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.

Investigation of extraterrestrial objects is habitually accompanied by the observation of extraterrestrial magnetic fields, which often have significant effects on the solar wind in the vicinity of the objects. Should we be surprised to find this experience repeated at an asteroid?

A recent study considered the conditions that would have to be satisfied by the surface magnetic field of a small planetary body at asteroidal distances so that the field would be an obstacle capable of stopping the solar wind or deflecting it sufficiently to generate a detectable magnetic interaction (Greenstadt, 1971).

A determination of the internal structure and composition of the asteroids will give us important information concerning the origin of the solar system and the formation of the planets. We can obtain this information by measuring the total mass and the internal mass distribution through the use of spacecraft missions to flyby or to rendezvous, orbit, and land on the asteroids. The Doppler tracking technique used for mass measurement on planetary probes is suitable only for a flyby mission of the larger asteroids (> 20 km) because the gravity force field of the smaller asteroids is not strong enough to appreciably affect the trajectory of the probe during a flyby. If a rendezvous mission is used and the spacecraft is placed in orbit about an asteroid, the mass can be determined from the orbital period, but the effect on the spacecraft orbit due to the mass anomalies under the surface will not be easily seen unless the anomaly is very large because again the gravity force field decreases rapidly with decrease in anomaly size.

The present lack of exact information about the distributions of asteroids and asteroidal meteoroids causes the largest uncertainty in the description of the interplanetary meteoroid environment between the orbits of Mars and Jupiter. Observed asteroids are inferred to have diameters from a few kilometers to a few hundred kilometers. The presence of these larger bodies suggests the presence of smaller, unobservable bodies. When asteroids collide, fragments are produced that eventually collide with other fragments. Because of this continuous collision process, much smaller asteroids most probably exist. Such asteroidal meteoroids, if present in sufficient number, could pose considerable danger to spacecraft.

A NASA Langley Research Center meteoroid detection experiment will be performed on both the Pioneer F and G missions. The objective of this experiment is to obtain data that will indicate the population of meteoroids in the 10-9 to 10-8 g mass range in interplanetary space and, in particular, the region of the asteroid belt, and establish a first indication of the meteoroid penetration hazard to spacecraft in the asteroid belt. Specifically, the experiment will detect meteoroid penetrations of stainless steel targets 25 and 50 µm (1 and 2 mils) thick as the Pioneer spacecraft travel in interplanetary space through the asteroid belt to Jupiter and beyond.

Astronomical measurements of the brightness, color, and polarization of light from solar system bodies at phases not visible from Earth are planned for Pioneers F and G. An imaging photopolarimeter (IPP) in the scientific payloads will make these measurements and produce line-scan color images of Jupiter. The IPP collects light with a 2.5 cm aperture telescope that is able to turn in the plane containing the spin axis. Data are taken in scans around the spin axis along a cone whose apex angle is periodically changed by stepping the telescope. The light is analyzed for polarization, divided into blue and red components, and detected by four photomultipliers having electron channel multipliers. Appendix A gives a summary of instrument characteristics.

One of the most extensive programs of systematic observations of minor planets currently being conducted is that at the Crimean Astrophysical Observatory. The observed positions, together with the residuals from the predictions in the annual Ephemeris volumes, have been listed in the Minor Planet Circulars for several years now; and inspection of the residuals yields information about the accuracy of minor planet ephemerides as a whole.

The observations are made using a 40 cmf/4 double astrograph, the limiting magnitude being about 18. Between June 1968 and October 1969, corresponding to one synodic period of an average minor planet, about 40 percent of all the numbered objects were observed. These observations may be regarded as reasonably representative of all the planets. Observations were made of 60 of the objects with numbers in the 100’s and only 29 of those in the 1100’s, but in general the distribution was surprisingly uniform.

Close-approach asteroids, members of the so-called Apollo and Amor groups, are of considerable current interest as potential targets for probes and also in connection with the question of identification of the parent bodies of the meteorites. The possibility that some of these asteroids may be surviving comet nuclei has been suggested earlier. (See, e.g., Öpik, 1963.) Relatively few objects of this type are known; all are small bodies found accidentally in the course of work not always related to investigations of minor planets. The known Apollo and Amor asteroids, and notes as to their present observational status, are listed in table I. (See also table II of Marsden.) Except when they are relatively close to Earth (many can approach within 0.10 AU), these objects are faint and often in very awkward positions as well, low in the evening or morning sky at twilight.

On October 7, 8, and 9, 1970, Charles Kowal, in search of fast-moving asteroids, exposed three photographic plates of an area of the sky near the ecliptic with the 122 cm Schmidt reflector of the Hale Observatories. Several weeks later Eleanor Helin of the California Institute of Technology examined the plates under a blink microscope and discovered a fast-moving object of about 15 mag. During the 15 min exposures the object had left a slightly bumpy trail indicating light variations with a period of a few minutes.

Four positions of the object were reported to the author, who derived the heliocentric orbit defined in table I. From table II it is seen that the residuals of that orbit are not very satisfactory. Note that the third and the fourth position represent the end points of the same trail. There is a clear indication that after October 9, the predicted heliocentric positions would soon diverge from the actual positions. This circumstance, and the fact that the orbit is so similar to that of Earth, suggested that the object could actually be moving in an Earth-centered orbit.

In the past decade the question has been raised, at IAU meetings, as to whether more asteroids should be discovered, and the answer now is: “Yes, indeed, do discover as many comets and asteroids as possible.” Marsden gives strong encouragement to search for “lost” comets and asteroids. The more ephemerides known the better physical and statistical studies we can make. We have completion of the asteroids now to about 14 mag. With existing patrol instruments of about 25 cm opening, for instance at Indiana and in South Africa, the limiting magnitude for an extended program is about 16.