by Paul Gilster | Apr 19, 2005 | Deep Sky Astronomy & Telescopes
More on the ‘fine structure constant,’ that fundamental number that seems to be crucial to our understanding of electromagnetism, and therefore the way the universe works. Our recent story on Michael Murphy and his Cambridge team discussed findings from the Keck I telescope on Mauna Kea that suggested subtle changes to the value of the fine structure constant since the earliest era of the universe. But those findings remain highly controversial, as was apparent on Monday the 18th.
That was the day that astronomer Jeffrey Newman (Lawrence Berkeley National Laboratory) presented data from the DEEP2 redshift project, a five-year survey of galaxies more than seven light years away. Speaking at the annual meeting of the American Physical Society (APS) in Tampa, Newman said his team’s results showed no change to the constant within one part in 30,000.
“The fine structure constant sets the strength of the electromagnetic force, which affects how atoms hold together and the energy levels within an atom. At some level, it is helping set the scale of all ordinary matter made up of atoms,” Newman said. “This null result means theorists don’t need to find an explanation for why it would change so much.”
Consider the fine structure constant a ratio of other constants. Designated by the Greek latter alpha, the constant is equal to the square of the charge of the electron divided by the speed of light times Planck’s constant. Some theories suggest that alpha would change only if the speed of light changed over time.
Image: From the Hubble Ultra Deep Field, a snapshot of galaxies in various ages, sizes, shapes and colors. Studying the light of such objects may help us discover whether or not the fine structure constant has changed over time. Credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team.
Although DEEP2 was not designed to probe such matters, Newman realized that a study of the oxygen emission lines from distant galaxies would provide suitable data to study the question. The team compared emission lines for galaxies at varying distances. Murphy’s team, on the other hand, had looked at absorption lines in the spectra of distant quasars. Astronomer John Bahcall (Institute for Advanced Study) pointed out some years ago that when it came to measuring the fine structure constant, studying emission lines from distant galaxies would be less error-prone than measuring absorption lines.
Centauri Dreams note: The fine structure constant remains controversial, but DEEP2 is provocative in more ways than this. This massive survey of distant galaxies should provide insights into how the universe itself has evolved. Moreover, its measurements will be a useful yardstick against which to measure various models of dark energy, that mysterious force that seems to be causing the expansion of the universe to accelerate. While Newman released data from the survey’s first year of operation (2002), the full release will have to wait until 2007. The survey finishes its observations this summer. You can read more in this UC Berkeley news release.
by Paul Gilster | Apr 18, 2005 | Sail Concepts |
Beamed propulsion is the classic solution to the mass ratio problem in interstellar flight. Rather than pushing more and more fuel to get your payload to another star system, you leave the fuel behind. Robert Forward’s vast lightsail proposals come immediately to mind, but in 2001 physicist Geoffrey Landis proposed propulsion by particle beam, with energy delivered from the Solar System to the departing spacecraft.
The notion is this: a charged particle beam is accelerated and focused, then neutralized in charge to prevent the beam from expanding as it travels due to electrostatic repulsion. When the particles reach their target, they are re-ionized and reflected by a magnetic sail, which Landis originally conceived as ‘a large superconducing loop with a diameter of many tens of kilometers.’
The particle-beam idea seems to solve two problems with lightsails: First, a light beam provides a relatively inefficient energy source, demanding huge power facilities and thus driving up the cost. Some of Forward’s proposals were mind-boggling, calling for laser power up to an astounding 7.2 terawatts. The latter was for Forward’s most audacious concept, a round-trip manned mission to Epsilon Eridani.
The second issue is that laser-pushed sails call for extremely large lenses to deliver a tightly collimated beam to the receding starship. The lens for Forward’s Epsilon Eridani mission would be fully a thousand kilometers in diameter, dwarfing our manufacturing capabilities into the forseeable future. Add to these problems the difficulty of manufacturing ultra-thin films in space, not to mention deploying them as sails, and the particle beam emerges as an increasingly attractive alternative.
Landis has now revised his original particle beam ideas to incorporate new technologies. Specifically, he proposes using techniques developed by Robert Winglee at the University of Washington to reduce the physical structure of the sail. The concept is to create a magnetic sail through the use of mini-magnetosphere plasma propulsion (M2P2). From the paper:
By shrinking the physical structure of a magnetic sail, the invention of the mini-magnetosphere plasma propulsion, or ‘M2P2’, has brought the idea of a particle-beam-pushed sail closer to reality. The particle-beam is reflected by a magnetic field. In the mini-magnetosphere, the magnetic field is inflated to a large area by injection of a plasma into the magnetic field, and hence large magnetic field areas are possible with only a small physical structure.
Winglee’s ideas, tuned for missions within the Solar System, assume an
on-board supply of propellant to replenish the plasma, which gradually leaks into space. Landis’ idea is to replace the plasma from the particle beam itself. ‘…Thus the mini-magnetosphere will self-inflate with the particle beam, resulting in no requirement for on-board propellant except, possibly, for initial inflation of the magnetic field.’
Image: An artist’s impression of a mini-magnetosphere deployed around a spacecraft. Plasma or ionized gas is trapped on the magnetic field lines generated onboard, and this plasma inflates the magnetic field much like hot air inflates a balloon. The mini-magnetosphere is then blown by the plasma wind from the Sun or, in Geoffrey Landis’ interstellar concept, by a particle beam sent from the Solar System. Credit: Robert Winglee, University of Washington.
Two more particle beam advantages: The magnetic sail provides braking by creating drag against the solar wind from the destination star, and also becomes a shield against interstellar dust.
For more on mini-magnetosphere plasma propulsion, see the M2P2 page at the University of Washington. The revised Landis paper is “Interstellar Flight by Particle Beam,” in Acta Astronautica Vol. 55, pp. 931-934 (2004). For useful background on magnetic sails and braking, see D. G. Andrews and R. M. Zubrin, “Magnetic Sails and Interstellar Travel,” International Astronautical Federation Paper IAF-88-5533, Bangalore, India, October 1988. A useful cost study of interstellar mission options was performed by Dana Andrews in “Cost Considerations for Interstellar Missions,” Paper IAA-93-706 (1993).
by Paul Gilster | Apr 16, 2005 | Astrobiology and SETI
“A recent book by the mathematician Amir Aczel makes the case for the probability of extraterrestrial life being 1. The physicist Lee Smolin wrote that ‘the argument for the non-existence of intelligent life is one of the most curious I have ever encountered; it seems a bit like a ten-year-old child deciding that sex is a myth because he has yet to encounter it.’ The late Stephen Jay Gould, referring to Tipler’s contention that ETCs would deploy probe technology to colonize the Galaxy, wrote that ‘I must confess that I simply don’t know how to react to such arguments. I have enough trouble predicting the plans and reactions of people closest to me. I am usually baffled by the thoughts and accomplishments of humans in different cultures. I’ll be damned if I can state with certainty what some extraterrestrial source of intelligence might do.’
“It is easy to sympathize with this outlook. When considering the type of reasoning employed with the Fermi paradox, I cannot help but think of the old joke about the engineer and the economist who are walking down a street. The engineer spots a banknote lying on the pavement, points to it, and says, ‘Look! There’s a hundred-dollar bill on the pavement.’ The economist walks on, not bothering to look down. ‘You must be wrong,’ he says. ‘If there were money there, someone would already have picked it up.’ In science it is important to observe and experiment; we cannot know what is out there unless we look. All the theorizing in the world achieves nothing unless it passes the test of experiment.”
Stephen Webb, from Where Is Everybody? (New York: Copernicus Books, 2002), p. 24.
Centauri Dreams note: Webb’s book is a great read and belongs on your shelf (though I’ve never seen such an exasperating method of footnoting). In the above, ETC means ‘extraterrestrial civilization.’ The Aczel book is Probability 1: Why There Must Be Intelligent Life in the Universe (New York: Harcourt Brace, 1998). Lee Smolin is quoted from The Life of the Cosmos (London: Weidenfeld and Nicolson, 1997). The Gould quote comes from The Flamingo’s Smile (London: Penguin, 1985); cf. the essay “SETI and the Wisdom of Casey Stengel.”
by Paul Gilster | Apr 15, 2005 | Culture and Society, Exoplanetary Science |
We’ve recently discussed habitable zones, normally defined as the area around a star where liquid water can exist on the surface. Thoughts on just how far the habitable zone around our own star extends vary, but the Carnegie Institution’s Maggie Turnbull pegs it at between .7 AU and 1.5 AU. To adjust the notion of habitable zone to other stars, Turnbull says, the same relationship can be scaled as the square root of the luminosity of the star.
Turnbull thinks about such things because she has created a database of stars that could have terrestial-type planets around them. This is clearly of significance to future missions like Terrestrial Planet Finder and makes me wish I had been able to attend the NASA Forum for Astrobiology Research in March, where Turnbull gave a talk called “Remote Sensing of Life and Habitable Worlds: Habstars, Earthshine and TPF.” The next best thing is an edited transcript of the lecture in Astrobiology Magazine, which is running it in a series of four parts under the title “Surfing the Wavelengths.”
While the goal of Terrestrial Planet Finder is to image planets between Mars and Earth size (and perhaps somewhat larger) that exist in their stars’ habitable zones, a second goal is to study the atmospheres of these worlds. The mission will work in the mid-infrared because in those wavelengths, planets emit their own light, even as light from the star prevents less glare. From the transcript:
So a planet’s mid-infrared light can give us a handle on the temperature of the planet, and tell us if that temperature is right for liquid water at the surface. Also in the mid-infrared, we can see some exciting signatures, such as carbon dioxide, water, and ozone. Since ozone is a proxy for molecular oxygen, it’s an indicator of life.
Ponder the challenge: the Earth is ten billion times less bright than the Sun at optical wavelengths, but in the mid-infrared, it’s ‘only’ a million times less bright. The Earth-Sun system at ten parsecs (30 light years) would have an angular separation between Earth and the Sun of 100 milliarcseconds, a separation so vanishingly small that it calls for interferometry — flying a number of vehicles in formation, or else a single, very large telescope. (Centauri Dreams continues to advocate Webster Cash’s New Worlds Imager for this mission — see the story Planetary Systems in the Billions from last September).
According to Turnbull, though, as we move toward efficient and practical interferometry in space, Terrestrial Planet Finder will start its work in the optical range, with a 2014 mission carrying a coronagraph that will suppress the light of the central star to allow any planets to be observed. And this is intriguing:
In the Earth’s optical spectrum, we see Rayleigh scattering in the blue part of the spectra – we’re seeing the blue sky of our planet. We also have signs of oxygen, ozone, and water. We may even be able to see signs of vegetation in the optical.
The interesting thing about observing the Earth in the optical is that you can see all the way through Earth’s atmosphere to the ground. The light that reflects back contains the spectrum of whatever is on the ground of that planet, whether it’s oceans, or soil, or plants. And plants happen to have a very distinctive spectrum that could be observable even across stellar distances.
What better way to close a discussion of observing terrestrial worlds than with a picture of our own? The ‘pale blue dot,’ perhaps the most stunning of space images, was a parting gift from Voyager 1 as it left the Solar System.
Here’s what Carl Sagan said about this image: “… We succeeded in taking that picture [from deep space], and, if you look at it, you see a dot. That’s here. That’s home. That’s us. On it, everyone you ever heard of, every human being who ever lived, lived out their lives. The aggregate of all our joys and sufferings, thousands of confident religions, ideologies and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilizations, every king and peasant, every young couple in love, every hopeful child, every mother and father, every inventor and explorer, every teacher of morals, every corrupt politician, every superstar, every supreme leader, every saint and sinner in the history of our species, lived there on a mote of dust, suspended in a sunbeam.” From a 1996 commencement address, drawing on Sagan’s Pale Blue Dot: A Vision of the Human Future in Space (New York: Random House, 1994).
by Paul Gilster | Apr 14, 2005 | Missions, Outer Solar System, Sail Concepts |
How do you build an interstellar solar sail? Back in the 1980s, two studies of sail design set parameters that before then had remained largely unanalyzed. Gregory Matloff and Eugene Mallove were able to show in their papers “Solar Sail Starships: Clipper Ships of the Galaxy” and the later “The Interstellar Solar Sail: Optimization and Further Analysis,” that a so-called ‘sundiver’ trajectory coud produce exit velocities from the Solar System on the order of 1000 kilometers per second, even for large payloads. Both papers appeared in the Journal of the British Interplanetary Society, which remains the leading venue for interstellar studies.
A sundiver maneuver is tricky stuff; the spacecraft is established on a hyperbolic solar orbit that swings close to the Sun; at perihelion (closest approach), the sail is exposed to sunlight (having, perhaps, been shielded until now by an occulting object, such as a small asteroid). Make the sail reflective enough and the accompanying linkages to its payload strong enough and you’ve got a vehicle that could reach Alpha Centauri in a millennium. This is more or less state of the art for manned starship conjecture — the classic paper is by Alan Bond and Anthony Martin, “Worldships: Assessment of Engineering Feasibility,” which appeared in JBIS in 1984 (Vol. 37, pp. 254-256).
These concepts have not been forgotten by interstellar theorists, but recent work has focused on missions to nearby interstellar space. Aurora, for example, was a sail design for a mission to the near interstellar medium at about 200 AU from the Sun; it followed previous studies by Claudio Maccone and colleagues of a probe to the Sun’s gravitational focus at 550 AU.
Image: An artist’s conception of an interstellar solar sail. Credit: Jet Propulsion Laboratory.
For its part, NASA began investigating a near-term solar sail mission to the heliopause in 1998, reaching speeds of 50 kilometers per second with a boost from a close pass by the Sun. And in a 2003 paper, Gregory Matloff and Travis Taylor presented a proposal for an Oort Cloud Explorer mission that would reach the domain of the long-period comets. The Matloff/Taylor sail would be a 681-meter object that would use four smaller, attached 5-meter sails for attitude control, with a 150 kilogram payload. The authors believe that a close solar approach could result in a mission time of roughly 100 years, taking the vehicle to the outer edge of the Oort Cloud.
From the paper:
In the forseeable future, the solar sail may…see application to the exploration of nearby interstellar space rather than neighboring stellar systems. As well as expanding the radius of human exploration to 1,000 — 10,000 AU, the interstellar solar sail can serve as a test bed for concepts relating to interstellar beamed sailing. Sailcraft materials and structures will have commonalities in both approaches and can be tested in solar sailors long before beam projectors and power stations are constructed and deployed in solar space.
Centauri Dreams note: The use of laser or other forms of beamed propulsion to drive a ‘lightsail’ takes us beyond the conventional solar sail and ramps up the possibilities for missions to other stars. Matloff notes that even low-power beam projectors may help with nearby interstellar missions like the Oort Cloud Explorer by providing an extra push to the vehicle as it approaches the Sun, thus boosting its exit speed out of the Solar System. Such missions would be valuable testbeds for the larger beamed energy facilities that may one day get a lightsail to Alpha Centauri.
Matloff and Taylor’s paper “From the Sun to Infinity” is found in CP664, Beamed Energy Propulsion: First International Syposium on Beamed Energy Propulsion, ed. A.V. Pakhomov (American Institute of Physics, 2003). Matloff and Mallove’s “Solar Sail Starships: Clipper Ships of the Galaxy” appeared in JBIS 34 (1981), pp. 371-380. Their “The Interstellar Solar Sail: Optimization and Further Analysis” appeared in JBIS 36 (1983), pp. 201-209. Both are now considered landmarks in the field of interstellar mission studies.
