B equivalent) passband. These data thus indicate a substantial reddening of the reflected light from Pluto. We find a weighted mean (B -- V) = 0.9540 ? 0.0010 that is considerably higher than the long-standing value of (B -- V) = 0.868 ? 0.003 most recently measured in 1992-1993. This change in color cannot be explained by the evolving viewing geometry and provides the strongest evidence to date for temporal changes on the surface of Pluto that are expected to be linked to volatile transport processes. We also report on the discovery of a new rotational modulation of Pluto's hemispherical color that ranges from 0.92 to 0.98 with the least red color at the longitude of maximum light and most red at minimum light. The phase coefficient of Pluto is nearly the same as measured in 1992-1993 with a value of Delta *b B = 0.0392 ? 0.0064 and Delta *b V = 0.0355 ? 0.0045 mag deg--1 for the F435W and F555W data, respectively. The Pluto phase curve is still very close to linear but a small but significant nonlinearity is seen in the data. In contrast, the light curve of Charon is essentially the same as in 1992/1993, albeit with much less noise. We confirm that Charon's Pluto-facing hemisphere is 8% brighter than the hemisphere facing away from Pluto. The color of Charon is independent of longitude and has a mean weighted value of (B -- V) = 0.7315 ? 0.0013. The phase curve for Charon is now shown to be strongly nonlinear and wavelength dependent. We present results for both Pluto and Charon that better constrain the single-particle scattering parameters from the Hapke scattering theory.
The highest resolution (pixel scale 30 km) images of Ceres to date have been acquired by the Advanced Camera for Surveys onboard Hubble Space Telescope, through three wide band filters, centered at ...535, 335, and 223 nm, covering more than one rotation of Ceres. The lightcurve at 535 nm agrees with earlier observations at V-band Tedesco, E.F., Taylor, R.C., Drummond, J., Harwood, D., Nickoloff, I., Scaltriti, F., Schober, H. J., Zappala, V., 1983. Icarus 54, 23–29 in terms of magnitude, amplitude, and shape. The 0.04 magnitude lightcurve amplitude cannot be matched by Ceres' rotationally symmetric shape, and is modeled here by albedo patterns. The geometric albedos at the above three wavelengths are measured to be
0.087
±
0.003
,
0.056
±
0.002
, and
0.039
±
0.003
, respectively. V-band geometric albedo is calculated to be
0.090
±
0.003
, consistent with earlier observations Tedesco, E.F., 1989. In: Binzel, R.P., Gehrels, T., Matthews, M.S. (Eds.), Asteroids II. Univ. of Arizona Press, Tucson, pp. 1090–1138. A strong absorption band (30%) centered at about 280 nm is observed, but cannot be identified with either laboratory UV spectra or the spectra of Europa or Ganymede. The single-scattering albedo has been modeled to be
0.070
±
0.002
,
0.046
±
0.002
, and
0.032
±
0.003
, respectively. The photometric roughness of Ceres' surface is found to be about
44
°
±
5
°
from photometric modeling using Hapke's theory, consistent with earlier radar observations Mitchell, D.L., Ostro, S.J., Hudson, R.S., Rosema, K.D., Campbell, D.B., Velez, R., Chandler, J. F., Shapiro, I.I., Giorgini, J.D., Yeomans, D.K., 1996. Icarus 124, 113–133. The first spatially resolved surface albedo maps of Ceres at three wavelengths have been constructed from HST observations, as well as the corresponding color maps. Eleven surface albedo features are identified, ranging in scale from 40–350 km. Overall the range of these albedo and color variations is small compared to other asteroids and some icy satellites.
We present the surface mapping of the southern hemisphere of Asteroid (4) Vesta obtained from Hubble Space Telescope (HST). From 105 images of Vesta through four filters in the wavelengths best to ...characterize the 1-μm pyroxene band, we constructed albedo and color-ratio maps of Vesta. These new maps cover latitudes −50° to +20°. The southern hemisphere of Vesta displays more diverse albedo and color features than the northern hemisphere, with about 15 new albedo and color features identified. The overall longitudinal albedo and color variations in the southern hemisphere are comparable with that of the northern hemisphere, with a range of about ±20% and ±10%, respectively. The eastern hemisphere is brighter and displays more diogenitic minerals than the western hemisphere. Correlations between 1-μm band depth and band width, as well as between 1-μm band depth and albedo, are present on a global scale, attributed to pyroxene composition variations. The lack of correlations between albedo and the spectral slope indicates the absence of globalized space weathering. The lack of a global correlation between 1-μm band depth and topography suggests that the surface composition of Vesta is not completely controlled by a single impact. The distribution of compositional variation on Vesta suggests a possible large impact basin. Evidence of space weathering is found in regions, including the bright rim of the south-pole crater where the steepest gravitational slope on Vesta is, and a dark area near a gravitationally flat area. We propose to divide the surface of Vesta into six geological units different from the background according to their 1-μm absorption features and spectral slopes, including two eucrite-rich units, a low-Ca eucrite unit, a diogenite-rich unit, a space weathered unit, and a freshly exposed unit. No evidence of olivine-rich area is present in these data.
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