New study shows why so many asteroids have their own moons / Nuevas investigaciones sobre asteroides binarios y como se forman

There's an article coming out in the next issue of Nature on a study that has revealed a lot about asteroid formation and why so many of them have their own moons. Their lack of strong gravity alone would dictate that so many asteroids shouldn't be able to have their own moons, and the study has concluded that the moons form from the asteroids themselves early on, which are first spun about by the sun's solar energy, which speeds it up to a point that bits of matter are thrown off, and sometimes these turn into moons around the main asteroid. Here's the link to the article
on spaceref.com.

In this week's issue of the journal Nature, a trio of astronomers from Maryland and France say the surprising answer is sunlight, which can increase or decrease the spin rate of an asteroid.

Derek Richardson, of the University of Maryland, his former student Kevin Walsh, now Poincare Fellow in the Planetology Group in the Cassiopee Laboratory of CNRS at the Cote d'Azur Observatory, France, and that group's leader, co-author Patrick Michel outline a model showing that when solar energy "spins up" a "rubble pile" asteroid to a sufficiently fast rate, material is slung off from around the asteroid's equator. This process also exposes fresh material at the poles of the asteroid.

If the spun off bits of asteroid rubble shed sufficient excess motion through collisions with each other, then the material coalesces into a satellite that continues to orbit its parent.

The article also points out the fact that learning more about these asteroids and the way they are formed can have immediate practical effects, such as saving humanity if any of these asteroids were to head towards Earth, because the more we know about the way they are formed the better:

Because the team's model closely matches observations from binary asteroids, it neatly fills in missing pieces to a solar system puzzle. And, it could have much more down-to-earth implications as well. The model gives information on the shapes and structure of near-Earth binary asteroids that could be vital should such a pair need to be deflected away from a collision course with Earth.

I'm curious what the theory is on the formation of trinary asteroids though. Are they formed in the same way? Here's one called 87 Sylvia that I'd love to see visited one day:



Not only is it a trinary system but it's particularly large as well, with a diameter of 385×265×230 km, and its moons about 18 km and 7 km in diameter. The Wikipedia page has some information on what the moons would look like from Sylvia:

Romulus, the first moon, was discovered on February 18, 2001 from the Keck II telescope by Michael E. Brown and Jean-Luc Margot. It is about 18 km in diameter and orbits at a distance of 1356±5 km, taking 3.6496±0.0007 days (87.59 h) to complete an orbit of Sylvia.

Remus, the second moon, was discovered over three years later on August 9, 2004 by Franck Marchis of UC Berkeley, and Pascal Descamps, Daniel Hestroffer, and Jérôme Berthier of the Observatoire de Paris, France. It is 7±2 km in diameter and orbits at a distance of 706±5 km, taking 1.3788±0.0007 days (33.09 h) to complete an orbit of Sylvia.

It is thought likely that both Sylvia and its moons are accretions of rubble from a past asteroid collision ^[1] Other, smaller moons formed in a similar way may also be found.

From the surface of Sylvia, Romulus and Remus would appear roughly the same size. Romulus, the outermost moon, would be about 0.89° across, slightly bigger than the closer but smaller Remus, which would be about 0.78° across. Because Sylvia is far from spherical, these values may vary by a little more than 10%, depending on where the observer is on Sylvia's surface. Since the two asteroidal moons appear to orbit (as best we can tell) in the same plane, they would occult each other once every 2.2 days. When the season is right, twice during Sylvia's 6.52 year orbital period, they would eclipse the Sun, which, at 0.15° across, is much smaller than when seen from Earth (0.53°). From Remus, the inner moon, Sylvia appears huge, roughly 30°×18° across, while its view of Romulus varies between 1.59 and 0.50° across. From Romulus, Sylvia measures 16°×10° across, while Remus varies between 0.62° and 0.19°.

The key number there is 0.53°, which is the size of the Sun from Earth, so each of the moons looks larger than the Sun. Now what does Sylvia look like when standing on one of them?