GASPRA AND IDA: IMPLICATIONS OF SPACECRAFT RECONNAISSANCE FOR NEO ISSUES

10 February 1997, revised 17 April 1997. Published in slightly revised form from that shown here (July 1997) in: Near-Earth Objects: The United Nations International Conference Edited by J.L. Remo, Annals of the New York Acad. Sci., 822, 227-235.

Introduction

On October 29, 1991, the Galileo spacecraft flew past the 18 x 9 km main-belt asteroid 951 Gaspra, and sent a stream of images and other data back to Earth. It was the first time that an asteroid had ever been seen "up close." For most of the nearly two centuries since the first asteroid was discovered, these small, distant objects had been nothing more than points-of-light even in our largest telescopes. During the decade before Galileo's encounter with Gaspra, groundbased radar had begun to give us delay-doppler maps of some small Earth-approaching asteroids, but they revealed little more than the objects' gross shapes. Even so, Gaspra is a considerably larger asteroid than most Near Earth Object's (NEO's), so it was a striking revelation to suddenly see images showing not only its extremely angular shape but an array of craters and other geological features on its surface.

Nearly two years later, on August 28, 1993, Galileo obtained even higher resolution images of the still-larger main-belt asteroid 243 Ida. In the years to come, we may expect analogous images to be obtained for some asteroids from the recently refurbished Arecibo radar observatory. We expect even more detailed data from 433 Eros (imaging, geochemical, and much more), which will be orbited by the Near Earth Asteroid Rendezvous (NEAR) mission in calendar year 1999. (NEAR will make a Gaspra-like fly- by reconnaissance of 253 Mathilde in late June 1997.) Additional asteroid missions are planned by several countries. However, until these new missions are developed, launched, and return data, our knowledge of the physical nature of asteroids -- a population already known from astronomical data to be extraordinarily diverse -- will be based largely on Galileo's observations of Gaspra and Ida.

The purpose of this short paper is not to repeat the summaries and detailed research papers describing these two asteroids that are already published in the widely available literature. Most of the detailed research results may be found in special issues of Icarus dealing with each asteroid: for Gaspra see Icarus, Vol. 107 (1), 1994; for Ida see Icarus, Vol. 120 (1), 1996. The images are all available over the Internet and in other ways from NASA's Planetary Data System.

Physical Traits and NEO Issues

In applying our knowledge about Gaspra and Ida to the NEO population, we must consider several different types of issues concerning NEO's. While many NEO's may be comets, the majority are believed to be fragments from the main asteroid belt. Hence the scientific investigation of NEO's, especially by spacecraft, can consider relevant attributes of Gaspra and Ida to be possibly pertinent to studies of some NEO's. In addition, NEO's are of particular interest as potential way stations for astronauts in a Mars exploration program, as sources of raw materials for deep space operations, and as potentially threatening bodies (if one were to be found to be on possible Earth-impact trajectory, we would want to know about the physical nature of such objects in order to better characterize the threatening body in order to plan effective mitigation).

Evaluation of the cosmic impact hazard depends, of course, on our perceptions of the nature of the threatening projectiles. The question of how hazardous asteroids and comets might be has evolved in recent decades due to new perceptions about how many projectiles (and resulting impacts) there are as well as about how serious the environmental consequences of impacts are. Because, to first order, the environmental consequences of an impact depend on the kinetic energy of the impactor rather than on the specifics of its physical nature, much of the discussion of comets and asteroids has, appropriately, centered on their numbers: how many are there of various masses and what are their orbits?

Nevertheless, a closer examination of the hazard issue reveals significant reasons for considering the physical natures of the threatening bodies. Most obviously, any attempt to deflect or destroy such a body may depend critically on its physical nature. For example, depending on such properties as strength and cohesiveness, one might not dare to give the object a single impulse (for purposes of deflection) if it might, instead, break the body up into a potentially more dangerous rain of fragments.

But even evaluation of the hazard itself is affected by the nature of the bodies. Small bodies have a very wide variety of material compositions (ranging from "snow" to iron-nickel alloy); furthermore, they probably vary widely in cohesion (ranging from solid objects to rubble piles bound together only by gravity). There are other possibly relevant physical parameters, as well, including shape (ranging from spherical to markedly asymmetric) and spin (fast or slow, principal axis rotation or tumbling). These can all have second-order (or occasionally first-order) effects on the consequences of an impact.

As I discuss below, Galileo's studies of Gaspra and Ida tell us, in some detail, about these particular bodies. In a more philosophical sense, Galileo's explorations -- particularly the unexpected discovery of Ida's satellite Dactyl -- also remind us of how little we really know about asteroids and comets. Since Gaspra and Ida are both S-type asteroids, we can readily imagine that other types of asteroids (and comets!) might be very different.

Let me amplify on the presence of Dactyl. It is one concrete example that asteroids and comets are not necessarily solitary bodies. There is, in fact, much observational and theoretical work suggesting that small asteroids may commonly be multi-component bodies held together only by gravity (i.e. rubble piles). The main element of the theoretical arguments (supported by experimental results scaled up to the asteroid case as well as hydrocode modelling) is that it takes far more collisional energy to disperse fragments from an impact than it takes simply to fracture and break-up the body. Indeed, some asteroids are known to be immersed in "dust" bands (discovered by the IRAS satellite) and some comets have been observed by groundbased radar to be immersed in debris trails composed of macroscopic objects (numerous particles that are large compared with the multi-cm wavelength of the radar). As I explain elsewhere (Chapman, 1997), groundbased astronomers are nearly blind to the presence of objects in the meter to kilometer size range, so we have little knowledge of how common they are. So-called dust bands could just be the small-particle component of an invisible population -- dominating the mass -- of much bigger objects, just as for Saturn's rings. The upper portion of this size range includes objects large enough to penetrate the Earth's atmosphere and produce severe local or regional damage.

Even if dangerous accompanying objects of these sizes are uncommon, the break up of Comet Shoemaker-Levy 9 reminds us that bodies which are, in fact, very weak (or are loose, gravitationally bound aggregates) can be tidally disrupted by close passages to larger objects, like Jupiter, the Earth, or the Sun. A spray of large objects -- whether generated by natural processes or by attempts to deflect a threatening solid object -- could be very damaging by distributing effects over a broader region of the Earth.

Studies of an asteroid also help us define the distinctions between observations of the surficial layer (optical surface, or somewhat deeper layers when investigated at long wavelengths) and the properties of the whole body. While surface properties are most readily characterized (and may be relevant, for example, for attempts to interact with the asteroid in in situ exploration, resource acquisition, or certain kinds of mitigation measures), the bulk of the body will dominate the consequences of an impact on the Earth as well as some kinds of mining or threat-mitigation measures. Some direct measurements (of the gravity field, for example) and indirect inferences from the geology of the surface tell us about the interiors of a small body. Galileo's observations of Ida and Gaspra provide clues about these issues. Even if the data fail to resolve them, they highlight the kinds of evidence that may be important to obtain in future exploration of asteroids and comets.

Gaspra and Ida

It is important to emphasize that virtually all of our rigorous knowledge about the natures of Gaspra and Ida pertain to their optical surfaces. Studies of Dactyl's motion have provided a crude estimate for the bulk density of Ida, but no instrument probed these objects more deeply than their superficial thermal skin depths. Nevertheless, photogeologists have honed their skills on the Earth (where there is the opportunity for ground truth reality checks), the Moon, and other planets. By interpretation of morphological surface features, some of which "dig" into the interior, combined with spatial correlations with compositionally-relevant remote-sensing data (e.g. infrared absorption bands indicative of mineralogy), geologists can make significant inferences about regolith depth, interior configuration, and other matters that are important to future exploration, mining, and hazard mitigation of asteroids.

Such inferences can be more powerful if data are obtained that are more comprehensive than were possible from the brief, serendipitous fly-bys of Gaspra and Ida. Nevertheless, it is interesting to consider some of the inferences made from the Galileo data.

Gaspra

Distant images of Gaspra show it, from some perspectives, to resemble a peanut-like object, as though composed of two objects stuck together with the contacts subsequently smoothed out. Despite Gaspra's generally lumpy appearance from various perspectives, the last several images (at highest resolution) portray a very angular, misshapen body, whose shape is prominently formed by planar elements (called "facets") connected by ridges. It remains possible that the narrow "end" of Gaspra is, indeed, a second component of the body, but the impression is stronger (at least to many members of the Galileo Imaging Team, cf. Thomas et al., 1994) that Gaspra is a monolithic object whose present shape has been formed by exogenic spallation or "chipping away" from its original form.

Following receipt of images from Ida, which show Ida to look very different from Gaspra in some important respects, I have become increasingly convinced that Gaspra is probably a strong, metallic object. (See Chapman, 1996, and Chapman et al., 1996, for brief earlier discussions of this idea.) Several independent pieces of evidence lend credence to this idea. First, it has long been known that Gaspra is unusual among S-type asteroids in having a reflectance spectrum that is dominated by olivine rather than pyroxene. This means that it probably is not chondritic in composition but, rather, is of geochemically differentiated composition. A portion of the interior of a large, differentiated precursor body, near the core-mantle interface, would plausibly exhibit a reflection spectrum like Gaspra's. So it is easy to draw an analogy between Gaspra and olivine-rich stony-iron meteorites (pallasites).

Another feature of Gaspra is its very low crater density compared with Ida. Since Gaspra and Ida are expected to be in roughly the same collisional environment, due to the appreciable eccentricities of asteroid orbits, one would ordinarily expect the crater populations to be similar. One possibility is that Gaspra's surface is, by chance, rather young; that is, it may be that a large impact event obliterated much of Gaspra's pre- existing impact record comparatively recently, and Gaspra's surface has not had time to accumulate a saturation density of subsequent craters. An alternative explanation is that Gaspra's surface is very strong (e.g. metallic) and thus a given projectile forms craters that are much smaller on its surface than on the surface of a rocky body.

Taken together, this combination of attributes of Gaspra -- its monolithic shape, its low crater density, and its apparently differentiated composition, plus the fact that Gaspra differs from Ida -- all suggest to me that Gaspra is a strong, essentially metallic object. Since there are alternative explanations for all of these attributes, my conclusion may be wrong. However, the possibility that an object as large as Gaspra might have this kind of physical make-up is sufficient to consider Gaspra as an example of one end of the spectrum of kinds of bodies that might potentially impact the Earth. Unlike bodies of weaker strength, monolithic metallic bodies of any size can penetrate the Earth's atmosphere and cause damage. Gaspra represents a starting point for modelling how mitigation measures might deal with objects like Gaspra, or with smaller versions of Gaspra.

Ida

Ida is a larger body than Gaspra and, in some ways, even more misshapen, with maximum and minimum dimensions of 56 x 15 km. Ida is not so angular and faceted as Gaspra, however, and Thomas et al. (1996) conclude that it is made predominantly of two different pieces. The general configuration of Ida can be described as two lobes separated by a "waist". The surface topography is notably different on the two portions. For instance, although both portions are saturated with small and moderate sized craters, one portion appears to have been heavily bombarded even by the rarer largest projectiles whereas the other portion lacks very large craters; a younger surface age for that portion could explain why it has not been bombarded so heavily.

It is uncertain whether Ida's gross shape and dichotomous geology reflects formation of Ida from two different objects or, instead, reflects inhomogeneities inherited from a precursor. Ida appears to be a coarse "rubble pile". I emphasize "appears" because, in fact, Ida's internal make-up remains conjectural. Ida has a deeper regolith than Gaspra (estimated at 50-100 m by Sullivan et al., 1996), and emplacement mechanisms have been modelled and described (Geissler et al., 1996). How the regolith grades into a coarse megaregolith or rubble-pile internal structure is difficult to assess from images of the exterior. The features discussed by Thomas and others in the special Ida issue of Icarus are suggestive of a multi-component internal structure, but do not prove it conclusively.

Indeed, Asphaug et al. (1996) propose a possible mechanism for the formation of grooves, observed on one part of Ida, that would preclude Ida's internal structure from being a classical rubble pile. Asphaug et al. note that the cracked or grooved terrain is roughly on the opposite side of Ida from a peculiar large feature called Vienna Regio. If Vienna Regio were caused by a large impact, then fractures in the opposing Pola Regio could result, according to hydrocode simulations, but only if elastic stresses could be propagated through the interior of the body, which seems unlikely for a rubble-pile.

Ida's low density implies that it has appreciable void space, since none of the possible mineralogical assemblages inferred from its reflectance spectrum have densities so low as Ida. But, within error bars, the void space could be small enough that a rubble pile structure would not necessarily be implied.

Unlike Gaspra, Ida appears to be composed of minerals like the ordinary chondrite meteorites, which have physical properties similar to ordinary terrestrial rocks. Thus Ida's strength is probably like rock -- whether solid rock or, as preferred, broken-up rubblized rock, overlain with a rocky soil (regolith) up to 100 m deep.

Conclusions

Following the Galileo encounters, we have far better ideas about the physical properties of asteroids than we had before...certainly better than "spherical cow" models from the epoch when we had astronomical observations only. Asteroids are clearly very irregular in shape, perhaps reflecting inhomogeneities in their interiors. Although it is not conclusive, these first two asteroids observed close up probably have a very wide range of bulk physical properties -- monolithic fragment with the properties of steel (Gaspra) or a multi-component, gravitationally bound assemblage of rocky materials, overlain with up to 100 meters of soil (Ida).

Because of its presumed strength, a small version of Gaspra would not be expected to break up by tidal disruption or stresses during atmospheric penetration. Such bodies would be very damaging in the vicinity of their impact point, but would not be as efficient at widely distributed damage as a body that might break up before or during entry and cause widely dispersed damage. Obviously, if the object were larger than a couple of kilometers in diameter, then worldwide effects would be expected from the great volume of ejecta alone (Chapman and Morrison, 1994; Morrison et al., 1994).

Ida, on the other hand, reminds us of the widespread evidence that many asteroids are probably rocky rubble piles. Because Ida is larger than any known Earth-approaching object, it is important to recognize that smaller rocky rubble piles may be different from Ida. A variety of studies, including radar observations, have indicated that small rubble piles may be common among Earth-approaching objects. In particular, it is expected that bodies a few km in diameter and smaller will tend to lack surface layers of soil or regolith. Such objects stand some chance of breaking up prior to entry and small ones (like the object that caused Tunguska) may explode entirely in the lower atmosphere, unlike a metallic object.

While Galileo studied main-belt asteroids, appreciably larger than the vast majority of potential Earth impactors, it has nevertheless provided a reality check to our conceptions of asteroids that were previously based on unresolved telescopic studies with a healthy dose of theoretical extrapolation. Forthcoming missions, some directed at actual near-Earth objects (both asteroids and comets, some missions in planning stages with others like the Near Earth Asteroid Rendezvous mission already launched) should greatly augment our understanding of various small bodies. Because of the diversity of small bodies, however, it will always be essential to perform detailed physical observations of any particular body slated for exploration, mining, or deflection, prior to undertaking such an interactive mission.

Acknowledgements

I thank David Morrison and Alan Harris for discussions, the Galileo Project and Galileo Imaging Team for accomplishing these encounters and interpretations, and John Remo for encouraging me to complete this manuscript.

References

Asphaug E., Moore J.M., Morrison D., Benz W., Nolan M.C., and Sullivan R.J. (1996). Mechanical and geological effects of impact cratering on Ida. Icarus 120, 158-184.

Chapman, C.R. and D. Morrison (1994). Impacts on the Earth by asteroids and comets: assessing the hazard. Nature 367, 33-40.

Chapman C.R., Ryan E.V., Merline W.J., Neukum G., Wagner R., Thomas P.C., Veverka J., and Sullivan R.J. (1996). Cratering on Ida. Icarus 120, 77-86.

Chapman, C.R. (1996). S-type asteroids, ordinary chondrites, and space weathering: the evidence from Galileo's fly-bys of Gaspra and Ida. Meteoritics & Plan. Sci. 31, 699-726.

Chapman, C.R. (1997). Asteroids: relationships with other small bodies. Presented at "Asteroids, Comets, Meteors 1996". Submitted to Plan. Spa. Sci.

Geissler P., Petit J-M., Durda D.D., Greenberg R., Bottke W., and Nolan M. (1996) Erosion and ejecta reaccretion on 243 Ida and its moon. Icarus 120, 140- 157.

D. Morrison, C.R. Chapman, P. Slovic 1994. The impact hazard. In Hazards Due to Comets & Asteroids (Ed. T. Gehrels, Tucson: Univ. of Ariz. Press), 59-91.

Sullivan R., Greeley R., Pappalardo R., Asphaug E., Moore J.M., Morrison D., Belton M.J.S., Carr M., Chapman C.R., Geissler P., Greenberg R., Granahan J., Head J.R. III, Kirk R., McEwen A., Lee P., Thomas P.C., and Veverka J. (1996) Geology of 243 Ida. Icarus 120, 119-139.

Thomas P.C., Veverka J., Simonelli D., Helfenstein P., Carcich B., Belton M.J.S., Davies M.E., and Chapman C. (1994) The shape of Gaspra. Icarus 107, 23-36.

Thomas P.C., Belton M.J.S., Carcich B., Chapman C.R., Davies M.E., Sullivan R., and Veverka J. (1996) The shape of Ida. Icarus 120, 20-32.

FIGURES NOT REPRODUCED HERE

Figure 1 caption. Galileo images of Ida and Gaspra are shown with the objects in approximately correct relative sizes.

Figure 2 caption. This portrait of Ida and its small moon, Dactyl, was taken by the solid state imaging system of the Galileo spacecraft during its encounter on 28 August 1993.

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