NASA is looking at the possibility of sending the new Orion manned space vehicle to a Near Earth Asteroid. While the Moon is frequently cited by advocates of a vigorous manned space program as a valuable material resource to be exploited, NEAs have several advantages. Some NEAs may be as easy to get to as the Moon, because although they do not orbit the Earth, they orbit the Sun on an orbit similar enough to that of the Earth that it doesn’t take much energy to get a spacecraft to one. NEAs are small and therefore have a weak gravitational acceleration. This makes landing on such an object more like docking with a space station, but that’s actually easier than landing on something with a non-negligible gravitational field like the Moon. And it makes it much easier to pull away when you’re done. Also, NEAs and small asteroids in general appear to be rubble piles, or loosely consolidated blocks of material. This could make retrieval of mineral resources much easier than from the Moon. These ideas have been explored for years in science fiction novels, and for the time being getting any practical benefit from an NEA certainly still qualifies as science fiction. However I find it encouraging that NASA is at least exploring some options for the manned program other than just a race back to the Moon.

The real immediate benefit of more detailed exploration of asteroids, however, is scientific, not economic. Asteroids are remnants of the early solar system that, aside from collisions with their neighbors, have been relatively inert for 4.5 billion years. That’s a significant fraction of the age of the universe, and as such they allow us an excellent opportunity to peak far back in time and learn about the conditions of the middle-aged universe and the formation of the planets in our system.

I’m obviously a big fan of the Cassini mission, so what could more satisfying than a contest of the best images of the year from Cassini? The contest is here. I voted for this amazing view of the Earth and the Moon (!) seen through the tenuous dust rings of Saturn. You and all of humanity are there on that fuzzy spot seen from about 930 million miles away.

Colleague and friend Dr. Jeff Cuzzi of NASA’s Ames Research Center is one of three authors of a book presenting some of the beautiful images and discoveries from the first part of Cassini’s mission at Saturn. Jeff is the Cassini Interdisciplinary Scientist for Rings and Dust. His role is to look at the ring observations with the big picture in mind and make sure that we provincial instrument scientists don’t let something valuable slip through the cracks as we pursue our own particular observations. Saturn: A New View, while sadly lacking my own contribution (just look at the header of this blog) to the Cassini Beautiful Image Database (not a real database, but come to think of it, maybe not a bad idea), is nevertheless an excellent survey of the Saturn system and how our view of it is being transformed almost daily by fresh results from the intrepid spacecraft. I like it so much I got three copies in time for Christmas.

A new picture of Saturn’s rings is emerging, and it has to do with moons. Cassini observations presented by Carolyn Porco, head of the Cassini imaging team, at the AGU meeting today show that the small moons within and near the rings are shaped like their Roche lobes. The Roche lobes are not the ears of some strange aliens, but rather are the region of space around an object where its gravity is dominant over the gravity of the planet or star that the object orbits. For example, the Moon orbits the Earth because the Moon is within the Earth’s Roche lobe: the Earth’s gravity controls the motion of the Moon, not the gravity of the Sun. If the Moon were much further from the Earth and outside its Roche lobe, then it would go into an independent orbit around the Sun instead of orbiting the Earth.

So what does this have to do with Saturn’s rings? The particles in Saturn’s rings have Roche lobes that are only a little bit bigger than the particles themselves. This is because they are so close to Saturn, so its gravity can easily dominate the gravity of the small ring particles or moons. Ring particles running into a moonlet can stick to the moonlet if they are within the Roche lobe, but fall off otherwise. The Roche lobe is not spherical, so if accreting ring particles fill the Roche lobe of a moonlet, they will create a low density moon with a particular shape: that of the Roche lobe. And here’s what that looks like.

Image credit: NASA/JPL/Space Science Institute
In this image of the small moon Pan, within the Encke gap of the A ring, the moon is seen to be elongated toward Saturn, and has the shape of the Roche lobe for an object at that location. Measurements of the densities of Pan and other moons show that the moons closest to Saturn are filling their Roche lobes. Any new particles hitting these moons cannot stick because they are pulled away by Saturn’s gravity, leaving the moons with a characteristic shape and density signaling that even within the rings their can be accretion. This may be a clue to the apparent youthful nature of the rings: in a relatively short time, say 10-100 million years from now, Pan will be hit by a small cometary object that will break it apart, creating a new ringlet.