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TABLE II.-Mass Estimates
System Mass, kg
SECOND GENERATION (1,6 - 2500-2750 s)
Hg ProPELLANT STORAGE
SECOND GENERATION tprESSURE REG.)
ROLLOUT SOLAR ARRAY
CENTRAL COMPUTER AND SEQUENCER
SECOND GENERATION iDATA Bus SYSTEM)
HiGH POwtR TRANSMITTER
Figure 9.-Technology status of critical subsystems as of January 1971.
improvements obtainable from second generation subsystems will add performance gains that are, however, not critical to mission accomplishment. As seen in the chart, the improved subsystems are well along in their development toward flight application.
Preliminary analysis and conceptual design study of a solar electric bus vehicle for an Eros sample-return mission show that no major obstacle exists today in terms of technical feasibility, design approach, and operational concepts to early adoption of a program aimed at exploring nearby asteroids such as Eros and returning soil samples. Solar electric propulsion provides basic advantages in payload capacity, mission flexibility, and operational convenience needed to make such a mission more cost effective, more reliable, and more exciting from a scientific exploration standpoint. However, more detailed study of vehicle design, mission implementation, mission timing, performance tradeoffs, and cost factors are required to further substantiate these predictions. It appears that even with an early start of such a program it would not be realistic to expect to meet a launch date prior to the 1977 opportunity. Subsequent launch opportunities for Eros sample-return missions occur about every 2 yr. These opportunities as well as missions to other nearby asteroids require further study.
Alfvén, H., and Arrhenius, G. 1970a, Structure and Evolutionary History of the Solar
POTENTIALS OF ASTEROID SPACE MISSIONS
There is an important relation between the development of solar electric spacecraft and the possible inclusion within the space program of new missions devoted primarily to asteroid investigations. The notion that space missions to asteroids have a high potential to return critically needed data will be illustrated in some detail.
One might wonder why there is so much material on solar electric spacecraft at an asteroid colloquium. For 2 yr, independent teams have been working on feasibility studies and preliminary plans for solar electric spacecraft. For the studies to be realistic, it is necessary to consider the missions on which these hypothetical spacecraft might be flown, and this is discussed, for example, in the previous papers in this session. In addition to missions, the scientific objectives must also be considered to bring into evidence the varieties of data to be acquired and thereby define the science instrument payload in some detail. All this activity has produced several kinds of results. We now have an understanding of technique and possible science return for several kinds of asteroid missions. The space science planners find a growing awareness that the asteroid exploration may provide a rich storehouse of clues as to the origin of the solar system. There is now a real possibility that some of these missions will become a reality. The present studies represent the final stage in the 10 yr technological development of solar electric spacecraft. (See Stuhlinger, in this volume.") The next step, if this 10 yr history, representing a sizable investment in technology, is to continue, is to put this technical knowledge into practice. If this happens, and if at the same time interest within what might be called the asteroid sector of the scientific community is sufficiently high, solar electric spacecraft will be built and asteroid missions will be among those on which they will be flown. In spite of the vast amount of facts known, it is not hard to find critically important knowledge gaps, weaknesses, and ambiguities that seriously hinder theoretical progress. The following is a partial list of knowledge gaps and
insecure facts that could be cleared up by a multiple asteroid flyby mission, such as that discussed by Brooks and Hampshire:”
(1) Asteroid size, shape, albedo, distribution of surface reflectance, phase function for angles up to 90°, surface composition by reflectance spectroscopy (McCord, Adams, and Johnson, 1970; Chapman, Johnson, and McCord”) for selected bodies down to absolute magnitude 14.5, surface temperature (in support of the work of Allen"), and mass of larger asteroids (Anderson*)
(2) The existence and nature of the small-body population, down to micrometeoroids, including orbital characteristics, cumulative space density including possible fine structures such as jetstreams (many papers on this latter subject are presented in this colloquium), the population index, and possibly the particle mass density and composition
(3) Facts relating to the space distribution of the particles giving rise to the zodiacal light
I should note that Pioneers F and G, bound for Jupiter in 1972 and 1973, will be the first spacecraft to gather data in the asteroid belt. I should like now to go into more detail regarding how the objectives previously mentioned might be achieved, illustrating this particularly with the use of television. I have made no attempt to optimize television equipment; I am merely examining what the planned Mariner Venus-Mercury television system could accomplish on an asteroid mission. Figure l is a composite of two graphs. The inner rectangle has been adapted from a portion of figure 6 in the report of the Palomar-Leiden survey of faint minor planets (van Houten et al., 1970). In the plot of the log of the number per half-magnitude interval against absolute magnitude of the asteroids in the distance group 2.6 to 3.0 AU, the circles are the Palomar-Leiden results and the x's are the older McDonald survey results. Note particularly the break in the curve from about 11 to a little over 12 in magnitude. The outer rectangle is miss distance b versus diameter d, both in kilometers. The abscissa of the two graphs are matched, assuming an asteroid albedo of 0.16. The symbol appearing in the lower center is not the letter L, but is an indicator of how far the inner graph would move to the right and the diagonal lines would move upward if the albedo were 0.10, a more favorable situation. The diagonals represent lines of constant angular disk size corresponding to asteroid diameters and miss distance. The camera and optics chosen for this discussion have a field of view of 1:1 by 1:4, and the format is a frame with 700 by 832 pixel, or image, elements. A reasonable accuracy requires 10 pixel elements. In the number pairs on the diagonal lines, the first
is the number of pixel elements across the diameter, and the second is the apparent magnitude as seen by the camera. Thus, the practical limit of 10 pixel elements corresponds to an apparent magnitude of -2.89. The horizontal lines concern the passage of the asteroid past the spacecraft: The first number in the number pair is the velocity in kilometers per second and the second number is the angular rate in degrees per minute. The television camera and other equipment can be mounted on a scan platform that is controlled by a star tracker. The practical upper limit for the scan rate of the platform and cameras for accurate pointing lies somewhere between these two lines. A study of figure 1 shows that it is possible to make adequate disk measurements of asteroids lying on both sides of the interesting break in the distribution curve. Judging by the magnitudes in this range of usefulness, it seems entirely possible to obtain reflectance spectra of the type that Chapman and McCord get for asteroids in this range. The same is true of other optical measurements, including infrared radiometry and zodiacal light photopolarimetry. The small-particle sensors are discussed by Kinard and O'Neal" and Soberman, Neste, and Petty” for Pioneers F and G. On an asteroid mission, these may be considerably enlarged. There are also other types of small-particle sensors such as the one described by Berg and Richardson (1969), which is an impact detector to determine velocity, energy, and direction. There are