by Paul Gilster | Jun 24, 2008 | Exoplanetary Science
The dusty disks around other stars can tell us much about how planets form, creating a catalog of systems in various stages of development. But some of the best evidence for our own system’s formation has to be dug out of the ground. It’s based on the chondrules found in certain meteorites that seem to have been formed in the earliest stages of its life. They’re small, round objects about a millimeter in size, made of glass and crystal and thought to have been formed by the flash heating of dust. We’re talking major heat here, up to 2000 degrees Celsius.
A new study of chondrules is unusual because it finds higher levels of sodium than ought to be there. That’s problematic because the heat of chondrule formation, under existing theories, should have boiled off volatile chemical elements. Here’s Conel Alexander (Carnegie Institution) on the matter:
“Chondrules formed as molten droplets produced by what was probably one of the most energetic processes that operated in the early solar system. You would expect all the sodium to evaporate and be lost from the chondrules under such conditions. Instead, the sodium was retained. The chondrules stayed as effectively closed systems throughout the heating and melting.”
What does the sodium mean? Only if the chondrule formation took place in a far denser dust cloud than we had previously imagined could this element have been retained. The researchers are talking about a dust density at least one hundred times the densities previously considered. At that density, intriguing small objects begin to form, collapsing under their own gravity. Chondrules, in other words, may have been linked to the formation of planetesimals, the kilometer-sized objects that are the precursors to larger, rocky planets like our Earth.
Image: False color micrograph of a chondrule from the Semarkona meteorite. Red indicates olivine crystals, blue and green are glass. High-density dust prevented sodium in the glass from evaporating during chondrule formation, despite high temperatures. Scale bar = 0.5 mm. Credit: Carnegie Institution.
The picture that emerges is one that can only be filled out by future work on how dust grains evolve into the clumps that become planetesimals. Huge issues remain. What was the cause of the heating that produced the chondrules in the first place? Note too that the team sees chondrule formation occurring only in small regions of the solar nebula, those with the highest densities of solids. If their theory is correct, this should explain how materials that were not processed the way the chondrules were can show up in the same chondritic meteorites. Here the mechanism seems reasonable, if highly complex. Quoting from the paper:
Given the high densities of solids, it is likely that whatever event was responsible for chondrule formation would have processed solids in that region to a different extent than materials further away… [A] low velocity shock ( 2-3 km/s) could suffice to melt the chondrules in the very dense clump, but minimally process materials outside the clump, allowing pre-solar grains or CAIs [calcium-aluminum-rich inclusions] located thousands to ten thousands of kilometers away to show no signs of thermal processing. If the chondrules then become gravitationally bound after their processing (due in part to the increased density that accompanies processing in shock waves) they will contract on timescales of a few orbits. This would allow the unprocessed materials to diffuse into the self-gravitating clump before it completely collapses.
Untangling that process is going to be no easy matter, but the presence of chondrules making up between twenty to eighty percent of this class of meteorite tells us their mixing with volatiles like sodium must be explained. So that primordial dust cloud must have had patches of considerable density, perhaps a clue as we try to identify other infant disks that may evolve into systems like our own. The paper is Alexander et al., “The Formation Conditions of Chondrules and Chondrites,” Science Vol. 320. No. 5883 (20 June 2008), pp. 1617-1619 (abstract).
by Paul Gilster | Jun 23, 2008 | Deep Sky Astronomy & Telescopes |
Have the laws of physics stayed the same throughout the history of the cosmos? It’s an interesting question because even minute changes to physical constants could imply the existence of extra dimensions, of the sort posited by string theorists. But that’s a big ‘could’, because despite earlier controversial findings, at least one cornerstone constant — the ratio of a proton’s mass to that of an electron — looks to be exactly the same in a galaxy some 6 billion light years away as it is when we measure it on Earth. A study led by Michael Murphy (Swinburne University) presents the result in a recent issue of Science.
The constant, known as mu, determines the value of the strong nuclear force, so it has everything to do with how atomic nuclei hold themselves together. No one can say why the mass of a proton should be 1836 times that of an electron. All we know is that it is. To be more precise, the value is 1836.15. The recently published research studied light from the quasar B0218+367, examining how it was partially absorbed by ammonia gas in an intervening galaxy on its way to Earth-based astronomers. It’s a useful measurement because the wavelengths at which ammonia absorbs energy from the quasar turn out to be quite sensitive to mu.
Take a look at the image below to get an idea of how key gravitational lensing has proven to be in this work. The quasar B0218+367 is about 7.5 billion years away. Two things are happening to its light as it moves toward us. First, its wavelength is being stretched, making it redder the farther it travels. And usefully for us, the light is being gravitationally lensed by the intervening galaxy six billion light years away. The result: Two quasar images and one extremely helpful set of data.
Image: Radio contour map of the quasar B0218+367 at about 7.5 billion light years distance. The galaxy containing absorbing ammonia molecules lies about 6 billion light years away and, though it is not seen in this radio map, gravitationally lenses the background quasar light to produce two bright quasar images on the sky (big red circles). The physical size of the image (at the distance of the absorbing galaxy) is about 19,000 light years across. Credit: Andi Biggs (MERLIN Image).
Christian Henkel (Max Planck Institute for Radio Astronomy) sees a clear result: “By comparing the ammonia absorption with that of other molecules, we were able to determine the value of the proton-electron mass ratio in this galaxy, and confirm that it is the same as it is on Earth.”
While we tend to assume that the laws of physics are the same everywhere, it’s an assumption that has to be verified by observations of different times and places in the cosmos. For that matter, the four fundamental forces of nature — gravity, electromagnetism, and the strong and weak nuclear forces — can’t be predicted from our theories, but can only be measured by experiment. That points to a major hole in our understanding of how physical constants govern the universe.
Finding out whether these constants remain the same is a prerequisite for deepening our understanding of how they emerge. And despite the earlier claim (in a Dutch study of 2006) that small differences in mu were observable, this new work is the first to use ammonia molecules, which turn out to be ten times more sensitive than any previous method (the Dutch team used molecular hydrogen). What’s next is to move the investigation beyond the confines of a single galaxy to weigh the value of mu in other eras, but as the details of the lensing involved in this experiment make clear, finding the right targets is a significant challenge.
The paper is Murphy et al., “Strong Limit on a Variable Proton-to-Electron Mass Ratio from Molecules in the Distant Universe,” Science Vol. 320. No. 5883 (20 June 2008), pp. 1611-1613 (abstract).
by Paul Gilster | Jun 20, 2008 | Asteroid and Comet Deflection |
Asteroid and comet impacts seem to be obvious culprits in mass extinctions on Earth. The heavily cratered Moon reminds us how severe earlier bombardments have been, and it’s an easy segue to note that 23 extinction events are now thought to have occurred since the beginnings of life on our planet. In the past 540 million years (the period during which abundant animal life has existed), we can identify five mass extinctions, with huge losses in particular to marine plants and animals.
The Chicxulub crater in the Yucatan is a striking piece of evidence for this scenario, but massive volcanic activity may well have played a role, and perhaps a major one. And what of the other extinctions? A new theory published in Nature seems to put a damper on the easy correlation of extinctions with impacts. Indeed, Shanan Peters (University of Wisconsin-Madison) argues that the largest factor may have been changes in ocean environments related to sea level. Says Rich Lane (National Science Foundation): “Impacts, for the most part, aren’t associated with most extinctions. There have also been studies of volcanism, and some eruptions correspond to extinction, but many do not.”
What Peters argues is that the expansion and contraction of the world’s oceans, caused by the shifting of tectonic plates and changes in climate, caused massive marine extinctions as sea levels declined. A case in point is the shallow sea that covered much of North America during the era of the dinosaurs. As it drained, animals like mosasaurs and giant sharks went extinct while life on the marine shelves changed irrevocably. Peters calls sea level changes a ‘forcing mechanism,’ one that correlates with many — not all — mass extinction events. “These results,” says the scientist, “argue for a substantial fraction of change in extinction rates being controlled by just one environmental parameter.”
All of which adds a cautionary note as we discuss the still vital need to catalog and perhaps one day intercept Earth-crossing objects. The danger they pose is real and one that should be a driver for new space technologies in support of a defensive mission we should hope we never have to fly. But in presenting that case, we should also be aware that extinction events have a varied and still unfolding set of causes, one in which impacts from the skies may play a smaller role than we had previously suspected. Good science demands that we get the facts right as we work to place impact events in a sound historical context.
The paper is Peters, “Environmental determinants of extinction selectivity in the fossil record,” published online in Nature (15 June 2008). Abstract available.
by Paul Gilster | Jun 19, 2008 | Culture and Society |
Projects that take more than a single generation to complete — the Ultimate Project that would build a multi-generational starship is a classic example — keep the issue of long-term thinking bubbling in these pages. The immense distances to the stars almost force the issue upon us. I’m reminded of something Hoppy Price told me at JPL five years ago. I was researching my Centauri Dreams book and we had been discussing the idea that scientists should see the end of the projects they start.
“Robert Forward talked about getting there in fifty years or less, a time scale that seemed to make sense because it would equal the possible lifetime involvement of a researcher,” Price said. “What may be more reasonable is to take a little more time. Because we’re also working on the beginnings of a program to build very long lifetime electronics, systems that can operate for up to two hundred years. If you let yourself take two, even three hundred years to get there, the problem of propulsion becomes a bit easier. We as a culture may have to start thinking in terms like that. The average worker on a medieval cathedral didn’t live to see it completed. My view is that the first time we send something to Alpha Centauri, it will probably take hundreds of years to get there.”
As NASA’s lead investigator on solar sails, Price had been thinking about these things for some time before I walked into his office that day. Note the medieval cathedral he mentions, a frequent reference in such discussions (and see the comments to yesterday’s story for more). It may seem hard to believe, but long-term missions to Alpha Centauri aren’t hugely beyond today’s technologies. Although the basics of the Innovative Interstellar Explorer design have changed, Ralph McNutt (Johns Hopkins Applied Physics Laboratory) has studied systems that could take a probe to 1000 AU in less than fifty years. Now imagine that system ramped up to move ten times faster. The resultant craft would reach Alpha Centauri in about 1400 years.
1400 years. Buildings on Earth — the Hagia Sophia in Constantinople, the Pantheon in Rome — have been maintained for longer than that.
Image: An 18th Century view of the Pantheon by Italian artist Giovanni Paolo Pannini. If we can keep a building alive for over a millennium, can we do the same with a spacecraft?
We all want to see faster propulsion technologies (this is why the Tau Zero Foundation is trying to support ongoing research through philanthropy). But it’s interesting to see how we cope with long-term solutions to things. In the business world as opposed to the ecclesiastical realm of cathedrals, we have abundant examples of companies that have been handed down for centuries within the same family. Construction firm Kongo Gumi, for example, was founded in Osaka in 578, and ended business activity only last year, being operated by the 40th generation of the family involved. The Buddhist Shitennoji Temple and many other well known buildings in Japanese history owe much to this ancient firm.
And here’s another, also from Japan. Hoshi Ryokan is an innkeeping company founded in Komatsu in 718 and now operated by the family’s 46th generation. If you’re ever in Komatsu, you can go to a hotel that has been operated on the site ever since. Nor do we have to stay in Japan. Fonderia Pontificia Marinelli has been making bells in Agnore, Italy since the year 1000, while the firm of Richard de Bas, founded in 1326, continues to make paper in Amvert d’Auvergne, providing its products for the likes of Braque and Picasso. I’m drawing this list from The 100 Oldest Companies in the World (thanks to Stewart Brand’s Long Now Foundation for the pointer) and continuing to muse on our species’ ability to work and preserve.
Image: Japan’s Ise Shrine (Mie prefecture, Japan). The wooden temple complex has been rebuilt every twenty years for the last thousand, a classic example of long-term effort in maintaining a structure.
We live in a world of immediate satisfaction, snapping up luxury goods with every tick of the eighteen month cycle of the digital revolution, but it’s good to step back and reflect on the things that last. Ultimately, exploration is less about the individual and more about the species, as is scientific discovery itself. And I would wager that if we do fail to find a means for shortening the interstellar trip in the next century or two, we’ll still make the journey. Maybe with generation ships, maybe with robotics, and using who knows which currently feasible system of propulsion. Let’s keep thinking about innovative technologies, but let’s remember that in the broader scheme, our species has shown it can go for the long haul.
by Paul Gilster | Jun 18, 2008 | Missions |
By Larry Klaes
Tau Zero journalist Larry Klaes takes on an old subject with a new twist: The multi-generational starship. It’s a familiar trope in science fiction (think Brian Aldiss’ Non-Stop or Heinlein’s ‘Universe’), but one given modern impetus in the hands of a small team of visionaries dedicated to making it happen. These guys think big, not just in terms of ship size but trip duration (ten thousand years!), and envision at least 500 years as the time needed to get their project ready to launch. Always a promoter of long-term thinking, Centauri Dreams follows the improbable tale with considerable interest.
Despite how they appear to us in the night sky and the relative ease and speed with which spaceships in most science fiction stories fly to them, the twinkling stars in the heavens are, in reality, immensely far away. The few robotic probes that have left our Solar System faster than any other vehicles yet built would not — if aimed in their direction — reach the the nearest stars for 77,000 years. Spaceships that could attain speeds approaching that of light (186,000 miles per second), while theoretically possible, have many technological and physics hurdles to overcome and are a long way from being built.
Due to this reality, scientists began contemplating in the last century how humans could reach other star systems alive aboard vessels that could become practical in the not too distant future.
Birth and Death Among the Stars
One idea that was quickly taken up by science fiction authors is the multigenerational starship. A large selection of people would be placed aboard a giant spacecraft with the necessary tools and resources to survive the many centuries it would take a relatively slow-moving vessel to reach another star system. The crew members who eventually arrive at the target sun and its circling worlds would be the distant descendants of the original explorers, ready to disembark from their ship and settle in these new lands.
Image: It’s a big sky. Can we do better than the 77,000 years it would take a Voyager-class spacecraft to reach the Centauri stars? In the image, Alpha Centauri is visible just left of center, a triple system whose light here appears as a single bright dot. Credit: Claus Madsen/ESO.
One group of individuals has been inspired enough by the multigenerational starship concept to begin the first serious efforts to make the idea not just a reality but to turn it into a purposeful focus for all of humanity – to expand into the Milky Way galaxy and save our species from any threats of extinction.
Grandly called The Ultimate Project, the plan calls for building a cylindrical starship over one mile long and one mile wide weighing 100 million tons that would carry one million people across interstellar space for 10,000 years or more to colonize an inhabitable Earthlike planet that astronomers hope to find within the next few decades.
Finding Another Earth
The original designer of The Ultimate Project is Dr. Steven Kilston of Ball Aerospace & Technologies Corporation in Boulder, CO. When Kilston became manager of Ball’s Terrestrial Planet Finder (TPF) program ten years ago, he began to wonder how humanity would respond to the discovery of a planet much like our world circling another star.
“My answer was that we should go there, and I began discussing with many aerospace industry colleagues the practical considerations that could enable that,” replies Kilston. In January of 1999, he presented his first poster on The Ultimate Project to the American Astronomical Society (AAS). Kilston has since presented his idea at JPL, MIT, and many other institutions.
Kilston envisions a project and ship that would take over 500 years and cost $50 trillion to complete before it even leaves the Solar System. Spinning at one revolution each minute to create a sense of gravity similar to Earth’s surface for the comfort of its one million residents, the ship would use nuclear fusion engines to eventually move at 373 miles per second through the Milky Way galaxy. In addition to the relatively slow speed being an attainable velocity target, a slower travel rate means there will be less chance of a dangerous impact with space debris. A layer of water in the ship’s outermost deck would provide protection for the crew from cosmic radiation.
With the rapid advancements in technology and our understanding of the Universe happening every day, some might wonder why humanity should invest in a project that will take centuries to build and thousands of years to transport people to another star system when a better means of reaching the stars might come along in the near future.
“If people thought like that about computers, they would never buy one,” says Kilston, who also cites Europe’s medieval cathedrals and the Great Wall of China as examples of some successful multigenerational projects. “Also, having experts, machines, and materials on board, together with maintaining communications with Earth throughout the voyage, would mean that any improved technology could be incorporated quickly in situ, possibly keeping this ship ahead of later ones.”
Surviving the Journey
Another concern regarding multigenerational starships is the possibility of the crew’s society degenerating into barbarism and forgetting its original purpose after centuries of isolation from Earth. Most of the science fiction stories that take place aboard such vessels assume this situation.
Sven U. Grenander, a senior engineer at JPL, a “self-assigned Rogue Technologist”, and manager of the formation and start-up of The Ultimate Project, thinks it is a certainly that factionalism and barbarism will flourish aboard the starship without a written constitution that can survive the long trip.
“The constitution has to serve as a fractal seed that can grow in a predictably orderly fashion and not be overtaken by chaos or lawlessness,” says Grenander. “The constitution is the most important element of the project as it is the only thing that will keep the human crew from spiraling off into any number of project-defeating directions.”
As for how the average citizen can become involved, Nancy J. Grenander, who manages the project’s operations and business structure, says she “would like to see The Ultimate Project stimulate young minds so that they can think about the possibilities of doing something that some may think impossible. I want to see them let their imaginations go forward even though they might not see the completed project.”
“If you want to build a ship, don’t drum up the men to gather wood, divide the work and give orders. Instead, teach them to yearn for the vast and endless sea.”
– Antoine de Saint Exupéry, author of The Little Prince.
