more binary asteroids: Sylvia
and 1998 WW_31
by Germano D'Abramo and Andrea Boattini - Copyright Tumbling Stone 2001
In the last few months two more binary systems have been discovered. Astronomers M. E. Brown and J. L. Margot, at California Institute of Technology, reported the discovery of a satellite to asteroid (87) Sylvia. The images were taken on Feb. 18, 2001 using the adaptive optics system on the 10-meter Keck II telescope in Hawaii. The brightness ratio between (87) Sylvia and its moon was measured to be of the order of 450, suggesting a ratio of radii of about 20:1. It is also estimated that the moon completes an orbit once every four days.
The latest discovery of a binary asteroid was reported by astronomer C. Veillet, at Canada-France-Hawaii Telescope (CFHT), on Apr. 16, 2001 (see IAUC 7610). The recovery images of the transneptunian object TNO (dict.) 1998 WW_31, taken by Veillet, A. Doressoundiram and J. Shapiro with the 3.6-meter CHFT, showed the presence of two close, but distinguishable objects moving together over the two nights of observation (Dec. 22 and 23, 2000). With the help of archival images of 1998 WW_31, taken nearly a year before, again at CHFT, it was possible to secure its binary nature. Hence, 1998 WW_31 is the second TNO (after Pluto) recognized to have a satellite. While the ratio of radii of the pair Pluto/Charon is 2:1, 1998 WW_31 is believed to be essentially a double object with comparable components: a preliminary analysis of Dec. 22, 2000 image shows that the two component differ of only 0.4 magnitudes (dict.) (R band) between each other.
Asteroids shapes as
a result of collisions
by Germano D'Abramo * - Copyright Tumbling Stone 2001
It is now well-established that mutual collisions have played and still play a fundamental role in the evolution of the whole asteroidal populations. During the first stages of the evolution of the Solar System, the debris constituting the primordial disk collide at very low relative velocity, allowing the accretion of the first small planetary bodies, called "planetesimals". Nowadays due to higher eccentricities and inclinations as a result of their dynamical evolution, asteroids collide at mean relative velocity from 5 up to 20 kilometers per second and the energies involved in these processes are usually higher than the energy needed to escape the gravitational well of such bodies. For this reason, the current phase of the collisional evolution is actually destructive.
A mosaic of
asteroids. From the left, two radar images of Castalia
Hudson and Ostro based on radar
Of the two colliding bodies, the smaller
one, which is usually called "projectile", is
completely shattered during the impact, while the bigger body,
called "target", can undergo a wide spectrum of
mechanical modifications: from surface craterization to complete
shattering and dispersion. Essentially it depends on a few
factors, such as the relative kinetic energy between the
projectile and the target, the strength of the material which
constitutes them, the internal structure and the target
self-gravitational potential energy.
Craterization is the most likely process. Indeed, it is the process of which we have the widest direct proof: every planet and asteroid keeps evident tracks of this kind of event. However, it must be said that almost all known asteroids, maybe with the exception of the biggest ones, are"fragments" of catastrophic collisions. For example, the Near Earth Asteroid 433 Eros, recently approached and studied by the NASA probe NEAR, is surely a huge impact rocky chunk.
The transition between craterization and catastrophic disruption (conventionally defined as the phase in which the mass of the impact largest remnant is less than half the mass of the target) is everything but sharp. What could happen is that the result of a collision can dangerously approach this limit without exceeding it. For example, asteroid 253 Mathilde shows an impressive impact crater (nearly 30 kilometers wide), which is comparable with the dimensions of the asteroid itself. Another remarkable example is crater Stickney on Phobos, the larger and innermost of the two moons of Mars, believed to be an asteroid captured by the Red Planet. It is not difficult to imagine that in these cases the energy released by the impact could have led to the total disruption of these bodies. Their internal structure (maybe porous aggregate) has probably damped the propagation of impact shock wave, sheltering the most distant regions of the target from disruption and dispersion.
When the impact energy exceeds the craterization limit, the target experiences a catastrophic break-up. In the case in which almost all the fragments have a velocity greater than the target escape velocity (dict.), they run away giving birth the to what is called an Asteroidal Family. It is possible that during the escape process two or more fragments have a relative velocity lower than the mutual escape velocity. In this case they could form binary asteroids or re-accumulated bodies, usually called "rubble-piles". Binary asteroids or rubble-piles can be also generated when the target is shattered but not completely dispersed, since some fragments cannot escape the target self-gravity. In the latter case this ensemble of fragments re-accumulates and, during the collapse, it begins to rotate faster (according to the conservation law of the total angular momentum (dict.), the initial one plus that imparted by the projectile) reaching a rotational equilibrium shape similar to those showed by a fluid in hydrostatic rotational equilibrium. They are, in order of increasing initial angular momentum, bi-axial ellipsoids (or Maclaurin spheroids), tri-axial ellipsoids (or Jacobi ellipsoids) and the balanced (equal masses) binary systems (or Darwin ellipsoids).
Recently radar-resolved Near Earth Asteroids 4179 Toutatis and 4769 Castalia are found to have very unusual shapes and they are probably contact binaries with monolithic lobes or even rubble-pile binaries, namely rubble-piles rotating so fast that their equilibrium configuration turns out to be a binary (i.e.contact Darwin ellipsoids).
Asteroid Ida and its
satellite Dactyl - The asteroid Ida was imaged by the
Galileo spacecraft on August 28th 1993.
Another well-known binary asteroid is 243 Ida-Dactyl. If we look at the sizes of its components we can see that this binary system is highly unbalanced: Ida is 58 kilometers long and 23 kilometers wide, while its satellite Dactyl is approximately egg-shaped, measuring about 1.2 x 1.4 x1.6 kilometers. Although Ida has a highly irregular shape it is believedto be a monolithic body, not a binary nor a rubble-pile. It is a member of the Koronis asteroidal family. The origin of the Ida-Dactyl binary system is probably different from the scenarios described above. Dactyl might have been originated from the re-accumulation of a ring of debris around Ida. This debris could have been injected in orbit as a consequence of a big impact of a rather massive asteroid on Ida. Its shape (quite regular, too regular to be a monolithic rocky body of such small dimensions), its surface spectrum (somewhat different from that of Ida, suggesting a younger age) and its short dynamical lifetime (surely shorter than the age of Ida) all give some support to this formation mechanism.
Germano D'Abramo (*) - Istituto di Astrofisica Spaziale IAS CNR - The Spaceguard Central Node