Lucy F. G. Lim

Multiple Asteroid Systems: Dimensions and Thermal Properties from Spitzer Space Telescope and Ground-based Observations

We collected mid-IR spectra from 5.2 to 38 lm using the Spitzer Space Telescope Infrared Spectrograph of 28 asteroids representative of all established types of binary groups. Photometric lightcurves were also obtained for 14 of them during the Spitzer observations to provide the context of the observations and reliable estimates of their absolute magnitudes. The extracted mid-IR spectra were analyzed using a modified standard thermal model (STM) and a thermophysical model (TPM) that takes into account the shape and geometry of the large primary at the time of the Spitzer observation. We derived a reliable estimate of the size, albedo, and beaming factor for each of these asteroids, representing three main taxonomic groups: C, S, and X. For large (volume-equivalent system diameter Deq > 130 km) binary asteroids, the TPM analysis indicates a low thermal inertia (C 6 � 100 J s � 1/2 K � 1 m � 2 ) and their emissivity spectra display strong mineral features, implying that they are covered with a thick layer of thermally insulating regolith. The smaller (surface-equivalent system diameter Deff < 17 km) asteroids also show some emission lines of minerals, but they are significantly weaker, consistent with regoliths with coarser grains, than those of the large binary asteroids. The average bulk densities of these multiple asteroids vary from 0.7–1.7 g/cm 3

OSIRIS-REx: Sample Return from Asteroid (101955) Bennu

In May of 2011, NASA selected the Origins, Spectral Interpretation, Resource Identification, and Security–Regolith Explorer (OSIRIS-REx) asteroid sample return mission as the third mission in the New Frontiers program. The other two New Frontiers missions are New Horizons, which explored Pluto during a flyby in July 2015 and is on its way for a flyby of Kuiper Belt object 2014 MU69 on January 1, 2019, and Juno, an orbiting mission that is studying the origin, evolution, and internal structure of Jupiter. The spacecraft departed for near-Earth asteroid (101955) Bennu aboard an United Launch Alliance Atlas V 411 evolved expendable launch vehicle at 7:05 p.m. EDT on September 8, 2016, on a seven-year journey to return samples from Bennu. The spacecraft is on an outbound-cruise trajectory that will result in a rendezvous with Bennu in November 2018. The science instruments on the spacecraft will survey Bennu to measure its physical, geological, and chemical properties, and the team will use these data to select a site on the surface to collect at least 60 g of asteroid regolith. The team will also analyze the remote-sensing data to perform a detailed study of the sample site for context, assess Bennu’s resource potential, refine estimates of its impact probability with Earth, and provide ground-truth data for the extensive astronomical data set collected on this asteroid. The spacecraft will leave Bennu in 2021 and return the sample to the Utah Test and Training Range (UTTR) on September 24, 2023.

Time - resolved Gamma Ray spectral analysis of planetary neutron and Gamma Ray instrumentation

The current gamma ray/neutron instrumentation development effort at NASA Goddard Space Flight Center¿s Astrochemistry Laboratory aims to extend the use of neutron interrogation techniques, using a 14 MeV Pulsed Neutron Generator (PNG) combined with neutron and gamma ray detectors, to probe the surface and subsurface of planetary bodies in situ without the need to drill. One aspect of the current work includes the development of taking timed tagged event-byevent data using our custom designed software with the Canberra Lynx Digital Signal Analyzer to provide a unique three-dimensional master data set with channel/energy, time, and intensity information. Since the master data set is not limited to predetermined coincidence timing gates set for a specific nuclear process, the user is allowed the flexibility to slice the data cube in a multitude of ways without loss of information or experimental time due to the need for additional acquisition windows. Time tagged event-by-event data allows the user to isolate a particular energy line from the spectrum over a specific window in time with respect to the PNG pulse, analyze a gamma ray spectrum resulting from either neutron capture, between the burst, or inelastic scattering events, during the neutron burst, and extract data for engineering purposes to optimize timing windows to look at specific elements in different environments. In this paper, we will present the results of our experimental data using the time tagged event-by-event data analysis technique compared with non-time-gated data taken at the test facility at NASA Goddard Space Flight Center. Comparison of these data will show the advantages and validity of this method to obtain more precise, sensitive, and accurate elemental composition measurements.

Development of the probing in-situ with Neutron and Gamma rays (PING) instrument for planetary science applications

This paper describes the testing of a prototype active neutron/ gamma ray instrument for use in planetary science space applications. The Probing In situ with Neutrons and Gamma rays (PING) instrument can measure the full bulk elemental composition of a planets surface over a 1 m2 area and down to 30 – 50 cm depth without the need to drill into the surface material. PING consists of three components: a pulsed neutron generator that emits 14 MeV neutrons that penetrate the surface and excite the nuclei of the planetary material; a gamma ray spectrometer that measures the energy and intensity of the gamma rays emitted by these nuclear reactions; and neutron detectors to measure the neutron moderation properties of the material. PING is tested on Earth at a unique facility near NASA/Goddard Space Flight Center where PING can be safely operated outdoors and unshielded sitting atop large (1.8m × 1.8m × .9m), well-characterized granite and basalt monuments. We will describe both this test facility and our experiments, and present gamma ray spectroscopy results that demonstrate PINGs capabilities.

Modeling orbital gamma-ray spectroscopy experiments at carbonaceous asteroids

To evaluate the feasibility of measuring differences in bulk composition among carbonaceous meteorite parent bodies from an asteroid or comet orbiter, we present the results of a performance simulation of an orbital gamma-ray spectroscopy (GRS) experiment in a Dawn-like orbit around spherical model asteroids with a range of carbonaceous compositions. The orbital altitude was held equal to the asteroid radius for 4.5 months. Both the asteroid gamma-ray spectrum and the spacecraft background flux were calculated using the MCNPX Monte-Carlo code. GRS is sensitive to depths below the optical surface (to ≈20-50 cm depth depending on material density). This technique can therefore measure underlying compositions beneath a sulfur-depleted (e.g., Nittler et al. 2001) or desiccated surface layer. We find that 3σ uncertainties of under 1 wt% are achievable for H, C, O, Si, S, Fe, and Cl for five carbonaceous meteorite compositions using the heritage Mars Odyssey GRS design in a spacecraft-deck-mounted configuration at the Odyssey end-of-mission energy resolution, FWHM = 5.7 keV at 1332 keV. The calculated compositional uncertainties are smaller than the compositional differences between carbonaceous chondrite subclasses.

Trajectory Optimization for Missions to Small Bodies with a Focus on Scientific Merit

Trajectory design for missions to small bodies is tightly coupled both with the selection of targets for the mission and with the choice of spacecraft power, propulsion, and other hardware. Traditional methods of trajectory optimization have focused on finding the optimal trajectory for an a priori selection of destinations and spacecraft parameters. Recent research has expanded the field to multidisciplinary systems optimization that includes spacecraft parameters. The logical next step is to extend the optimization process to include target selection based not only on engineering figures of merit but also scientific value. This article presents a new technique to solve the multidisciplinary mission optimization problem for small-body missions, including classical trajectory design, the choice of spacecraft power and propulsion systems, and the scientific value of the targets. This technique, when combined with modern parallel computers, enables a holistic view of the small-body mission design process that previously required iteration among several different design processes.