Ultraviolet

8 02 2012

The effect of the gravities of two stars upon Fram’s orbit was quite pronounced.

Fram’s orbit was highly eccentric, meaning that it was a not a simple circle around Alpha B, but rather an elongated ellipse. Traced simply, Alpha B sat inside one end – called an apsis – of that ellipse, while the other apsis stretched away toward Alpha A. But neither Alpha A nor B were stationary, and themselves orbited a mutual barycentre. Their own orbit greatly complicated Fram’s orbit. The apsides changed with each orbit relative to the position of both stars. This was called apsidal precession. Each time Fram completed an orbit of Alpha B, Alpha A had moved relative to its binary partner, and its gravity tugged at Fram. As a result, each completed ellipse reorientated itself toward Alpha A.

Astronomers explained apsidal precession by tracing a complex spiral that represented Fram’s orbit. The lines of that spiral converged at periapsis – the apsis that coincided with perihelion, Fram’s closest approach to Alpha B – but diverged in wandering arcs near aphelion, as each orbit traced a different apoapsis. Fram was moving away from periapsis and, slowly, methodically, irrevocably, gliding toward apoapsis. Fram had only in the last week passed periapsis, and took roughly three Earth years to complete an orbit, meaning that the apoapsis of that orbit was about eighteen months away.

There were many concerns about the habitability of Fram, almost all of which had been foreseen and discussed long before the Quoqasi left Sol. Only one among these was its wandering orbit, which itself posed the major problem of exposure to ultraviolet light.

Alpha B was less of a concern than Alpha A. Alpha A was larger, hotter and brighter than Sol, while Alpha B was similarly smaller and cooler. We knew that hot objects preferentially emitted radiation at shorter wavelengths and higher frequencies. Wien’s displacement law described the relationship between temperature and peak frequency. The hotter the object, in this case a star, the shorter the wavelength at which it emitted radiation. This was why hot, A-type stars like Sirius tended to emit blue light in the visible spectrum, while cooler M-type stars like Proxima tended toward red light.

But visible light was only one component of the electromagnetic spectrum. Higher up the spectrum according to frequency were ultraviolet, x-rays and gamma rays. Hotter stars not only pumped out more light, but also preferentially emitted these higher frequency, shorter wavelength types of electromagnetic radiation.

Not only did Alpha A pump out more energy than both Sol and Alpha B, but it also preferred to emit more dangerous energy.

Humans had evolved while sheltered from most ultraviolet radiation by Earth’s ozone layer, a belt of the stratosphere where the interaction of molecular oxygen and solar ultraviolet light continuously interconverted oxygen from O2 to O3; the process also converted ultraviolet radiation into thermal energy. But Fram, of course, possessed no ozone layer. Almost all of the oxygen in Fram’s atmosphere was covalently bonded with a carbon atom to form carbon dioxide, which was not only poisonous to breath but offered no shelter from ultraviolet light.

From Earth, it was easy to miss some of the features of Fram that would challenge our early efforts, such as the damage that the regolith would pose to our vehicles and equipment. But it was comparatively simpler to understand the stellar system, and we were prepared for the worst of the ultraviolet light. Equipment at risk of UV degradation, like synthetic polymers, had been reinforced with stabilisers and absorbers – such as benzophenones, zinc oxide, and titanium dioxide. Moreover, the same lack of independent oxygen in the atmosphere that prevented the development of an ozone layer also suppressed the reaction between ultraviolet rays and free radicals that led to the worst of polymer degradation.

All of these measures would be tested at apoapsis, when Fram was suspended between the two stars and at its closest approach to the more threatening Alpha A. At that point there would be no real night, but rather a bright twilight, the sky dark blue and the terrain of the planet lit with a quality of light like the totality of a solar eclipse on Earth. Exposure to ultraviolet would be highest at apoapsis.

Some of the biologists noted the similarity of the colonisation of Fram to the emergence of life on Earth. Early prokaryotes approached the surface of Earth’s oceans billions of years ago, before Earth had developed an ozone layer, and, exposed to the worst UV light, promptly died out. Those that survived had developed the necessary enzymes, base excision repair enzymes, which identified and corrected the genetic damage caused by exposure to ultraviolet.

And so, as Fram continued gracefully along its complex orbit, we began to study how the methanogens had evolved to tolerate such intense ultraviolet light…

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19 02 2012
A, G, U and X « Orbital Shipyards: Alpha Centauri System

[…] the bonds between nucleobases and distort the macromolecule. Because of the direct exposure to ultraviolet light, we think the methanogens have evolved without thymine and […]

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