Rigel Kentaurus

13 05 2007

Our new home was a world named Fram. It orbited the dimmest of the close binary Alpha Centauri A and Alpha Centauri B. It was one of three planets we had found in the system. Maud and Belgica were small balls of iron that orbited close to Alpha Centauri A, while Fram orbited Alpha Centauri B alone.

Fram was a small world, no larger than Saturn’s moon Titan; Fram possessed a similarly thick atmosphere, although less exotic. An atmosphere of mostly carbon dioxide (65%) and hydrogen (15%), with lesser and trace gases (methane 8%, argon 7%, nitrogen 5%) enshrouded the planet.

The two stars orbited a mutual barycentre, and took just under eighty years to complete orbits of one another. Alpha A and Alpha B at their closest were 11.4 AUs apart, still farther than the distance from Sol to Saturn. At their farthest, they were 36 AUs apart, greater than the orbit of Neptune.

Fram was in a highly elliptical orbit of Alpha Centauri B, which at its perihelion was 0.75 AU from Alpha B; its orbit stretched away toward Alpha Centauri A with an aphelion of 1.3 AU. This looping orbit wobbled with each revolution, as its aphelion was tugged toward Alpha A by the competing gravities of the binary and the rotation of Alpha A around the barycentre. Fram completed an orbit of Alpha Centauri B every three years and five Earth months.

Even at its closest point to Alpha B, less than the distance between Earth and Sol, Fram only skirted the outer edge of Alpha B’s habitable zone. Fram was thus a cold world. When it grazed the HZ around perihelion, Fram’s surface temperature hovered between 5 and 15 degrees Celsius. At aphelion, methane would bond with water ices and would snow from the sky and settle into the craters that pockmarked Fram’s surface; thin sheets of methane and water ice were thus frozen beneath the regolith at the basins of many craters.

Alpha Centauri B was a dim main sequence, orange red dwarf, about eighty-six to ninety percent Sol’s diameter and mass, but only forty-two to fifty-two percent its luminosity. Because of Fram’s elliptical orbit, which was at its closest to the bright Alpha Centauri A when at aphelion to Alpha Centauri B, the planet was at its coldest when the two stars were at their brightest. From Fram, Alpha A appeared to brighten as the two stars approached and dimmed as they receded. Under our e-suits we would wear thermals and coats and scarves.

Much of the matter that, given the stable conditions of Sol, might had agglomerated into large planets had instead been scattered and distributed by the competing gravities of the binary stars; in the Alpha Centauri system there had formed no Jovians, no gas giants, no large terrestrial planets. The entire system was composed of comets and asteroids and lumps of planetesimals too small to form into spherical shapes under their own gravity, all churned about in complex orbits. Fram was the largest of these rich, metallic lumps; it had three satellites that we named Nansen, Sverdrup and Amundsen.

Fram had an extensive ring system, despite being a much smaller world than the magnificent, ringed giants of distant Sol. Its largest moon, Amundsen had been disintegrating for about six million years: struck directly by a planetesimal likely flung by Proxima, Amundsen had shattered and now swam through a complex ring system formed from the debris. It was likely that all of Fram’s ring system had, once, been a part of Amundsen. This ring would, as with the formation of the Moon around Earth, clear in a few billion years, as the deformed remains of Amundsen and the small shepherd moons Nansen and Sverdrup consumed the debris.

But not all of Amundsen had settled so easily into a new orbit. There were strings of fresh craters across the scarred face of Fram, and impact sites across the leading hemispheres of Nansen and Sverdrup. Alarmingly, in the twenty-five years since the first automated probes from Sol had shot through the system at relativistic speeds, a new and massive impact site had formed on Fram’s northern hemisphere.

Fram’s surface had been weathered as had Earth’s Moon by regular impacts for billions of years. But unlike the Moon, Fram had an atmosphere, through which small meteors quickly burned up, and wind and weather fronts and dust storms, which moved the dusty regolith around and disguised all but those enormous craters that had geologically altered the landscape.

Nonetheless, there had been no peak period of bombardment for any of the planets of the system, as there had been in distant Sol; rather, bombardment was a geologically regular occurrence. Impacts from comets had given Fram what little water ice there was on its surface and the lesser gases in its atmosphere.

Fram was a forbidding place. It was cold and dry and its atmosphere poisonous, and yet, a form of life was found here. In the deepest basins of craters and in rilles between uplifted basalt sheets there existed a kind of translucent vegetation – anaerobic methanogens, which sought out the volatile ices frozen here and converted these to methane. It was to these methanogens that Fram owed the methane in its atmosphere.

There was a certain desolate beauty to Fram. At perihelion, Alpha A would disappear for months at a time behind Alpha B, while at aphelion both stars would be opposite one another, and would banish night entirely. Proxima, a flare star, could dramatically brighten and in moments appear as bright as Jupiter from Earth. All three of Fram’s moons could go into eclipse simultaneously, a sight bisected by Fram’s elegant ring. The zodiacal light was bright and intense, even long past sunsets; aurorae filled the night sky, sometimes from two directions, varying in colour; the planet’s ring cast a band of light close to the horizon and divided the hemispheres; and asteroids looped about solar system, brighter than artificial satellites in low-orbit.

From Alpha Centauri, Sol was a bright yellow speck, maybe the magnitude of Capella seen from Earth, far away in the constellation Cassiopeia; and it transformed that constellation’s w shape into a less precise zig-zag…




One response

24 08 2012
Epilogue « Orbital Shipyards: Alpha Centauri System

[…] came to Alpha Centauri and made Planetfall upon Fram in 2084; in the following years, we explored farther and farther afield, and as we did so, our […]

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