Colonizing the Galaxy Using World Ships

The British Interplanetary Society’s Kelvin Long is no stranger to these pages, perhaps best known as the founder and first leader of Project Icarus, but an indefatigable writer on interstellar topics as well. Kelvin’s first book, Deep Space Propulsion: A Roadmap to Interstellar Flight, is scheduled for publication by Springer later this year. Fellow writer Pat Galea has a background in electronic engineering and physics, and has been a professional software engineer since 1993. As well as contributing to the Project Icarus starship design, he is a supporter of Monkey World in the UK, and a staunch advocate for preserving Bletchley Park, home of World War 2 code breaking. Both Long and Galea are Tau Zero practitioners as well. The duo here offer us an overview of a symposium Kelvin recently organized in London that as far as I know was a first: A conference entirely devoted to the breathtaking concept of interstellar colony craft potentially hundreds of kilometers long.

by Kelvin F. Long & Pat Galea, Symposium Chairmen

On the 17th of August 2011, a gathering devoted to the colonisation of space assembled at the headquarters of the British Interplanetary Society (BIS) in London. The society has been the home of visionary research for many years and has never been afraid to allow free thinking speculation on the future of man in space. The topic of conversation for this symposium was the concept of a World Ship. The reference is to a very large vehicle many tens of kilometres in length and having a mass of millions of tonnes, moving at a fraction of a percent of the speed of light and taking from hundreds of years to millennia to complete its journey.

A worldship is self-contained and self-sufficient, carrying a crew that may number hundreds to thousands and might even contain an ocean, all directed towards an interstellar colonisation strategy. Several seminal papers on the topic were published in 1984 by Anthony Martin and Alan Bond. They developed a ‘wet’ World Ship concept 10 km in diameter, nearly 200 km in length, on the order of a peta-kilogram (1015 kg) habitat mass and an equivalent mass in propellant. The biggest of the ‘dry’ World Ship concepts they developed had a diameter of 15-20 km, and was around 220 km in length. The habitat mass was an order of magnitude lower than the wet concept but the propellant mass was similar in magnitude. These are obviously massive vessels and they would travel to the stars with cruise velocities of around 0.5% of the speed of light, taking hundreds of years to reach the nearest stars.

Image: Conference organizer Kelvin Long.

In one of these papers, Martin had this to say:

“The World Ship concept has now evolved to a stage where it needs to be subjected to more searching analysis. The technological and engineering aspects need to be re-examined in more detail… If the World Ship concept survives this scrutiny intact, even if modified in detail, then the fundamental questions raised by the possibility of interstellar travel and colonisation about the apparent absence of extraterrestrial intelligence in the Solar neighbourhood, about the future of mankind, and about its place in the Universe will become more demanding of an answer.”

The recent conference at the BIS included seven presentations discussing the technology, motivations, financing and cultural interactions of communities aboard such a large space vessel. In this article a brief synopsis of the presentations is given, all of which will shortly be submitting for publication in a special issue of the Journal of the British Interplanetary Society (JBIS), further contributing to the important literature of interstellar studies.

The Enzmann Starship

Kelvin Long, a Project Icarus designer, gave a presentation on “The Enzmann Starship: History & Engineering Appraisal,” on behalf of his co-authors Adam Crowl and Richard Obousy. The history of the concept was clarified and it was revealed that a communication with Robert Enzmann had been established. Most remarkable was the fact that Enzmann claims he first thought of the concept in 1949. Every drawing or painting known to exist on the concept was presented to the audience and some of the subtle design changes pointed out. The important role that Rick Sternbach and Don Davis had played in developing the engineering concept, and G. Harry Stine’s in publicising it, was emphasised. Some preliminary engineering calculations were shown on the authors’ understanding of the Enzmann Starship (or Slow Boat): A 30,000-tonne habitat structure with 3 million tonnes Deuterium fuel, travelling a cruise speed of 0.09c (27,000 km/s) on a 60-year journey to the stars. Large scaled versions were also shown as developed by the authors for the symposium, including a 300,000-tonne Slow Ship travelling at a cruise speed of 0.045c (13,500 km/s) on a 150-year journey, and a 3,000,000-tonne World Ship travelling at a cruise speed of 0.014c (4,200 km/s) on a 350-year journey.

The three concepts would start with 200, 2,000 and 20,000 people respectively, but grow by the end of the journey to 2,000, 20,000 and 200,000 people. The mass distribution with the growing population throughout the trip (i.e. ship mass per person) varied from 150 tonnes/person to 15 tonnes/person at the journey’s end, emphasising the need to attempt planetary settlement in some form. Long pointed out that previous NASA space settlement studies had indicated a distribution of around 65 tonnes/person was desirable. The Enzmann Starship was seen as a fascinating idea and the presenter claimed that Robert Enzmann deserved equal recognition with the “other Bobs” in interstellar studies, namely Robert Forward, Robert Bussard and Robert Frisbee.

Image: The October, 1973 issue of Analog featured a gorgeous Rick Sternbach cover of two Enzmann starships to illustrate G. Harry Stine’s article, “A Program for Star Flight.”

Communicating with the Diaspora

Pat Galea, also a Project Icarus designer, gave a presentation on “Communications Between Worldships” or ‘building the Diasporanet’ as the speaker called it. He firstly gave an overview of the Project Daedalus communications systems and described the historical Project Cyclops which was intended for SETI research, but which the Daedalus designers proposed to use for long distance signal reception. Using a 40 m diameter transmitting antenna, with a 1 MW power output, Daedalus could download data to Earth at a rate of 864 kbps (kilobits per second) over a distance of 6 light years. Modern lasers over this distance would have a data rate equivalent to around 10 bps using a 20 W power output, although this could be increased to around 500 kbps using a 1MW power output, but the major advantage of using lasers is that the physical sizes of the transmitting and receiving elements could be much smaller than those used for Radio Frequency signals.

Two network topology configurations were described for World Ships moving outwards radially from a single point (i.e. Earth): multiple connection, in which each World Ship could communicate with Earth and other World Ships directly, and the ‘Star’ topology, in which each World Ship can communicate only with Earth. Galea then described the exciting possibility of using the Sun as a gravitational lens, with a signal relay craft located beyond the minimum ‘focusing’ distance at 550 AU (based on an idea extensively studied by Claudio Maccone). This provides a way of improving the reception of signals from distant World Ships significantly, drastically improving the data rate. To tackle the severe challenge of keeping the relay craft on track (on the order of meters of accuracy), each World Ship would send extremely bright—but very short—‘sync pulses’ back to a constellation of Navigational Satellites (NavSats) in orbit around the Sun. By comparing the very slight timing differences of the pulse arrival at each satellite, the NavSats could assist the relay craft in maintaining its track, and thus keeping a lock on the signal being received from the World Ship. As the Earth is acting as a router, passing messages back and forth between the World Ships, this system forms the basis of an interstellar internet, or “DiasporaNet”.

Image: Project Icarus designer Pat Galea.

The Worldship and the Sail

Galea then gave a presentation on behalf of Greg Matloff, who could not make it to the meeting. The presentation was on “World Ships: The Solar-Photon Sail Option.” It was argued that an ideal propulsion option for World Ships is the hyper-thin solar-photon sail, and that this is the only known ultimately feasible interstellar propulsion system that can also be applied for en-route galactic-cosmic ray shields as well as for acceleration and deceleration. The author reviewed three options for sail configurations known as Parachute, Hollow-body and Hoop Sails.

It was argued that although World Ship expeditions may not be likely from the present day Sun using solar-photon sail technology, they would be more feasible as the Sun evolves on and beyond the main sequence. Finally, the author raised the tantalising possibility that future space telescopes may be capable of imaging habitable planets circling near stars which have Solar-photon sail propulsion craft departing the system. The solar-mass type star Pollux at 35 light years was suggested as one location where we could search for evidence of a migration underway. In discussions after the talk, Alan Bond made the comment that using sails for World Ships is prohibitively difficult due to the issues of controlling the large number of oscillations in the sail while in flight. Kelvin Long argued that artificial intelligence technology could take care of that issue, though Bond countered that any smart technology in the sail would add to the mass, making the sail less effective.

Reliability in Deep Space

Andreas Hein, another Project Icarus designer, gave a presentation on “World Ships – Architectures & Feasibility Revisited.” The paper was co-authored with Mikhail Pak, Daniel Pütz, Christian Bühler and Philipp Reiss. The speaker first considered the mean distance to habitable planets, including a 140-light year estimate by Alan Bond, 88-light year distance by Claudio Maccone, and 33-light year distance from Kepler mission data. He considered the definition of a World Ship, and defined three criteria: self-sufficiency (thousands of years), population size (>100,000) and speed (<0.01c). One interesting aspect of the authors’ work was the consideration of reliability and mission success, with modelling performed to address this question. Hein concluded that 99.99% reliability was achievable provided sufficiently large scrap stock was made available for the mission. If the ship had a low reliability, then this could be compensated by sending many such vessels together.

Hein then considered migration push and pull factors. Push factors were defined to be scenarios such as war, genocide and poverty. Pull factors were defined to be education, job opportunities, security, stability, wealth. Finally, potential mission architectures were then considered including a complete trade space of population size, trip time and energy requirements. Hein said that one of the things which would determine the launch of a World Ship was the testing of several prototype vehicles first. He concluded that the mission architecture strongly depended on the habitable planet distance and that self-replication factories and advanced artificial intelligence would likely be requirements for World Ship technology.

The Worldship Rationale

Gerry Webb, a member of the original Project Daedalus Study Group, gave a presentation on “Why World Ships? An Enquiry into the Idea of World Ships and the Nature of Their Sponsoring Cultures.” The speaker first discussed definitions of World Ships and explored the science fiction literature. He said that the 1984 Anthony Martin and Alan Bond definition (see above) was the best one to adopt. He described World Ship concepts such as those presented in Arthur C. Clarke’s novel Rendezvous with Rama. He said it was important that every few decades we discuss the margins of World Ship definitions, looking for show-stoppers to ensure they are not neglected. The evolution of the World Ship concept was described and three reasons for their construction given: (1) saving the human race, (2) inevitable expansion of living space, and (3) actively driven colonization plans. Webb made the important point that humans began to envisage the concept of a World Ship as soon as our knowledge of the physical universe allowed us to do so.

Webb next considered the question of who would sponsor a World Ship, and why they would do it. He said that any society interested in building a World Ship will begin to do so when they have enough materials, energy, information and motivation for doing so. He added: “once the solar system economy is big enough, only the last [motivation] will present a problem for World Ship fulfilment, since ‘starship building man’ is in a very small minority now and may always be”. Threats to long-term human spaceflight prospects were defined to be collision risks, gravity, radiation risks, infections and social disharmony. He considered the factors that make humans what they are, and defined us as Genes (G), Culture (C) and Ideology (I) which are the hardware, operating system and program respectively. He said that all three of these elements would be of equal importance in attaining a human success in World Ship building, and hence our survival and expansion into the galaxy.

An important point made by Webb was that it had been found that approximately 1% of every nation on earth was made up of so-called psychologically difficult types, meaning manic depressives or those suffering from mental disease. This remarkable constancy, it was claimed, may have contributed to human cultural and ideological advances. In discussions, Kelvin Long pointed to the Arthur C. Clarke story The City & the Stars, which had a small instability element to the population. Webb argued that the government of a World Ship was very important in order to maintain stability. He referred to the Aristotelian organization of states in order of decreased stability as Tyranny (monarchy or totalitarian), Oligarchy (aristocracy or elitist) and Democracy (polity or republican). Those sponsoring a World Ship would be natural expansionists—ark builders—and the pressures to do so would be government funded programs, private funded, or internal pressures such as victimization or resource depletion. Finally, he said that given a big enough economy, World Ships would be inevitable, and each ship may be very different, based on different beliefs and ideologies. Darwinian natural selection would ensure that only the fittest would reach the destination stars. The others would simply not make it there. He ended with a cautionary note relating to the Fermi Paradox which may become more topical by the time that a World Ship is built and said that if alien World Ships exist, they may not be sympathetic to us.

Estimating a Launch Date

Stephen Ashworth gave a presentation on “A Development Roadmap for the World Ships.” The speaker raised the possibility that we may become extinct before we have a chance to build a World Ship. Ashworth thought that the construction of free-orbiting space colonies would be prototypes for World Ship living quarters. He discussed current propulsion technology such as that used by the DAWN spacecraft travelling at around 30 km/s, and speculated on future propulsion systems.

Ashworth strongly disagreed with the picture painted by some of an ‘Independence Day’ scenario where colonies hop from one world to the next to extract vital resources. He displayed graphs illustrating the sum of journey and exploration time against habitability time (the duration upon which we could inhabit artificial space colonies for a continuing duration). The point at which the figures cross over was said to be an estimate of the time at which we could likely launch the first World Ship. 2357 was estimated initially assuming progress in both life support and propulsion. However, if progress in propulsion is more rapid, Ashworth believes the first World Ship launch date could be brought forward to a much earlier date. If life support technologies progress but propulsion does not, then the launch date would be pushed back to around 2450. Alternatively, if progress in life support technologies and propulsion declined then the two curves may never cross each other, meaning we may never launch a World Ship.

Ashworth concluded by saying that much attention was being placed on finding an Earth-analogue planet, and although he thought this was a worthwhile pursuit of scientific study and recreation, he did not think it was the object of resource extraction or colonisation. In all likelihood, he said, humans would prefer to live in artificial orbiting space colonies rather than living on another planet. This view was not shared by all present. In particular Martyn Fogg (of terraforming fame) said that he would like to see the habitable planets and this would be a reason for making the journey. Ashworth ended by saying that World Ships would be a logical end point to the colonisation of the solar system — they are inevitable.

The Economics of Interstellar Growth

Finally, Frederik Ceyssens gave a presentation titled “On the Financing of World Ships & Other Gigascale Space Projects.” His co-authors were Maarten Driesen and Kristof Wouters. Ceyssens discussed the new age of discovery, in which astronomers are finding hundreds of exoplanets constantly. He argued that there will be more interest in exploring and settling on another habitable world. Such settlements may have greater potential for grand scale colonisation than a colony in our own solar system and also provide long-term survival and knowledge-generation benefits. The question for him was whether this interest is sufficiently strong to attract funding.

Ceyssens defined some of the challenges facing current space exploration, namely low-cost access to space, very high specific impulse propulsion, high reliability and lifetime for equipment, and creation of life support systems. His conclusion was that a World Ship would be an ultra long-term project and at high cost. He said that a World Ship would be equivalent to a ‘beefed up’ megaproject such as Project Apollo, megaprojects being defined as involving a steep threshold unsurpassable by normal economic or scientific development. As well as Project Apollo ($100bn) he discussed other megaprojects including the Manhattan Project ($22bn), ITER ($20bn) and the construction of either an interstellar probe ($1,000bn) or a World Ship ($10,000bn). He considered projections for World Ship investments assuming set annual growth figures.

Ceyssens’ proposal for achieving the eventual launch of a World Ship was the formation of a ‘Fourth Millennium Fund’ which he argued would be transparent, have global outreach, be lean with a low cost structure and have a single well defined goal, such as finding a second home for humanity. He was looking for people to invest today and to commit to such long term investments to ensure that a World Ship could be built in the future, once we could afford it. It is worth noting that Alan Bond made a comment that, in his opinion, money as we know it in the future will not be the economic system by which things are constructed and resources are allocated.

A Fallback Plan for Earth

During the subsequent discussions many competing views emerged from the attendees relating to World Ships. One of these was the view that before any World Ship would depart it would first identify a habitable planet to head towards. However, several attendees thought that the advanced crew (so called Starship Man) would no longer be interested in settling another planet but would instead be content to live in artificial structures in orbit.

The question of when such a mission would be launched was also debated. Some saw it as an inevitable gradual process of solar system colonisation placing it many centuries into the future, whilst others thought that circumstances on Earth (e.g. catastrophe, war, famine) may give rise to the construction and launch of such a mission much earlier than people think. This would also determine the stability of the crew aboard the World Ship. A mission planned centuries in advance with great care would have a well organised community on board paying attention to population control, policing and resource usage. But a World Ship constructed in a hurry may have less detail in the planning, with the result that only a dictatorial style rule on board the vessel would be capable of maintaining social stability. These sociological factors would likely have a significant impact on the success or failure of such a mission.

As we consider the future prospects of mankind on Earth and in space, it is important that we keep our options open. From the perspective of today’s society, with a large population, poor resource distribution, and in financial crisis, the prospects for building a World Ship some day seem at first sight to be pure science fiction. However, there is nothing in the laws of physics or in the many text books on engineering that suggest a World Ship is impossible to build. As indicated by Alan Bond during the discussions, a future society, perhaps in a better position than we are in today and with a more positive outlook on the Universe, may look upon such an enterprise as an equivalent undertaking to Project Apollo in the 1960s. Given the potential threats — internal and external — that humanity may face in the future, it is important we keep our options open and continue to strive for expansion into space. Designing World Ships is a lot of fun in the practice of extreme aerospace engineering, but it is also a possible fall-back plan should the future turn out to be bleak for humanity on Earth.

Image: Kelvin Long (left) and Alan Bond at the conference.

On a final note, we have to accept that if humans can conceive of a World Ship, then so could an extraterrestrial and they may be out there now star hopping, perhaps on their way here. Alan Bond pointed out that a nuclear pulse engine would be detectable from large interstellar distances and would appear to be a double-ping milli-second pulsar running down on the timescale of decades as it moves away from us. He said that our machines will colonize the stars before us, and so might the machines of another race. Perhaps we should be more concerned if the starships were coming towards us.

It is worth ending this article on a cautionary note from Arthur C. Clarke: “I can never look now at the Milky Way without wondering from which of those banked clouds of stars the emissaries are coming.”

The two original papers referenced in this article by Anthony Martin and Alan Bond are “World Ships – Concept, Cause, Cost, Construction and Colonisation” and “World Ships – An Assessment of the Engineering Feasibility,” both published in JBIS, volume 37, number 8, June 1984. Kelvin Long has also published a separate conference report on the Project Icarus site.


Entering the Age of Sail

I see that the new agenda for the 100 Year Starship Study symposium has now been posted. The meeting will be held in Orlando in about a month, set up along a number of parallel tracks from interstellar destinations to propulsion options and habitats, a wide-ranging set of sessions that will allow many in the far-flung interstellar community to exchange ideas in person for the first time. DARPA’s intention is to spur research and select an organization that will sustain and develop interstellar ideas over the next century, an exciting long-term prospect indeed.

That interstellar flight demands long-term thinking should be obvious given the state of propulsion research today. Over the last sixty years, numerous ideas on how to drive a vehicle to a substantial percentage of the speed of light have been advanced, but almost all of these remain no more than concepts in journals. We’re not remotely at the stage where we can choose a single option as the likely propulsion choice for development. Rather, we’re theorizing and experimenting and seeing where many different strands of thinking lead, all of which takes me to the solar sail.

Jim Essig was kind enough to pass along a recent story in Science News dealing with current experimentation and planning on solar sail ideas. Here we’re dealing with a technology that, unlike almost all the other interstellar options, has actually flown, even if we’re still in the preliminary phases of evaluating the technology’s performance. One day it may be that we’ll push enormous sails with laser beams or microwaves, but for now we’re talking about small attempts to measure the effect of momentum transfer by solar photons to push a spacecraft.

Image: A close-up of the fully deployed IKAROS sail. Credit: JAXA.

The Japanese success with the IKAROS sail has been marvelous to watch as the 20-meter sail became operational in interplanetary space, demonstrating an innovative liquid crystal technology that allowed mission scientists to change the sail’s reflectivity and affect its course. Bear in mind that IKAROS is to be the first of a family of Japanese sails, the next model being 50-meters across and intended for launch to Jupiter and the asteroids. That mission will focus on propulsion systems that combine the solar sail with an ion drive for interplanetary journeys.

We’ve talked extensively in these pages about IKAROS as well as NASA’s NanoSail-D, the latter whimsically called LunchSat because the budget-strapped team could only find time to work on it during lunch. Nonetheless, the doughty engineers under Dean Alhorn built two 3-meter sails that fit into a CubeSat, attempting to launch the first without success because of the failure of the Falcon 1 rocket that would have taken it into space. The second was launched last November, went silent for weeks, and suddenly deployed and reported in for duty. Alhorn has now set his sights on a larger sail to follow on to NanoSail-D, which will re-enter the atmosphere within months.

The NASA effort had downsized since the days when the agency was testing two 20-meter x 20-meter solar sails at a research facility in Ohio [although see Jack Crawford’s comment below], but the fact that NanoSail-D could become operational is a testament to the enthusiasm of scientists working the mission. And the new sail possibility is a 38-meter x 38-meter sail demonstrator, as discussed in this news release from the Office of the Chief Technologist.

The Science News story is well worth your time for its encapsuled analysis of where we are in solar sail development. We now look toward the Planetary Society, where Lou Friedman brings his own extensive sail experience to the building of three potential designs. From the story:

Closest in concept to the original grand dreams about solar sailing, yet freighted with the memory of a recent failure, is the LightSail project of the Planetary Society. Friedman, its architect, has seen pretty much everything in the world of solar sailing; he worked on the original Halley proposal in the 1970s and spearheaded the society’s drive to fly a privately funded sail in the early 2000s. That effort, paid for mainly by an entertainment company led by Carl Sagan’s widow, ended with a splash in 2005 when the Russian rocket it was supposed to ride from a nuclear submarine failed to reach orbit.

After licking his wounds, Friedman decided to work with NanoSail-D in its initial stages. That restored his enthusiasm and inspired LightSail. “We got so interested in the design that we said we’ll go further: We’ll instrument the craft and build in attitude control and a telemetry system,” says Friedman. Thanks to CubeSats, the sail could be built for less money than the society’s last, failed attempt.

Image: Chris Biddy (left) and Lou Friedman at Stellar Exploration (San Luis Obispo, CA) for the first full-scale deployment test of the sail on LightSail-1. Credit: The Planetary Society Credit: The Planetary Society.

Like Alhorn’s NASA team, the Planetary Society is building two sails in case something goes wrong, and envisions future versions that will not only achieve higher Earth orbit but travel to the L1 Lagrangian point between the Earth and the Sun. Other sail projects to watch with interest include a consortium from the University of Surrey and aerospace firm Astrium, an aerospace subsidiary of the European Aeronautic Defence and Space Company (EADS) that is building a 5-meter sail for CubeSat deployment for launch some time in 2012. The same team is studying sails as a way to deorbit satellites, removing space debris that poses a collision hazard in orbit — this is also part of the thinking behind the NanoSail-D concept.

And we’re still not through. The German space agency DLR and the European Space Agency are working on a series of solar sails called Gossamer that would experiment with larger sails as technology demonstrators of increasing size and complexity, while engineers at the University of Illinois at Urbana-Champaign are studying new deployment methods that involve spinning deployment of sail blades that could one day lead to a spinning sail with rotating blades as large as 10 kilometers long. As Science News points out, what will move the solar sail idea forward is not just this early experimentation, but the follow-on missions that target specific tasks that are optimized for sails and defy other propulsion techniques. In such ways does a promisng space technology begin to get the shakeout in space it needs to mature.

Image: ESA & DLR Project Gossamer in orbit. Credit: DLR.


Cassini’s Latest from Hyperion

We can thank the British astronomer John Herschel (1792-1871) for giving the moons of Saturn their classical theme, resulting in the familiar names Mimas, Dione, Enceladus, Tethys, Titan, Rhea, and Iapetus for the seven moons known to him when he wrote his Results of Astronomical Observations Made at the Royal Observatory, Cape of Good Hope (1847). It was a natural, then, that the moon discovered shortly thereafter would get a name like Hyperion (although it wasn’t Herschel but merchant and astronomer Willam Lassell who suggested the name). Hyperion was an elder brother of Cronos (Saturn), and was associated with watchfulness and observation.

Only recently have we discovered just how unusual Hyperion turns out to be, as the unprocessed image from the Cassini mission below clearly demonstrates. Its shape is irregular, indicating it may be the remnant of a larger body broken by some ancient impact. Its low density indicates large amounts of water ice mixing with small amounts of rock. Its low albedo suggests a layer of dark material that may be associated with Saturn’s moon Phoebe, much of which seems to have wound up on Iapetus. And it resembles nothing so much as an enormous sponge.

Image: NASA’s Cassini spacecraft obtained this unprocessed image of Saturn’s moon Hyperion on Aug. 25, 2011. Image credit: NASA/JPL-Caltech/Space Science Institute.

You can see more images of Hyperion from the encounter here. The image above was made as the spacecraft flew past Hyperion at some 25,000 kilometers. We’re looking at a very small place, some 270 kilometers across, and a world that tumbles chaotically in its orbit around Saturn, the motion making it difficult for scientists to predict what Cassini would see as it set about imaging the moon on this flyby. Color measurements made during this observation run will tell us much about the moon’s brightness as lighting changes, which in turn should offer some insight into its surface texture.

Back in 2007 the analysis of data from a 2005 Cassini flyby pointed to a surface composition of water and carbon dioxide ices along with the hitherto mentioned dark material that Dale Cruikshank (NASA Ames), lead author of the paper on that study, found interesting:

“Of special interest is the presence on Hyperion of hydrocarbons — combinations of carbon and hydrogen atoms that are found in comets, meteorites, and the dust in our galaxy. These molecules, when embedded in ice and exposed to ultraviolet light, form new molecules of biological significance. This doesn’t mean that we have found life, but it is a further indication that the basic chemistry needed for life is widespread in the universe.”

Many of Hyperion’s craters appear to be crisply defined, evidently the result of the moon’s low density. Just half as dense as water, Hyperion is porous enough to compress when struck by debris, but the ejecta of an impact often fails to return to the surface because of the moon’s low gravity. Denser worlds usually lack a large population of craters with such crisp visual definition.

Two papers on that earlier Hyperion work go into detail on all this. They are Cruikshank et al., “Surface composition of Hyperion,” Nature 448 (5 July 2007), pp. 54-56 (abstract) and Thomas et al., “Hyperion’s sponge-like appearance,” Nature 448 (5 July 2007), pp. 50-56 (abstract). A slightly later summary of the dark coating we see on various Saturnian moons can be found in Saturn’s Dark Materials, a Centauri Dreams article in which I look at research from 2008.


In the Sky with Diamonds

The idea of a planet around a pulsar is so bizarre that we often forget that three planets around the pulsar PSR B1257+12 were the first exoplanets ever detected. This pulsar is the remnant of a once massive star in the constellation Virgo that became a supernova, and the planets there — detected by Alex Wolszczan (Penn State) — were the first new planets discovered since the era when Clyde Tombaugh was putting the blink comparator through its paces at Lowell Observatory, an effort that led to the discovery of Pluto in 1930. And these are tiny worlds at that. A newly found fourth planet in the B1257+12 system is thought to be no more than one-fifth the mass of Pluto itself. We can find worlds like this because the beam of electromagnetic radiation pulsars emit is extraordinarily regular, making planetary signatures apparent.

Now another pulsar — PSR J1719-1438, some 4,000 light years away in the constellation Serpens (the Snake) — is in the news because of the discovery that its own pulses are being affected by the gravitational pull of a small planet. What we are learning about the new planet is highly interesting. It is slightly more massive than Jupiter, and orbits the pulsar at a distance of about 600,000 kilometers, racing around its primary in a scant two hours and ten minutes. The pulsar rotates more than 10,000 times per minute and has a mass about 1.4 times that of our Sun, but is only 20 kilometers in radius. At 600,000 kilometers out, a planet larger than 60,000 kilometers (five times Earth’s diameter) would be pulled apart by the pulsar’s gravity.

This must be, then, a small planet with a great deal of mass, which is why this story stands out. For what Matthew Bailes (Swinburne University of Technology, Melbourne) and colleagues are reporting is a planet that may itself be the remains of a massive star. The pulsar and its companion are close enough that the planet must in fact be what’s left of a white dwarf that has lost over 99.9 percent of its original mass. And that leaves us with an interesting relic, a remnant of carbon and oxygen at such high density that the star may be made largely of diamond.

The paper on this work states the matter clearly:

PSR J1719?1438 demonstrates that special circumstances can conspire during binary pulsar evolution that allows neutron star stellar companions to be transformed into exotic planets unlike those likely to be found anywhere else in the Universe. The chemical composition, pressure and dimensions of the companion make it certain to be crystallized (ie diamond).

Image: Artistic reproduction of an extrasolar planet around a pulsar. Copyright : Paris Observatory/UF.

Most of the mass of the so-called ‘diamond planet’ would have been drawn toward the pulsar. Interestingly enough, very fast-spinning pulsars like this one — astronomers call them millisecond pulsars — normally have companions of some kind. In fact, as many as 70 percent of them do. According to this CSIRO news release, some astronomers believe that such companions, when burning as a star, would be responsible for transferring matter to the pulsar and spinning it up to its high speed. The result over time: A millisecond pulsar keeping company with a white dwarf.

This configuration of pulsar and white dwarf makes sense, but finding former white dwarfs that have survived destruction only to become crystalline planets is not likely to be common:

“The ultimate fate of the binary is determined by the mass and orbital period of the donor star at the time of mass transfer. The rarity of millisecond pulsars with planet-mass companions means that producing such exotic planets is the exception rather than the rule, and requires special circumstances,” said Dr. Benjamin Stappers (University of Manchester).

We should learn a great deal more about pulsars from the project this work grew out of, a search for pulsars that is the largest and most sensitive of its type ever attempted. It will doubtless identify more pulsar planets, and probably more intriguing circumstellar disks of the kind already found around the pulsar 4U 0142+61. Planets, as we’re realizing more and more, seem to find myriad ways to form even after events as massive as a supernova. The paper is Bailes et al., “Transformation of a Star into a Planet in a Millisecond Pulsar Binary,” published online in Science Express August 25 2011 (abstract).


WISE: Coolest Brown Dwarfs Yet

The WISE mission has again come through, this time in the form of a discovery we’ve been more or less anticipating but now see confirmed. The Wide-field Infrared Survey Explorer works at infrared wavelengths ideal for spotting things we just can’t find with ground-based telescopes. WISE has now turned up six Y dwarfs, stars so cool that you could set your office thermostat to match them without real discomfort. The Y dwarfs range from nine to 40 light years away.

Consider them the coldest class of brown dwarfs, completely incapable of reaching the temperatures needed to induce stable fusion at the core, their light gradually fading with time. And if the line between gas giant planets and brown dwarfs was ever malleable, it’s here. The atmosphere of these stars is similar to that of Jupiter, and one of them, WISE 1828+2650, now becomes the coldest brown dwarf known, its estimated atmospheric temperature something less than 25 degrees Celsius. Says WISE science team member Davy Kirkpatrick (Caltech):

“The brown dwarfs we were turning up before this discovery were more like the temperature of your oven. With the discovery of Y dwarfs, we’ve moved out of the kitchen and into the cooler parts of the house.”

Look closely at the center of the image below and you’ll see WISE 1828+2650.

Image: NASA’s Wide-field Infrared Survey Explorer, or WISE, has uncovered the coldest brown dwarf known so far (green dot in very center of this infrared image). Called WISE 1828+2650, this chilly star-like body isn’t even as warm as a human body, at less than about 80 degrees Fahrenheit (25 degrees Celsius). Like other brown dwarfs, it began life like a star, collapsing under its own weight into a dense ball of gas. But, unlike a star, it didn’t have enough mass to fuse atoms at its core, and shine steadily with starlight. Instead, it has continued to cool and fade since its birth, and now gives off only a feeble amount of infrared light. WISE’s highly sensitive infrared detectors were able to catch the glow of this object during its all-sky scan, which lasted from Jan. 2010 to Feb. 2011. WISE 1828+2650 is located in the constellation Lyra. The blue dots are a mix of stars and galaxies. Credit: NASA/JPL-Caltech/UCLA.

But it’s Michael Cushing (JPL), who is lead author of the Y dwarf paper in the Astrophysical Journal, who gets my attention. He’s taking note of the fact that one of the Y dwarfs, WISE 1541-2250, may move past Ross 154 to become the seventh closest star system known, at approximately nine light years out. And Cushing is thinking the data harvest is hardly over:

“Finding brown dwarfs near our sun is like discovering there’s a hidden house on your block that you didn’t know about,” Cushing said. “It’s thrilling to me to know we’ve got neighbors out there yet to be discovered. With WISE, we may even find a brown dwarf closer to us than our closest known star.”

There’s that thought again, a brown dwarf closer than Proxima Centauri, and it’s still a possibility. But whether such a star exists or not, the Y dwarfs we’re now finding should be useful in their own right. From the paper:

Independent of their spectral morphology, the study of these ultracool brown dwarfs will provide important insights into both ultracool atmospheric physics and the low-mass end of the stellar mass function. Because brown dwarfs and exoplanets have similar atmospheric conditions, ultracool brown dwarfs are also excellent exoplanet analogs that can be used as benchmarks for model atmospheres. The study of these ultracool brown dwarfs will therefore directly inform the interpretation and characterization of exoplanets detected with the next generation of high-contrast imagers…

It’s interesting to reflect on how the brown dwarf story has developed. The existence of this category of star was predicted in the early 1960s, but it took projects like the Two Micron All Sky Survey (2MASS), the Sloan Digital Sky Survey and the the Deep Near-Infrared Southern Sky Survey to start turning them up in bulk. But as the paper on the Y dwarf discovery notes, these successes still left a gap of almost 400 K between the coolest brown dwarfs then known (with an effective temperature of 500 K) and Jupiter (approximately 124 K). The existence of a cooler Y class to follow on to the brown dwarf spectral classes L and T seemed more and more likely, and now we have hard evidence for objects too cool to be detected by the earlier surveys.

The paper is Cushing et al., “The Discovery of Y Dwarfs Using Data from the Wide-field Infrared Survey Explorer (WISE),” accepted for publication in the Astrophysical Journal (preprint). You’ll also want to see Kirkpatrick et al., “The First Hundred Brown Dwarfs Discovered by the Wide-field Infrared Survey Explorer (WISE),” accepted for publication in the Astrophysical Journal Supplement Series (preprint).