Engineering topics

Degewij, J.

Publications and source records attributed to Degewij, J..

14 records

NASA NTRS1982

Remote comets and related bodies - VJHK colorimetry and surface materials

VJHK colors for a number of asteroids and eight comets at various solar distances and levels of activity were obtained, and the observations are interpreted in terms of a two-component mixing model in which outer solar system interplanetary bodies are viewed as mixtures of ice and dark carbonaceous-type (RD and C) dirt. It is inferred that the observed comets have comae, and perhaps surfaces, of dirty ice or ice dirt grains colored by an RD-dirt component. This inference is supported by systematics of an 'alpha index' based on VJHK colors and empirically correlated with albedo and ice/dirt ratio. Among comets the alpha index correlates with solar distance in a way that suggests comets emit dirty ice grains which are stable at large solar distance but from which the ice component sublimes and leaves dirt grains at small solar distance.

Hartmann, W. K.↗

NASA NTRS1982

uvby photometry in McCormick proper motion fields

The Danish 50 cm telescope at the European Southern Observatory was used to obtain high-precision uvby photometry for 50 F2 to G2 stars, with V values in the 9.4-12.3 mag range, which were selected in the southern galactic polar regions of the McCormick proper motion fields and measured on six different nights. The brighter stars are found to systematically exhibit smaller m(1) indices, of about 0.02 mag, upon comparison with the earlier data of Blaauw et al (1976). Single measurements are given for 98 stars in eight McCormick fields at intermediate southern galactic latitudes.

Degewij, J.↗

NASA NTRS1982

Do comets evolve into asteroids - Evidence from physical studies

Physical observations of low-activity periodic comets and asteroids in distant and peculiar orbits are reviewed. In particular, magnitude studies and reflection spectra at visible and infrared wavelengths are discussed. While it may never be possible to prove unambiguously that a cometary nucleus has been observed, the fainter a given comet is, the more likely it is that some light reflected from a solid surface will be observed. Studies of magnitudes obtained by Roemer suggest that at certain periods the nuclei of P/Arend-Rigaux and P/Neujmin 1 may be photometrically resolved. It is shown that this may also be the case for P/Schwassmann-Wachmann 1, but that it is less likely for P/Tempel 2. At times of minimum activity, the optical spectra of P/Arend-Rigaux and P/Schwassmann-Wachmann 1 resemble those of RD-type asteroids like 944 Hidalgo. In order to progress with physical studies of cometary nuclei, an increased effort should be made to obtain magnitudes and other physical observations of comets during stages of minimal activity.

Degewij, J.↗

NASA NTRS1982

The outer satellites of Jupiter

Photometry and spectrophotometry of the Jovian outer satellites are used to throw light on the problem of their formation. The main plausible mechanisms for the formation of these satellites are discussd, including capture and breakup in the proto-Jovian nebula, collision within Jupiter's gravitational sphere of influence, and capture through the Jovian Lagrangian points after dissipation of the proto-Jovian nebula. UBV, VRI, and JHK color diagrams for various asteroids are used to show their similarity to the outer Jovian satellites. The evidence suggests that at least two outer Jovian satellites are comparable to C-type asteroids, and that two others are more nearly comparable to RD-type bodies. The apparent C-type satellites are both in the prograde cluster and the apparent RD-types are in the retrograde cluster. The Gradie and Veverka (1980) hypothesis that the reflectances and albedo characteristics of RD bodies may be due to aromatic kerogenlike compounds is discussed.

Cruikshank, D. P.↗

NASA NTRS1981

Spectrophotometry of faint comets: The asteroid approach

Observing programs at optical (0.35-0.8 micron) and near-infrared (1.1-2.4 micron) wavelengths, directed at the acquisition of reflection spectra of faint and distant comets, are described. The ultimate goal is to obtain spectrophotometric measurements of comets for which a significant part of the light is expected to be reflected by the solid surface of the nucleus.

Degewij, J.↗

NASA NTRS1981

Surface materials on unusual planetary object Chiron

JHK near-infrared colorimetry of the surface of the planetary object 2060 Chiron has yielded colors consistent with those of outer solar system asteroids, which have: (1) albedos of only a few percent, (2) C-, RD-, or DM-type spectra, and (3) no known H2O ice absorption features. The colors are also in keeping with theoretical colors for certain size distributions of dirty ice grains. Along with VJHK colorimetric data, results suggest that the spectrally dominant surface is probably dark, carbonaceous-like silicate dust with a possible, microscale admixture of ice grains. It is concluded that, if Chiron has the low albedo common to such materials on known interplanetary bodies, its diameter may lie in the 310-400 km range and therefore place it among the eight largest asteroids.

Hartmann, W. K.↗

NASA NTRS1980

Photometric properties of outer planetary satellites

Optical broadband photometry for the satellites J6, J7, J8, S7, S9, U3, U4, and N1 and polarimetry for J6, obtained between 1970 and 1979, are presented. The outer Jovian satellites resemble C-type asteroids; J6 has a rotational lightcurve with a period of approximately 9.5 hr. The satellites beyond Jupiter also show C-like colors with the exception of S7 Hyperion. S9 Phoebe has a rotational lightcurve with period near either 11.25 or 21.1 hr. For U4 and N1 there is evidence for a lightcurve synchronous with the orbital revolution. The seven brighter Saturnian satellites show a regular relation between the ultraviolet dropoff and distance to the planet, probably related with differences in the rock component on their surfaces.

Degewij, J.↗

NASA NTRS1980

Near-infrared colorimetry of J6 Himalia and S9 Phoebe - A summary of 0.3- to 2.2-micron reflectances

VJHK measurements of J6 Himalia and S9 Phoebe, using the new NASA IRTF telescope, show that these objects have carbonaceous chondritic type colors in the 0.5- to 2.2-micron region. For Phoebe, this is in contrast to the JHK colors published by Cruikshank (1980), which indicated that the satellite's surface was unlike the material found on asteroids and on the dark side of Iapetus. J6 is known to have a low albedo from thermal infrared studies (Cruikshank, 1977), and the new VJHK observations of S9 imply that it also has a low albedo. The H and K reflectances of S9 are slightly lower than those of J6, suggesting some slight difference in surface composition or a contamination by foreign material. The conjectured low albedo of S9 can be tested with measurements in the thermal infrared.

Degewij, J.↗

NASA NTRS1980

Spectroscopy of faint asteroids, satellites, and comets

Nineteen asteroids with orbital elements comparable to those of short-period comets and the outer Jovian satellites J6 Himalia and J7 Elara have been observed with the 228-cm telescope and image-tube spectrograph of Steward Observatory. No activity indicating cometary outgassing was detected. If comets are being perturbed into asteroidal orbits, then this lack of activity can be explained by an apparently short transition time between active and extinct phase. In addition, spectra of the faint comets Sanguin 1977p, Chernykh 1977l, Arend-Rigaux 1950 VII, West 1978a, and van Biesbroeck 1954 IV were obtained, showing CN(0,0) and in some cases C2(1,0) and C3 emission. Comet Arend-Rigaux was active again in the fall of 1977 and Comet West showed on May 31, 1978 a weak tail at a distance from the sun of 6.0 AU.

Degewij, J.↗

NASA NTRS1980

Imaging photopolarimeter on Pioneer Saturn

Results of Pioneer 11 imaging photopolarimeter observations of Saturn, its rings, and Titan are presented. The imaging photopolarimeter is a pointable telescope with an aperture of 2.5 cm and passbands of 390 to 500 to 720 nm which uses the spin of the spacecraft to scan across an object. Images of the Saturn system and of the rings are presented, and the absence of a D ring, structures in the C, B and A rings and the Cassini division and the discoveries of the F ring and the provisionally named Pioneer division separating it from the A ring are reported. A mean particle size less than 15 meters is estimated from estimates of total ring mass and the optical depth of the B ring. The discovery of the satellite 1979 S 1 at 2.53 Saturn radii is also noted. Models of the vertical aerosol structure of Saturn's atmosphere are compared with the polarization data, and it is indicated that the density of cloud particles decreases with altitude with a scale height about one fourth that of the gas, and that an optical depth of one is to be found at 750 mbar.

Gehrels, T.↗

NASA NTRS1979

Albedo and color contrasts on asteroid surfaces

Asteroids in general display only small or negligible variations in spectrum or albedo during a rotational cycle. Color variations with rotation are described in the literature but are usually comparable to the noise in the measurements. Twenty-four asteroids have been systematically monitored for such color changes. Only 3 Juno, 4 Vesta, 6 Hebe, 71 Niobe, 349 Dembowska, and 944 Hidalgo display color variations larger than 0.03 mag. In each of these cases the asteroid appears redder near maximum brightness. Of seven asteroids monitored polarimetrically, only 4 Vesta shows a convincing variation, attributed to an albedo change with rotation. The lightcurve can be explained by albedo differences alone; Vesta apparently has a nearly spheroidal shape. Nothwithstanding the above results, the degree of uniformity of most asteroid surfaces is remarkable. If asteroids exist with large discrete domains of ferrosilicate, metallic, and/or carbonaceous material together on their surfaces, they have not yet been identified.

Degewij, J.↗

NASA NTRS1979

Distant asteroids and outer Jovian satellites

Sixty percent of the sampled objects in the Hilda, Trojan and outer Jovian satellite locations belong to C-type and another 30% belong to a new group called RD-type (reddish and dark), sometimes referred to simply as D-type. Objects in this group have low albedo values between 2 and 4% and steep reflection spectra between 0.7 micron and 0.9 micron. Furthermore, 944 Hidalgo belongs to this group but shows color variation over its surface. Meteoritic minerals with similar optical reflection spectra are discussed. Trojans with sizes down to 15 km in the cloud preceding Jupiter are about 3.5 times more numerous than those in the following cloud. RD-type Trojans appear more often in the preceding cloud. There is a resemblance of spectrum, albedo and phase relation among the majority of Trojans and the outer Jovian satellites.

Degewij, J.↗

NASA NTRS1978

Minor planets and related objects. XXV - UBV photometry of 145 faint asteroids

Magnitudes and colors on the UBV system are presented for 145 minor planets, including 31 objects in the Eos family, 14 in the Koronis family, 6 in the Nysa family, 11 in the Themis family, 4 Hungarias, 7 Hildas, 8 Trojans, and several objects in unusual orbits. Clearly defined color groups for the Eos, Koronis, Nysa, and Themis family members are strongly suggestive of origin from discrete parent bodies. The Nysas apparently show large light-curve amplitudes. The Mars-orbit crossers 1977RA and 1980 = 1950LA, the earth-orbit crosser 1976UA, and several main-belt asteroids are found to show peculiar colors of unknown significance. The Mars-orbit crosser 1916 = 1953RA appears to be a typical S object, while 1474 Beira and 1977VA show neutral colors. Asteroids at semimajor axis in excess of 4 AU (Trojans preceding and following Jupiter, 279 Thule, and 944 Hidalgo) all belong to a color group centered at B-V = 0.72, U-B = 0.24.

Degewij, J.↗

NASA NTRS1977

Lightcurve analyses for 170 small asteroids

Small asteroids, with sizes of about 1 km, appear to rotate faster than do the larger, about 200 km, asteroids. There is a lack of highly elongated objects, probably due to a collisional erosion process in the main asteroid belt.

Degewij, J.↗