Engineering topics

J R Spencer

Publications and source records attributed to J R Spencer.

3 records

NASA NTRS2019

The HST Lightcurve of (486958) 2014 MU69

We report Hubble Space Telescope (HST) lightcurve observations of the New Horizons spacecraft encounter Kuiper Belt object (KBO) (486958) 2014 MU69 acquired near opposition in July 2017. In order to plan the optimum flyby sequence the New Horizons mission planners needed to learn as much as possible about the target in advance of the encounter. Specifically, from lightcurve data, encounter timing could be adjusted to accommodate a highly elongated, binary, or rapidly rotating target. HST astrometric (Porter et al., 2018) and stellar occultation (Buie et al., 2018) observations constrained MU69's orbit and diameter (21–41 km for an albedo of 0.15–0.04), respectively. Photometry from the astrometric dataset suggested a variability of ≥0.3 mags, but they did not determine the period or provide shape information. To that end we strategically spaced 24 HST orbits over 9 days to investigate rotation periods from approximately 2–100 h and to better constrain the lightcurve amplitude. Until New Horizons detected MU69 in its optical navigation images beginning in August 2018, this HST lightcurve campaign provided the most accurate photometry to date. The mean variation in our data is 0.15 magnitudes which suggests that MU69 is either nearly spherical (a:b axis ratio of 1:1.15), or its pole vector is pointed near the line of sight to Earth; this interpretation does not preclude a near-contact binary or bi-lobed object. However, image stacks do conclude that MU69 does not have a binary companion ≥2000 km with a sensitivity to 29th magnitude (an object a few km in size for an albedo of 0.04–0.15). Our data are not of sufficient signal to noise to uniquely determine the period or amplitude, however, they did provide the necessary information for spacecraft planning. We report with confidence that MU69 is not both rapidly rotating and highly elongated (which we define as a lightcurve amplitude ≥ 0.5 magnitude). Since this paper is being published post fly-by, we note that our results are consistent with the fly-by imagery and orientation of MU69 (Stern et al., 2019). The combined dataset also suggests that within the KBO lightcurve literature there are likely other objects which share a geometric configuration like MU69 resulting in an underestimate of the contact binary fraction for the Cold Classical Kuiper Belt.

S D Benecchi↗

NASA NTRS2014

Thermophysical Property Variations Across Dione and Rhea

Maps of the variation in bolometric Bond albedo and thermal inertia across Rhea and Dione have been produced using various day and nighttime observations, taken by Cassini's Composite Infrared Spectrometer (CIRS). The albedo maps show the same trend that has been previously observed on these satellites from reflected sunlight and thermal observations: a higher albedo on their leading hemispheres. The thermal inertia maps show two previously unknown anomalous high thermal inertia regions: low latitudes on Dione's leading hemisphere and the bright ejecta blanket of Rhea's Inktomi crater. The thermal inertia on Dione increases modestly from a background value of 8 J m(exp -2) K(exp -1) s(exp -1/2) to 11 J m(exp -)2 K(exp -1) s(exp -1/2) in a region preferentially bombarded by high-energy electrons. We believe that this region on Dione is probably analogous to the thermally anomalous regions recently discovered at equivalent locations on Mimas and Tethys, dubbed Pac-Man features. The smaller magnitude of Dione's thermal anomaly, compared to that of Mimas and Tethys, provides additional evidence that surface alteration by high-energy electrons produces these anomalies, as the high-energy electron flux decreases with increasing distance from Saturn. However, unlike on Mimas and Tethys, the thermally anomalous region on Dione does not display a spatially correlated decrease in the IR/UV (0.930 micrometers/0.338 micrometers) color ratio, implying that the minimum electron energy threshold of the IR/UV ice surface darkening mechanism is not met on Dione. The average of the mapped bolometric Bond albedos on the leading and trailing hemispheres of Dione are 0.49 +/- 0.11 and 0.44 +/- 0.13 respectively. On Rhea the thermal inertia increases from a background value of 10 J m(exp -2) K (exp -1) s(exp -1/2) to 19 J m(exp -2) K(exp -1) s(exp -1/2) on the ejecta blanket of Inktomi crater, probably due to a mixture of high and low thermal inertia material in this region, such as a fine- and large-grain ice mixture. This is the first time a thermal inertia anomaly has been associated with an impact crater on an icy saturnian satellite. On Rhea the average of the mapped bolometric Bond albedos on the leading and trailing hemispheres are 0.59 +/- 0.11 and 0.56 +/- 0.13 respectively.

Saturn↗

NASA NTRSDate not supplied

Lessons From Pluto for Uranus Moons

Introduction: Small patches of starkly different albedo on both Oberon and Umbriel may be evidence for frost segregation and volatile migration. The high obliquity of the Uranian system makes it a good candidate for volatile transport activity. CO 2 and ammonium species have been detected on some Uranian satellites, and CH 4 is at least theoretically viable as a resident volatile. The OWL-recommended UOP Orbiter spacecraft should be equipped with instruments capable of evaluating the nature and extent of volatiles in the Uranian satellite system.

J M Moore↗