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NASA NTRS1976

On the detection of a cometary mass distribution

The problem of detecting a possible cometary distribution on the fringes of the solar system is examined. The acceleration of a space probe due to a hypothetical cometary mass distribution with the surface density rising to a maximum and subsequently falling off with increasing distance from the sun is analyzed. The total minimum detectable cometary mass for the Pioneer and Mariner spacecraft is estimated on the basis of this model to be on the order of 1000 earth masses. Precision tracking of deep space probes is less sensitive by three orders of magnitude for the detection of an unseen cometary mass distribution at the fringes of the solar system than are the secular perturbations of long-period comets.

Boss, A. P.↗
NASA NTRS1977

On the early scattering processes of the outer planets

The gradual scattering of small bodies by the Jovian planets in the late stage of their accretion is simulated by Monte Carlo calculation. The effects of collisional interaction of the scattered planetesimals with the inner planets and asteroidal belt are estimated. The total mass influx injected into the terrestrial zones from the outer planetary zone could be appreciable; however, if the damping effects due to the mutual inelastic or catastrophic collisions among the small bodies were significant, the total mass influx would be greatly reduced. The origin of the cometary Oort cloud in relation to these scattering processes is also discussed. The calculation indicates that, in addition to Jupiter, Uranus and Neptune were likely to be the major contributors to the population of long-period comets if they originated from such scattering processes.

Ip, W. H.↗
NASA NTRS1987

The study of the physics of cometary nuclei

The numerical calculations of stability for many possible orbits of the double nucleus for P/Holmes showed that the likelihood of such a precollision history was quite high. A number of investigations were made of hypothetical orbits for particles about the asteroid Amphitrite to test for stability. The purpose was to establish more favorable fly-by orbits close to the asteroid for the Galileo missions en-route to Jupiter, reducing the collisional hazards. A statistical study was made of the orbits of long-period comets with small original semi-major axes recently perturbed from the great Opik-Oort Cloud. The results from the space missions to Halley's comet are partially reported in the two papers in the appendices.

Whipple, Fred L.↗
NASA NTRS1990

The Oort cloud

Although the outermost planet, Pluto, is 6 x 10 to the 9th km from the sun, the sun's gravitational sphere of influence extends much further, out to about 2 x 10 to the 13th km. This space is occupied by the Oort cloud, comprising 10 to the 12th-10 to the 13th cometary nuclei, formed in the primordial solar nebula. Observations and computer modeling have contributed to a detailed understanding of the structure and dynamics of the cloud, which is thought to be the source of the long-period comets and possibly comet showers.

Wessman, Paul R.↗
NASA NTRS1992

About distribution and origin of the peculiar group of sporadic meteors

A particular group of sporadic meteors are picked out from analysis of meteor catalogs derived from results of radar observations in Mogadisho and Kharkov. The semi-major axes are equal or more than 1.73 AU and inclinations of orbits are equal or more than 90 degrees for these meteors. The distributions of radiants, velocities, and elements of orbits were derived. The probable source of meteor bodies of this peculiar group is the long-period comets, in particular, the comets of the Kreutz's group.

Andreev, V. V.↗
NASA NTRS1991

The forest and the trees

Comments on the nature of cometary nuclei, some problems regarding cometary origin, and some correlations among cometary data are presented. Comparisons with an earlier report on cometary nuclei are noted, and most of the earlier advances in concept are substantiated. The mean density of the Halley nucleus may have been underestimated, while the nature of the rotation remains uncertain. The dust/gas ratio apparently needs to be increased by as much as two times, perhaps to unity or higher. CHON grains appear to be important sources of gas. Evidence is presented to support the thesis that aging among long-period comets increases statistically as the periods decrease. Data on the orientation of cometary axes with respect to the Galaxy and the properties of clusters defined by these axes are presented.

Whipple, Fred L.↗
NASA NTRS1997

The Cretaceous-Tertiary Impact Crater and the Cosmic Projectile that Produced it

Evidence gathered to date from topographic data, geophysical data, well logs, and drill-core samples indicates that the buried Chicxulub basin, the source crater for the approximately 65 Ma Cretaceous-Tertiary (K/T) boundary deposits, is approximately 300 km in diameter. A prominent topographic ridge and a ring of gravity anomalies mark the position of the basin rim at approximately 150 km from the center. Wells in this region recovered thick sequences of impact-generated breccias at 200-300 m below present sea level. Inside the rim, which has been severely modified by erosion following impact, the subsurface basin continues to deepen until near the center it is approximately 1 km deep. The best planetary analog for this crater appears to be the 270 km-diameter Mead basin on Venus. Seismic reflection data indicate that the central zone of downward displacement and excavation (the transient crater is approximately 130 km in diameter, consistent with previous studies of gravity anomaly data). Our analysis of projectile characteristics utilizes this information, coupled with conventional scaling relationships, and geochemical constraints on the mass of extraterrestrial material deposited within the K/T boundary layer. Results indicate that the Chicxulub crater would most likely be formed by a long-period comet composed primarily of nonsilicate materials (ice, hydrocarbons, etc.) and subordinate amounts (less than or equal to 50 percent) primitive chondritic material. This collision would have released the energy equivalent to between 4 x 10(exp 8) and 4 x 10(exp 9) megatons of TNT. Studies of terrestrial impact rates suggest that such an event would have a mean production rate of approximately 1.25 x 10(exp -9) y(exp -1). This rate is considerably lower than that of the major mass extinctions over the last 250 million years (approximately 5 x 10(exp -7) y(exp -1). Consequently, while there is substantial circumstantial evidence establishing the cause-effect link between the Chicxulub basin forming event and the K/T biological extinctions, the results of our analysis do not support models of impact as a common or singular causative agent of mass extinctions on Earth.

Sharpton, Virgil L.↗
NASA NTRS1997

Origin and Evolution of the Unusual Object 1996 PW: Asteroids from the Oort Cloud?

The unusual object 1996 PW was discovered on 1996 August 9 by the Near-Earth Asteroid Tracking automated search camera operating from Haleakala in Hawaii. Although asteroidal in appearance, it was soon determined that the object is in a near-parabolic orbit similar to that of a long-period comet. No object that was not an active comet has ever been discovered on such an eccentric orbit. The discovery of 1996 PW prompted us to examine and evaluate its possible origins, including the intriguing possibility that it is an asteroid from the Oort cloud. Current models for the formation of the Oort cloud argue that most of the material there should be from the Uranus-Neptune region and thus cometary, not asteroidal, in composition. We better quantify these models and show that approximately 1% of the Oort Cloud population should be asteroids. We find that 1996 PW has almost certainly been a resident of the Oort cloud. However, we find it equally likely that 1996 PW is an extinct comet or an asteroid. Although not conclusive, our results represent a significant change in our understanding of the Oort cloud, because they suggest that the ejection process sampled material from as close to the Sun as the asteroid belt in the primordial solar nebula and hence much warmer formation temperatures than previously thought. This diverse sample is preserved in the Oort cloud.

Weissman, Paul R.↗
NASA NTRS1994

The Contemporary Hazard of Cometary Impacts

Cosmic impacts pose a continuing hazard of loss of human life and property. Significant contemporary risk is associated with projectiles in the energy range from about 10 megatons of TNT up to the size of the K/T impactor. The lower threshold for damage is defined by the atmosphere of the Earth, which effectively shields us From smaller projectiles. Up to energies of about a gigaton of TNT, the effects are local or regional for impacts on the land, or coastal for ocean impacts, which can generate large tsunamis. A greater risk is associated with still larger impacts, which are capable of causing global ecological catastrophe, possibly leading to mass mortality From starvation and epidemics. If such a impact took place anywhere on Earth during our lifetimes, we would each be in danger, independent of where the projectile struck. Statistical estimates indicate that each human on this planet runs a risk of roughly 1 in 20,000 of dying from this cause. Prudence suggests that we should be concerned about such impacts and seek ways of avoiding them or mitigating their consequences. The primary objective of any program to deal with this hazard is to determine whether or not such a near-term impact is likely. The best approach for the asteroidal component is a comprehensive telescopic survey, which can discover all Earth-crossing asteroids larger than 1 km in diameter and provide decades of warning in which to plan ways to deflect or destroy a threatening object. Long-period comets, however, pose a much greater challenge, since they cannot be discovered long in advance of a possible impact, their orbits are harder to predict, and they are significantly more difficult to deflect or destroy.

Morrison, David↗
NASA NTRS1993

(abstract) The Impactor Flux in the Pluto-Charon System

The impactor flux on Pluto and Charon consists of long-period comets from the Oort cloud passing through the region, and intermediate-period comets evolving inward to (or outward from) short-period orbits. The source of the intermediate-period comets is likely an extended disk of remnant planetesimals beyond the orbit of Neptune, commonly referred to as the Kuiper belt, though the Oort cloud may be an additional source of some short-period comets. The Pluto-Charon system actually circulates within the Kuiper belt, and with an aphelion of 49.3 AU, moves through a region where remnant planetesimal orbits would likely be stable over the age of the solar system. The recent discovery of two objects at Kuiper belt distances, 1992 QB 1 and 1993 FW, and the fact that the orbit determined for 1992 QB 1 is likely of low eccentricity and low inclination, have provided strong observational evidence for the Kuiper belt. Weisman et al (1989) showed that, for the expected number of objects in the Kuiper belt necessary to provide the short-period comet flux, cometary cratering on Pluto and Charon is dominated by Kuiper belt comets. We will provide updated estimates of the numbers, based on an improved understanding of Oort cloud and Kuiper belt dynamics since 1989, and on improved estimates for the radii and masses for Pluto and Charon. Because Pluto and Charon actually penetrate the stable region of the Kuiper belt beyond 40 AU, expected cratering rates for the pair are substantially higher than for Triton, which is located at the inner edge of the Kuiper belt. However, the Pluto-Charon system is tidally evolved and this may have led to resurfacing events at various times in its history. Thus, detailed predictions of the integrated crater density or comparisons with Triton are not very meaningful without a knowledge of that history. Differences in the suspected compositions of Pluto and Charon may also lead to different surface rheologies and thus, different retention times for cratered surfaces.

Pluto↗
NASA NTRS1992

The activities of comets related to their aging and origin

Attention is given to two indices developed for the purpose of comparing the natures of various classes of comets: the Activity Index (AI), which measures the inherent magnitude increase in brightness from great solar distances to maximum near the perihelion, and the Volatility Index (VI), which measures the variation in magnitude near the perihelion. AI determinations are made for 32 long-period (LP) comets and for 14 short-period (SP). The range of values of AI is of the order of 3 to 10 magnitudes with a median of about 6. No fading with aging among the LP comets was found. No systematic change with period occurs for the VI index. Weak correlations are found with the gas/dust ratio of comets. No correlations are discovered between the two indices, nor of either index with near-perihelion magnitudes or orbital inclination.

Whipple, Fred L.↗
NASA NTRS1992

A volatility index for comets

The variations in total brightness of a comet when it is most active, near perihelion, are presently used as the bases of a volatility index (VI) for short-period (SP) and long-period (LP) comets. Volatility does not correlate with period among the LP comets, and thereby shows no 'aging' effect; similarly, the VI measurements are the same for SP and for LP comets and exhibit no correlation with (1) absolute magnitude near perihelion, (2) orbital inclination, or (3) activity index measuring the intrinsic brightness change from great solar distances to the maximum near perihelion. Active comets are shown to be basically alike irrespective of their orbits or 'ages'.

Whipple, Fred L.↗
NASA NTRS1992

Long-term orbital evolution of short-period comets found in Project Cosmo-DICE

Orbital evolutions of about 160 short-period (SP) comets are numerically integrated for 4400 years in the framework of a realistic dynamical model. By the round-trip error in closure test, a reliable time space of the integrated orbits is estimated for each comet. Majority of the SP comets with their Tisserand's constant(J) between 2.8 and 3.1 are found to evolve within the past 1000-2000 years from the orbits whose perihelia are near the Jovian orbit to the orbits with perihelia of 1-2 AU. This evolution is much more rapid than that expected from Monte Carlo simulations based on symmetric distribution of planetary perturbations, thus suggesting that asymmetry of perturbation distribution play an important role in cometary evolution. Several comets are shown to evolve from the near-Saturn orbits and then to be handed over under the control of Jupiter. We also find that a few comets were captured from long-period orbits (a = 75-125 AU) via only a few close encounters with Jupiter. It is confirmed that the captured SP comets of low-inclination with 2.7 less than J less than 3.1 show more or less strong chaotic behavior. On the other hand, comets with longer orbital period and/or of high inclination reveal slow or quasi-periodic orbital evolution.

Nakamura, Tsuko↗
NASA NTRS1991

A new activity index for comets

An activity index (AI) is derived from observational data to measure the increase of activity in magnitudes for comets when brightest near perihelion as compared to their inactive reflective brightnesses at great solar distances. In all, AI determinations are made for 32 long-period (L-P) comets and for 14 short-period (S-P). The range of values of AI is the order of 3 to 10 magnitudes with a median about 6. An expected strong correlation with perihelion distance, q, was found to vary as approx. q(-2.3). Residuals from a least-square solution, delta AI, were used for comparing comets of different orbital classes, the standard deviation of a single value of delta AI was only plus or minus 1(m).1 for L-P comets and plus or minus 1(m)2 for S-P comets. Among the L-P comets, 19 of Period, P greater than 10(exp 4) yr yielded delta AI greater than or equal to -0(m).27 compared to +0(m).39 for 13 of 10(exp 4) greater than P greater than 10(exp 2) yr. This denies any effect of aging among the L-P comets. The 14 S-P comets yielded (delta AI) less 0(m).3 plus or minus 0(m).3 than the 32 L-P comets. The results suggest a common actively level, nature and probably origin for comets of all orbits, sizes and ages.

Whipple, Fred L.↗
NASA NTRS1976

Effects of solar radiation on the orbits of small particles

A modification of the Robertson (1937) equations of particle motion in the presence of solar radiation is developed which allows for partial reflection of sunlight as a result of rapid and varying particle rotations caused by interaction with the solar wind. The coefficients and forces in earlier forms of the equations are compared with those in the present equations, and secular rates of change of particle orbital elements are determined. Orbital dimensions are calculated in terms of time, probable sizes and densities of meteoric and cometary particles are estimated, and times of infall to the sun are computed for a particle moving in an almost circular orbit and a particle moving in an elliptical orbit of high eccentricity. Changes in orbital elements are also determined for particles from a long-period sun-grazing comet. The results show that the time of infall to the sun from a highly eccentric orbit is substantially shorter than from a circular orbit with a radius equal to the mean distance in the eccentric orbit. The possibility is considered that the free orbital kinetic energy of particles drawn into the sun may be the energy source for the solar corona.

Lyttleton, R. A.↗
NASA NTRS1992

A new activity index for comets

An activity index, AI, is derived from observational data to measure the increase of activity in magnitudes for comets when brightest near perihelion as compared to their inactive reflective brightness at great solar distances. Because the observational data are still instrumentally limited in the latter case and because many comets carry particulate clouds about them at great solar distances, the application of the activity index is still limited. A tentative application is made for the comets observed by Max Beyer over a period of nearly 40 years, providing a uniform magnitude system for the near-perihelion observations. In all, 32 determinations are made for long-period (L-P) comets and 15 for short-period (S-P). Although the correlations are scarcely definitive, the data suggest that the faintest comets are just as active as the brightest and that the S-P comets are almost as active as those with periods (P) exceeding 10(exp 4) years or those with orbital inclinations of i less than 120 deg. Comets in the range 10(exp 2) less than P less than 10(exp 4) yr. or with i greater than 120 deg appear to be somewhat more active than the others. There is no evidence to suggest aging among the L-P comets or to suggest other than a common nature for comets generally.

Whipple, Fred L.↗
NASA NTRS1992

A model of the galactic tidal interaction with the Oort comet cloud

A model of the in situ Oort cloud which is isotropic with a random distribution of perihelia directions and angular momenta is considered. The energy distribution adopted has a continuous range of values appropriate for long-period (greater than 200 yr) comets. The evolution of 1,000,000 comets are tracked as they are made observable by the galactic tidal torque. Detailed results are obtained for the predicted distribution of new comets. Predictions of observed distributions are presented and compared with the random in situ results as well as with the actual observed distributions of class I comets. The predictions are in reasonable agreement with actual observations and, in many cases, are significantly different from random when perihelia directions are separated into galactic northern and southern hemispheres.

Matese, John J.↗
NASA NTRS1975

Development of a comet as it pursues its orbit

The properties of cometary dust-swarms in almost parabolic long-period orbits are examined. The effect of passage of the solar system through interstellar gas clouds is shown to be capable of affecting substantially the angular momentum of a comet about the sun, thus accounting for the existence of comets with high values of perihelion distance. The same process would enable cometary particles to adsorb interstellar gases and regenerate their gas content.

Lyttleton, R. A.↗