Mark R. Lankton

The MESSENGER mission to Mercury: scientific payload

Abstract The MErcury, Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) mission will send the first spacecraft to orbit the planet Mercury. A miniaturized set of seven instruments, along with the spacecraft telecommunications system, provide the means of achieving the scientific objectives that motivate the mission. The payload includes a combined wide- and narrow-angle imaging system; γ-ray, neutron, and X-ray spectrometers for remote geochemical sensing; a vector magnetometer; a laser altimeter; a combined ultraviolet-visible and visible-infrared spectrometer to detect atmospheric species and map mineralogical absorption features; and an energetic particle and plasma spectrometer to characterize ionized species in the magnetosphere.

Assessment of Localized Deformations in Sand Using X-Ray Computed Tomography

The internal fabric and localized deformation patterns of triaxial sand specimens were investigated using computed tomography (CT). Three displacement-controlled, conventional, drained axisymmetric (triaxial) experiments were conducted on dry Ottawa sand specimens at very low effective confining stresses (0.05, 0.52, and 1.30 kPa) in a microgravity environment aboard the Space Shuttle during the NASA STS-79 mission. CT scanning was performed on these flight specimens, as well as on an uncompressed specimen and a specimen tested in a terrestrial laboratory at 1.30 kPa effective confining stress. CT demonstrated high accuracy in detecting specimen inhomogeneity and localization patterns. Formation of deformation patters is dependent on the effective confining stress and gravity. Multiple symmetrical radial shear bands were observed in the specimens tested in a microgravity environment. In the axial direction, two major conical surfaces were developed. Nonsymmetrical spatial deformation was observed in the 1-G specimen. Analysis tools were developed to quantify the spatial density change. Void ratio variation within and outside the shear bands is calculated and discussed.

Shear Band Characterization of Triaxial Sand Specimens Using Computed Tomography

A thorough quantitative analysis of the internal density distribution and strain localization of axisymmetric triaxial sand specimens is presented. Computed tomography technique was used to acquire detailed three-dimensional images of a series of Ottawa sand specimens subjected to Conventional Triaxial Compression (CTC) conditions at very low effective stresses in microgravity and terrestrial laboratories. Analysis tools were developed to quantify the distribution of local void ratio, track the onset, propagation, thickness, and inclination angle of shear bands, and calculate the variation of void ratio within and outside shear bands. It has been found that shear bands initiate in the post-peak strength regime in CTC specimens, where a rather complex pattern of shear bands develops such that behavior is highly influenced by large-scale kinematics of the specimen. Four main deformation patterns were identified and their contribution to the overall volume change of the specimens was quantified.

Hot Times at Mercury: Mission Operations for the Mercury Atmospheric and Surface Composition Spectrometer on MESSENGER

The thermal environment of the planet Mercury requires constant monitoring by the MESSENGER mission operations team during the planning of spacecraft and instrument operations. During the peak heating periods of each Mercury year (88 Earth days), the Mercury Atmospheric and Surface Composition Spectrometer (MASCS) takes advantage of seasonal spacecraft off-pointing, in combination with instrument power cycling, to maintain the temperature of sensitive detectors within operational limits. Spacecraft and instrument thermal models, in combination with recorded flight data, have been used to update operational plans seasonally so as to maximize scientific return while staying within instrument operational limits. As the year of primary mission operations drew to a close, thermal management procedures already in place were updated, as a result of expected changes in the thermal environment, to enable MASCS to maximize measurement opportunities for a second Earth year of extended mission operations.

Prometheus's Challenge: Scheduling MASCS Observations Using SciBox for Orbital Operations at Mercury

Performing scientific observations of a planet from orbit is a complicated endeavor for a spectrograph with a small field of view. Adding in a second spectrometer with a different field of view, and attempting to observe three different aspects of the planet with a total of five detectors, constrained by severe orbital, pointing, and downlink limitations, increase the challenge. On board the MESSENGER spacecraft, the Mercury Atmospheric and Surface Composition Spectrometer (MASCS), which consists of two separate instruments (the Visible and Infrared Spectrograph – VIRS – and the Ultraviolet and Visible Spectrometer – UVVS), will be facing precisely that challenge during the orbital operation phase of the mission to Mercury. As the cruise operations and three successive flybys of Mercury have demonstrated, manually sequencing observations for these two instruments is a labor-intensive task. In order to help schedule MASCS observations of both the surface and the planetary exosphere more efficiently, a planning tool called SciBox will be employed to generate the initial observation suite for each orbital period and coordinate MASCS observations with the other science instruments aboard the spacecraft.

BEHAVIOR OF PARTICULATE MATERIALS AT VERY LOW PRESSURES

Three separate sets of cyclic compression and extension experiments on loose and dense specimens of cohesionless assemblies of subangular quartz particles were conducted under displacement control at very low effective pressure levels. The first two sets were conducted under drained conditions on separate flights of the NASA Space Shuttle (STS-79, 1996; and STS89, 1998) under microgravity conditions, and the last set was recently conducted on Columbia (STS-107) in January, 2003. The Columbia experiments were designed both to verify earlier data as well as to expand into new areas. Large portions of the STS-107 data were collected via telemetry during flight, although the on-board stored data records, the specimens and experiment hardware were lost. Overall experiment technique, observations and data will be presented for all sets of experiments, with a focus placed on the planned and achieved goals of the Columbia experiments.

Computed tomography investigation of microgravity-tested sand samples

Computed Tomography (CT) is being used to investigate the complex internal structure of axisymmetric (triaxial) sand specimens. A series of triaxial experiments was conducted on dry Ottawa sand specimens at very low effective confining stresses in a microgravity environment aboard the Space Shuttle during two missions. Post-flight analysis includes studying the internal fabric and failure patterns using CT. In addition ground-tested specimens subjected to different compression levels are scanned to investigate the evolution of instability patterns, quantify void ratio variation, and provide a direct comparison with microgravity specimens. For an upcoming Shuttle mission, trial specimens are scanned to investigate an experimental reforming method for flight and evaluate techniques for reconstituting specimens. The CT technique demonstrates good ability to detect specimen inhomogeneities and localization patterns, and quantify void ratio variation within sand specimens.