Saturday, October 27, 2012

A Planet of Alpha Centauri

The nearest stellar system to our own, as Sky and Telescope reports, has a planet. Which is a vindication of sorts for rocketpunk-era science fiction, which made our neighbor a favored location.

The newly discovered world's name, at least for now, is distinctly unromantic: Alpha Centauri Bb. The capital B signifies that it orbits the slightly less luminous of the main Alpha Centauri pair. (Alpha Centauri C, AKA Proxima Centauri, is a very distant companion of the AB pair, and slightly closer to good old Sol.) The lower-case b marks the first planet to be discovered in the system.

Its mass is roughly comparable to Earth's, but Bb is almost certainly not inhabited by tall blue cat people, and even less likely to ever be inhabited by us. It is a 'hot Earth,' or at any rate just plain hot, orbiting Alpha Centauri B at a distance of 0.04 AU. This fact is in no way discouraging. Our search methods heavily favor planets in furnace orbits, and where there is one planet there is a decent chance of more.

In fact, as extrasolar planets go there is nothing special about Alpha Centauri Bb except for its address. But its address - Alpha Centauri - is special indeed.

When interstellar settings first developed back in the Golden Age, the preference was for familiar named stars, as I noted in the Tough Guide entry on Nomenclature. Thus notable SF worlds circled such dubious candidate stars as Sirius or even Betelgeuse.

By around the 1950s, as the theory of stellar evolution made these locations problematic, attention turned to nearby sunlike stars. Alpha Centauri ruled this list - with some hesitancy, ironically, because of doubts whether planets in double-star systems could have stable orbits, or for that matter form in the first place.

These doubts were only laid to rest when we started actually finding planets in double-star systems.

A few things distinguish Alpha Centauri. It is the third brightest star in the night sky, after Sirius and Canopus. But it lies far in the southern sky, thus invisible to observers in most of the northern hemisphere. Hence its lack of a familiar given name. Though in fact it does have a given name, Rigilkent, known mainly to the dwindling ranks of people familiar with celestial navigation.

According to Wikipedia, Alpha Centauri A is about 1.5 times solar luminosity, while B is about half solar luminosity. Component A, with a spectral type of G2, is often described as a near-twin of Sol, though in mass and luminosity, though not spectral type, Sol is more nearly intermediate between the two. (B, the one now known to have a planet, is spectral type K1.) They orbit each other every 80 years, ranging from a little more than Saturn distance to considerably more than Neptune distance.

The third component, Proxima, is dim and distant, nearly a quarter of a light year from the bright pair (and somewhat closer to Sol). Travel between them would thus be more like a short interstellar voyage than a long interplanetary trip. The system is probably somewhat older than the Solar System - what that means for life, if it has a lifebearing planet, is anyone's guess.

Alpha Centauri is an interesting stellar (and planetary!) system, but of course by far the most interesting thing about it - to us - is its distance, 4.37 light years to the AB pair. Which invites speculation about going there, or at least sending a probe. Proxima is closer, 4.21 light years, but a mission would almost certainly be aimed at the main pair, possibly with a secondary probe to be deflected to Proxima.

So ...

Nuclear electric propulsion, of the sort I have often discussed here, should be good for on order of 300 km/s if tuned for maximum specific impulse. Since Alpha Centauri is itself approaching Sol at about 25 km/s, it will be appreciably closer by the time the mission gets there, in about 4000 years. Which is an awfully long duration for mission planning, not to mention getting it funded.

Getting there sooner requires some admixture of handwavium to the spacecraft specifications. But there is handwavium and there is handwavium. And this is probably as good a place as any to link the Engine List page at Atomic Rockets.

The short form is that, for credible mass ratios, high-end fission technologies can potentially reach on order of 0.03 c, about 10,000 km/s, for travel times on order of 100 years. Fusion can, in principle, bump you up to around 0.1 c, and you get there in not too much over 40 years. And there are other propulsion options.

Bear in mind that you will probably want to slow down, not flash through the Alpha Centauri system at an appreciable fraction of the speed of light. So mission delta v must be twice the travel speed, plus a bit of margin for local exploration once you get there.

By the way, I am thinking here mainly of robotic missions. Human missions involve a whole 'nother set of complications, the first two of which are long term life support and a much heavier payload.

And even for a robotic mission there will be engineering complications. For fusion, these begin with getting it to work at all, in a form that won't vaporize the spacecraft. (We have already accelerated appreciable masses - in the kilograms - to a few thousand km/s. Unfortunately, these masses were internal components of nuclear weapons.) Probably you should expect realistic technologies to achieve around 1 percent  of theoretical propulsive performance, or at best 10 percent  - thus, a tenth to a third of ideal mission speed.

Those provisos made, no heroic/fanatical effort should be required for a mission to reach Alpha Centauri at some date within the plausible midfuture. The difficulty level is on the same order as a substantial permanent base (and incipient colony) on Mars, outposts in the outer system, and regular travel among them. Which, to be sure, is a far more demanding order than people thought at midcentury. But it is doable.


Note: Just when I was trying to get back into a more frequent posting rhythm here, all the usual excuses raised their ugly heads. But at least the World Series will end this coming week, one way or the other. (No one knows better than SF Giants fans that it really ain't over till it's over. But it will be over soon.)

The image comes from the Sky & Telescope article link at the beginning of the post.