by Paul Gilster | Aug 18, 2005 | Sail Concepts
James Benford’s JPL experiments pushing an ultralight carbon sail with a microwave beam were the first solid demonstration that the beamed sail concept would work. Both James and brother Gregory were deeply involved in the design of the Cosmos 1 solar sail mission, and understandably disappointed that its microwave experiment — aimed at demonstrating a microwave push on the orbiting craft from the Deep Space Network’s Goldstone antenna on Earth — was never completed.
But an interesting offshoot of the JPL study was that while photon pressure on the sail was clearly demonstrated, the power of the beam did not account for all the observed acceleration. Something else was clearly at work, evidently the evaporation of absorbed molecules from the hot side of the sail, a phenomenon known as desorption. In a March, 2005 paper for Acta Astronautica, the Benford brothers suggest using this effect to achieve additional thrust over conventional solar sail designs. In fact, a microwave sail designed around these principles could have advantages over solar and laser sails.
The reason: microwaves do not damage sail materials and can heat them less destructively than lasers. So why not paint a sail with a compound that can sublime away with heating, giving the sail a powerful boost early in the mission and allowing it, once the coating has been expended, to function as a standard solar sail? Such a combination would allow a beamed sail in low-Earth orbit to be boosted quickly into an interplanetary trajectory; it would then use solar photon pressure for the duration of the journey, having started out at much higher speeds than would have been possible without the coating.
“Solar sails are plagued in mission plans by low accelerations, which dictate long orbital times,” the authors say. “Laser sails have problems with atmospheric distortion if the laser beam is fired from the ground, which microwave beams do not. A natural collaboration emerges between subliming sails driven by beams in LEO, converting to greatly accelerated solar sails for the long mission.”
The coating itself needs to be easy to apply to a sail surface, and one that sublimes readily when heated by microwaves. Finding candidate compounds — and the Benfords discuss the possibilities in this paper — would allow remarkable advantages. “The upper temperature range of thermal desoption-driven sails,” they write, “promises higher specific impulse than liquid rockets…A major thrust of future work should be to study such embedding and the resultant thermal desorption rates of both painted materials and desorption of embedded atoms.”
The paper is Gregory Benford and James Benford, “Acceleration of Sails by Thermal Desorption of Coatings,” in Acta Astronautica Vol. 56, No. 6 (March, 2005), pp. 593-599. James Benford’s thoughts on the design for a future Cosmos-series sail can be found here. An earlier Centauri Dreams article discussing thermal desorption is also available.
by Paul Gilster | Aug 17, 2005 | Deep Sky Astronomy & Telescopes |
Getting an overview of our own galaxy is tricky work. After all, we live in one of its spiral arms, so we see through a swarm of surrounding stars that mask the true galactic shape. Astronomer Ed Churchwell at the University of Wisconsin describes the effort as an attempt to define the boundaries of a forest from a vantage point deep within the woods.
But when it comes to stars, changes of wavelength can help. Working in the infrared, the Spitzer Space Telescope can see through intervening clouds of interstellar dust to the Milky Way’s dazzling center. Churchwell and team’s latest work is a survey of 30 million stars using Spitzer data that has revealed details about what the Milky Way looks like from the outside. The picture, as shown in the illustration, is a bit different than we had been led to expect.
For cutting through the galactic center is what Churchwell calls a “long central bar.” Other galaxies have been observed with stellar bars — large bodies of gas, dust and stars. In fact, approximately one-third of all spiral galaxies are barred. No one is exactly sure how the bars form, but they seem to rotate as if they were solid objects and have the effect of channeling interstellar gas inward toward galactic center.
Within a bar, stars no longer follow circular orbits but move in more radial orbits that pass through the center of the galaxy. Suspicions had been growing that the Milky Way was a barred galaxy, but this new study seems to confirm the theory. The Milky Way’s bar consists of relatively old and red stars in a band perhaps 27,000 light years in length, longer than any previous estimates. In this era, the bar is tilted at about a 45 degree angle relative to a line between our own Sun and the center of the galaxy.
Churchwell’s team will publish its work in an upcoming edition of Astrophysical Journal Letters. Image credit: NASA/JPL-Caltech/R. Hurt (SSC/Caltech).
by Paul Gilster | Aug 16, 2005 | Exoplanetary Science
We now have 156 confirmed extrasolar planets orbiting 133 stars, making for 17 multiple planet systems. Keeping up with these fast-breaking discoveries is a challenge, but Julia Espresate at the Instituto de Astronomía (Ciudad Universitaria, Mexico) has produced two useful catalogs now available on the arXiv site. The first lists stellar data including spectral type, luminosity, rotation period, stellar metalicity, age and other factors for the 133 stars. The second provides a breakdown of data for the 156 extrasolar planets so far detected.
Good information on extrasolar planets has been available on the Internet for a while, as witness the Extrasolar Planets Encyclopaedia. What has been lacking is a source that ties together the planetary information with data on the characteristics of the parent stars — the latter tend to be widely scattered in the scientific literature. Thus the utility of Espresate’s work, which also reveals how great are our gaps in knowledge of these systems. Almost 40 percent of the stars examined have no reported rotation period, for example, while 10 percent have no reported luminosity class.
We are, in short, finding planets much faster than we can study them in detail. Espresate’s paper “Catalog of 156 Confirmed Extrasolar Planets and Their 133 Parent Stars” is to be published in Revista Mexicana de Astronomía y Astrofisíca.
by Paul Gilster | Aug 15, 2005 | Astrobiology and SETI, Autonomy and Robotics |
How did the mechanism for protein synthesis — the ribosome — come into being? Answering that question would be useful not just in the study of life on Earth, but also in learning where else in the universe we might expect to find life. Intense work on the subject is ongoing at the University of Houston, where a team led by George E. Fox, a professor of biology and biochemistry, is studying how protein synthesis began and evolved.
Protein synthesis happens when RNA copies genetic information from DNA and turns that raw data into proteins that are essential to the functioning of living cells. “Since many of the components of the ribosome are shared by all organisms, we know this machinery is very, very old,” Fox said. “If we can discover the earliest aspects, then scientists may be able to devise experiments to see how simple RNAs might have given rise to this machinery. This information would help us to better understand how life evolved on Earth and how ribosomes actually work, which remains a fundamental problem in biochemistry.”
One way you can study these things is by examining ‘extremophiles,’ forms of life that exist in environments once thought too extreme to sustain life. “All known living organisms on Earth share various biochemical properties, such as the same genetic code, the same major amino acids in proteins, and – with minor exceptions – the use of DNA or occasionally RNA as genetic material,” Fox said. “This suggests life had a single origin from an earlier ‘prebiotic’ world…”
In a separate endeavor, partnering with the Houston Museum of Natural Science, Fox and team have created a movie called “Fantasy Worlds: Exploring the Limits of Life,” which looks at known extremophiles and uses animations to explore the kind of planets on which such organisms might thrive. To this point, the movie is available only through showings at the museum in Houston, but one hopes for wider distribution.
Meanwhile, the final stage of a project to develop a robotic astrobiologist is on track. Zoë is an autonomous rover that runs on solar power and is equipped to detect micro-organisms, which it will do during a two-month stay in Chile’s barren Atacama Desert. Last year, the robot traveled 55 kilometers autonomously and found living organisms using its Fluorescence Imager (FI) to locate chlorophyll and other organic molecules.
Image: Zoë in Chile’s Atacama Desert. Finding life in Earth’s most inhospitable terrain helps us tune up techniques of robotic autonomy that may one day uncover living organisms on other worlds. Credit: Carnegie Mellon University.
“This is the first time a robot is looking for life,” said Carnegie Mellon associate research professor David Wettergreen, who leads the project. “We have worked with rovers and individual instruments before, but Zoë is a complete system for life seeking. We are working toward full autonomy of each day’s activities, including scheduling time and resource use, control of instrument deployment and navigation between study areas.”
Zoë’s exploits in the Atacama have been the subject of an earlier story in Centauri Dreams. The desert’s interior is so dry that parts of it receive no precipitation for decades at a time. The robot will be guided remotely from an operations center in Pittsburgh as it looks for proof of life and maps the distribution of the various habitats. More on Zoë’s mission can be found here.
by Paul Gilster | Aug 12, 2005 | Astrobiology and SETI, Culture and Society |
Enrico Fermi’s famous question “Where are they?” continues to resonate among scientists and laymen alike. After all, shouldn’t the universe be teeming with life, and hasn’t intelligent life had enough time to spread through our own galaxy? Some estimates put the average age of Earth-like planets in the Milky Way at 6.4 billion years, whereas our own Earth is 4.5 billion years old. Some biospheres, in other words, may have had a two billion year jump on us. Shouldn’t we be seeing signs of extraterrestrial life?
One intriguing solution to the Fermi paradox appears in Karl Schroeder’s novel Permanence (New York: Tor Books, 2002). Using a hypothesis from evolutionary biology called ‘adaptationism,’ Schroeder’s protagonist argues that consciousness is not necessarily required for toolmaking. “In fact, consciousness appears to be a phase. No species we have studied has retained what we could call self-awareness for its entire history. Certainly none has evolved into some state above consciousness.”
This view is so strikingly at variance with our conventional view of the universe that it brings most readers (including this one) up short. Astrophysicist Milan M. ?irkovi? (Astronomical Observatory of Belgrade), who discusses Schroeder’s work in a recent issue of the Journal of the British Interplanetary Society, puts the case this way: “…our estimates and expectations of the phenomenon of intelligence…are wrong. Intelligence is significant only insofar as it offers an evolutionary advantage, a meaningful response to the selective pressure of the fluctuating environment. Only so far, and no further is the ‘selfish gene’ willing to carry that piece of luggage.”
Image: The Sombrero Galaxy, some 50 million light years from Earth. According to some theories, galaxies like this may be aswarm with life, but lack long-term technological cultures. Credit: NASA/Hubble Heritage Team.
?irkovi?, it should be noted, is not promoting Schroeder but simply arguing that the ideas found in Permanence deserve wider scrutiny, a fact that seems undeniable given how unsuccessful we have been at resolving the Fermi question. Adaptationism itself is drawn from the work of biologists like Richard Dawkins and John Maynard-Smith. The argument implies that adaptive traits disappear once the environment changes enough that the advantage gained by those traits is no longer functional. Extending adaptationism, then, we can say of intelligence that it too will disappear once its selective advantage is changed by local conditions.
Here’s Schroeder’s protagonist again, working these themes to their logical conclusion:
It’s the same with consciousness. We know now that it evolves to enable a species to deal with unforeseen situations. By definition, anything we’ve mastered becomes instinctive. Walking is not something we have to consciously think about, right? Well, what about physics, chemistry, social engineering? If we have to think about them, we haven’t mastered them — they are still troublesome to us. A species that succeeds in really mastering something like physics has no more need to be conscious of it. Quantum mechanics becomes an instinct, the way ballistics already is for us.
The result is that we make a supreme adaptation to our own technology, in essence being swallowed up in it. Schroeder continues:
Originally, we must have had to put a lot of thought into throwing things like rocks or spears. We eventually evolved to be able to throw without thinking — and that is a sign of things to come. Some day, we’ll become…able to maintain a technological infrastructure without needing to think about it. Without need to think, at all…
And there’s your solution to the Fermi paradox: we don’t find evidence of a galactic civilization because technological culture is transient. Technical societies eventually move into a state of fragmented habitats as they expand into their local stellar neighborhoods. Ultimately, they revert to direct biological adaptation. Life persists but in the form of species adapted to their environment in ways that move them away from higher-order intelligence.
In this view, intelligence is no more exceptional than the color of a butterfly’s wings. In the long run, it is completely irrelevant. These challenging and in some ways aggravating thoughts are explored much more fully in Schroeder’s book and in ?irkovi?’s excellent essay. His paper is “Permanence — An Adaptationist Solution to Fermi’s Paradox?” in JBIS 58 (January/February 2005), pp. 62-70. Even the casual reader will find material for any number of further science fiction stories in these pages.
