The Mercury Dual Imaging System on the MESSENGER Spacecraft

The Mercury Dual Imaging System (MDIS) on the MESSENGER spacecraft will provide critical measurements tracing Mercury’s origin and evolution. MDIS consists of a monochrome narrow-angle camera (NAC) and a multispectral wide-angle camera (WAC). The NAC is a 1.5° field-of-view (FOV) off-axis reflector, coaligned with the WAC, a four-element refractor with a 10.5° FOV and 12-color filter wheel. The focal plane electronics of each camera are identical and use a 1,024×1,024 Atmel (Thomson) TH7888A charge-coupled device detector. Only one camera operates at a time, allowing them to share a common set of control electronics. The NAC and the WAC are mounted on a pivoting platform that provides a 90° field-of-regard, extending 40° sunward and 50° anti-sunward from the spacecraft +Z-axis—the boresight direction of most of MESSENGER’s instruments. Onboard data compression provides capabilities for pixel binning, remapping of 12-bit data into 8 bits, and lossless or lossy compression. MDIS will acquire four main data sets at Mercury during three flybys and the two-Mercury-solar-day nominal mission: a monochrome global image mosaic at near-zero emission angles and moderate incidence angles, a stereo-complement map at off-nadir geometry and near-identical lighting, multicolor images at low incidence angles, and targeted high-resolution images of key surface features. These data will be used to construct a global image base map, a digital terrain model, global maps of color properties, and mosaics of high-resolution image strips. Analysis of these data will provide information on Mercury’s impact history, tectonic processes, the composition and emplacement history of volcanic materials, and the thickness distribution and compositional variations of crustal materials. This paper summarizes MDIS’s science objectives and technical design, including the common payload design of the MDIS data processing units, as well as detailed results from ground and early flight calibrations and plans for Mercury image products to be generated from MDIS data.     

Non-evidence of lightning and associated volcanism at Venus

Reports of unpredicted lightning and its spatial association with mountains of possible volcanic origin are provocative features of the 1980's literature on Venus. These reports are based upon interpretation of low-frequency 100 Hz electric field noise observed from the Pioneer Venus Orbiter during 1978–1986. These speculations have been repeatedly challenged in the literature. Even though explosive volcanism, like lightning, is discounted in the literature, researchers have been prompted to believe in present-day eruptions by the suggestion that volcanic plumes might stimulate the otherwise unexpected lightning. Recent introductions of a distinct set of higher-frequency electric field noise has resulted in further claims for lightning, but these results, like those derived from the 100 Hz data are discounted be several independent studies. Commenting on the large body of 100 Hz data, Russell (1991) abandons earlier reports of the planetographic clustering of this noise, and states that active volcanoes are not the source of the Venus lightning. This welcome acknowledgement leaves unresolved problems. First, this brief comment is quite insufficient to correct the widespread and flawed perception that Venus is currently experiencing widespread lightning, stimulated by volcanic disturbances. Second, this admission leaves unexplained the origin of the voluminous 100 Hz data set. The foregoing problems, combined with negative results of recent independent studies, indicate strongly that the Pioneer Venus results provide no reliable evidence of either lightning or volcanism at Venus.     

The measured radiation environment within Spacelabs 1 and 2 and comparison with predictions

To measure the radiation environment in the Spacelab (SL) module and on the pallet, a set of passive and active radiation detectors was flown as part of the Verification Flight Instrumentation (VFI). SL 1 carried 4 passive and 2 active detector packages which, with the data from the 26 passive detectors of Experiment INS006, provided a comprehensive survey of the radiation environment within the spacecraft. SL 2 carried 2 passive VFI units on the pallet. Thermoluminescent dosimeters (TLDs) measured the low linear energy transfer (LET) dose component; the HZE fluence and LET spectra were mapped with CR-39 track detectors; thermal and epithermal neutrons were measured with the use of fission foils; metal samples analyzed by gamma ray spectroscopy measured low levels of several activation lines. The TLDs registered from 97 to 143 mrad in the SL 1 module. Dose equivalents of 330±70 mrem in the SL 1 module and 537±37 mrem on the SL 2 pallet were measured. The active units in the SL 1 module each contained an integrating tissue-equivalent ion chamber and two differently-shielded xenon-filled proportional counters. The ion chambers accumulated 125 and 128 mrads for the mission with 17 and 12 mrads accumulated during passages through the South Atlantic Anomaly (SAA). The proportional counter rates (1 cps at sea level) were 100 cps in the middle of the SAA (mostly protons), 35 cps at large geomagnetic latitudes (cosmic rays) and 100 cps in the South Horn of the electron belts (mostly bremsstrahlung). Detailed results of the measurements and comparison with calculated values are described.     

Energy deposition in the upper atmosphere in the EXCEDE III experiment

The EXCEDE III sounding rocket flight of April 27, 1990 used a 18 Ampere 2.5 keV electron beam to produce an artificial aurora in the region 90 to 115 km. A “daughter” sensor payload remotely monitored the low-energy X-ray spectrum while scanning photometers measured the spatial profile of prompt emissions of N₂⁺ (1N) and N₂ (2P) transitions (3914Å and 3805Å, respectively). Two Ebert-Fastie spectrometers measured the spectral region from 1800 to 8000Å. On the “mother” accelerator payload, the return current electron differential energy spectra were monitored by an electrostatic analyzer (up to 10 keV) and by a retarding potential analyzer (0 eV to 100 eV). We present an overview of the results from this experiment.     

Reduced-order H/sub /spl infin// and L/sub 2/-L/sub /spl infin// filtering via linear matrix inequalities

Necessary and sufficient conditions are derived for the existence of a solution to the continuous-time and discrete-time reduced-order H/sub /spl infin// and L/sub 2/-L/sub /spl infin// filtering problems. These conditions are expressed in terms of linear matrix inequalities (LMIs) and a coupling nonconvex matrix rank constraint. Convex LMI problems are obtained for the full-order and the zeroth-order filtering. An explicit parametrization of all reduced-order filters that correspond to a feasible solution is derived in terms of a contractive matrix, and iterative algorithms are proposed to solve the reduced-order filtering problems using alternating projections.     

ISEE plasma observations near the subsolar magnetopause

The early ISEE orbits provided the opportunity to study the magnetopause and its environs only a few Earth radii above the subsolar point. Measurements of complete two-dimensional ion and electron distributions every 3 or 12 s, and of three-dimensional distributions every 12 or 48 s by the LASL/MPI instrumentation on both spacecraft allow a detailed study of the plasma properties with unprecedented temporal resolution. This paper presents observations obtained during four successive inbound orbits in November 1977, containing a total of 9 magnetopause crossings, which occurred under widely differing orientations of the external magnetic field. The main findings are: (1) The magnetosheath flow near the magnetopause is characterized by large fluctuations, which often appear to be temporal in nature. (2) Between 0.1 and 0.3R _E outside the magnetopause, the plasma density and pressure often start to gradually decrease as the magnetopause is approached, in conjunction with an increase in magnetic field strength. These observations are in accordance with the formation of a depletion layer due to the compression of magnetic flux tubes. (3) In cases where the magnetopause can be well resolved, it exhibits fluctuations in density, and especially pressure and bulk velocity around average magnetosheath values. The pressure fluctuations are anticorrelated with simultaneous magnetic field pressure changes. (4) In ope case the magnetopause is characterized by substantially displaced electron and proton boundaries and a proton flow direction change from upwards along the magnetopause to a direction tranverse to the geomagnetic field. These features are in agreement with a model of the magnetopause described by Parker. (5) The character of the magnetopause sometimes varies strongly between ISEE-1 and -2 crossings which occur 1 min apart. At times this is clearly the result of highly non-uniform motions. There are also cases where there is very good agreement between the structures observed by the two satellites. (6) In three of the nine crossings no boundary layer was present adjacent to the magnetopause. More remarkably, two of the three occurred while the external magnetic field had a substantial southward component, in clear contradiction to expectations from current reconnection models. (7) The only thick (low-latitude) boundary layer (LLBL) observed was characterized by sharp changes at its inner and outer edges. This profile is difficult to reconcile with local plasma entry by either direct influx or diffusion. (8) During the crossings which showed no boundary layer adjacent to the magnetopause, magnetosheath-like plasma was encountered sometime later. Possible explanations include the sudden formation of a boundary layer at this location right at the time of the encounter, and a crossing of an inclusion of magnetosheath plasma within the magnetosphere. (9) The flow in the LLBL is highly variable, observed directions include flow towards and away from the subsolar point, along the geomagnetic field and across it, tangential and normal to the magnetopause. Some of these features clearly are nonstationary. The scale size over which the flow directions change exceeds the separation distance (several hundred km) of the two spacecraft.     

Meteoroids and space debris hypervelocity impact penetrations in LDEF MAP foils compared with hydrocode simulations

The continued analyses of penetrating impacts on MAP foils of Aluminium and Brass have produced data for several LDEF faces, i.e., Space, West, and East. These data have immediate bearing on the interpretation and design of devices to detect the penetration of a thin metallic film by a dust grain which have been tested both in the laboratory and in space. A crucial component of the analysis has been the theoretical calculation utilizing CTH, a Sandia National Laboratory Hydrodynamic computer code /1/ to assess the parameters of the hypervelocity penetration event. In particular theoretical hydrodynamic calculations have been conducted to simulate the hypervelocity impact event where various cosmic dust grain candidates, e.g., density = 0.998, 2.700, 7.870 (gm/cm³), and velocities, i.e., 7 - 16 km/s, have been utilized to reproduce the events. Theoretical analyses of hypervelocity impact events will be reported which span an extensive matrix of values for velocity, density and size. Through a comparison between LDEF MAP foil measurements and CTH hydrocode calculations these analyses will provide an interpretation of the most critical parameters measured for space returned materials, i.e., for thin films, the diameter of the penetration hole, D_h, and for semi-infinite targets, the depth-to-diameter ratio of craters, . An immediate consequence of a comparison of CTH calculations with space exposed materials will be an enhancement of the coherent model developed by UKC-USS researchers to describe penetration dynamics associated with LDEF MAP foils.     

A high-resolution Ge spectrometer for Gamma-Ray burst astronomy

The Transient Gamma-Ray Spectrometer (TGRS) to be flown aboard the WIND spacecraft is primarily designed to perform high resolution spectroscopy of transient -ray events, such as cosmic -ray bursts and solar flares over the energy range 25 keV to 8.2 MeV with an expected spectroscopic resolution of 3 keV at 1 MeV. The detector itself consists of a 215 cm³ high purityn-type Ge crystal kept at cryogenic temperatures by a passive radiative cooler. The geometric field of view defined by the cooler is 1.8 steradian. To avoid continuous triggers by soft solar events, a thin BeCu Sun-shield around the sides of the cooler has been provided. A passive Mo/Pb occulter, which modulates signals from within ±5° of the ecliptic plane at the spacecraft spin frequency, is used to identify and study solar flares, as well as emission from the galactic plane and center. Thus, in addition to transient event measurements, the instrument will allow the search for possible diffuse background lines and monitor the 511 keV positron annihilation radiation from the galactic center. In order to handle the typically large burst count rates, which can be in excess of 100 kHz, burst data are stored directly in an onboard 2.75 Mbit burst memory with an absolute timing accuracy of ±1.5 ms after ground processing. The memory is capable of storing the entire spectral data set of all but the largest bursts. WIND is scheduled to be launched on a Delta II launch vehicle from Cape Canaveral on November 1, 1994. After injection into a phasing orbit, the spacecraft will execute a double lunar swing-by before being moved into a controlled halo orbit about theL1 Lagrangian point (250R _e towards the Sun). This will provide a 5 light-second light travel time with which to triangulate gamma-ray burst sources with Earth-orbiting systems, such as those on-board the Gamma-Ray Observatory (GRO). The response of instrument to transient -ray events such as GRB's and solar flares will be presented as well as the expected response to steady state point sources and galactic center line emission.