IV-3

An Assessment of Near-Earth Asteroid Resources

MICHAEL J. GAFFEY, ELEANOR F. HELIN, and BRIAN O'LEARY

The very large number of small asteroids in near-Earth space provide many opportunities to transfer material to the Earth-Moon system at low cost. On the basis of remote-sensing evidence and meteorite studies, a substantial fraction of these objects should contain abundant volatile (water, carbon, carbon compounds) and/or free metal (nickel-iron) materials. Although there are only about 40 known Earth-approaching asteroids, this number could be increased by at least an order of magnitude in a few years by a dedicated search program with a large telescope (48-in. Schmidt). A discovery rate of 20 objects/yr could be expected from a facility costing about $3 million to establish.

The material these objects contain should be determined, if possible, during the discovery opposition, because of their long synodic periods. Relatively high-resolution, broad-wavelength coverage (0.3-2.0 m) reflectance spectra could be made for many of these objects with a large (~100 in.) telescope and. instruments underdevelopment. Such spectra are necessary for detailed mineralogic, characterizations of these asteroids. Low-resolution survey spectra with similar wavelength coverage (UBVRIJHK filters) would provide useful characterizations of fainter objects.

A spacecraft mission should be planned to one or more of these asteroids. Support for programs to locate and characterize the nearEarth asteroids is essential to any program of large-scale space operations since asteroidal bodies appear to be the least expensive source of certain needed raw materials.

INTRODUCTION

In addition to the four terrestrial planets and the Moon, a very large number of minor objects, ranging in size from single dust motes to bodies more than 10 km in diameter, are present in the inner solar system. A small fraction of these asteroids follow orbits that intersect the Earth's orbit, and when collisions occur, meteors, meteorites, and impact craters result (depending on the size of the object).

These asteroids may offer the nearest (in terms of energy) source of raw materials, certainly the nearest beyond the Moon, for any large-scale space activities. The suitability of these asteroids as a resource base will depend on (1) what types of needed materials are available, (2) how the material can be transferred in a useful form to the required area, and (3) how many of these objects have orbits favorable for efficient transfer.

This paper evaluates the potential resources available among the Apollo-Arnor objects (those asteroids that cross or approach the Earth's orbit). The orbital distribution of the known Apollo-Amor objects (about 40) and of the expected population (about 200,000 with diameters greater than 100 m) are discussed from both empirical (i.e., observation) and theoretical perspectives. Several programs aimed at locating these bodies are outlined, and their relative cost and discovery rate discussed.

The chemical and physical nature of these bodies can be deduced from an analysis of remote-sensing data, from analysis of meteorites, and from a study of their source regions. A program of telescopic and spacecraft observations to determine the chemical composition and physical characteristics of these objects is outlined. The transfer (delta V) requirements for several members of the Apollo-Amor population were investigated in a separate paper (ref. 1). A scenario for moving a 106 - 107 -metric-ton portion of such an object into Earth-Moon space and the requirements for such a transfer were investigated in a second companion paper (ref. 2).

APOLLO ASTEROID ORBITS AND POPULATIONS

Introduction

The distribution of asteroids in general and of the Apollo-Amor group in particular can be determined from either observational or theoretical considerations. The distribution of known asteroids as a function of distance from the Sun (semimajor axis a) is shown in figure 1. Between 1.8 and 3.5 AU (asteroid belt), this distribution is probably a reasonable representation of the actual distribution, subject to corrections for observational bias against the more distant and lower albedo (fainter) objects in the outer asteroid belt (ref. 3). The population of known objects with a semimajor axis <1.6 AU (mostly Apollo-Amor objects) is probably not representative of the population as a whole because of the small number of known objects (about 40) and because of observational selection effects that have favored the discovery of certain types of objects. Increasing the number of known Apollo objects to several hundred in a systematic search program will provide a secure basis for evaluating population patterns and for identifying the most suitable bodies for material return.

Based on the discovery rate of Apollo objects in three systematic search programs, it is estimated that the number of bodies larger than 1 km should be 800 400 (ref. 4).This number is comparable to that estimated from rediscovery statistics (refs.5, 6), impact statistics for large craters on the Earth and Moon (refs.6, 7), and from the mass yield of meteorites from Apollo objects (refs.6, 8). Based on the observed scaling law for asteroidal objects and collisional fragments (refs.9), the number of objects larger than 100 m should exceed 100,000 (2.0 LOX 105). These objects are unstable against perturbations by, close approaches to, and collisions with the inner planets on a time scale of 107 years and must be replenished from an external reservoir.

History and Discovery Circumstances of Apollo Asteroids

Max Wolf, who had actively pursued a photographic search of asteroids at Heidelberg during the first three decades of the twentieth century, was succeeded by Karl Reinmuth. In 1932, Reinmuth found, on a 2.5-hr expo sure plate, a long trail of a close, fast-moving asteroid The new asteroid was named "Apollo" and became the prototype of a new class of asteroids characterized by a perihelion distance of 1 AU. After the discovery of Apollo, the next 5 years witnessed the discovery of two additional members, Adonis and Hermes.

The next decade was a quiet time in the realm of Apollo discoveries. However, in the late 1940s, two systematic programs were started: one at Mt. Palomar, the National Geographic Sky Survey (1949-1965), and the other, the Lick Observatory Proper Motion Survey. As by-products of these two surveys, several more Apollo asteroids were discovered. It should be understood that any asteroid trails found on these plates were considered undesirable, flawing the otherwise acceptable photographic plate. It was primarily the diligence of certain individuals, who were sufficiently intrigued by the long trails, that led to recovery and followup of the new objects.

After these two surveys were completed, there was another lull in discovery activity. From the early 1950s through the early 1970s, four other Apollo objects were found incidentally in other observational programs. The first systematic search for Earth-crossing asteroids was initiated in 1973 using the 46-cm (18-in.) Schmidt at Mt. Palomar. The results of this ongoing study have been the discovery of four Apollos, five Mars crossers, and another dozen asteroids from inner regions of the main belt. Figure 2 shows the discovery plate for one of the Apollo objects (1976 AA). Discovery plates (X15) for two recent Apollo objects, (a) 1976 AA, January 7, 1976; (b) 1976 UA OCtober 25, 1976 (Hale Observatory Photographs).

This dedicated program has had an influence on other observers in the astronomical community. By acquainting other observers (particularly the regular observers using the 122 cm (48-in.) Schmidt) with the nature and rationale of the search program and its significance, another four Apollos have been discovered. It should be pointed out that, before the 46-cm Schmidt planet-crossing asteroid search, these asteroid trails were ignored. These observers, with a better understanding of the importance of reporting these trailed objects, have been responsible for doubling the total number of Apollo asteroids discovered in the last 4 years.

ASTEROIDAL MATERIALS

Introduction

While a good case can be made that any mass in space is useful (e.g., reaction mass, radiation shielding, etc.), certain raw materials will probably be more in demand than others (e.g., metals, volatiles, carbon, etc.) (ref. 2). It is therefore appropriate to consider the availability of such materials in objects of the near-Earth population. Several perspectives can be used to characterize the material resources on these bodies. The meteorites represent direct samples of extraterrestrial materials that can be studied in the laboratory. Orbital data from atmospheric meteor paths provide clues concerning the membership of materials in a near-Earth population. Studies of meteor trails provide chemical and physical data for materials that do not survive atmospheric entry. Collections of meteoric dust from high altitudes provide similar information.

Interpretation of reflectance spectra (fraction of sunlight reflected as a function of wavelength) can provide mineralogical characterizations of the surface materials of objects large enough to be measured through a ground-based telescope (refs. 10, 11). This information is available for several of the brighter Apollo-Amor asteroids (refs. 12-14) and indicates the distribution of material types in the Earth-approaching population.

Since the lifetime of an object in the near-Earth population is short (about 107 years), these bodies must be continuously replenished from some external reservoir of material (e.g., main-belt asteroids, comets, etc.) (refs. 6, 15). Studies of the nature of these reservoirs and the source mechanisms provide clues to the general nature of the material types present in the near-Earth population.

Availability of Raw Material

Supplies of certain specific raw materials will be required to build or maintain an inventory for processing purposes. The replenishment requirements for basic raw materials consumed (e.g., incorporated in long-lived products, expended, or lost) in large quantities will determine which types of asteroidal material are most useful. The minerals of major significance found in meteorites (refs. 16, 17) and which should be present on Apollo asteroids include: olivine (iron-magnesium silicate), pyroxene (iron-magnesium-calcium silicate; low calcium pyroxenes are termed "orthopyroxenes" because of their crystal structure), feldspar (calcium-sodium-potassium alurnino-silicate), clay minerals (hydrated iron-magnesium silicate), and metals (nickel-iron alloys). The various meteorite types contain different abundances and compositions of these minerals in a range of structural types which reflects their mode of formation. Four general classes of meteorites are recognized: chondrites, achondrites, stony-irons, and irons. Chondrites were named for the presence of small spherical inclusions (chondrules). These meteorites contain approximately solar abundance of the nonvolatile elements (e.g., iron, magnesium, silicon, etc.) and apparently represent (essentially) unaltered material formed during the accretion of the solar system. The ordinary chondrites are composed of olivine, pyroxene, and feld-spar. spar with varying amounts of NiFe metal (about 15-19 percent H (high iron)-type; about 4-9 percent L (low iron)-type; about 0.3-3 percent LL (low iron, low metal)type). The carbonaceous chondrites (c. chondrites), despite their name, are not necessarily characterized by abundant carbon. Type Cl-C2 (CI-CM or type I-II) meteorites do contain relatively abundant carbon (about 1-5 percent) as graphite and tarlike organic compounds. The bulk of the Cl meteorites consists of a complex clay mineral assemblage and the C2 meteorites contain 15-70 percent olivine as inclusions in a clay mineral matrix. These clay minerals contain about 20 percent water in their structure. Type C3 meteorites consist mostly of olivine and contain only traces of water and carbon (<1 percent). The C4 meteorites consist mainly of olivine and magnetite and are essentially devoid of either carbon or water.

The achondrites are silicate-rich meteorites apparently formed by igneous activity in their parent bodies. These consist of various mixtures of olivine, pyroxene, and feldspar and include eucrites and howardites or basaltic achondrites (pyroxene, feldspar) diogenites (pyroxene), nakhlites (olivine, calcic pyroxene), chassignites (olivine) and ureilites (olivine, carbon).

The stony-irons consist of mixtures of nickel-iron metal (30-70 percent) with various silicates: pallasites (olivine), mesosiderites (pyroxene, feldspar), and several other mixtures.

The iron meteorites consist of alloys of nickel and iron ranging from 5 percent to 50 percent nickel with an average near 10 percent Ni. Cobalt (about 0.5 percent) and other siderophile elements (Ga, Ge, etc.) are enriched in many of these meteorites. The irons often contain several . percent silicate inclusions. Mason (ref. 16) provides a good introduction to the meteorites. Wasson (ref. 17) has reviewed and summarized recent work.

Two lines of direct evidence define the range of materials available in near-Earth space. First, the meteorites provide direct samples of such material. Wetherill (ref. 6) argues persuasively that most of the meteorite specimens recovered at the Earth's surface must derive from Apollo-Amor group asteroids. Certain select main-belt asteroids may also be supplying a component of the meteorite flux (refs. 18-20). The pre-encounter orbits of at least three recovered meteorites (Pribram and Lost City, refs. 21-23 and Innisfree, 1977) are consistent with the orbits of members of the Apollo population.

A second line of direct evidence involves mineralogical characterizations of Apollo-Amor group asteroids from remotely obtained spectral evidence. Many important minerals exhibit characteristic features in their reflectance spectra which can be used to identify their presence, composition, and abundance from telescopic spectra (refs. 11, 14, and 24-26). Observational data of varying mineralogical significance are available for 12 Earth-approaching or Mars-approaching asteroids. These bodies and types of materials associated with each are listed in table 1. With one exception, these are consistent with mafic-silicate (olivine, pyroxene) or mafic-silicate-metal assemblages. The available spectral and meteorite evidence indicates that the near-Earth population is dominated by ordinary chondritic-type materials.

However, in the observational case, one is dealing with the statistics of small numbers where a selection effect of unknown magnitude is operating (i.e., one tends to preferentially discover and observe brighter objects such as those with the highest albedo in any given size range). Anders (ref. 27) noted that it is possible to supply the ordinary chondrites from a very small number of favorably situated parent bodies. There is an additional bias introduced by atmospheric entry which operates against weak structures, such as would be expected for CI chondrites or cometary debris (ref. 23). Therefore, while it is evident that ordinary chondritic assemblages are present in the near-Earth population, their relative abundance and the character and Abundance of other types of meteoritic materials cannot be ascertained on the basis of available data.


TABLE 1 .- CHARACTERIZATIONS OF APOLLO-AMOR OBJECTS
NumberNameNote a
AU
qaSurface type bAlbedocDiameter, c
km
433Eros1.4581.13Olivine, pyrozene, metal (~H Chon)
(ref. 13)
0.1723
887Alinda2.5161.15Olivine, carbon (~C3)(ref.26).174
1011Laodamia2.3941.56"S"-probably silicate or metal-rich
assemblage
.167
1036Ganymede2.6661.22"S"-probably silicate or metal-rich
assemblage
---(~35)
1566Icarus1.078.19Pyrozene (olivine, metal)(ref.26).171
1580Betulia2.1961.12"C"-opaque-rich assemblage, possibly carbonaceous-6
1620Geographos1.244.83"S"-probably silicate or metal-rich
assemblage
.183
1627Ivar1.8641.12"S"-probably silicate or metal-rich
assemblage
--(~7)
1685Toro1.368.77Pyrozene, olivine (ref. 12).123
1864Daedalus1.461.56"O"-probably silicate or metal-rich asssemblage--(~2)
1960 UA-2.261.05"U" ?----
2062Aten.97.79"S"-probably metal-rich assemblage.171

aPerihelion distance for each object. By definition, Apollo objects have q< 1.0 and Amor objects , 1.00<q<1.38 1011 Ladamia is included even though it only approaches the orbit of Mars.
bWhere adequate spectral data are available, mineralogical characterizations and meteorite equivalents are given. Bracketed numbers indicate the appropriate reference. Where only UBV colors are available, the Chapman-Morrison-Zellner classification of the object is summarized by Zellner and Bowell is given. Underlined classification symbols indicate those based on a single classification criterion. Probable mineral assemblages are indicated.
cAlbedos and diameters as summarized by Morrison (ref.36). The diameters in parenthesis were derived assuming an average albedo for the "C-S" class of the object and should be considered as indicative only.


It is clear, however, that the material-population pattern of the main asteroid belt is not applicable to the Apollo population. Chapman et al. (ref. 28) evolved an asteroid classification scheme that utilizes observational properties (UBV colors, polarization, albedo) which, in modified form, has been used to define general population patterns in the asteroid belt (ref. 3). The types described in this system (C = carbonaceous, S = silicaceous or stony-iron, M = metal-rich, etc.) should not be taken as compositional descriptions. Most main-belt C-type asteroids turn out to be similar in spectral signature to carbonaceous type I and II meteoritic materials, but a significant minority of these C-type asteroids are a quite different assemblage (opaque-rich mafic silicate assemblages) (ref. 26).

However, as a classification scheme of limited compositional significance, the C-S system provides some important information concerning asteroid distributions. The relative abundance of S objects increases to nearly 50 percent at the inner edge of the belt, while the C objects constitute about 76 percent of all main-belt asteroids (ref. 3). Of the 12 objects thus far observed that either cross or approach the Earth-Mars orbits, only one (1580 Betulia) has the low albedo and neutral colors of the C group. It is possible, but by no means certain, that this asteroid is made up of material similar to Cl or C2 meteorites. Higher resolution, broader-wavelength spectral data are needed before the composition of this object can be established.

Source Regions and Mechanisms for Near-Earth Asteroids

The two major candidates for source regions for nearEarth asteroids are the main-belt asteroids and the comets. The major difficulty for an asteroidal source for meteorites (and Apollo asteroids) is to modify the orbit from that of a main-belt object to an Earth-approching object in as short a time as indicated by the cosmic-ray exposure age of the meteorites, or at a rate sufficient to maintain the Apollo population. The problem is less severe for small (~1 m) bodies than for larger bodies. Collisional injection of material from a body adjacent to a Kirkwood Gap into the gap (ref. 19) or into a secular resonance with Jupiter (ref. 19) provides a way to transfer this material into Earth-crossing orbits. Momentum transfer from sunlight by an absorption -re-radiation mechanism (Yarkovsky effect) may transfer small objects (~1 m) into Earth-crossing orbits in relatively short time intervals (~107 yr) and might move larger objects into a resonance with Jupiter where relatively rapid orbital evolution can occur (ref. 29).

Asteroids that approach the orbit of Mars can be subjected to small cumulative perturbations and converted into Earth-crossing objects on a time scale of ~1X109 to 2X109 yr (ref. 30, 31) This rate is apparently too low to replenish the Apollo-Amor group from the available number of Mars-approaching or Mars-crossing bodies (ref. 6) Thus, while material equivalent to most meteorite types (except ordinary chondrites) is present in the asteroid belt and mechanisms for modifying these orbits exist, questions can be raised as to the relative importance of this source.

The capture of comets into the inner solar system by Jupiter is a well-known phenomenon. Wetherill (ref.6, 20) concludes that, on the basis of dynamical considerations (refs. 15, 32), most (>90 percent) of the Apollo-Amor objects are of cometary origin. In this model, to account for the type of assemblages inferred to be present on the Apollos, it is plausible to assume that some portion of the material in cometary orbits represents the return of inner solar-system in material that was expelled by Jupiter during the early post-accretionary period of the solar system. This cometary hypothesis is not unanimously accepted (ref.33).

If we assume that these two hypotheses are both correct and that only the relative rates of each are in doubt, then we can define a reasonable qualitative model of the near-Earth population. The asteroid belt should provide material from objects adjacent to resonance surfaces which include achondrite, stony-iron, and Cl-C2 and C3 type assemblages. The Mars-approaching asteroids may provide ordinary chondritic or stony-iron assemblages based on the relative increase of S-type objects toward smaller semimajor axes. The cometary source could provide ordinary chondritic, achondritic, and stony-iron materials (ejected inner-solar-system material) and C1 material (devolatilized cometary nuclei). A degassed cometary nucleus should retain a significant amount of trapped volatiles in its deep interior (water, carbon dioxide, methane, and ammonia ices) (ref.33). The relative abundance of these types cannot be established until a significant number of Apollo-Amor objects have been discovered and observed with sufficient spectral resolution and wavelength coverage to make mineralogical characterizations.

Physical Characteristics and Mechanical Properties

The mechanical properties of an Apollo-Amor object are determined by its composition and previous history. Most of the meteorite types exhibit a degree of coherency, that is, they tend to have reasonable physical strength and are hard to crush. The irons and stony-irons have the greatest strength. Ordinary chondrites tend to become stronger with increasing metamorphic grade. The C1 chondrites are quite friable (weak or easily disaggregated), the C2 reasonably coherent, and the C3 reasonably hard.

How the characteristics of a small specimen scale up to an object 100 in or larger is somewhat difficult to evaluate. Generally, any large fragment resulting from an impact event is likely to be extensively fractured and weak on a large scale. This is supported by meteorite fall evidence which indicates that most large (e.g., 2 m) bodies break up into many smaller ones (~20 cm) very soon after initial atmospheric contact. Typically, a large fall will consist of hundreds of individual objects, each with its own fusion crust, indicating breakup early in reentry. On a larger scale, doublet impact craters on Mars and Earth are more common than impact statistics would suggest, and this could be attributed to pre-atmospheric breakup under differential gravitational stress near the planet. It seems reasonable to assume that an object 100 m or larger will not withstand significant differential forces or high accelerations.

Devolatilized cometary nuclei are likely to be very porous "fairy-castle" materials with relatively little physical strength. The low density and low physical strength of fireball meteors, generally associated with extinct comets, are evidence of these properties (ref. 23). While there may be relatively dense regions in the interior as a result of weak compressional loading, these objects can generally be considered fragile.

RECOMMENDATIONS FOR GROUND-BASED TELESCOPE SEARCH FOR ASTEROIDS

Introduction

Statistically, it is estimated that there are about 1000 Earth-crossing asteroids larger than 1.0 km, many of which would be of interest for either missions or as the sources of raw material. Yet, as of July 1977, only 24 Apollo-type asteroids (Earth crossers) with well-defined orbits are known. Included in this small number, asteroids 1976 AA, 1976 UA, and 1977 HB are potentially attractive mission objectives. Asteroids 1976 AA and 1976 UA, actually the first of a new orbital class, were discovered in the past year. Besides orbit and delta-V considerations, the physical properties of a particular asteroid (e.g., abundance of volatiles and metals) will play a role in the selection process. Therefore, we need to know about the physical and chemical properties of the Apollo group.

A planned search program, using a 46-cm (184n.) Schmidt telescope, and the subsequent increased interest generated in the astronomical community has doubled in 4 years the number of known Apollo asteroids. It took 40 years to discover the first 12 members. Since it has been estimated that at least 10 times as many Apollo asteroids should be observed to enable a judicious mission/resource selection, then one would estimate that, at the current rate of discovery, approximately 50 years of diligent effort would be required. This problem can be overcome by employing larger telescopes with more observing time. This approach is addressed here.

A goal of an order-of-magnitude increase in known objects in 5 to 10 years - a reasonable period for planning optimum use of present Space Shuttle capabilities - can be achieved by use of a 122-cm (48-in.) Schmidt telescope 10 days/month with appropriate support at other facilities.

Asteroid Detection Probabilities

The probability of detecting an asteroid in a search program depends on the apparent magnitude or brightness of the object, on the limiting magnitude of the search telescope, and on the amount of time committed to search. Figure 3 shows the surface corresponding to an l8th-magnitude detection limit as a function of opposition angle and object size for two reasonable albedos (0.15 and 0.04) and corresponding phase functions (0.025 and 0.038 magnitude/deg). Figure 4 shows the detection distances for a 1-km object with an albedo of 0.15 and a phase function of 0.023 magnitude/deg. The advantage of searching at or near opposition is evident. The disadvantage of a search at a high opposition angle (Sun-object-Earth angle ) is that the volume searched per square degree of plate area decreases substantially with increasing opposition angle. For example, at a limiting 17th magnitude for = 0o - 5.6X 10-5 (AU3 /deg2 ), = 0o - 3.8X10-5, = 50o - 2.1X10-5, = 70o - 1.6X10-5, and = 90o - 1.2X10-5,

The probable discovery rate of Apollo asteroids as a function of telescope size and the characteristics. and distribution of appropriate telescopes are given in table 2. Projected discovery rates increase dramatically with increasing telescope size and limiting magnitude. The 46-cm (18-in.) Palomar Schmidt is a telescope currently committed to a search for planet-crossing asteroids. The search program has led to the discovery of four Apollo asteroids over a 4-year period. Other Apollo asteroid discoveries have been made with large aperture Schmidt telescopes (100-cm (40-in.) and 122-cm (48-in.)) plates taken for other purposes. Observations with the 100-cm (40-in.) ESO Schmidt have concentrated on the Fast-Blue Survey (FBS) of southern skies, extending the Palomar National Geographic Sky Survey to declinations below - 23o . The 122-cm (48-in.) Palomar Schmidt is used for many purposes, but the ongoing monthly program plates that have recorded the Apollo asteroids are the Supernovae Program and the Solar-System Search. Four Apollo asteroids have been incidentally discovered on 122-cm (48-in.) Palomar Schmidt plates. The ESO Schmidt (operated by R. M. West and H. E. Schuster on the FBS) has found one Apollo, but only after several had been lost because the plates were not examined soon enough for followup observations. Too much time had elapsed since the plates were taken, and the fleet object was lost.

TABLE 2 .- DISCOVERY OF RATE OF APOLLO ASTEROIDS VERSUS TELESCOPE
Telescope aperature size, cm (in.)Discovery rate (current)Potential discovery rate by increasing to 10 nights/dark runThreshold magnitude/size for moving objectsTelescopes in U.S.Telescopes outsideAvailability of equipment nights/yrCost of new instruments, $ millions
46 (18)1 per year;
48 nights/year
>215m = 0.5 kma2-50-600.25
50 (20)1 per year;
48 nights/year
>215m5-1-.3
61 (24)1 per year;
48 nights/year
2-315m5-2-.4
81 (32)1 per year;
48 nights/year
3-416m5-110-15 nights/year but not now in operation.9
100 (40)1-2 per year (incidental)>416-17m 0.3 km-1ESO Southern Sky Survey- by-product2.25
105 (42)-5-6--1-2.75
122 (48)1 per year (incidental)>817-17m5 0.2 km12Standard program byproduct3

aSize estimate corresponds to detection limit of discovery telescope.


If one or two moderate-sized Schmidt (apertures 24, 32, and 36 in.) were dedicated to search systematically during a 7-10-day period each dark of the Moon or if sufficient time were allocated on existing Schmidt telescopes (24- to 48-in. aperture), the current yearly discovery rate could be increased 5 to 10 times (in the last 2 years, three or four Apollo asteroids have been discovered each year). Only four of these asteroids were discovered in a search program designed specifically for their detection. The other discoveries were made on plates taken for other purposes. Hence, if one or two telescopes devoted to serious search were available, one could very conservatively estimate an order-of-magnitude increase in discovered objects. If one uses the present known number of Apollo asteroids (24), it is reasonable to estimate that the number could be doubled in a year of concentrated effort. B. G. Marsden (Smithsonian Astrophysical Observatory, Harvard University, personal communication, 1975) pointed out that there should be two Apollo asteroids discovered each month with the 122-cm (48-in.) Palomar Schmidt telescope during dark runs. With this increase in known objects, it is reasonable (based on the discovery of (1943), 1976AA, 1976UA, 1977 HB, and somewhat less suitable, 1975 YA) that 5 to 10 percent of the new discoveries would prove to have orbital characteristics that would qualify them as favorable mission candidates (ref. 35). Asteroid Search Recommendations - Observation

The search efforts must be accelerated to ensure a dramatic increase in the discovery rate of planet-crossing asteroids. The commitment of any instrument for a dedicated asteroid search should include the detection of all classes of asteroids up to the magnitude limit of the search instrument. To optimize such a search, a strong recommendation would be to secure the use of or financial support for the construction of a large Schmidt telescope (I 22-cm (48-in.)). The objective of this search, however, would be to detect objects of moderate to fast motion (1/30 per day, upward) and therefore generally close to the Earth. Using the 46-cm (18in.) Schmidt telescope, in which 1/30 per day motion was used as a lower limit of motion for followup work, has produced 20 discoveries: 4 Apollos, 5 Mars crossers, 3 asteroids in the Phocaea regions, 2 asteroids in the Hungaria region, and 7 others from the edge of the main-belt region. The limiting factor for increasing the discovery numbers is the detection limit of a given telescope. The limiting magnitude of the 18-in. Schmidt is about magnitude 15 for a moving object.

Based on current numbers, one might expect that about 5 to 10 percent of the new objects would be of interest for rendezvous or sample return missions. This is a conservative estimate, based on only the 24 known Apollos. By committing a large-aperture Schmidt for a 10-day/month search, one could optimally expect to discover about 250 Earth crossers in 10 years or about 500 Earth crossers in 20 years.

Proposed System of Search

A search plan should consist of photographing a series of standard fields distributed along the ecliptic and extending to moderate latitudes north of the ecliptic. A 122-cm (48-in.) Schmidt camera plate is 35.5 X 35.5 cm (14 X 14 in.) and is 6.6 degrees in effective diameter. Monthly (each dark run), a series of fields should be photographed clustered around opposition. Two exposures should be made of each standard field, one of 12-min duration and one of 6 min. Typical planet crossing objects can be recognized on the 12-min exposures by the amount of elongation or trailing of their images. The 6-min exposures would be used to confirm the detected objects and to determine their direction of motion. Under good seeing conditions, stars to 21m are recorded in 6-7 min exposures with a variety of plate emulsions (e.g., 103 a-o, a-e). The magnitude limit for detecting moving objects is lower; however, fast-moving planet-crossing asteroids fainter than apparent magnitude 17 to 17.5 would only be marginally discernible.

Exposed plates should be developed immediately and scanned within 24 hr by microscopic examination. Generally, three observer teams would work shifts through an observing run; this intensive effort requires participation by several persons and, if continued through a 10-14-day dark run, would require two or three team shift rotations. Fast-moving objects must be detected promptly and followed for several days or they will be lost. Followup observations should also be obtained at least through the following lunation, if possible, so that the orbits can be computed with sufficient accuracy to recover these objects on their second apparition.

With the immediate goal of an expanded search program to accumulate sufficient observations to derive reliable Orbits, the discovery telescope must continue to follow the new discovery. But it is critical that other observatories better located for followup observation be willing to collaborate on securing needed observations.

Asteroid Search Recommendations - Telescopic and Manpower Support

With a large Schmidt telescope and with adequate support, data necessary for the mission/natural resource program can be obtained within an acceptable time. Table 2 shows the presently available telescopes and projected rate of discovery when applied 10 nights/ month to this program. To produce the data within a feasible period of time (5 to 10 years), time on a 122-cm Schmidt telescope is required. Table 3 shows a cost breakdown for constructing and supporting such a facility.

TABLE 3 .- COST ESTIMATES FOR DEDICATION OF A 122-CM (48-IN.) SCHMIDT
AND ASSOCIATED EQUIPMENT
Estimated cost,
$ millions
Setup cost:
122-cm (48-in.)
3.0
Measuring machine
(14 X 14 in. plate, custom made size)
.04
Automatic blink machine
(14 X 14 in. plate size)
.6
Computer (or buy time)
Establish international clearinghouse to record all astronomical observations and distributing costs
.1
Subtotal3.86
One-year operation cost
Photographic plates (14 X 14 in.) plus chemicals (1-year supply)
0.02
Guest observing charges and reimbursement to other observing facilities.01
Communication (telex, telegram, telephone, etc).005
Travel (U.S. and foreign) for 3-4 people.03
Personnel support
2-3 full-time astronomical observers
.06
3-4 support technical assistants.05
Subtotal (plus overhead)0.175
Total4.035

Once a principal telescope is available, there is a related significant problem, that is, backup and followup by other astronomical facilities. This support is necessary to secure orbits, particularly when weather or other difficulties render the discovery telescope inoperable on subsequent nights or when the motion of the object carries it beyond the observational range of the discovery instrument (e.g., to high negative declinations). With the discovery of relatively large numbers of objects, this could require a significant effort rather than the present somewhat informal effort, often the result of personal contact.

An observational clearinghouse should be established to collect monthly telescope logs from observatories with suitable telescopes. This could be handled simply the monthly plates of each telescope could be reproduced, punched on cards, and collated on printouts and distributed to interested observers. From this information on known existing plates and their positions, it is possible to check for additional observations of a new discovery either before or after discovery. Locating plates that contain needed positions from a review of Palomar logbooks has revealed additional positions of new objects, extending the arc from a few days to a few months. A substantial increase in secure orbits could be obtained at relatively low cost by the coordinated use of such a clearinghouse for long-exposure, wide-angle plates.

To determine surface compositions, it would be most desirable to discover these bodies before opposition and, ideally, with several weeks or months of lead time so that. observation time could be requested on large telescopes. To do this, it would be necessary to search a region near the ecliptic about 90o from the opposition point (forward for objects beyond 1 AU, backward from objects inside 1 AU). This requirement would decrease somewhat the efficiency of a search program. From the point of view of obtaining compositional characterizations of these objects (spectrophotometric metric or radiometric observations), it would seem worthwhile to partition a portion of any general search effort into a survey of the = 0 to = 90o directions. The value of early detection and discovery opposition spectral work is high for interesting objects whose synodic periods are long (>5 years). Failure to achieve a compositional characterization during the discovery opposition means a wait equal to the synodic period before such a study can be made.

ASTEROID MATERIAL CHARACTERIZATIONS - RECOMMENDATIONS

Any reasonable scenario for asteroidal resource utilization will require a spacecraft survey mission to a likely candidate body selected on the basis of a preliminary material characterization from Earth-based studies, Even if nonspecific mass is desired, a characterization will be necessary to define the most appropriate recovery program (e.g., type of processing and mining equipment,type and amount of expendables for system operations, etc.). The available techniques for studying these objects can be divided into ground-based (or near-Earth) and space craft.

Earth-based characterizations must depend on information contained in reflected sunlight (reflectance spectroscopy) or reflected man-made radiation (radar) or in emitted thermal radiation (thermal infrared, thermal microwaves). Reflectance spectroscopy is most sensitive to surface mineralogy, but a certain minimum spectral resolution and wavelength coverage is necessary to obtain such characterizations. A spectrum with 1-percent resolution over the interval 0.3-5 m could provide detailed information (mineral chemistry, abundance, and distribution) on surface materials. In practice, one almost certainly would not be able to obtain such spectral data for the asteroids in near-Earth space. The photon flux from the observed object (magnitude, telescope size, detector sensitivity, the flux onto detector and atmospheric transmission) are the main factors that control the resolution and coverage of spectral data. Low-resolution data (e.g., broad band-pass UBV filters) can be obtained for very faint objects (i.e., 18 magnitude), but are not particularly sensitive to surface composition. High-resolution spectra cannot be made for very faint objects unless longer integration times or lower spectral precision is accepted. Simultaneous detection over a broad spectral region (e.g., dispersion of the spectrum across an array of detector elements - vidicon or CCD spectrometer or interferometer) can alleviate this discrepancy somewhat. Wavelength coverage depends on the additional factors of spectral flux and atmospheric transmission. The spectral flux of almost any object reflecting sunlight peaks in the mid-visible and decreases rapidly toward the UV and IR (e.g., At 1.0-m flux, it is down to a third of the peak flux and at 2.0 m, down to 5 percent) following a blackbody curve for ~6000 K object (Sun). This flux loss must be compensated for by increasing integration times or decreasing resolution. Atmospheric absorptions in the near IR (e.g., H2O vapor at 1.4 and 1.9 m) effectively block these spectral regions for faint objects.

We recommend a multiple approach to ground-based studies of the near-Earth population of asteroids:

  1. A low-resolution, wide-coverage spectral program (UBVRIJHK). These data can provide some indicative compositional information on very faint objects (~17-19 magnitude on large (~100-in. or 2.5 m) telescopes) in an observing period of several days. The IR filters (RIJHK) are essential since they can provide criteria for discriminating metal-rich assemblages from silicate-rich assemblages and may provide a preliminary subdivision of the opaque-rich assemblages.
  2. A moderately high-resolution (~3 percent), wide-coverage (0.3-2.0 m) spectral program. This program can provide detailed mineralogical characterizations for reasonably faint asteroids (~15-16 magnitude) observed with a large telescope (~100 in. or 2.5 m). The instrumentation to make these measurements (vidicon or CCD arrays) are being developed as astronomical instruments.
  3. Measurements of the thermal IR can be used to determine the diameter and albedo of faint asteroids (~16-17 magnitude) on the large telescopes. The albedo provides an additional compositional discriminant.
  4. Radar reflectances of these asteroids can be obtained only when the object is near Earth and when it is in a portion of the sky accessible to the large radio telescopes, such as Arecibo.
  5. Photopolarimetry measurements can be made of reasonably bright objects (~14magnitude). Measurements near opposition which can determine the shape and depth of the negative branch of the phase-polarization curve can provide clues on the structure and optical density of the surface material.
  6. High-resolution spectroscopy (~3 percent) of the thermal infrared should provide indications of silicate chemistry, but previous attempts to apply this to telescopically observed objects have not been successful. Limiting magnitude (~12 magnitude) is probably not faint enough to be of general interest to this program.
  7. The compositional significance of thermal microwaves and the limiting magnitude is unclear.
A program of low- and high-resolution, wide-coverage reflectance spectroscopy coupled with broadband thermal IR radiometry should provide a basis for a reasonably precise characterization of asteroid materials. The high-re solution, wide-coverage reflectance spectroscopy is the essential component of such a characterization.

The design of a spacecraft system to characterize an asteroid or several asteroids depends rather critically on the mission profile. Mission options include single- or multiple-asteroid flybys or rendezvous with either ballistic or low-thrust spacecraft. The asteroid studies that can be done with available spacecraft instrumentation and general mission profiles are discussed separately. Experience derived from the design of a state-of-the-art mission for studying the Moon (CPO) is helpful in a discussion of possible asteroid missions.

Experiments that characterize the "chemical" properties of a planetary surface remotely include reflectance, x-ray, gamma-ray, and alpha-particle spectrometers. A spacecraft reflectance spectrometer can provide detailed mineralogical characterizations of each spatial resolution element on the surface of the asteroid. An x-ray spectrometer detects the x-ray emission lines of lighter elements (Z < 16 and, under favorable conditions, Z 30) that have,been excited by solar x-rays to obtain elemental ratios such as Mg/Si. A gamma-ray spectrometer detects gamma-ray emission from (1) the decay of natural radioactive elements, (2) the decay of radioisotopes induced by galactic cosmic-ray bombardment, and (3) decay of excited nuclei in atoms subjected to galactic cosmic-ray bombardment. A gamma-ray instrument will obtain elemental ratios for the naturally occurring radioisotopes (U, Th, K40) and the abundant elements (e.g., Fe, Mg, Si, O, etc.) in the surface material. A gamma-ray spectrometer can also measure the content of water (H) in the surface material of an asteroid. An alpha-particle spectrometer measures the energy of alpha particles emitted by the decay of daughter products of the U-Th series.

Other experiments that remotely characterize surface properties include thermal infrared and microwave radiometers and radar sounders and altimeters. An infrared radiometer measures the temperature distribution across the asteroid surface, which provides an indication of surface (~1-10cm) conductivity (thermal inertia, texture), while a microcrowave radiometer can characterize the temperature distribution with depth for the top meter or so of the surface material. A radar sounder/altimeter can measure the texture (particle size) and conductivity of the top meter or several meters (depending on wavelength) of the surface material and can measure the shape of the asteroid with precision.

An imaging system is essential both for terminal guidance during approach to the asteroid and in obtaining high-resolution (~1-10 m) photographs of the body. Photographs are important for locating the ground tracks of high-resolution instruments (e.g., reflectance spectrometer, thermal infrared, and radar) to relate their results to surface features. Photographs also provide size and shape information which can be combined with mass determination to obtain bulk density.

Spacecraft tracking during a flyby can provide a mass determination and, during the orbit about an asteroid, can provide a very precise mass determination, moment of inertia, and density distribution measurements that provide information on the internal structure of the asteroid.

In situ measurements can be made by experiments delivered to the surface either by a soft lander or a penetrator. Chemical (alpha-particle backscattering, neutron activation analysis, x-ray fluorescence) and mineralogical analysis (reflectance spectrometer, x-ray diffraction) can be carried out on the surface. Monitoring the deceleration of the penetrator on impact provides a measure of physical strength of materials. A soft lander would be needed to acquire a sample for any sample return mission.

The scientific payload on any asteroid mission will depend on the propulsion systems available and the survey goals. For a ballistic trajectory and given propulsion system (e.g., Shuttle plus interim upper stage (IUS)), the total spacecraft mass launched to an asteroid (spacecraft bus, additional propulsion requirements, scientific payload) will depend on the orbit of the target asteroid. A low-thrust propulsion system (after trajectory insertion) can increase payload capacity or enable multiple asteroid rendezvous missions. An ideal mission (neglecting sample return) would probably involve insertion of the spacecraft into an orbit (or station-keeping location) near the asteroid for an extended survey mission (months). With a low-thrust propulsion system, the spacecraft can be inserted into a new trajectory to a second or third asteroid.

We recommend planning studies to optimize a series of spacecraft missions based on the propulsion systems available in the near term (1978-1985) and far term (post-1985) for various asteroid targets.

References

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