by Paul Gilster | Dec 2, 2009 | Exoplanetary Science |
To the two ongoing hunts for planets around the Alpha Centauri stars we can now add a third. John Hearnshaw (University of Canterbury, Christchurch) reports in a recent post on Cosmic Diary that the university’s Mt. John Observatory has begun a program to search for Earth-mass planets around Centauri A and B. Although the observatory is heavily invested in microlensing technologies (working with the Microlensing Observations in Astrophysics collaboration), the new efforts will put radial velocity methods to work using the Hercules spectrograph.
The program is a joint effort with Stuart Barnes at the Anglo-Australian Observatory and Mike Endl at the University of Texas (Austin). And as Hearnshaw notes, the problem is a formidable one, given that an Earth-mass planet in the habitable zone around Centauri A creates a ‘wobble’ of only 10 cm/s (slightly larger for the less massive Centauri B). Yet the observatory is banking on Hearnshaw’s statement that 30,000 spectra of Centauri A or B over three years can detect a habitable zone ‘Earth.’
The habitable zone around Centauri A should be found at about 1.2 AU, while 0.75 AU is calculated for Centauri B. What else do we know about the primary Centauri stars? Earlier work has demonstrated that no gas giants as massive as Jupiter can exist there — Doppler studies would have found them by now. But the case for Earth-mass planets remains open. Hearnshaw notes the more positive findings that have emerged in the past decade or so:
We are encouraged to undertake these key observations for several reasons. First, theoretical studies by Javiera Guedes et al. in 2008 showed that Earth-mass planets are likely to have formed in the alpha Centauri system. In their simulations, planets of mass 1 to 2 Earth masses always form in the habitable zone around alpha Cen B, no matter what the initial conditions. What’s more, Paul Holman and Matt Wiegert found that stable orbits are possible in this binary provided they are within about 3 A.U. of either star. The orbits are almost certain to be coplanar with the binary star orbit, which has a semi-major axis of 23 A.U. What is more, the binary orbit is tilted at 79 degrees to the line of sight, so any putative planetary orbits are likely to be at that same favourable angle for detecting Doppler shifts (if the angle is small, the orbits would be close to face on and no Doppler shifts are then detectable).
We’ve looked at the studies mentioned above repeatedly in these pages (run a search for past articles), but we’ve also noted recent work by Philippen Thébault (Stockholm Observatory), Francesco Marzari (University of Padova) and Hans Scholl (Observatoire de la Côte d’Azur), who question whether the accretion process of planetary formation would allow such planets to form. The heartening thing is that this dispute is likely to be settled one way or the other within a few short years. Mt. John Observatory already has several thousand spectra in its new campaign and intends to intensify the search, an effort Hearnshaw calls “a realistic target within our grasp.”
And note this:
One other fortunate circumstance makes this the ideal programme for Mt John Observatory. Being the world’s southernmost optical observatory (at 44ºS), we are able to observe alpha Centauri for 12 months of the year, even in November and December. The star is circumpolar and passes the southern horizon at lower culmination at an altitude of some 15 degrees, when it is still readily observable. No other observatory can see alpha Centauri all year, and a periodic gap in the data every year can be disastrous when trying to detect periodic signals which may well have around a one-year period. We therefore plan to press ahead with the alpha Centauri campaign during 2010 and 2011, with the hope of making the historic discovery of an Earth-like analogue orbiting our nearest star, at just 4.3 light years distance.
Image: The night sky above Mt. John Observatory. Note the Southern Cross (just above and to the right of the dome), with Alpha and Beta Centauri the two stars to its left. Alpha Centauri (the leftmost bright star) is a triple system made up of Centauri A, Centauri B and Proxima Centauri. Credit: Fraser Gunn.
We should know something, and relatively soon. Debra Fischer’s work at the Cerro Tololo Inter-American Observatory (Chile) is ongoing, as is that of Michel Mayor and Stéphane Udry using the High Accuracy Radial velocity Planet Searcher (HARPS) at the European Southern Observatory facilities at nearby La Silla. The betting here has been that while the Mayor team may be the first to find a larger world, an Earth-mass planet is likely to be claimed by Fischer, whose resources, unlike HARPS, are totally committed to the Centauri search. Now we add this interesting New Zealand campaign as excitement grows that whatever Centauri planets there be may soon be discovered.
by Paul Gilster | Dec 1, 2009 | Culture and Society |
Back in 1999, with NASA’s Interstellar Probe Science and Technology Definition Team investigating the possibility of reaching another star, then administrator Dan Goldin exhorted the agency to push its limits. “We have to set goals so tough it hurts — that it drives technology — in semiconductors, materials, simulation, propulsion,” he told reporters, and later that year he described a new kind of space vehicle, one that taps advances in genetic algorithms, neural nets and nanotechnology. Those were breathtaking days, if short-lived. We were in a new intellectual space, a long way from Apollo.
Reconfiguring the Metaphor
Is a reconfigurable probe the size of a Coke can, one that taps local materials to adapt to a remote star system, what we might call a ‘starship’? Charles Stross asks this question in a recent essay, noting that the Pioneers, Voyagers and New Horizons we’ve sent on missions that will reach interstellar space are starships, but not in the popular sense. The star ‘ship’ metaphor calls up vessels the size of the Queen Mary, populated with Star Trek-like crews. And as Stross notes, the new kind of interstellar thinking isn’t remotely like that. Hence his title, ‘The Myth of the Starship.’
We have a long tradition of nautical baggage. Seafaring ships of the great age of exploration were largely wooden, and — with the aid of their human crew — self-repairing; subject to the availability of raw materials, there wasn’t much aboard a 16th or 17th century sailing ship that couldn’t be made on board. Aside from carpentry, the inhabitants of even a relatively small port could make the necessities to keep a ship at sea on a voyage of years — a smithy, a pottery, a glass-blower, weavers of sailcloth and makers of hardtack. Shipbuilding was by no means easy (it was an economic activity born on the backs of the large numbers of peasant farmers and fisherfolk it took to provide the surplus to feed the workers in the shipyards) but it wasn’t anything like the Apollo project, which sucked up the labour of a third of a million skilled engineers and technicians for a decade.
We don’t, then, call the Apollo spacecraft ‘Moon-ships’ because the word isn’t applicable to their mission. Nor is the sense of destination the same. Our first space colonists, no matter where they go, will have to build and sustain their own biosphere or, if they’re robotic, their own ‘mechanosphere’ (in Stross’ terms) to sustain themselves. The basic infrastructure has to be packaged to go along with the space travelers, and in interstellar terms, that’s quite a demand. One way to proceed is through nanotechnology coupled with highly advanced artificial intelligence, leaving the humans at home.
Not Your Grandfather’s Starship
But build a craft along these lines and you don’t wind up with anything remotely resembling the Enterprise. Instead, it’s going to look like a minimalist ‘DVD balanced on a microwave beam, or a can of beans hanging below a light sail energized by lasers.’ Nanotech assemblers will build the necessary remote station upon arrival at another star system to transmit information back to Earth. Such probes will probably be very small, possibly quite numerous (Stross doesn’t mention them, but Robert Freitas’ ‘needle’ probes, scattered into space in the millions, come to mind) and they will build their remote scientific bases out of the materials at hand.
This isn’t a ship, surely:
If anything, it’s going to resemble a seed pod for a different kind of life, and on arrival it’s going to hatch and grow into a tree, or a forest, or a manufacturing-industrial complex. Finally, long after arrival, it might have sufficient resources to divert from homeostasis and growth to construct a biosphere, open communications with home, and prepare to download digitized colonists — if the whole uploading concept doesn’t prove to be chimerical, and if there’s something to be done with the serialized primate core-dumps at the other end.
Although Stross keeps referring to ‘starwisp’ as the operating mode here, Geoffrey Landis has done a thorough job demonstrating that the Robert Forward Starwisp probe wouldn’t work — in fact, the moment the microwave beam Forward envisioned as powering this spider web-thin structure illuminated it, Starwisp would vaporize. Forward himself acknowledged this and was deep into other concepts when he died. But the general formula of a sail-like craft riding a beam of some kind is what Stross sees, correctly I think, as the most likely technology for our first probes to other stars.
Generations Among the Stars
As to ‘generation ships,’ in which people are born and live out their lives without ever reaching the destination, Stross points to the huge problems of biology, environmental engineering and sociology that these invoke. There goes that word ‘ship’ again, which Stross prefers to replace with the term ‘interstellar transportation system.’
Such a system needs to provide not only a mechanism for sending a self-replicating technosphere across interstellar distances; it needs to be able to produce a habitable space at the destination, and provide a return option (for data, if nothing else).
But a different kind of generation-spanning technology may eventually emerge. Create stable space habitats in which life is minimally acceptable for colonists and the refinement of these technologies will eventually produce generations that spend their lives off a planetary surface, even if they’re still located in the Solar System. It’s not at all beyond the imagination to suggest that such an environment might, at least in a few cases, be populated with people who saw no particular reason to stay within the system’s boundaries.
But I wouldn’t call that a ship either. It’s an enclosed approximation of a planet, one on which almost every environmental variable has been regulated to produce as close to a planet-like experience as possible. “The whole reference frame we instinctively assume when we hear the word “ship” is just so wrong it’s beyond wrong-ness: it’s on a par with Baron Munchausen’s lunar exploits as seen in light of the Apollo Program,” says Stross, who argues that the ship metaphor is doomed.
Survival of the Myth
I doubt it. I like the metaphor and think of ‘starship’ as the logical statement of what we’d like to achieve. The fact that the term does not describe our current technological capabilities doesn’t concern me. If we never develop interstellar craft with human crews, so be it, we’ll call them something else, just as no one actually calls New Horizons a ‘starship.’ On the other hand, there is an aspirational value in our choice of words, one that the word ‘starship’ picks up on quite nicely.
If it’s reminiscent of the great sailing vessels that once took colonists from one side of the world to another on largely one-way voyages that built new lives, so much the better. Given our choice of imaginings, this is how we, adapted to our planetary surface, would like to reach new star systems if all things were possible. And can we be sure that the aspiration built into the word ‘starship’ won’t trigger some far future breakthrough that really does make the metaphor into reality? No one can know, but what seems impossible is almost always worth attempting if only to understand our limits.
No, I doubt we’ll lose the term ‘starship’ even if its only actual appearance is in fiction. We string words together to create concepts that defy the limitations that surround us, and have been doing so since the days of the Gilgamesh epic and Homer’s enchanted tales. Charles Stross, one of science fiction’s brightest lights, is no stranger to the magic of word-weaving, and surely knows that the human experience is a potent mix of hard realism with an inescapable counterpart, a ‘what if’ that challenges all our assumptions. Resonant mythic echoes alone will keep ‘starship’ a viable term, an object of dreaming that encapsulates what we would do if the universe will let it be possible.
