Multi-Spectral Imager on the Near Earth Asteroid Rendezvous Mission

A multispectral imager has been developed for a rendezvous mission with the near-Earth asteroid, 433 Eros. The Multi-Spectral Imager (MSI) on the Near-Earth Asteroid Rendezvous (NEAR) spacecraft uses a five-element refractive optical telescope, has a field of view of 2.93 × 2.25°, a focal length of 167.35 mm, and has a spatial resolution of 16.1 × 9.5 m at a range of 100 km. The spectral sensitivity of the instrument spans visible to near infrared wavelengths, and was designed to provide insight into the nature and fundamental properties of asteroids and comets. Seven narrow band spectral filters were chosen to provide multicolor imaging and to make comparative studies with previous observations of S asteroids and measurements of the characteristic absorption in Fe minerals near 1 µm. An eighth filter with a much wider spectral passband will be used for optical navigation and for imaging faint objects, down to visual magnitude of +10.5. The camera has a fixed 1 Hz frame rate and the signal intensities are digitized to 12 bits. The detector, a Thomson-CSF TH7866A Charge-Coupled Device, permits electronic shuttering which effectively varies the dynamic range over an additional three orders of magnitude. Communication with the NEAR spacecraft occurs via a MIL-STD-1553 bus interface, and a high speed serial interface permits rapid transmission of images to the spacecraft solid state recorder. Onboard image processing consists of a multi-tiered data compression scheme. The instrument was extensively tested and calibrated prior to launch; some inflight calibrations have already been completed. This paper presents a detailed overview of the Multi-Spectral Imager and its objectives, design, construction, testing and calibration.     

Diversity of Comets: Formation Zones and Dynamical Paths

The past dozen years have produced a new paradigm with regard to the source regions of comets in the early solar system. It is now widely recognized that the likely source of the Jupiter-family short-period comets (those with Tisserand parameters, T > 2 and periods, P, generally < 20 years) is the Kuiper belt in the ecliptic plane beyond Neptune. In contrast, the source of the Halley-type and long-period comets (those with T < 2 and P > 20 years) appears to be the Oort cloud. However, the comets in the Oort cloud almost certainly originated elsewhere, since accretion is very inefficient at such large heliocentric distances. New dynamical studies now suggest that the source of the Oort cloud comets is the entire giant planets region from Jupiter to Neptune, rather than primarily the Uranus-Neptune region, as previously thought. Some fraction of the Oort cloud population may even be asteroidal bodies formed inside the orbit of Jupiter. These comets and asteroids underwent a complex dynamical random walk among the giant planets before they were ejected to distant orbits in the Oort cloud, with possible interesting consequences for their thermal and collisional histories. Observational evidence for diversity in cometary compositions is limited, at best. This revised version was published online in June 2006 with corrections to the Cover Date.     

Scanning with charge-coupled devices

The 0.91-m Spacewatch Telescope of the Steward Observatory of the University of Arizona on Kitt Peak is dedicated to scanning with charge-coupled devices (CCDs) during the dark half of the month. We explored six modes of using CCDs for searches of gamma-ray bursters, debris in geosynchronous space, satellites of asteroids, brown dwarfs, the tenth planet, comets, cometesimals, and various types of asteroids. In the process, we gained experience with cosmic rays and artifacts in CCD observations. Each of these topics is described. I especially note that the existence of cometesimals has not been confirmed by the Spacewatch Telescope, contrary to reports published by others.This paper describes a new discipline in astrophysics, scannerscopy, of surveying with a CCD rather than with photography at a Schmidt telescope. It uses the CCD in scanning rather than in sequencing of stare exposures as is done at most observatories. This may save telescope time, and flat-fielding is rarely needed. Usually we turn the drive off, but the scanning can be done with the telescope moving. In any case, the motion on the sky is precisely followed by slaving the charge transfer of the CCD to the drift rate of the image, while the CCD is read out continuously during the observing. Our primary application of CCD scanning is on moving objects such as comets and asteroids. We also do routine astronomy with CCDs, in a transit method, and this yields a precision of better than ±0.7 arc sec.We presently use a Tektronix 2048×2048 CCD, 38 arc min wide, to a limiting magnitude ofV=20.5 (6 detection). This is successful even for discovering rare and small near-Earth asteroids. 1990 UN with a diameter of 90 m and 1991 BA at 9 m are the smallest natural objects observed outside the Earth's atmosphere to date. In a month with good conditions we find typically 2000 new main-belt asteroids and, on average, nearly two near-Earth asteroids. Only the latter are followed up with astrometry. The goal is to study magnitude-frequency relations, as well as to complete the inventory of dangerous impactors on Earth. We are designing a new CCD-scanning telecope to become an order-of-magnitude more effective in the discovery of elusive objects than the Spacewatch Telescope. The paper also describes possibilities with cameras on spacecraft that pass through the asteroid belt; thousands of small asteroids can be observed with the CCDs of CRAF and CASSINI.     

The Origin and Evolution of the Asteroid Belt��Implications for Vesta and Ceres

Vesta and Ceres are the largest members of the asteroid belt, surviving from the earliest phases of Solar System history. They formed at a time when the asteroid belt was much more massive than it is today and were witness to its dramatic evolution, where planetary embryos were formed and lost, where the collisional environment shifted from accretional to destructive, and where the current size distribution of asteroids was sculpted by mutual collisions and most of the asteroids originally present were lost by dynamical processes. Since these early times, the environment of the asteroid belt has become relatively quiescent, though over the long history of the Solar System the surfaces of Vesta and Ceres continue to record and be influenced by impacts, most notably the south polar cratering event on Vesta. As a consequence of such impacts, Vesta has contributed a significant family of asteroids to the main belt, which is the likely source of the HED meteorites on Earth. No similar contribution to the main belt (or meteorites) is evident for Ceres. Through studies of craters, the surfaces of these asteroids will offer an opportunity for Dawn to probe the modern population of small asteroids in a size regime not directly observable from Earth.     

The observational data base on the motion and evolution of comets and asteroids

Current observational data base on the motion of comets and asteroids is reviewed. Particular attention is paid to the absolute and relative abundances of different dynamical types of objects, and to the time intervals between their first and last observations. The latter quantity, ranging from two days to two milliennia for individual objects, is the dominant measure of the accuracy of the orbit determination. Distribution of the tracking times of comets (distinguished by dynamical age: new, long-period, Halley type, Jupiter family) and asteroids (distinguished by stability: Apollos, Amors, main-belt asteroids, outer librators, outer unstable objects) are reconstructed. The peculiar shapes of individual distributions can be explained by the complex mechanisms of discoveries, rediscoveries, orbit computations, follow-up observations and backward identifications. A comparison is also made with the dynamical data base on meteoroids, as regards the accuracy of their orbits.The cumulative tracking times (170000 yr for all 7600 objects with known orbits taken together) are compared with the lifetimes and occurrence rates of different events of evolutionary significance. Only in the case of short-period comets the evolution is rapid enough to render observable a variety of important changes, ranging from drastic transformations of orbits to disruption or total outgassing. For asteroids, only minor cratering collisions which do not result in detectable changes of their orbits are covered by the whole observational history.Expected future improvements of observing and data-handling techniques are outlined. With these in view, the size and character of the data to become available by the end of this century are predicted. Dynamical types of objects, which are currently known in only one or a few examples, are pointed out. Apparently, other types of rare occurrence and short survival time still escape detection. A list of easiest targets of short-duration spacecraft missions is presented.The deficiencies of current statistics due to observational selection; the broad variety of regimes of motion occupied by widely differing proportional representations of the known objects; and demands for suitable targets of future spacecraft missions make it highly desirable to maintain the present rapid rate of augmentation of the data base for the years to come.Recent passages of two comets — 1983d IRAS-Araki-Alcock and 1983e Sugano-Saigusa-Fujikawa — near the Earth indicate that both the collision rate given in Table VIII and the contribution of long-period comets to it may have been slightly underestimated. The appropriate adjustment of the log-t values by less than — 0.10 has no effect of the general conclusions, however.The success of the orbiting observatory IRAS in detecting faint interplanetary objects lends better promises for the increase of the number of known objects (in particular comets) than anticipated in Section 6 and estimated in Table IX. Obviously, the outcome will largely depend on the implementation, time coverage and degree of exploitation of similar projects in the near future.     

Virtis: An Imaging Spectrometer for the Rosetta Mission

The VIRTIS (Visual IR Thermal Imaging Spectrometer) experiment has been one of the most successful experiments built in Europe for Planetary Exploration. VIRTIS, developed in cooperation among Italy, France and Germany, has been already selected as a key experiment for 3 planetary missions: the ESA-Rosetta and Venus Express and NASA-Dawn. VIRTIS on board Rosetta and Venus Express are already producing high quality data: as far as Rosetta is concerned, the Earth-Moon system has been successfully observed during the Earth Swing-By manouver (March 2005) and furthermore, VIRTIS will collect data when Rosetta flies by Mars in February 2007 at a distance of about 200 kilometres from the planet. Data from the Rosetta mission will result in a comparison – using the same combination of sophisticated experiments – of targets that are poorly differentiated and are representative of the composition of different environment of the primordial solar system. Comets and asteroids, in fact, are in close relationship with the planetesimals, which formed from the solar nebula 4.6 billion years ago. The Rosetta mission payload is designed to obtain this information combining in situ analysis of comet material, obtained by the small lander Philae, and by a long lasting and detailed remote sensing of the comet, obtained by instrument on board the orbiting Spacecraft. The combination of remote sensing and in situ measurements will increase the scientific return of the mission. In fact, the “in situ” measurements will provide “ground-truth” for the remote sensing information, and, in turn, the locally collected data will be interpreted in the appropriate context provided by the remote sensing investigation. VIRTIS is part of the scientific payload of the Rosetta Orbiter and will detect and characterise the evolution of specific signatures – such as the typical spectral bands of minerals and molecules – arising from surface components and from materials dispersed in the coma. The identification of spectral features is a primary goal of the Rosetta mission as it will allow identification of the nature of the main constituent of the comets. Moreover, the surface thermal evolution during comet approach to sun will be also studied.     

Water Reservoirs in Small Planetary Bodies: Meteorites,Asteroids, and Comets

Asteroids and comets are the remnants of the swarm of planetesimals from which the planets ultimately formed, and they retain records of processes that operated prior to and during planet formation. They are also likely the sources of most of the water and other volatiles accreted by Earth. In this review, we discuss the nature and probable origins of asteroids and comets based on data from remote observations, in situ measurements by spacecraft, and laboratory analyses of meteorites derived from asteroids. The asteroidal parent bodies of meteorites formed \(\leq 4\) Ma after Solar System formation while there was still a gas disk present. It seems increasingly likely that the parent bodies of meteorites spectroscopically linked with the E-, S-, M- and V-type asteroids formed sunward of Jupiter’s orbit, while those associated with C- and, possibly, D-type asteroids formed further out, beyond Jupiter but probably not beyond Saturn’s orbit. Comets formed further from the Sun than any of the meteorite parent bodies, and retain much higher abundances of interstellar material. CI and CM group meteorites are probably related to the most common C-type asteroids, and based on isotopic evidence they, rather than comets, are the most likely sources of the H and N accreted by the terrestrial planets. However, comets may have been major sources of the noble gases accreted by Earth and Venus. Possible constraints that these observations can place on models of giant planet formation and migration are explored.     

Infrared astronomy on the midcourse space experiment

The Midcourse Space Experiment (MSX) is a multiple objective experiment scheduled to fly by the end of 1994. Infrared photometry and interferometry will be obtained by a solid hydrogen cooled, off-axis telescope of 35 cm unobscured primary aperture. The sensitivities of the line scanned arrays are comparable to IRAS bands 1 and 2 but the spatial resolution is some 30 times better. Nine broadly defined astronomy experiments are planned for the 18 month cryogen phase of the mission. Four of these experiments survey regions not adequately covered by previous infrared missions: the zodiacal cloud near the sun and the anti-solar direction, the Galactic Plane where IRAS sensitivities were limited by confusion and the gaps left by the IRAS survey. The higher sensitivity obtained from raster scans will probe Galactic structure and create intermediate spatial resolution maps of extended sources such as HII regions, the Magellanic Clouds and nearby galaxies. Measurements are also planned on a number of solar system objects such as planets, asteroids, the dust bands, comets and cometary debris trails. Moderate resolution spectra of a number of bright, discrete, extended sources will be obtained as well as low resolution spectral mapping along the Galactic Plane and Zodiacal dust cloud.     

Review,based on particles and fields observations made aboard the Giotto spacecraft,concerning the nature of the solar wind interaction with comets P/Halley and P/Grigg–Skjellerup

A key driver underlying the decision to command the Giotto spacecraft on to an encounter with comet P/Grigg-Skjellerup following its highly successful encounter with P/Halley, was the unique scientific opportunity this provided to compare measurements made using the same suite of plasma and fields instruments at (a) very active and 'fresh' comet P/Halley and (b) at a weakly outgassing object (P/Grigg-Skjellerup). In the present paper an overview is provided of the complementary observations resultingly made aboard Giotto during each encounter, with special emphasis on measurements made by the energetic particles instrument EPONA (range E∼60–≥260 keV). Differences identified between the two complementary data sets in the nature of the Solar Wind interaction with the individual comets investigated are discussed and shown to be associated with (a) basic differences between the comets themselves and (b) differences in the interplanetary circumstances characterising each encounter This revised version was published online in June 2006 with corrections to the Cover Date.     

Expectations for Infrared Spectroscopy of 9P/Tempel 1 from Deep Impact

The science payload on the Deep Impact mission includes a 1.05–4.8 μm infrared spectrometer with a spectral resolution ranging from R∼200–900. The Deep Impact IR spectrometer was designed to optimize, within engineering and cost constraints, observations of the dust, gas, and nucleus of 9P/Tempel 1. The wavelength range includes absorption and emission features from ices, silicates, organics, and many gases that are known to be, or anticipated to be, present on comets. The expected data will provide measurements at previously unseen spatial resolution before, during, and after our cratering experiment at the comet 9P/Tempel 1. This article explores the unique aspects of the Deep Impact IR spectrometer experiment, presents a range of expectations for spectral data of 9P/Tempel 1, and summarizes the specific science objectives at each phase of the mission.     

Perspectives on the Comet-Asteroid-Meteorite Link

We discuss the possibility that CI and CM carbonaceous chondrites are fragments of extinct cometary nuclei. Theoretical and observational work suggests that comets evolve into asteroids, and several extinct cometary nuclei are now suspected to be among the near Earth object population. This population is the most likely source of meteorites and consequently, we may expect that some meteorites are from extinct comets in this population. The mineralogy and chemistry of CI and CM chondrites is consistent with the view that they originate from asteroidal objects of carbonaceous spectral classes, and these objects in turn may have a cometary origin. We do not suggest that CI or CM chondrites are directly delivered by active comets during perihelion passage or that these chondrites come from cometary debris in meteor streams. Instead, we summarize arguments suggesting that CI and CM chondrites represent fragments of cometary nuclei which evolved into near Earth asteroids after losing their volatiles. This revised version was published online in June 2006 with corrections to the Cover Date.     

Advanced Technologies Demonstrated by the Miniature Integrated Camera and Spectrometer (MICAS) Aboard Deep Space 1

MICAS is an integrated multi-channel instrument that includes an ultraviolet imaging spectrometer (80–185 nm), two high-resolution visible imagers (10–20 μrad/pixel, 400–900 nm), and a short-wavelength infrared imaging spectrometer (1250–2600 nm). The wavelength ranges were chosen to maximize the science data that could be collected using existing semiconductor technologies and avoiding the need for multi-octave spectrometers. It was flown on DS1 to validate technologies derived from the development of PICS (Planetary Imaging Camera Spectrometer). These technologies provided a novel systems approach enabling the miniaturization and integration of four instruments into one entity, spanning a wavelength range from the UV to IR, and from ambient to cryogenic temperatures with optical performance at a fraction of a wavelength. The specific technologies incorporated were: a built-in fly-by sequence; lightweight and ultra-stable, monolithic silicon-carbide construction, which enabled room-temperature alignment for cryogenic (85–140 K) performance, and provided superb optical performance and immunity to thermal distortion; diffraction-limited, shared optics operating from 80 to 2600 nm; advanced detector technologies for the UV, visible and short-wavelength IR; high-performance thermal radiators coupled directly to the short-wave infrared (SWIR) detector optical bench, providing an instrument with a mass less than 10 kg, instrument power less than 10 W, and total instrument cost of less than ten million dollars. The design allows the wavelength range to be extended by at least an octave at the short wavelength end and to ∼50 microns at the long wavelength end. Testing of the completed instrument demonstrated excellent optical performance down to 77 K, which would enable a greatly reduced background for longer wavelength detectors. During the Deep Space 1 Mission, MICAS successfully collected images and spectra for asteroid 9969 Braille, Mars, and comet 19/P Borrelly. The Borrelly encounter was a scientific hallmark providing the first clear, high resolution images and excellent, short-wavelength infrared spectra of the surface of an active comet’s nucleus.     

Orbital maneuver strategy design based on piecewise linear optimization for spacecraft soft landing on irregular asteroids

Recently, asteroid exploration becomes an important branch of human’s deep space activities. In this paper, a piecewise linear optimal orbital maneuver strategy is designed for a spacecraft soft landing on irregular-shaped asteroids. First, the space around an irregular asteroid is converted into several grid units, and the gravitational field of the asteroid is linearly fitted in each unit. Then, the soft-landing orbital maneuver strategy design problem is formulated as a piecewise linear optimal problem, and further transferred into a family of two-point boundary value problems, which can be solved using collocation method. Finally, a corresponding algorithm is developed to obtain the piecewise linear optimal maneuver strategy, which is proved to be able to achieve the soft-landing mission well. Simulation results show that the error of the model linearization is small enough, while the calculation efficiency is remarkably improved, and the robustness of maneuver strategy is also improved.     

The OSIRIS-REx Visible and InfraRed Spectrometer (OVIRS): Spectral Maps of the Asteroid Bennu

The OSIRIS-REx Visible and Infrared Spectrometer (OVIRS) is a point spectrometer covering the spectral range of 0.4 to 4.3 microns (25,000–2300 cm^?1). Its primary purpose is to map the surface composition of the asteroid Bennu, the target asteroid of the OSIRIS-REx asteroid sample return mission. The information it returns will help guide the selection of the sample site. It will also provide global context for the sample and high spatial resolution spectra that can be related to spatially unresolved terrestrial observations of asteroids. It is a compact, low-mass (17.8 kg), power efficient (8.8 W average), and robust instrument with the sensitivity needed to detect a 5% spectral absorption feature on a very dark surface (3% reflectance) in the inner solar system (0.89–1.35 AU). It, in combination with the other instruments on the OSIRIS-REx Mission, will provide an unprecedented view of an asteroid’s surface.     

The light of the night sky: Astronomical,interplanetary and geophysical

Summary We bring together our general results in two figures. Figure 14 portrays the resolution of the light of the night sky into its three principal components based on a series of zenith observations extending over a year at the two stationse: Fritz Peak in Colorado, U.S.A., (latitude N 39°.9, longitude W 105°.5) and Haleakala in Hawaii, U.S.A. (latitude N 20°.7, longitude W 156°.3). The observations are from a current study by Roach and Smith (1964a) using photometers centered on wavelength 5300 Å. With respect to sidereal time the airglow continuum is a constant. The two Milky Way traverses are conspicuous features of the integrated starlight curves. The variation of the zodiacal light is the result of the variable ecliptic latitude of the zenith throughout the year. A refined analysis of the data, not shown in the plot, gives a further variation of the zodiacal light as a function of - _bd, the differential ecliptic longitude between the zenith and the sun. The zodiacal light is the brighter of the three components except when the Milky Way is in the zenith. The zodiacal light tends to be systematically brighter toward the horizon so that it is definitely the most prominent of the three for the sky as a whole.The interrelationships of the constituents of the light of the night sky are shown from a different point of view in Figure 15 where the ordinate is logarithm of the surface brightness and the abscissa is logarithm of the distance or extent. Moving downward in the plot the features of the night sky appear below the line corresponding to the end of twilight. The brightness of the nightglow, the zodiacal light and gegenschein, the integrated starlight and galactic light are comparable (on the logarithm scale) but one is impressed with the vastly different linear distances in connection with the several phenomena. The nightglow is a terrestrial phenomenon having a thickness of about one atmospheric scale height (log R 7). The zodiacal light is an interplanetary phenomenon with a characteristic dimension of one astronomical unit (log R 13). The integrated starlight from our galaxy has a characteristic maximum dimension of some 30 kpc (log R 23). Finally the extra galactic nebulae which collectively contribute much less than 1% of the light of the night sky are at distances as much as log R 28. They can be photographed individually in spite of the competition of the sky background and in spite of the hazard of extinction by intervening dust.In the preparation of this report the writer has been impressed with the confluence of several circumstances that make possible the observation of the universe in the visible part of the spectrum. Any one of several contingencies might have made such observations impossible.Let us consider the matter of contrast. The prime example here is the bright (but beautiful!) day sky which prohibits serious daytime study of the astronomical sky. There follows, during a diurnal terrestrial rotation the period of twilight which under the best of circumstances lasts a little less than 1 1/2 hours but which, during the local summer, in the vicinity of polar regions persists all night. The obliquity of the ecliptic is sufficient to make a stimulating annual sequence of seasons but small enough to keep the twilight period of reasonable duration over a good portion of the earth.A hazard narrowly averted is that due to the interplanetary dust cloud leading to the zodiacal light. The concentration of dust is very small indeed (Figure 10) so that an increase by a factor often would be trivial in terms of the constitution of the solar system. But such an increase would result in a night sky so bright (average zodiacal light 2000 S₁₀ (vis) instead of 200) that the Milky Way would be difficult to see and the airglow difficult to measure. The aesthetic gain in a rather spectacular zodiacal light pattern over the sky would hardly compensate for the loss from the absence of the details of our galactic universe. The effect of such an enhanced zodiacal light would correspond to that experienced in a planetarium when the operator adjusts the rheostats to bring on dawn and the celestial objects disappear.A permanent twilight that would have the same effect would be due to the hydroxyl nightglow if (a) it were concentrated in the visible part of the spectrum rather than in the near infra red or if (b) the human eyes were sensitive in the near infrared.The narrow escape from the cosmic ignorance that would have resulted from a situation in which the observer found himself in a less favorable environment is well illustrated by the zone of avoidance of extra galactic nebulae in the vicinity of the Milky Way plane. If our galaxy were not highly flattened so that its extent perpendicular to the plane is sufficiently small to permit an observational window outward we would not have been able to photograph the extra-galactic objects and we would have been content with a rather restricted concept of a universe consisting of a single galaxy. The same dire result would have occurred if the sun to which our planet is attached were more deeply embedded in the galactic dust near the galactic center. Thus we find compensation for our non-central location.There can be little doubt that human ingenuity would in time have overcome any or all of the above circumstances as the radio astronomers have done by changing the exploring frequency so as to avoid the difficulties. But this would have taken time, especially in the absence of the stimulation of the knowledge gained by visual and photographic observations. It is likely that the time lag would have been sufficient that the present review could not have been written by the present author. It may be conjectured whether other astronomers on other planets are as fortunate or whether, after all, this is the best of all possible worlds.Contribution number 73. The report was written while the author was a Senior Specialist at the East-West Center of the University of Hawaii — on leave of absence from the Central Radio Propagation Laboratory of the U.S. National Bureau of Standards, Boulder, Colo., U.S.A.

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一种载人小行星探测目标星初选方法

针对载人小行星探测任务目标选取问题,通过对目标星选取影响因素的定量分析,提出了目标星初选方案。首先根据火箭的运载能力提出目标星轨道参数的筛选条件,然后根据地面观测和尺寸约束提出目标星绝对星等的要求,再根据结构的稳定性和温控要求提出目标星自旋周期的范围,最后按照科学价值并综合考虑其他因素建立目标星选取优化模型,得出目标星优选序列。根据提出的目标星初选方案和具体的任务需求进行载人小行星探测目标星选取,得出了目标星优选序列,证明了该方案合理、有效,为载人小行星探测提供一定的技术参考。     

Near Infrared Spectrometer for the Near Earth Asteroid Rendezvous Mission

The Near-Infrared Spectrometer (NIS) instrument on the Near-Earth Asteroid Rendezvous (NEAR) spacecraft is designed to map spectral properties of the mission target, the S-type asteroid 433 Eros, at near-infrared wavelengths diagnostic of the composition of minerals forming S asteroids. NIS is a grating spectrometer, in which light is directed by a dichroic beam-splitter onto a 32-element Ge detector (center wavelengths, 816–1486 nm) and a 32-element InGaAs detector (center wavelengths, 1371–2708 nm). Each detector reports a 32-channel spectrum at 12-bit quantization. The field-of-view is selectable using slits with dimensions calibrated at 0.37° × 0.76° (narrow slit) and 0.74° × 0.76° (wide slit). A shutter can be closed for dark current measurements. For the Ge detector, there is an option to command a 10x boost in gain. A scan mirror rotates the field-of-view over a 140° range, and a diffuse gold radiance calibration target is viewable at the sunward edge of the field of regard. Spectra are measured once per second, and up to 16 can be summed onboard. Hyperspectral image cubes are built up by a combination of down-track spacecraft motion and cross-track scanning of the mirror. Instrument software allows execution of data acquisition macros, which include selection of the slit width, number of spectra to sum, gain, mirror scanning, and an option to interleave dark spectra with the shutter closed among asteroid observations. The instrument was extensively characterized by on-ground calibration, and a comprehensive program of in-flight calibration was begun shortly after launch. NIS observations of Eros will largely be coordinated with multicolor imaging from the Multispectral Imager (MSI). NIS will begin observing Eros during approach to the asteroid, and the instrument will map Eros at successively higher spatial resolutions as NEAR's orbit around Eros is lowered incrementally to 25 km altitude. Ultimate products of the investigation will include composition maps of the entire illuminated surface of Eros at spatial resolutions as high as 300 m.     

Cometary Refractory Grains: Interstellar and Nebular Sources

Comets are heterogeneous mixtures of interstellar and nebular materials. The degree of mixing of interstellar sources and nebular sources at different nuclear size scales holds the promise of revealing how cometary particles, cometesimals, and cometary nuclei accreted. We can ascribe cometary materials to interstellar and nebular sources and see how comets probe planet-forming process in our protoplanetary disk. Comets and cometary IDPs contain carbonaceous matter that appears to be either similar to poorly-graphitized (amorphous) carbon, a likely ISM source, or highly labile complex organics, with possible ISM or outer disk heritage. The oxygen fugacity of the solar nebula depends on the dynamical interplay between the inward migration of carbon-rich grains and of icy (water-rich) grains. Inside the water dissociation line, OH^? reacts with carbon to form CO or CO₂, consuming available oxygen and contributing to the canonical low oxygen fugacity. Alternatively, the influx of water vapor and/or oxygen rich dust grains from outer (cooler) disk regions can raise the oxygen fugacity. Low oxygen fugacity of the canonical solar nebula favors the condensation of Mg-rich crystalline silicates and Fe-metal, or the annealing of Fe-Mg amorphous silicates into Mg-rich crystals and Fe-metal via Fe-reduction. High oxygen fugacity nebular conditions favors the condensation of Fe-bearing to Fe-rich crystalline silicates. In the ISM, Fe-Mg amorphous silicates are prevalent, in stark contrast to Mg-rich crystalline silicates that are rare. Hence, cometary Mg-rich crystalline silicates formed in the hot, inner regions of the canonical solar nebula and they are the touchstone for models of the outward radial transport of nebular grains to the comet-forming zone. Stardust samples are dominated by Mg-rich crystalline silicates but also contain abundant Fe-bearing and Fe-rich crystalline silicates that are too large (?0.1 μm) to be annealed Fe-Mg amorphous silicates. By comparison with asteroids, the Stardust Fe-bearing and Fe-rich crystalline silicates suggests partial aqueous alteration in comet nuclei. However, aqueous alteration transforms Fe-rich olivine to phyllosilicates before Mg-rich olivine, and Stardust has Mg-rich and Fe-rich olivine and no phyllosilicates. Hence, we look to a nebular source for the moderately Fe-rich to nearly pure-Fe crystalline silicates. Primitive matrices have Mg-Fe silicates but no phyllosilicates, supporting the idea that Mg-Fe silicates but not phyllosilicates are products of water-rich shocks. Chondrule-formation is a late stage process in our protoplanetary disk. Stardust samples show comet 81P/Wild 2 formed at least as late to incorporate a few chondrules, requiring radial transport of chondrules out to perhaps >20 AU. By similar radial transport mechanisms, collisional fragments of aqueously altered asteroids, in particular achondrites that formed earlier than chondrules, might reach the comet-forming zones. However, Stardust samples do not have phyllosilicates and chondrules are rare. Hence, the nebular refractory grains in comet 81P/Wild 2, as well as other comets, appear to be pre-accretionary with respect to asteroid parent bodies. By discussing nebular pathways for the formation of Fe-rich crystalline silicates, and also phyllosilicates and carbonates, we put forth the view that comets contain both the interstellar ingredients for and the products of nebular transmutation.     

The X-ray/Gamma-ray Spectrometer on the Near Earth Asteroid Rendezvous Mission

An X-ray/gamma-ray spectrometer has been developed as part of a rendezvous mission with the near-Earth asteroid, 433 Eros, in an effort to answer fundamental questions about the nature and origin of asteroids and comets. During about 10 months of orbital operations commencing in early 1999, the X-ray/Gamma-ray Spectrometer will develop global maps of the elemental composition of the surface of Eros. The instrument remotely senses characteristic X-ray and gamma-ray emissions to determine composition. 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 elements O, Si, Fe, H and naturally radioactive elements K, Th, U to surface depths on the order of 10 cm. The X-ray spectrometer consists of three gas-filled proportional counters with a collimated field of view of 5° and an energy resolution of 850 eV @ 5.9 keV. Two sunward looking X-ray detectors monitor the incident solar flux, one of which is the first flight of a new, miniature solid-state detector which achieves 600 eV resolution @ 5.9 keV. The gamma-ray spectrometer consists of a NaI(Tl) scintillator situated within a Bismuth Germanate (BGO) cup, which provides both active and passive shielding to confine the field of view and eliminate the need for a massive and costly boom. New coincidence techniques enable recovery of single and double escape events in the central detector. The NaI(Tl) and BGO detectors achieve energy resolutions of 8.7% and 14%, respectively @ 0.662 MeV. A data processing unit based on an RTX2010 microprocessor provides the spacecraft interface and produces 256-channel spectra for X-ray detectors and 1024-channel spectra for the raw, coincident, and anti-coincident gamma-ray modes. This paper presents a detailed overview of the X-ray/Gamma-ray Spectrometer and describes the science objectives, measurement objectives, instrument design, and shows some results from early in-flight data.     

World Space Congress: a vision quest

The World Space Congress (WSC) in October, 2002, brought together luminaries, aerospace engineers, students, and scientists to discuss strategies for reviving the world's space agency. WSC lectures and plenary sessions focused on future research in space. Among topics discussed are the use of the Hubble Space Telescope to scan for habitable planets and obtain data about the beginning of the universe, new weather satellites, planetary protection from comets or asteroids, exploration and establishment of colonies on the Moon and Mars, medical advances, the role of space exploration in the world economy.