Interplanetary dust can be defined as solid particles outside Earth's atmosphere in the size range larger than a molecule and smaller than an asteroid. It is studied by a number of quite different techniques. For Earth-based observers, these techniques include measurement of the brightness and polarization of the interplanetary light, optical radar studies of particles entering the upper atmosphere, photographic and radar meteor observations, study of meteorites, and various methods of collecting dust particles in the atmosphere, in ice cores, and in deep sea sediments. Observations made from spacecraft include some interplanetary light observations and measurements of individual particles by means of microphones, penetration sensors, and collection experiments. These observational techniques are described by Millman (1969) and Bandermann (1969).

The interplanetary dust may be composed of cometary material, interstellar grains, debris from asteroidal collisions, primordial material formed by direct condensation, or contributions from all of these sources. Before we can determine the origin of the dust, we need to know its physical nature, spatial distribution, and the dynamical forces that act on the particles. The spatial distribution and dynamics are separately treated in this symposium by Roosen. We discuss here the physical characteristics of the dust particles: their size distribution, chemical composition, physical structure, and optical properties.

Calculations of upper limits to the quantity of small particles in the asteroid belt are based on (1) the brightness of the counterglow coupled with observations and theory for the zodiacal cloud near Earth's orbit and (2) the destruction and erosion of asteroidal particles as they spiral toward the Sun because of solar radiation via the Poynting-Robertson effect. These calculations place the likely upper limit on asteroidal space particle density at the order of 5 to 10 times and the hazard to space vehicles at 2 to 4 times those near Earth's orbit. No such evidence indicates, however, that the hazard from small particles is actually much greater in the asteroid belt.

The bulk of the comments I made at the colloquium are to be found in two recent papers (McCrosky and Ceplecha, 1970; McCrosky et al., 1971). The following summary is offered in place of a full text.

Most interpretations of all meteor data rely on knowledge of the relationship (luminous efficiency) between mass and luminosity. Discussions of recent experiments made to determine the luminous efficiency are found in papers by Ayers et al. (1970) and Becker and Friichtenicht (1971). Faint meteor phenomena can be understood if the meteoroids are weak, low-density (pm ≈ 0.25 g/cm3) “fluff balls” such as can be expected on the basis of Whipple's comet model (Jacchia, 1955; Jacchia et al., 1967). Alternative explanations, not requiring low densities, have been offered. Jones and Kaiser (1966) propose thermal shock of strong, high-density material as a fragmentation mechanism. Allen and Baldwin (1967) and Baldwin and Allen (1968) have reanalyzed Jacchia's data in terms of phenomena observed in the laboratory in simulated reentry experiments. Here, a high-density meteoroid froths and thereafter behaves as a low-density body. They also revise the luminosity law to account for blackbody radiation of refractory material. McCrosky and Ceplecha (1970) show that neither of these alternative explanations can apply to large bodies and, using photographic observations of bright fireballs, they defend Jacchia's original proposal for all meteors.

The Martian satellites are some of the most interesting and accessible objects in the solar system, and therefore worth studying or even visiting. As small bodies, on the order of kilometers in diameter (see table I), they may have experienced no internal heating or volcanism, and could therefore be original condensations in the solar system. They may thus be the only remnants of the original planetesimals.

An additional point of interest is that they are the only satellites in the inner part of the solar system–Earth's Moon is generally assumed to be a sister planet. As such, their study could illuminate one of the important differences between the inner and outer parts of the solar system.

In a recent paper (Rabe, 1970), I suggested that at least some comets of the Jupiter group may have originated from the relatively dense Trojan clouds that, according to a recent survey by the van Houtens and Gehrels (1970), seem to be associated with the equilateral points L4 and L5 of the Jupiter orbit. This suggestion was inspired by the finding that the periodic comet Slaughter-Burnham has been captured (or perhaps recaptured) into unstable or temporary “Trojan” librations lasting approximately 2500 yr and by the circumstance that the Jacobi “constants” of most Jupiter group comets have values between 3.0 and 2.5, or just in that range which would also be occupied by all known and unknown Trojans associated with Jupiter in stable or unstable librations involving heliocentric eccentricities up to about 0.5 and inclinations as large as 30°. Moreover, it is well known that nearly all comets of the Jupiter group are able to approach Jupiter rather closely, thus providing the possibility for such drastic orbital changes as temporary capture into, or escape from, librational motion of the Trojan type. All, presumably stable, orbits of actual Trojan planets presently known have eccentricities e not exceeding 0.15, but we also know that stable short-period librations with much larger e values do exist in the restricted Sun-Jupiter problem, so that even in the real, nonrestricted situation there should be a possibility for corresponding librations with these more substantial short-period components. These librations may be unstable but may have long lifetimes nevertheless. The case of P/Slaughter-Burnham has proved that such motions are indeed possible, even with an e as large as 0.52.

There has long been speculation as to whether comets evolve into asteroidal objects. On the one hand, in the original version of the Oort (1950) hypothesis, the cometary cloud was supposed to have formed initially from the same material that produced the minor planets; and an obvious corollary was that the main physical difference between comets and minor planets would be that the latter had long since lost their icy surfaces on account of persistent exposure to strong solar radiation (Öpik, 1963). However, following a suggestion by Kuiper (1951), it is now quite widely believed that, whereas the terrestrial planets and minor planets condensed in the inner regions of the primordial solar nebula, icy objects such as comets would have formed more naturally in the outer parts, perhaps even beyond the orbit of Neptune (Cameron, 1962; Whipple, 1964a). Furthermore, recent studies of the evolution of the short-period comets indicate that it is not possible to produce the observed orbital distribution from the Oort cloud, even when multiple encounters with Jupiter are considered (Havnes, 1970). We must now seriously entertain the possibility that most of the short-period orbits evolved directly from low-inclination, low-eccentricity orbits with perihelia initially in the region between, say, the orbits of Saturn and Neptune, and that these comets have never been in the traditional cloud at great distances from the Sun.

Arguments for a long time have been brought forward in support of the idea that the minor planets with orbits approaching Earth's orbit might be of cometary origin. Our feeling is that before such hypotheses are considered for any particular object, it is necessary to prove that differences in physical appearance and dynamical behavior between a typical asteroid of the Apollo or Albert types and a typical short-period comet can be interpreted in terms of cometary evolution.

In this paper, we discuss models of cometary nuclei, transition of an object from cometary phase into asteroidal phase, and specific asteroidal objects that may be of cometary origin.

Most meteorites come from a small number of parent bodies (6 to 11), with radii mainly between 100 and 300 km. The most likely sources of meteorites are seven asteroid families between 1.9 and 2.8AU, whose eccentricities and inclinations are high enough to permit their collisional debris to cross the orbit of Mars. Comets are too small and too numerous to serve as a source of the major meteorite classes, but they may well be an important source of micrometeorites and carbonaceous chondrites.

Much of what we know about the early history of the solar system has been learned from the study of meteorites. This results from the fact, demonstrated by isotopic age measurements, that all of the various classes of stone and iron meteorites were formed 4.6 X 109 yr ago within a short period of time, probably less than 100 million yr in duration. This is also the age of Earth and the Moon and may be presumed to be the time of formation of the solid bodies in the solar system. Measurements of the products of the decay of the extinct radioactive isotopes 129Xe and 244Pu show, furthermore, that the formation of these solid bodies occurred within 100 million yr of the time of separation of the solar nebula from interstellar matter. Except for physical fragmentation into smaller bodies, the chemical and mineralogical composition of most meteorites has been essentially unaltered since this time during the formation interval of the solar system.

The nature of the volatile phase in comets has never been established from observations. Although water was likely to be its major constituent, evidence was still circumstantial. It is shown here that water evaporation quantitatively explains not only the brightness of the hydrogen and hydroxyl halos observed by the OAO for the two bright comets of 1970, but also, which is much more convincing, it explains their brightness dependence on the heliocentric distance.

It is shown that very old and still active icy-conglomerate or “clathrate” cometary nuclei may exist at the outer boundary of the asteroidal belt and belong to the group of relatively stable “outer” short-period comets of which only a small fraction have been discovered or recognized as cometary objects.hich is much more convincing, it explains their brightness dependence on the heliocentric distance.

The search for these comets among very faint asteroids with the mean motion μ ≤ 600”, and the study of their behavior could be very important to our knowledge about the final stage of cometary nuclei.

With respect to their orbital parameters, the known asteroids fall into different groups:

(1) Main belt asteroids orbit between Mars and Jupiter. Most of them have semimajor axes a in the range 2.0 < a > 3.5 AU.

(2) Asteroids outside the main belt form a number of different groups, such as the Trojans, Hildas, etc.

(3) Mars-crossing asteroids are bodies with low a values and perihelia inside the Martian orbit; in several cases they are inside Earth’s orbit. They are the closest neighbors in space of the Earth-Moon system. A list of them is given by Marsden, table II. We select from these the ones with eccentricities in the range 0.2 to 0.4; they will be referred to as the Eros group, named after its largest member. The arguments for a mission to an asteroid refer especially to the Eros group as a first target.

Let me first emphasize the area of agreement with Professor Alfvén. I, too, believe that the asteroids are of very great scientific interest; great enough to justify space missions some day. What we differ on is the timing and target selection for these missions (Alfvén and Arrhenius, 1970, and in this volume). I look upon space missions as a tremendously expensive way of obtaining scientific data, which should not be attempted until all available alternatives are exhausted. Ground-based research on asteroids and meteorites is nowhere near exhaustion; on the contrary, it is moving at an impressive pace. If we maintain this pace for another decade or two, we will not only have answered most of the questions posed for an early mission, but will be able to come up with a more worthwhile, more informative mission.

The landings of instrumented probes and astronauts on the Moon and the short glimpses at Venus and Mars that distinguished the spaceflight program of the last decade yielded such an impressive wealth of new knowledge that the President, in his programmatic speech of 1970, mentioned the continuing exploration of the solar system as one of the national goals during the decade of the seventies.

This exploration will be accomplished with unmanned spacecraft, except for the remaining three Apollo flights in 1971 and 1972 and Skylab in 1973. Planetary exploration will include photographic coverage of the surfaces of the celestial bodies; closeup pictures of specific surface features; magnetic and gravitational measurements; observations of atmospheres, ionospheres, and radiation belts; analysis of surface material in situ; and, as far as possible, the return of surface samples for careful chemical and mineralogical analyses and for age determinations.

The earliest flights to concentrate solely on the nature of particular asteroids will probably be of the simple flyby or rendezvous type. That is, data will be obtained by close investigation of these bodies without actually coming in contact with their surface. The technology of flyby missions to bodies at planetary distances is well established; there have been five successful Mariners to date and more flights are scheduled. Unfortunately, ballistic flyby missions suffer from the fact that the spacecraft is near the body of interest for a short period of time (relative velocities at asteroid encounters are 5 to 12 km/s). This problem is acute because spatial resolution of the onboard instruments must be very high, and typical flyby velocities will preclude detailed observations.

Of the many asteroids that are cataloged, selection of a target planetoid was narrowed to the Mars-crossing asteroids. The main belt and Trojan asteroids are much farther away and require longer voyage times and propulsive energy to accomplish sample returns. Within the Mars-crossing group, those asteroids were surveyed that have orbits inclined less than 15°, perihelia within 0.2 AU of Earth’s orbit (and therefore require less propulsive energy), and diameters greater than 1 km (to assist terminal observation and rendezvous). Eros was chosen as representative of this class.

The use of spacecraft for studying the physical properties of the asteroid belt can be approached in several ways. Certainly the simplest approach is to send a spacecraft into the asteroid belt and measure the effects of the environment encountered; this has the advantage of not requiring a vehicle to be targeted to any particular destination. With such an approach, properties may be obtained for those classes of objects that are populous enough to provide a significant number of encounters within the measurement range of the spacecraft. Unfortunately, the large asteroids do not constitute such a class of objects; the probability of an undirected spacecraft passing within measurement range of an asteroid having a diameter of 1 or more km is negligible.

If manned missions to the planets are to be considered at some time in the future, as has long been imagined, it appears to be reasonable to explore the feasibility of carrying out a manned mission to an asteroid as an intermediate step between lunar and planetary missions. If study shows that it is possible to conduct such a mission with Apollo-type hardware, and if the answer to the first question is yes, then an asteroid flight should probably be considered while Apollo-type facilities are still in existence. Once these facilities have been dismantled, it may be many years, perhaps a generation, before a new era in space exploration would make such a manned flight possible.

Unmanned missions to the asteroids have been proposed and investigated as part of the overall plan of exploration of the solar system. A principal incentive for landing on an asteroid and retrieving a surface sample for return to Earth is the expectation that detailed laboratory analysis of the sample material’s chemical composition, crystal structure, surface texture, magnetic characteristics, radioactive state, and age can provide essential clues, not available by other means, to the origin of asteroids and possibly the history and formative processes of the solar system (Alfvén and Arrhenius, 1910a,b; Bratenahl; Friedlander and Vickers, 1964; IIT Research Institute, 1964; Öhman, 1963). The results may indicate, for example, to what extent accretion or fragmentation processes have been involved in the formation of asteroids.