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usually in vast disagreement with distributions determined by other techniques because different assumptions have been made. Nevertheless, recent spatial densities from penetration satellites, when extrapolated, are consistent with the zodiacal light results. The large meteoroids ranging in mass from a few kilograms to 10° kg may be of asteroidal origin. It is difficult to derive the meteoroid flux from “falls.” The flux value depends on the probability of seeing the “fall” and establishing the relation between the mass found and the original mass. Both cometary and asteroidal meteoroid orbits contain selection effects. The photographic measures show two peaks in a typical distribution relative to Earth. The second peak is attributed to meteoroids in retrograde orbits because their higher rate of entry is more easily detected than the slower moving, direct orbits. This selection effect distorts meteor numbers in both distance and velocity and is inherent in the photographic technique. Among the average velocities so determined are 20 km/s by Dohnanyi (1966), 17 km/s by Kessler (1969) for a gravitational Earth and 15 km/s for a nongravitational one, 19 km/s by Dalton (1965), 22 km/s by Whipple (1963), and 30 km/s by Burbank et al. (1965). Radar measurements do not exhibit the bimodal shape of the velocity distribution of the photographic measures. The high-velocity peak is not attained because the more numerous small meteors have a diffuse, ionized wake. Before removing selection effects, the investigators at the Smithsonian Astrophysical Observatory get a higher average velocity from radar measurements. Kessler computes the probability of finding an asteroid at a given distance from the Sun. He has also flux levels for calculating the hazard to interplanetary flight; he gives the flux for interplanetary missions as 10−16 g/cm2/s. Near Earth, protection from 0.02 g particles is required, although encounters with particles as large as 200 g are possible. Kessler has used the counterglow to place an upper limit on the spatial density of the asteroidal debris and he gives the flux measured by Pegasus and Explorer satellite penetration experiments as reported by Naumann as the best estimate. The density of debris may be enhanced in the asteroid belt, but at 2.5 AU the lower velocity causes the penetration flux to be comparable to that at Earth. (See also the paper by Whipple.**) According to Opik (1968), the origin of those centimeter- to meter-size stony and nickel-iron fragments of interplanetary stray bodies, which have survived the passage through Earth's atmosphere and are now preserved in museums, is at present most commonly ascribed to the asteroid belt. The Lost City meteorite (McCrosky, 1970), whose orbit was calculated from photographic observations to have an aphelion of 2.35 AU, supports this assumption. If indeed the collision probability in the asteroid belt is high enough, a large number of fragments of all sizes result. With an orbital change, presumably obtained in the collision, these might be diverted to Earth's space.
Öpik (1968) discussed the statistics of inclination and eccentricity for several classes of small bodies. The statistics of true meteorite orbits are very incomplete because of the difficulty of obtaining satisfactory observations. The existing evidence shows that when the meteorite orbits are compared to belt asteroids and the ones that cross Mars’ orbit, they have inclinations too small for their high eccentricities. The Mars and belt asteroids, on the other hand, have average eccentricities that are too small for their corresponding inclinations. Absence of dilution (equipartition) among the Mars-orbit-crossing asteroids implies insignificant perturbations during the age of the solar system.
The periodic comets set a lower limit to meteorite debris input. Their orbital elements have remarkable similarity and their repeated revolutions in a short period must make a considerable contribution to the debris in the solar system. This may even exceed that from the “almost parabolic” members of the cometary cloud that are not periodic; these surround the solar system. For meteoroids it is worth noting that because of their ablation and breakup in the atmosphere, the low-velocity objects are strongly favored by the selection process. But, for the fireballs this selection effect does not work and their low relative velocities cannot be explained solely by this means; they may form a real, physically unique population.”
LAUNCH VEHICLE AND PROPULSION REQUIREMENTS FOR
A program for exploration of an asteroid may be as important as exploration of the planets in order to study a primitive stage in the development of the solar system. As an aside, the satellites of Mars, Phobos and Deimos, comparable in size to asteroids, also are interesting objects in their own right:70 Hills” made the prediction that few impact craters will be found on the satellites of Jupiter. A great debate” ensued over the timing of asteroid missions as there are so many preparatory ground-based studies still to be performed.
A flight to a near asteroid might be a flyby,73 a rendezvous,” an orbiter, or a sample-return” mission. The mission might involve a man” or it might be completely automated. The precise launch vehicle and propulsion requirements will vary as a function of the mission objectives and the weight of the scientific package required to obtain the objectives. Some of the planning aspects for an asteroid mission are reviewed?7 and a specific mission is described.” A few examples of scientific experiments in the Pioneer program are reviewed” and a beginning with specific suggestions was made.”
The difficulty with all ground-based observations of the asteroids is the lack of resolution on the surface so that the need is obvious for flyby missions to
take detailed pictures and to make photometric and polarimetric measurements over a wide spectral range. The range of phase angle attained during a flyby is much greater than that from Earth. A space-probe landing should be instrumented to study the surface in detail and collect samples that give precise information on the structure and composition of the asteroid. An unmanned flyby of a near asteroid would be the least demanding of the various asteroid missions and could be accomplished with presently available launch vehicles (e.g., Atlas/Centaur) and a Pioneer-type spacecraft” weighing approximately 200 kg. The flight time for this mission would be approx. imately 100 days and the communication distance at encounter would be about 0.5 AU. This flyby mission would have a 40 day launch window and would be a relatively inexpensive space mission. The unmanned rendezvous and/or orbiter mission” would require addi. tional propulsion capability beyond that indicated for a flyby. This increased propulsion capability could be supplied by either a high-performance chemical-propulsion stage or a solar electric-propulsion” system utilized as the final stage for the Atlas/Centaur, Titan IIIC, or Titan IIID/Centaur launch vehicle. An asteroid rendezvous mission to Icarus or Geographos could be accomplished using a solar electric-propulsion system optimized for use with a Titan IIID/Centaur launch vehicle. The net spacecraft mass for a rendezvous with Geographos would be 1800 kg. The departure date for this Geographos rendezvous could be in August 1977 and the related flight time would be about 650 days. The reference power for the solar electric-propulsion system would be 40 kW and rendezvous would take place at about 1.1 AU. The departure date for an Icarus rendezvous could be in September 1978 with a flight time of some 670 days. An asteroid rendezvous mission** is a relatively high-energy mission and would cost an order of magnitude more than a simple flyby of either Icarus or Geographos. An unmanned sample-return mission” would require a launch vehicle of even higher performance than for the rendezvous mission (e.g., Saturn V with appropriate upper stage). The amount of scientific information that could be collected from such a mission would, of course, be considerably greater in kind and quantity of data obtained. A manned expedition36 to a near asteroid would undoubtedly benefit from the availability of a space nuclear-power capability. A nuclear electric capability could be used to provide the power needed to propel an electric-propulsion spacecraft and/or to meet the onboard power requirements for the astronauts and their scientific instruments. Thermal nuclear rocket propulsion, when available, should provide an increase in performance over that presently obtainable from a comparable chemical stage. This increased performance would be most useful in accomplishing a manned exploration mission to an asteroid.
Any mission to an asteroid in this decade will probably use the current U.S. space transportation system. (See fig. 1.) At the end of this decade, however, if an operational space shuttle is available, missions to selected asteroids could be accomplished in part using this potentially cost-effective mode of transportation. When the space shuttle mode of transportation is used, appropriate upper stages will be required to operate in conjunction with the shuttle to accomplish an asteroid mission. These upper stages might be presently available ones (e.g., Centaur, Agena, Transtage, etc.), or entirely new stages designed to be used with the space shuttle (e.g., space tug, versatile upper stage, etc.). Mission requirements and the availability of an operational shuttle will dictate the appropriate transportation system to be used in accomplishing a mission to an asteroid.
In summary, it is seen that the initial mission to an asteroid might be a flyby of Eros or Geographos. This flyby could be followed by an automated orbiter or rendezvous mission making television or spin-scan imaging and photopolarimetric reconnaissance of the asteroidal surface. The orbiter could be appropriately followed by an automated sampling and return to Earth of selected asteroidal rocks. This automated mission could be followed by a manned mission to a suitable asteroid. Such a mission approach would represent a logical step-by-step sequence of exploration.
HEIGHT, METERS HEIGHT, - 1 10 FEET 2003 2:03 j 2003 _l 150100- | A fl 303 - o 0- - Éd 4 - - rotax in:: | * * *t, *|"or"---|"**|o]*****: oft 180 || 1130 || 3810 || 5100 || 4100 || 12 200 || 13 600 || 15 900 T 18 600 || 1 18 000 ^o - 230 || 320 | 730 || 410 || 1270 || - 3860 || - || 36 300
PAYLOAD VALUES ARE FOR EAST LAUNCH FROM ETR ExCEPT FOR SCOUT WHICH IS FROM WALLOPS
*no LAUNCH FActuries AvAILABLE FOR TITAN IIIB An Err
Figure 1.-Current U.S. space transportation system.
Toward the end of the book, the topics return to the ones with which it started—ephemerides” and telescopic observation**—but this time more specifically having the future needs of the space program in mind. Finally there is a summary of the colloquium in terms of what appears urgent and interesting to do in the future.8%
SOURCE BOOKS ON ASTEROIDS
Astronomical textbooks generally have only a few pages on minor planets; the asteroids are treated somewhat as “vermin of the sky.” To a stellar astronomer who gets trails made by moving asteroids on his long-exposure plate they, indeed, must be a nuisance. Incidentally, space junk is also becoming an increasing problem?" an extension of the National GeographicPalomar Atlas to the Southern Hemisphere, for instance, will be seriously hampered by long trails.
The new book by Hartmann (1972) has a good review chapter on asteroids. Krinov (1956), Watson (1962), and Roth (1962) have written brief semipopular reviews; Roth's historical section is a delight. There are articles written by Harwood (1924) and Arend (1945), but they are out of date. As inaccessible to readers in the United States as Arend's writing is the book by Putilin (1953), which is not too serious because it is mostly a review of certain procedures in positional work.
A semipopular introduction to the Trojan planets has been made by Wyse (1938) and Nicholson (1961). Short articles on asteroids have been written by Nicholson (1941), Porter (1950), Struve (1952), Miller (1956), and Ashbrook (1957). A splendid article on asteroids was written by Richardson (1965). The literature on asteroids and comets was reviewed recently (Gehrels, 1971). Summary reports of this colloquium have been made by Matthews (1971) and Hartmann (1971).
Arend, S. 1945, Quelques Aspects du Problème des Astéroïdes. Editions l'Avenir.
Brussels. Also, Ciel Terre 9-12, 1945. Ashbrook, J. 1957, Naming the Minor Planets. Sky Telescope 17, 74-75. Bos, W. H. van den, and Finsen, W. S. 1931, Physical Observations of Eros. Astron. Nachr. 241, 329-334. Burbank, P. B., Cour-Palais, B. G., and McAllum, W. E. 1965, A Meteoroid Environment for Near Earth, Cislunar and Near-Lunar Operations. NASA TN D-2747. Dalton, C. C. 1965, Statistical Analysis of Photographic Meteor Data, Part II: Verniani's Luminous Efficiency and Supplemented Whipple Weighting. NASA TN-X-53360. Dohnanyi, J. S. 1966, Model Distribution of Photographic Meteors, pp. 340-341. Bellcomm, Inc. Gehrels, T. 1970, Photometry of Asteroids. Surfaces and Interiors of Planets and Satellites (ed., A. Dollfus), ch. 6. Academic Press, Inc. London and New York.