Centauri Dreams

With the Dawn spacecraft on its approach to Vesta, I’ve been scouting around for science fiction that involves this interesting asteroid. The one story that stands out is famous for its author more than its quality. It’s “Marooned Off Vesta,” which turns out to be Isaac Asimov’s first published story. John Campbell rejected it at Astounding Science Fiction, so it was left to Amazing Stories‘ Ray Palmer to publish the Good Doctor’s first, written at the age of 18. “Marooned Off Vesta” appeared in Amazing‘s issue of March 1939 and would have faded into obscurity if its author hadn’t gone on to his spectacular career in fiction and non-fiction.

The era when Asimov didn’t make the cover of a magazine he was writing for didn’t last for long. I don’t particularly recommend you hunt this story down, although it appears (for sentimental reasons, I suppose) in 1973’s The Best of Isaac Asimov. Here a trio of space travelers in trouble look out at the surface of Vesta from their crippled craft. The dialogue positively clanks:

Brandon stared bitterly at the globe that filled almost the entire porthole, so Moore continued, “Watching Vesta won’t do you any good either.”

Mike Shea lumbered up to the porthole. “We’d be safe if we were only down there on Vesta. There’re people there. How far away are we?”

“Not more than three or four hundred miles judging from its apparent size,” answered Moore. “You must remember that it is only two hundred miles in diameter.”

“Three hundred miles from salvation,’~ murmured Brandon, “and we might as well be a million. If there were only a way to get ourselves out of the orbit this rotten fragment adopted. You know, manage to give ourselves a push so as to start falling. There’d be no danger of crashing if we did, because that midget hasn’t got enough gravity to crush a cream puff.”

Suffice it to say that “Marooned Off Vesta” is a bit of a slog. Nonetheless, the problem — how to get their shattered vehicle to safety using the limited supplies aboard the ship, is the heart of a tale whose problem solving and analysis of the physics involved reminds me of countless, more sophisticated works by a mature Asimov.

The embedded video below takes us to the post-Asimovian present. We can imagine Asimov’s crew looking out of their porthole at something vaguely like this. The video is actually a looping set of 20 images from Dawn taken for navigation purposes on June 1. One pixel here corresponds to 45 kilometers on the asteroid’s surface — the resolution approaches our best Hubble images of Vesta. Note the dark feature, about 100 kilometers in diameter, near the asteroid’s equator. It will be thrilling to see things like this resolve as Dawn draws ever closer to its rendezvous.

Dawn’s other target is Ceres, which it will travel to after completing its Vesta mission in 2012. Unlike Vesta, Ceres has a robust history in science fiction, from early tales like Garrett Serviss’ Edison’s Conquest of Mars (1898) through Larry Niven’s Known Space stories and novels like Joe Haldeman’s Buying Time (1989) and Bob Shaw’s The Ceres Solution (1981). The two asteroids could not be more different. Vesta has a differentiated structure, perhaps built around once radioactive materials in its core. We have much to learn about the place, but current thinking is that a layer of frozen lava covers deeper layers of rock and an iron/nickel core. The Vestoids, asteroids with similar orbital properties to Vesta, may have been created in the same impact that created a huge crater on Vesta’s south side, an event that also produced meteorites whose mineralogical composition leads researchers to associate them with Vesta.

Ceres, on the other hand, reminds us how much variety exists in the asteroid belt (a fact that Kevin Walsh offers an interesting hypothesis about in his current work on planetary migration). Ceres is larger than Vesta and almost spherical, with a density low enough to make it unlikely to have a metallic core. It’s now believed that Ceres is a cold body that never experienced internal heating. Frozen and perhaps liquid water — conceivably in copious amounts — may lurk below its surface, and there are some indications of frozen polar caps that evaporate in summer. The two asteroids are survivors of the planet-formation process, frozen in their development for 4.5 billion years. Dawn will be giving us a priceless look at the Solar System’s earliest era.

Our Voyager spacecraft are in a fascinating place indeed, where the stream of charged particles flowing out from the Sun — the solar wind — bumps up against what we might call the ‘interstellar wind,’ the tenuous material expelled from other stars in our neighborhood. We’ve looked at the solar wind’s possibilities for propulsion many times, pondering whether it could push a ‘magsail’ at high velocity to the outer system, and whether such a push would be controllable (it looks to be a turbulent ride indeed). But new work reminds us that the Sun’s magnetic field lines are likewise pushed outward by the solar wind. Merav Opher (Boston University) comments:

“The sun’s magnetic field extends all the way to the edge of the solar system. Because the sun spins, its magnetic field becomes twisted and wrinkled, a bit like a ballerina’s skirt. Far, far away from the sun, where the Voyagers are, the folds of the skirt bunch up.”

We’re learning from those same Voyagers as they move through the boundary region at system’s edge that the distant magnetic field is a place where magnetic field lines re-orient themselves, forming bubbles approximately 160 million kilometers wide. At this distance, the bubbles have become self-contained structures disconnected from the solar magnetic field. They’re large enough — a bit larger than the Earth’s distance from the Sun — that it would take weeks for the Voyagers to cross even one of them. For those of you keeping count, Voyager 1 is now traveling at just over 17 km/sec relative to the Sun, while Voyager 2 is at 15.5 km/sec.

Image: Red and blue spirals are the gracefully curving magnetic field lines of orthodox models. New data from Voyager add a magnetic froth (inset) to the mix. Credit: NASA

Once again Voyager overturns established ideas. The idea that the Sun’s magnetic field curved around to re-connect gracefully with the Sun now gives way at least partially to the idea of what University of Maryland physicist Jim Drake calls ‘a foam at the edge of the Solar System.’ What the Voyagers’ energetic particle sensor readings suggest is that the spacecraft are moving in and out of this foam in an area where the Solar System interacts with the rest of the galaxy. This region, the heliosheath, is our system border, and the characteristics of that border should affect how everything from galactic magnetism to cosmic rays fare as they try to enter the system.

Opher points out that the magnetic bubbles may be a major line of defense against galactic cosmic rays — sub-atomic particles accelerated to near light-speed — but we don’t yet have a handle on how efficient the bubble structure really is. Cosmic rays could become trapped inside the bubbles, but they may be too porous to maximize this effect. “We’ll probably discover which is correct as the Voyagers proceed deeper into the froth and learn more about its organization,” says Opher. “This is just the beginning, and I predict more surprises ahead.” With the Voyagers, we’ve learned to expect nothing less.

The paper is Opher et al., “Is the magnetic field in the heliosheath laminar or a turbulent bath of bubbles?” accepted by the Astrophysical Journal (preprint).