Engineering topics

Karttunen, H.

Publications and source records attributed to Karttunen, H..

5 records

NASA NTRS1990

A spherical harmonics method for asteroid pole determination

A new and general method is presented for determining pole orientations of unresolved solar system bodies that exhibit brightness variation with rotation. The accuracy of the method is limited only by the number of available apparitions, which imposes the degree where the Legendre expansion must be truncated. It is shown that power spectra of lightcurves provide enough information to extract pole coordinates from the data, independent of body shape and albedo variegation. The method has been tested using several computer-generated synthetic light-curves, and the accuracy is found to be about 5 deg in both the longitude and latitude of the pole, which is considerably better than that obtained from the standard triaxial ellipsoid model. Finally, the new spherical harmonics method has been applied to six well-observed asteroids: 16 Psyche, 22 Kalliope, 39 Laetitia, 44 Nysa, 45 Eugenia, and 349 Dembowska.

Lumme, K.↗

NASA NTRS1989

Modelling asteroid brightness variations. I - Numerical methods

A method for generating lightcurves of asteroid models is presented. The effects of the shape of the asteroid and the scattering law of a surface element are distinctly separable, being described by chosen functions that can easily be changed. The shape is specified by means of two functions that yield the length of the radius vector and the normal vector of the surface at a given point. The general shape must be convex, but spherical concavities producing macroscopic shadowing can also be modeled.

Karttunen, H.↗

NASA NTRS1989

Modelling asteroid brightness variations. II - The interpretability of light curves and phase curves

Light curves and phase curves have been computed for various asteroid models using the Lumme-Bowell (1981) scattering law. The effects of the scattering parameters on light curves were found to be almost negligible for homogeneous surfaces. The effects on phase curves were more distinct, but changing any of the scattering parameters affects the phase curves in a very similar way, making it impossible to find a unique set of parameter values corresponding to a given phase curve. Light curve amplitudes, on the other hand, depend very strongly on body shape. At least in the case of a triaxial ellipsoid it is possible to determine the axial ratios. Some observed irregularities of light curves can also be modelled easily, but the uniqueness of such models is far from obvious.

Karttunen, H.↗

NASA NTRS1985

Roughness of the lunar soil

Three stereoscopic close-up photographs taken of the lunar surface by Apollo 11 and 12 astronauts were analyzed to investigate the roughness of the lunar soil at millimeter resolution. Most existing measurements of regolith roughness were based on radar measurements at wavelengths of about 8.6 mm. Several statistical parameters were determined for they landing site. The surace roughness has been found to be nearly Gaussian. The rms slopes were obtained over scales between 0.5 mm and 5 cm. They steadily decrease with increasing scale length from 58 deg to 2 deg and are in reasonable agreement with radar-measured values. The autocorrelation coefficient of the height distribution has also been obtained; it has a scale length of 0.7 mm.

Lumme, K.↗

NASA NTRS1983

The wake model for azimuthal brightness variations in Saturn's A-ring

A dynamical model offering a possible explanation for azimuthal brightness variations in Saturn's ring A is considered. The orbits of ring particles encountering a massive moonlet (radius = about 100 m) are integrated numerically. It is seen that a density wake is produced if the random velocities of the ring particles are much smaller than 1 cm/s.

Karttunen, H.↗