25 Feb. 2011

Asteroids: Kleopatra and her twins

In this month's issue of the journal Icarus, the results of a long study of asteroid 216 Kleopatra, and other large asteroids with small satellites, are published. The main authors of the study are Franck Marchis (University of California, Berkeley) and Pascal Descamps (IMCCE, Observatoire de Paris).
The reason that these two astronomers and their team focused their attention on asteroids with satellites, is that if one can calculate the orbital parameters of these satellites then the mass of the main asteroid can be found. Knowing the shape, and therefore the volume, of the asteroid, the mean density can then be calculated, and this gives valuable information about the internal structure of the body.
To study 216 Kleopatra in detail, Marchis e Descamps, who discovered its two satellites in 2008 (image above), made use of the adaptive optics system of the Keck II telescope in Hawaii and the Spitzer space telescope. To help determine the size and shape of the asteroid they also used data from stellar occultations during the last 30 years, often made with small telescopes.
After five years of research, that also included the study of the asteroids 87 Sylvia, 90 Antiope, 121 Hermione e 22 Kalliope, the Franco-American team managed to paint a very detailed picture of 216 Kleopatra. The main body has a maximum diameter of 217 km and is dumbbell shaped, while the small moons are roughly spherical, both with diameters of approximately 8 km.
The density of 216 Kleopatra is about 3.6 g/cm³, which implies that much of its volume is actually empty space. To be a little more precise, if the asteroid has an iron-rich, rocky composition, as is normal, and given this has a density of around 5 g/cm³, then 30-50% of the volume is empty. This is actually what is expected for many asteroids, given their history of collisions since the formation of the solar system. 216 Kleopatra is the result of this kind of destructive evolution, and is actually an accumulation of rocky material held weakly together by its self gravity.
The two satellites (called Alexhelios and Cleoselene, after Alexander Helios and Cleopatra Selene II, the twins of Kleopatra and Mark Antony) are probably two large fragments from different impacts that happened about 100 and 10 million years ago. The more recent impact giving birth to the inner satellite.