Herbert A. Zook

A source for hyperbolic cosmic dust particles

Abstract Earlier analyses of the Pioneer 8 and 9 experimental meteoroid data have shown that the detectors on these two spacecraft are intercepting meteoroids with hyperbolic orbital parameters. It is shown in this paper that these results are entirely consistent with and, indeed, to be expected from other observations of the interplanetary meteoroid complex. Collisional breakup of meteoroids and post-collision radiation pressure modification of their orbits is found to be a sufficient cause for the observed results. Details of the calculations as well as of the results are presented.

Orbital evolution of dust particles from comets and asteroids

Abstract In a computer simulation, dust grains of radius 10, 30, and 100 μm were released at perihelion passage from each of 35 different celestial bodies: 15 main belt asteroids, 15 short period comets with perihelion greater than 1 AU, and 5 short period comets with perihelion less than 1 AU. The evolving orbit of each of the 105 released dust grains was then continuosly computed with the Everhart numerical integrator until the orbit aphelion passed inside of 0.387 AU, or the dust grain had been ejected from the Solar System. The forces due to the gravity of the Sun and the planets as well as radiation pressure, Poynting-Robertson drag, and solar wind drag were all included in these numerical simulations. It is found that when dust grains evolve to intersection with the Earths orbit, they nearly always retain orbital characteristics indicative of their origins; grains from main belt asteroids differ significantly in orbital characteristics, especially in orbital eccentricity, from grains that evolve from comets. Average intersect velocities with the Earth, before the Earths gravitationa acceleration is taken into account, are about 5 km/sec for asteroidal and in excess of 12 km/ sec for cometary-derived grains. Average orbital eccentricities are about 0.1 for asteroidal grains and usually in excess of 0.4 for cometary grains when their orbits can intersect the Earths orbit. These results mean that accurate trajectory measurements of meteoroids collected with a near-Earth space platform would make it possible to distinguish asteroidal grains from cometary grains.

In situ measurements of cosmic dust

In-situ measurements of cosmic dust provide information on the spatial and orbital distributions, and on the physical and chemical properties of dust in interplanetary space. Pioneers 8 through 11, Helios, Galileo and Ulysses spaceprobes measured interplanetary dust from 0.3 to 18 AU distance from the Sun. The Earth satellites HEOS, Hiten, as well as other spacecraft, determined the flux of micrometeoroids at 1 AU. The size distribution from a few micrometer to millimeter range was also characterized by analysis of lunar micro craters and later verified by near-Earth satellites like LDEF. Distinctly different populations of dust particles exist throughout the solar system. In the inner solar system, out to about 3 AU, zodiacal dust particles are observed by in-situ detection from spaceprobes. These particles orbit the Sun on low inclination (i 10 μm) outside about 3 AU. The dust detectors onboard the Ulysses and Galileo spaceprobes identified micrometer sized interstellar dust sweeping through the solar system. Within a distance from Jupiter of about 2 AU Ulysses and Galileo observed streams of tiny grains originating from within the jovian system.

Meteoroid impacts on the Gemini windows

Abstract Fourteen windows of the Gemini spacecraft were closely examined after return from space flight for evidence of meteoroid impact. Although a number of microscopic pits were found on each window, only one of these pits appears to have been caused by a meteoroid impact. A meteoroid flux-mass relation calculated from this single hit is found to be in close agreement with Naumanns (1966) analysis of the Explorer and Pegasus meteoroid penetration experiments. Particle, mass, and area distribution curves are derived from the flux-mass relation and are presented in graphical form. Problems in interpreting the data because of contaminants on the windows are also discussed.

Hyperbolic cosmic dust: Its origin and its astrophysical significance

Abstract The heliocentric radial distribution of the flux of hyperbolic cosmic dust particles, as measured by the Pioneer 8 and 9 spacecraft, is closely related to the radial variation of the spatial density of source or “parent” meteoroids. Within the limits of the experimental and theoretical uncertainties the spatial density of parent meteoroids, as deduced from the hyperbolic cosmic dust data, is found to be increasing with increasing heliocentric distance in the neighborhood of one a.u. Other recent experimental evidence confirms this result. The new results also suggest that the ratio of the areal density of submicron sized craters to the areal density of millimeter sized craters will be less on the north-south faces of lunar rocks than on the east-west faces of the same rocks. The changeinratio is not as large as previously thought, however. Finally it is noted that the solar system is not presently contributing significant amounts of dust to the interstellar medium though it may once have done so.

Analysis of orbital perturbations acting on objects in orbits near geosynchronous earth orbit

Orbital evolution has been numerically simulated for objects started in geosynchronous Earth orbit (GEO) or in orbits near GEO, during a project to study potential orbital debris problems in this region. Perturbations simulated include nonspherical terms in the Earths geopotential field, lunar and solar gravity, and solar radiation pressure. Objects simulated include large satellites, for which solar radiation pressure is insignificant, and small particles (a few microns in diameter), for which solar radiation pressure is an important force. Results for large satellites are largely in agreement with previous GEO studies that used classical perturbation techniques; orbital evolution studies were extended to possible storage orbits slightly above or below GEO. One notes that while the orbit planes of GEO satellites initially placed in equatorial orbits precess, so that those orbits reach inclinations of 14° to 15° to the equator, a “stable plane” exists inclined approximately 7.3° to the equator. The orbit planes of GEO satellites placed in such a stable plane orbit experience very little precession, remaining always within 1.2° of their initial orientation. Solar radiation pressure generates two major effects on small particles. One is an orbital eccentricity oscillation anticipated from previous research. The other is an oscillation in orbital inclination. This orbital inclination pattern is due to a precession of the small particles orbital angular momentum vector about an axis offset from the Earths polar axis. The magnitude of the precession axis offset angle depends on the particles cross-sectional area to mass ratio. The rate for this precession differs greatly from the precession rate predicted in a previous study using perturbation techniques. This difference points up the inadequacy of those perturbation techniques for orbits with large eccentricities. For one sequence of runs with small particles, Poynting-Robertson drag was added to the simulation in order to slowly reduce the orbital semimajor axis and probe for possible orbital period resonances near the GEO distance. A significant resonance was found at the geosynchronous distance, where small grains are trapped into a 1:1 resonance with the Earths daily rotation.

Solar flare activity: Evidence for large-scale changes in the past

Abstract An analysis of radar and photographic meteor data and of spacecraft meteoroid penetration data indicates that there probably has not been a large increase in meteoroid impact rates in the last 10 4 yr. The solar flare tracks observed in the glass linings of meteoroid impact pits on lunar rock 15205 are therefore reanalyzed assuming a meteoroid flux that is constant in time. Based on this assumption, the data suggest that the production rate of Fe-group solar flare tracks may have varied by as much as a factor of 50 on a time scale of about 10 4 yr. No independently obtained data are known to require conflict with this interpretation. Confidence in this conclusion is somewhat qualified by the experimental and analytical uncertainties involved, but the conclusion nevertheless remains the present “best” explanation for the observed data trends.

Reduced debris hazard resulting from a stable inclined geosynchronous orbit

Abstract A stable geosynchronous orbit with an initial inclination of approximately 7.4° will drastically reduce relative encounter velocities between objects in such an orbit, compared to relative velocities between objects in an initially equatorial geosynchronous orbit. Without north-south station keeping, the inclinations of objects placed into an equatorial geosynchronous orbit will increase to a maximum of 15°, giving rise to encounter velocities of up to 800 meters/second. Collisions at these velocities are likely to produce fragments large enough to contribute to collisional cascading. However, the inclination of objects in the stable geosynchronous orbit will change very little, not only with respect to the equator, but also with respect to other objects in that orbit. Careful initial placement can limit collision velocities between objects in such an orbit to less than 5 meters/second. This reduced collision velocity is expected to reduce significantly or eliminate the production of debris fragments large enough to contribute to collisional cascading.

Results in orbital evolution of objects in the geosynchronous region

The orbital evolution of objects at or near geosynchronous orbit (GEO) has been simulated to investigate possible hazards to working geosynchronous satellites. Both large satellites and small particles have been simulated, subject to perturbations by nonspherical geopotential terms, lunar and solar gravity, and solar radiation pressure. Large satellites in initially circular orbits show an expected cycle of inclination change driven by lunar and solar gravity but very little altitude change. They have little chance of colliding with objects at other altitudes, provided that the initial eccentricities of their orbits are small. However, if such a satellite is disrupted, debris can reach thousands of kilometers above or below the initial satellite altitude. Small particles in GEO experience two cycles driven by solar radiation: an expected eccentricity cycle and an unexpected inclination cycle. This inclination cycle results from a precession of the orbit plane driven by asymmetric torque effects of the radiation pressure on an eccentric orbit. Particles generated by GEO insertion stage solid rocket motors typically hit the Earth or escape promptly; a small fraction remain in orbits that persist longer than 10 years.