Application to 433 Eros

Our success in suggesting an explanation for Geographos has led us to consider the next most elongated asteroid, S-class asteroid 433 Eros, the target of the NEAR mission. Eros has many of the same distinguishing characteristics as Geographos (and our B- and M-class remnant rubble piles). Visual and radar observations taken during a 0.15 AU pass near Earth in 1975 report that Eros has a short rotation period (5.27 hours) and a highly elongated shape ( 36 x 15 x 13 km; 2.77 x 1.2 x 1.0, normalized; ellipticity $\varepsilon_{{\rm rem}} = 0.61$) ([Zellner 1976,McFadden et al., 1989,Mitchell et al., 1998]). Both values are comparable to those recorded for Geographos and with 15% (30 out of 195) of our S-, B-, and M-class disruption cases.

Even more intriguing, however, is Eros's pole-on silhouette, which, after modeling the older Goldstone radar data, looks something like a kidney bean (Fig. 5) ([Mitchell et al., 1998]). One must be careful not to overinterpret this shape, since it is based on data that has a signal to noise ratio of $\sim 70$ while the shape has been ``fit'' to a reference ellipsoid which can eliminate discriminating features. In fact, the concave side of the ``kidney bean'' shape may not be a single concavity, but several adjacent ones. Still, we believe it plausible that Eros's arched back and tapered ends are analogous to similar features on Geographos, themselves produced by spiral deformations associated with tidal forces. Images from the NEAR spacecraft should readily resolve this issue.

fig5

The NEAR spacecraft will offer several additional ways to test our hypothesis. Regardless of whether Eros is covered by regolith or bare rock, spectroscopic measurements will suggest a surface composition which can be directly compared to terrestrial rock samples. If the densities of these samples are substantially larger than Eros' bulk density, we can infer that Eros is probably a rubble pile. While observations of large craters would support the rubble pile scenario, too many would weigh against the tidal disruption scenario; global landslides caused by a relatively recent tidal disruption event should modify or bury craters. For this reason, we expect most tidally distorted objects to have relatively young and spectroscopically uniform surfaces. As we will describe below, however, the unknown dynamical history of Eros makes any prediction problematic. Landslides also sort debris as it goes downhill; high resolution images near the ends of Eros may not only show cusp-like features but a prevalence of small fragments. An estimate of the spatial distribution of block sizes inside Eros may come from NEAR's gravity field maps. Finally, the results of Bottke and Melosh (1996) and Richardson et al., (1998) show that asteroids affected by tides may often have small satellite companions which were torn from the original body. Thus, the presence of a small moon about Eros would be a strong indication that it had undergone tidal fission.

A possible problem, dynamically-speaking, is that Eros is currently an Amor asteroid on a solely Mars-crossing orbit (a = 1.46 AU, e= 0.22, $i= 10.8^\circ$). Test results show that tidal disruption events occur relatively infrequently near Mars, since it is a weak perturber (Bottke and Melosh 1996). Studies of Eros's orbital evolution, however, suggest that it may have been on a low-inclination, deeply Earth-crossing orbit in the past ([Michel et al., 1998]). Numerical integrations of Eros-type clones show that secular resonances $\nu_4$ and $\nu_{16}$ probably modified Eros's orbital parameters, decreasing its eccentricity enough to place it out of reach of the Earth, while increasing Eros's inclination to its current value ([Michel et al., 1996,Michel 1997,Michel et al., 1998]). If true, Eros would have been prone to low velocity Earth encounters (and tidal disruption) in some past epoch.