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At least 271 records · Page 15
All sourcesNASA NTRSDOE OSTI
NASA NTRS1978

Ion drive performance and trajectories

The use of Solar Electric Propulsion (SEP) for the Solar Probe Mission is addressed. The dependence of the payload mass on the site of the SEP and the flight time are described on the basis of preliminary data. The range of hardware expected to be available in the 1980's is summarized. There are several classes of optimal low-thrust trajectories for the Solar Probe Mission. These are trajectory types A, B, C corresponding to one, two, or three orbital revolutions, respectively. Plots of transfer trajectories corresponding to type-A, type-A with a Venus Swingby, type-B, and type-C with ab Icarus Rendezvous are shown. A summary of the SEP performance is given.

Yen, C. L.↗
NASA NTRS1986

Interplanetary and interstellar dust

Several processed polycyclic aromatic hydrocarbons (PAH) samples were produced to date. The Raman spectra and D/H ratios of these samples will be studied in the near future. A manuscript was sent to Icarus which gives a detailed discussion of the possibility of using solar flare tracks in interplanetary dust particles (IDPs) to determine a cometary vs. asteroidal source of the IDPs. The technique looks promising and a program was started to measure the large number of IDP track density necessary to address the issue of the source(s) of the dust. The results could potentially tell a great deal about comets and/or asteroids.

Sandford, S.↗
NASA NTRS1987

Definition and testing of the hydrologic component of the pilot land data system

The specific aim was to develop within the Pilot Land Data System (PLDS) software design environment, an easily implementable and user friendly geometric correction procedure to readily enable the georeferencing of imagery data from the Advanced Very High Resolution Radiometer (AVHRR) onboard the NOAA series spacecraft. A software subsystem was developed within the guidelines set by the PLDS development environment utilizing NASA Goddard Space Flight Center (GSFC) Image Analysis Facility's (IAF's) Land Analysis Software (LAS) coding standards. The IAS current program development environment, the Transportable Applications Executive (TAE), operates under a VAX VMS operating system and was used as the user interface. A brief overview of the ICARUS algorithm that was implemented in the set of functions developed, is provided. The functional specifications decription is provided, and a list of the individual programs and directory names containing the source and executables installed in the IAF system are listed. A user guide is provided for the LAS system documentation format for the three functions developed.

Ragan, Robert M.↗
NASA NTRS1988

Speckle interferometry of asteroids

This final report for NASA Contract NAGw-867 consists of abstracts of the first three papers in a series of four appearing in Icarus that were funded by the preceding contract NAGw-224: (1) Speckle Interferometry of Asteroids I. 433 Eros; (2) Speckle Interferometry of Asteroids II. 532 Herculina; (3) Speckle Interferometry of Asteroids III. 511 Davida and its Photometry; and the fourth abstract attributed to NAGw-867, (4) Speckle Interferometry of Asteroids IV. Reconstructed images of 4 Vesta; and a review of the results from the asteroid interferometry program at Steward Observatory prepared for the Asteroids II book, (5) Speckle Interferometry of Asteroids. Two papers on asteroids, indirectly related to speckle interferometry, were written in part under NAGw-867. One is in press and its abstract is included here: Photometric Geodesy of Main-Belt Asteroids. II. Analysis of Lightcurves for Poles, Periods and Shapes; and the other paper, Triaxial Ellipsoid Dimensions and Rotational Pole of 2 Pallas from Two Stellar Occultations, is included in full.

Drummond, Jack↗
NASA NTRS1991

Future applications of artificial intelligence to Mission Control Centers

Future applications of artificial intelligence to Mission Control Centers are presented in the form of the viewgraphs. The following subject areas are covered: basic objectives of the NASA-wide AI program; inhouse research program; constraint-based scheduling; learning and performance improvement for scheduling; GEMPLAN multi-agent planner; planning, scheduling, and control; Bayesian learning; efficient learning algorithms; ICARUS (an integrated architecture for learning); design knowledge acquisition and retention; computer-integrated documentation; and some speculation on future applications.

Friedland, Peter↗
NASA NTRS1992

Asteroidal versus cometary meteoroid impacts on the Long Duration Exposure Facility (LDEF)

Meteoroids that enter the Earth's atmosphere at low velocities will tend to impact the apex side (that surface facing the spacecraft direction of motion) of a spacecraft at a very high rate compared to the rate with which they will impact an antapex-facing surface. This ratio -- apex to antapex impact rates -- will become less as meteoroid entry velocities increase. The measured ratio, apex to antapex, for 500 micron diameter impact craters in 6061-T6 aluminum on LDEF seems to be about 20 from the work of the meteoroid SIG group and from the work of Humes that was presented at the first LDEF symposium. Such a ratio is more consistent with the meteoroid velocity distributions derived by Erickson and by Kessler, than it is with others that have been tested. These meteoroid velocity distributions have mean entry velocities into the Earth's atmosphere of 16.5 to 16.9 km/s. Jackson and Zook (in a paper submitted to Icarus) have numerically simulated the orbital evolution of small dust grains emitted from asteroids and comets. For those asteroidal grains small enough (below about 100 microns diameter) to drift from the asteroid belt to the orbit of the Earth, under P-R and solar wind drag, without suffering collisional destruction, the following results are found: as their ascending or descending nodes cross the Earth's orbit (and when they might collide with the Earth), their orbital eccentricities and inclinations are quite low (e less than 0.3, i less than 20 degrees), and their mean velocity with respect to the Earth is about 5 or 6 km/s. When gravitational acceleration of the Earth is taken into account, the corresponding mean velocities relative to the top of the Earth's atmosphere are 12 to 13 km/s. This means that, at best, these small asteroidal particles can not comprise more than 50 percent of the particles entering the Earth's atmosphere. And when gravitational focusing is considered, they cannot comprise more than a few percent of those in heliocentric orbit at 1 AU. The rest are presumably of cometary origin.

Zook, Herbert A.↗
NASA NTRS1992

SO2 on Venus: A final cross-calibration with Pioneer Venus

In our previous report, we noted that we had successfully completed all of the IUE observations which were proposed. A total of 7 Venus spectra were obtained. In the present reporting period, essentially all of the associated data reduction and analysis activities were completed, and Drs. Stern, Barker, and Na met to collaborate on data interpretation. The model atmosphere used in this analysis is similar to the one used in the analysis of the Pioneer Venus observations (Esposito, et al. 1979, 1988) and the previous IUE observations (Na, et al. 1990). In this model, sulfuric acid aerosols with a radius of about 1 micron are mixed uniformly with Rayleigh scattering gases throughout the cloud layer. There are two wavelength-independent absorbers. First, the aerosols are assumed to have a single scattering albedo of 0.98, and second a pure absorbing layer with an optical depth of 0.2 is added at an altitude of 75 mbar. The variable parameters in this model are the mixing ratios of SO2 at the cloud top (40 mbar level), and the scale height of SO2 at the same altitude. Model spectra were calculated using a radiative transfer code utilizing a Markov Chain method. Each model calculation takes into account the multiple scattering and the vertical inhomogeneity of the atmosphere. We reported those initial results at the 1992 meeting of the AAS Division of Planetary Sciences (Barker, Stern, and Na 1992) The important finding we have reached is that the 1991 IUE observations indicate that the long-term decline in Venus SO2 abundance has either halted or reversed. Our simultaneous groundbased measurements made on the McDonald Observatory 2.7 m corroborate this result. We are now preparing a paper describing these IUE results for publication in Icarus. With the IUE results reduced, we are also beginning a project to compare the 1991 IUE results to Pioneer Venus UV spectra made on the same dates.

Stern, S. Alan↗
NASA NTRS1992

Numerical simulations of cometary dust

Most observations of comets are done photometrically or spectrophotometrically. The interpretation of the aperture-averaged flux is relatively simple for an isotropic, radially expanding coma of infinite extent - the canonical model. However, the interpretation of the observations is not so clear when the motion of the dust is affected by radiation pressure, or when the emission is time-varying and anisotropic. For example, in a sample of CCD images of 10 comets, Jewitt and Meech (1987, Ap.J. 317, 992) found that the photometric profiles of only three comets were consistent, within the observational errors, with the profiles predicted form the canonical model. Photometric observations with large apertures, however, seem to suggest that the canonical model may be quite adequate (c.f. Osip, Schleicher, and Millis, 1992, Icarus 98, 115). The dust itself is characterized by a size distribution, with size dependencies on the expansion velocity, the scattered and thermal radiation, the response to radiation pressure, and probably the density. How good then are the approximations normally used in determining the production rates of the dust when these effects are present? As part of a program to better understand the dynamics of cometary dust and gas, a computer program has been developed which numerically simulates the emission of both dust and gas from a tilted rotating nucleus.

Lien, David J.↗
NASA NTRS1994

Quasi-periodic atmosphere-regolith-cap CO2 redistribution in the martian past

Our earlier Mars regolith-atmosphere-cap CO2 distribution model (Fanale et al., 1982, Icarus 50, 381-407) has been improved, revised, and extended back over Mars' mid to late history. The present model takes into account four new factors: (1) a more realistic long-term obliquity cycle, (2) thermal conduction as it affects the surface energy balance, (3) the changing solar constant, and (4) atmospheric erosion 3.5 byr ago to the present. Solar insolation and temperatures are computed for the full range of obliquities, latitudes, and epochs, and a CO2 adsorption relation is used, together with a conservation of mass constraint, to calculate atmospheric pressures and exchangeable CO2 mass as functions of obliquity and epoch for the regolith, atmosphere, and polar caps for two assumed thicknesses of a basalt regolith. It is found that the heat conduction term in the surface boundary condition has an important effect in reducing the range of atmospheric pressures over the obliquity cycle at all epochs.

Fanale, Fraser P.↗
NASA NTRS1996

Mid-IR Spectroscopy of the Jovian Stratosphere perturbed by comet P/Shoemaker-Levy 9 Impact

The objective was to obtain spectra from 5 to 14 micron before, during, and after the impacts. We were awarded 2 flights of the KAO which was based in Melbourne, Australia for the events. Impacts R and W were covered, and the observations were completely successful. A paper reporting the observation of water vapor is in press, and other work is in progress as limited funding permits. The text of this report is adapted from that of the Icarus paper.

Hunten, Don M.↗
NASA NTRS1997

Coronagraphic Observations of Lunar Sodium

This grant supported an investigation of lunar sodium by our coronagraph and spectrograph on nearby Mount Lemmon. We report successful operation and data analysis during International Lunar Atmosphere Week, September 15 - 22, 1995, and submittal of a paper to Icarus. The core of the proposed work was to observe the lunar sodium atmosphere with our classical Lyot coronagraph and specially-built grating spectrograph on Mount Lemmon, a 9400-foot peak about an hour's drive from Tucson. It is optimized for low scattered light and for observing from the Moon's limb to an altitude of approx.1 lunar radius. The grating has 600 lines/mm and a blaze angle of 49 deg., and is used with a somewhat wide slit at a resolving power of about 5000. It is called DARRK for the initials of the people who designed it. The rejection of stray light from the Moon's disk is spectacularly good: when the sky is clear this light is absent right up to a few arcsec from the limb. We use an excellent 1024 by 1024 pixel CCD camera, operated at -100 C; the exposures are 10 to 30 min. Data reduction is done with IRAF running on a Sun Sparcstation.

Hunten, D. M.↗
NASA NTRS1998

Stellar Occultation Probe of Triton's Atmosphere

The goals of this research were (i) to better characterize Triton's atmospheric structure by probing a region not well investigated by Voyager and (ii) to begin acquiring baseline data for an investigation of the time evolution of the atmosphere which will set limits on the thermal conductivity of the surface and the total mass of N2 in the atmosphere. Our approach was to use observations (with the Kuiper Airborne Observatory) of a stellar occultation by Triton that was predicted to occur on 1993 July 10. As described in the attached reprint, we achieved these objectives through observation of this occultation and a subsequent one with the KAO in 1995. We found new results about Triton's atmospheric structure from the analysis of the two occultations observed with the KAO and ground-based data. These stellar occultation observations made both in the visible and infrared, have good spatial coverage of Triton including the first Triton central-flash observations, and are the first data to probe the 20-100 km altitude level on Triton. The small-planet light curve model of Elliot and Young (AJ 103, 991-1015) was generalized to include stellar flux refracted by the far limb, and then fitted to the data. Values of the pressure, derived from separate immersion and emersion chords, show no significant trends with latitude indicating that Triton's atmosphere is spherically symmetric at approximately 50 km altitude to within the error of the measurements. However, asymmetry observed in the central flash indicates the atmosphere is not homogeneous at the lowest levels probed (approximately 20 km altitude). From the average of the 1995 occultation data, the equivalent-isothermal temperature of the atmosphere is 47 +/- 1 K and the atmospheric pressure at 1400 km radius (approximately 50 km altitude) is 1.4 +/- 0.1 microbar. Both of these are not consistent with a model based on Voyager UVS and RSS observations in 1989 (Strobel et al, Icarus 120, 266-289). The atmospheric temperature from the occultation is 5 K colder than that predicted by the model and the observed pressure is a factor of 1.8 greater than the model.

Elliot, James L.↗
NASA NTRS1996

EUVS Sounding Rocket Payload

During the first half of this year (CY 1996), the EUVS project began preparations of the EUVS payload for the upcoming NASA sounding rocket flight 36.148CL, slated for launch on July 26, 1996 to observe and record a high-resolution (approx. 2 A FWHM) EUV spectrum of the planet Venus. These preparations were designed to improve the spectral resolution and sensitivity performance of the EUVS payload as well as prepare the payload for this upcoming mission. The following is a list of the EUVS project activities that have taken place since the beginning of this CY: (1) Applied a fresh, new SiC optical coating to our existing 2400 groove/mm grating to boost its reflectivity; (2) modified the Ranicon science detector to boost its detective quantum efficiency with the addition of a repeller grid; (3) constructed a new entrance slit plane to achieve 2 A FWHM spectral resolution; (4) prepared and held the Payload Initiation Conference (PIC) with the assigned NASA support team from Wallops Island for the upcoming 36.148CL flight (PIC held on March 8, 1996; see Attachment A); (5) began wavelength calibration activities of EUVS in the laboratory; (6) made arrangements for travel to WSMR to begin integration activities in preparation for the July 1996 launch; (7) paper detailing our previous EUVS Venus mission (NASA flight 36.117CL) published in Icarus (see Attachment B); and (8) continued data analysis of the previous EUVS mission 36.137CL (Spica occultation flight).

Stern, Alan S.↗
NASA NTRS1997

Planetary Aeronomy and Related Studies

Mercury atmosphere - Sprague and Hunten, in collaboration with Katharina Lodders of Washington University, proposed, mainly on cosmochemical grounds, that S atoms are an important constituent of the atmosphere (30 times more abundant than sodium). This paper has appeared in Icarus. We also suggest that condensed sulfur is an excellent candidate for the radar-bright polar caps, more plausible than water ice because the latter is only barely stable even in permanently-shadowed craters. The best prospect for detection of the vapor is through its resonance lines, a triplet near 1814 A. Mercury is too close to the Sun to be observed by any existing space telescope, but there is some prospect that the search could be made from a Shuttle-based spectrograph such as Lyle Broadfoot's USTAR. Sprague and Hunten have completed an elaborate data analysis of over 100 measurements of the Na D lines, obtained with the 61-inch telescope and our echelle spectrograph. Full account has been taken of the radiative-transfer problem that arises because the Na atmosphere is not optically thin. The output of this code is used in another program that makes an elaborate inverse interpolation in two angles and optical depth and computes the effect of the seeing (always bad for Mercury). The seeing is determined by fitting cuts across a computed image to part of the spectrum adjacent to the sodium lines, and typically ranges from slightly less than 4 arcsec to worse than 6 (diameter at l/e of a Gaussian). The final result is a list of Na abundances, with some information on spatial distribution. One particularly interesting result of further analysis is a strong abundance maximum in the morning relative to the afternoon, confirming an earlier result for potassium, based on much fewer measurements. The analysis are completed during the extension of the present grant. This work depends heavily on the Hapke parameters used to estimate the reflectance of Mercury's surface. The paper by Domingue et al. examines the credibility of the available parameters, which are derived from disk-unresolved photometry, and concludes that errors in the derived Na abundances could be as great as 30%.

Hunten, D. M.↗
NASA NTRS1998

Laboratory Evaluation and Application of Microwave Absorption Properties Under Simulated Conditions for Planetary Atmospheres

Radio absorptivity data for planetary atmospheres obtained from spacecraft radio occultation experiments, entry probe radio signal absorption measurements, and earth-based radio astronomical observations can be used to infer abundances of microwave absorbing constituents in those atmospheres, as long as reliable information regarding the microwave absorbing properties of potential constituents is available. The use of theoretically-derived microwave absorption properties for such atmospheric constituents, or using laboratory measurements of such properties taken under environmental conditions which are significantly different than those of the planetary atmosphere being studied, often leads to significant misinterpretation of available opacity data. For example, laboratory measurements completed recently by Kolodner and Steffes (ICARUS 132, pp. 151-169, March 1998, attached as Appendix A) under this grant (NAGS-4190), have shown that the opacity from gaseous H2SO4 under simulated Venus conditions is best described by a different formalism than was previously used. The recognition of the need to make such laboratory measurements of simulated planetary atmospheres over a range of temperatures and pressures which correspond to the altitudes probed by both spacecraft entry probe and orbiter radio occultation experiments and by radio astronomical observations, and over a range of frequencies which correspond to those used in such experiments, has led to the development of a facility at Georgia Tech which is capable of making such measurements. It has been the goal of this investigation to conduct such measurements and to apply the results to a wide range of planetary observations, both spacecraft and earth-based, in order to determine the identity and abundance profiles of constituents in those planetary atmospheres.

Steffes, Paul G.↗
NASA NTRS1999

The Spatial Variation of Water and Ammonia near Jupiter's Great Red Spot

The CSHELL spectrometer at NASA's Infrared Telescope Facility was used in August 1996 to observe Jupiter at 5.18 microns. This wavelength sounds the 3-8 bar region in Jupiter's deep troposphere. A 1-arcsec-wide slit was aligned east-west on Jupiter and stepped from north to south across the Great Red Spot (GRS). Within our spectral bandpass are absorption lines of NH3 and a hot band of CH4. Radiative transfer models indicate that the strength of the CH4 feature is anti-correlated with gaseous H2O between 3 and 6 bars. The CH4 feature is predicted to be very strong for H2O abundances less than 10ppm and it should vanish when H2O > 300ppm. The depths of the observed CH4 and NH3 absorption features varied dramatically near the GRS. The center and east side (planetocentric) of the GRS is dry in both volatiles as indicated by strong CH4 absorption and a weak NH3 line. The CH4 line vanishes and the NH3 feature grows stronger on the west side of the GRS. We interpret this as due to a real variation in both volatiles - H2O and NH3 - due to a common dynamical mechanism. Water clouds are expected to be accompanied by saturated gaseous H2O profiles between 3 and 5 bars. The Galileo imaging team (Banfield et al 1998 Icarus 135, p230) deduced the presence of a cloud near the 4-bar level northwest of the GRS. Our data indicate that this same region is volatile rich; thus the combination of the two datasets provides a compelling case for a water cloud at this location. The deep volatile abundance does not correlate with 5-micron continuum opacity near the GRS. This suggests that the spatial variation of the 5-micron flux near the GRS is due primarily to NH3 clouds, rather than H2O clouds.

Bjoraker, G. L.↗
NASA NTRS1999

A New Scientific use of Total Eclipses of the Moon: Studies of the Generation and Loss of Atmospheres of Primitives Bodies

This grant supported observational campaigns to record the size and brightness of the lunar atmosphere as seen in sodium gas (Na) emissions during the totality phase of lunar eclipses. Three eclipse events were attempted, two from the Mc Donald Observatory in Fort Davis Texas, and one from the site of Italy's Galileo National Telescope (GNT) in La Palma, in the Canary Islands. In all three cases, clear skies prevailed and excellent datasets were obtained. Following the observational component of the grant, a period of detailed processing and analysis began. Eclipse events were chosen for study because when the moon is in full phase it has been within the terrestrial magnetosphere for a few days, thereby shielded from solar wind impact upon its surface. Since sputtering of Na from the lunar regolith by solar wind particles had been proposed as a source of the Moon's atmosphere, this was a test of the mechanism. If the lunar Na appeared to be diminished in comparison to abundances seen at other phases (e.g., at quarter phase when the moon is directly in the solar wind), the solar wind sputtering would indeed be a major source of lunar Na. These experiments could not be conducted during any full moon night because scattering of bright moonlight is so strong that low-light-level imaging of the lunar atmosphere could not be achieved. Hence, the use of eclipses. The final result of these experiments was, for once, clear and unambiguous. The robust size and Na brightness levels measured during all of the eclipses showed that solar wind sputtering could not be a major source of the lunar atmosphere. A major paper on this conclusion was published in ICARUS, and an oral presentation of partial results given at the The Three Galileos conference in Padova (Italy) in January 1997 and at the DPS meeting in Cambridge, MA, in July 1997.

Mendillo, Michael↗
NASA NTRS1999

Water Ice on Triton

We report the spectroscopic detection of H2O ice on Triton, evidenced by the broad absorptions in the near infrared at 1.55 and 2.04 micron. The detection on Triton confirms earlier preliminary studies (D. P. Cruikshank, R. H. Brown, and R. N. Clark, Icarus 58, 293-305, 1984). The spectra support the contention that H2O ice on Triton is in a crystalline (cubic or hexagonal) phase. Our spectra (1.87-2.5 micron) taken over an interval of nearly 3.5 years do not show any significant changes that might relate to reports of changes in Triton's spectral reflectance (B. Buratti, M. D. Hicks, and R. L. Newburn, Jr., Nature 397, 219, 1999), or in Triton's volatile inventory (J. L. Elliot et al., Nature 393, 765-767, 1998).

Cruikshank, Dale P.↗