S.R. Floyd

Compositional mapping with the NEAR X ray/gamma ray spectrometer

The X ray/gamma ray spectrometer (XGRS) instrument on board the Near Earth Asteroid Rendezvous (NEAR) spacecraft will map asteroid 433 Eros in the 0.7 keV to 10 MeV energy region. Measurements of the discrete line X ray and gamma ray emissions in this energy domain can be used to obtain both qualitative and quantitative elemental compositions with sufficient accuracy to enable comparison to the major meteorite typies. It is believed that Eros is an S-type asteroid, the most common of the near-Earth asteroids. The determination of whether Eros consists of either differentiated or undifferentiated materials is an important objective of this mission. Observations of Eros during the NEAR mission will contribute significantly to our understanding of the structure and composition of this asteroid. The NEAR spacecraft was successfully launched on February 17, 1996. The NEAR XGRS was turned on during the week of April 7, 1996, and all detector systems operated nominally. Background spectra have been obtained.

Radiation effects in the Si-PIN detector on the Near Earth Asteroid Rendezvous mission

Abstract A Si-PIN photodiode is being used as a solar X-ray monitor on the X-ray/gamma-ray spectrometer experiment which is flying on the Near Earth Asteroid Rendezvous spacecraft. Since its launch in February 1996 this photodiode has experienced several brief failures. These anomalies and other performance characteristics will be described. Efforts to reproduce these failures in ground tests with flight spare equipment will also be discussed.

Data management and analysis techniques used in the near X-ray and gamma-ray spectrometer systems

Abstract The NEAR Earth Asteroid Rendezvous (NEAR) spacecraft will encounter the 433Eros asteroid for a one year orbital mission in December 1998. Its on-board remote sensing instrumentation includes X-ray and gamma-ray (XGRS) spectrometers. NEAR is an orbital mission and long integrations over spatially specific asteroid regions are generally not possible. A methodology for simulating longer integrations has been developed for XGRS and uses unique management, correlative and analytical ground systems to render mapping data products. Evaluation of the spatial environment is accomplished through virtual renderings of the asteroid surface giving incidence, emission and surface roughness factors. Extended computer plate modeling information is employed to optimize ground computer systems processing time. Interactive visualization systems have been developed to manage close to a million spectra that will be collected during the encounter. Feedback systems are employed to inspect, tag and calibrate spectral data products. Mission planning, systems development and managerial responsibilities have been distributed to cooperating science organizations at The Goddard Space Flight Center, The University of Arizona, Cornell University, The Applied Physics Laboratory and The Max Plank Institute.

Future planetary X-ray and gamma-ray remote sensing system and in situ requirements for room temperature solid state detectors

Abstract X-Ray and gamma-ray remote sensing observations find important applications in the study of the development of the planets. Orbital measurements can be carried out on solar-system bodies whose atmospheres and trapped radiation environments do not interfere significantly with the emissions. Elemental compositions can be inferred from observations of these line emissions. Future planetary missions also will involve landing both stationery and roving probes on planetary surfaces. Both X-ray and gamma-ray spectrometers will be used for performing elemental analysis of surface samples. These future planetary missions will impose a number of constraints: the flight instruments must be significantly reduced in weight from those previously flown; for many missions, gravity assist will be required, greatly increasing mission duration, resulting in the passage of several years before the first scientific measurement of a solar system body. The detector systems must operate reliably after years of cosmic-ray irradiation. Both spectroscopy and imaging detection systems are required. Room temperature systems show great promise for application to planetary X-ray and gamma-ray remote systems. A number of laboratory and sub-orbital, orbital, and planetary flight mission investigations have been and will be carried out in order to develop room temperature solid state detector systems for space flight.

Data processing system for the Near-Earth Asteroid Rendezvous (NEAR) x-ray and gamma-ray spectrometer (XGRS) ground system

An x-ray and gamma-ray spectrometer (XGRS) is onboard the Near Earth Asteroid Rendezvous (NEAR) spacecraft to determine the elemental composition of the surface of the asteroid 433 Eros. The Eros asteroid is highly non-spherical in physical shape and the development of data management and analysis methodologies are in several areas a divergence from traditional remotely sensed geographical information systems techniques. Field of view and asteroid divergence from traditional remotely sensed geographical information system techniques. Field of view and asteroid surface geometry must be derived virtually and then combined with real measurements of solar, spectral and instrument calibration information to derive meaningful scientific results. Spatial resolution of planned geochemical maps will be improved from the initial conditions of low statistical significance per integration by repeated surface flyovers and regional spectral accumulation. This paper describes the results of a collaborative effort of design and development of the NEAR XGRS instrument ground system undertaken by participants at the Goddard Space Flight Center, University of Arizona, Cornell University, Applied Physics Laboratory, and Max Planck Institute.

Cruise measurements taken with the near XGRS

Abstract The X-ray/Gamma-ray Spectrometer on the Near Earth Asteroid Rendezvous spacecraft will remotely detect characteristic x-ray and gamma-ray emissions from the surface of 433 Eros to develop global maps of the elemental composition of the asteroid surface. Solar excited x-ray fluorescence in the 1 to 10 keV range will be used to measure the surface abundances of Mg, Al, Si, Ca, Ti, and Fe with spatial resolutions down to 2 km. Gamma-ray emissions in the 0.1 to 10 MeV range will be used to measure cosmic-ray excited emissions from such elements as O, Si, Fe, and H as well as naturally radioactive elements K, Th, and U to surface depths on the order of 10 cm. In-flight calibrations are essential to the understanding and analysis of data collected at Eros.

Spectral analysis and compositing techniques for the Near Earth Asteroid Rendezvous (NEAR Shoemaker), X-ray and Gamma-Ray Spectrometers (XGRS)

Abstract An X-ray and Gamma-Ray Spectrometer (XGRS) is on board the Near Earth Asteroid Rendezvous (NEAR) spacecraft to determine the elemental composition of the surface of the asteroid 433 Eros. The Eros asteroid is highly oblate and irregular in shape. As a result, analysis methodologies are in many ways a divergence from comparable techniques. Complex temporal, spatial and instrument performance relationships must be accounted for during the analysis process. Field of view and asteroid surface geometry measurements must be modeled and then combined with real measurements of solar, spectral and instrument calibration information to derive scientific results. NEAR is currently orbiting 433 Eros and is in the initial phases of its primary data integration and mapping phases. Initial results have been obtained and bulk chemistry assessments have been obtained through specialized background assessment and data reduction techniques.

NEUTRON-INDUCED GAMMA-RAY MEASUREMENTS ON SOFT LANDERS

Abstract Gamma-ray spectra provide useful data in the study of the development of planets. Measurements made on landers overcome many of the limitations of orbital measurements, especially for solar-system bodies where atmospheres interfere with obtaining orbital data. Elemental compositions can be inferred from the spectra, produced by neutron interactions with nuclei of the planetary body. The cosmic ray flux yields a relatively low neutron flux requiring long integration times (sometimes as long as a year) to obtain a statistical accuracy allowing elemental compositions to be inferred to determine different mineral compositions and to characterize the nature of the area studied. A recent advance is the combination of a gamma-ray spectrometer with a compact, pulsed neutron generator, which increases the neutron flux by orders of magnitude. Such a device is used on Earth and its capability is being extended to operate within the low weight and power requirements of typical lander missions. A pulsed source permits data accumulation by type of reaction, increasing the sensitivity for detecting many elements. The enhanced speed of measurement makes many new studies possible, such as time-dependent measurements to detect seasonal or diurnal changes from active processes. Where motion is provided, either around the lander or on a rover, data can be obtained at different positions to evaluate heterogeneities. The use of a pulsed neutron generator provides increased quality of elemental data and provides many additional options for system design to provide more useful information for understanding the creation and evolution of our solar system.

The Analog System for the Gamma Ray Spectrometer on_Mars Odyssey

The Gamma Ray Spectrometer instrument on board the Mars Odyssey spacecraft was powered up for the second time since launch in April 2001. High voltage was applied to the Ge detector in February 2002. The system is performing well, but the detector is showing resolution degradation from solar particles and cosmic ray exposure during the long cruise. The analog system and high voltage part of the system were designed and built at NASA’s Goddard Space Flight Center. This portion of the instrument system will be presented. Of the many design challenges, three stand out on this mission. The prohibited materials and elemental mass budget for everything used in the construction was defined and constrained. This type of constraint was unique and challenging. Signal overload recovery from cosmic ray and high‐energy solar proton hits was a challenge because the environment could not be well defined and varies throughout the Sun cycle. The design effort to prevent microphonic problems in the very sensitive front‐end...

A Ground Systems Template for Remote Sensing Systems

Spaceborne remote sensing using gamma and X‐ray spectrometers requires particular attention to the design and development of reliable systems. These systems must ensure the scientific requirements of the mission within the challenging technical constraints of operating instrumentation in space. The Near Earth Asteroid Rendezvous (NEAR) spacecraft included X‐ray and gamma‐ray spectrometers (XGRS), whose mission was to map the elemental chemistry of the 433 Eros asteroid. A remote sensing system template, similar to a blackboard systems approach used in artificial intelligence, was identified in which the spacecraft, instrument, and ground system was designed and developed to monitor and adapt to evolving mission requirements in a complicated operational setting. Systems were developed for ground tracking of instrument calibration, instrument health, data quality, orbital geometry, solar flux as well as models of the asteroid’s surface characteristics, requiring an intensive human effort. In the future, missions such as the Autonomous Nano‐Technology Swarm (ANTS) program will have to rely heavily on automation to collectively encounter and sample asteroids in the outer asteroid belt. Using similar instrumentation, ANTS will require information similar to data collected by the NEAR X‐ray/Gamma‐Ray Spectrometer (XGRS) ground system for science and operations management. The NEAR XGRS systems will be studied to identify the equivalent subsystems that may be automated for ANTS. The effort will also investigate the possibility of applying blackboard style approaches to automated decision making required for ANTS.