by Paul Gilster | Feb 22, 2006 | Sail Concepts
A solar sail competition to drive research? It’s a great idea, and one that has been explored in the past. Indeed, a whole variety of groups have looked into the possibility, from France’s Union pour la Promotion de la Propulsion Photonique (U3P) to Russia’s Space Regatta Consortium and the Aero-Club de France. And official rules for the Luna Cup were approved by the International Astronautical Federation at the World Space Congress in August of 1992, outlining a solar sail race to the Moon.
Now I’m looking at a NASA announcement passed along by James Benford that outlines prize competitions to be conducted under the agency’s Centennial Challenges umbrella. To quote from the document, “By making awards based on actual achievements instead of proposals, Centennial Challenges seeks novel and lower-cost solutions to engineering obstacles in civil space and aeronautics from new sources of innovation in industry, academia, and the public.”
The challenge possibilities are outlined in a NASA Request for Comments (RFC) document that explores competitions in a number of areas, ranging from low-cost space suits to lunar night power sources. And the one that has caught Benford’s eye involves solar sails. Here’s the relevant information:
The Station-Keeping Solar Sail Challenge is designed to promote the development of solar sail technology and the commercial services that may result from the ability to operate in novel orbits such as artificial Lagrange points.
The Station-Keeping Solar Sail Challenge has two prizes. To win Prize One and the $2,500,000 purse, a Team must be the first to deploy a solar Sailcraft, demonstrate a resultant trajectory acceleration change of at least .05 millimeters per second squared, and fly along a trajectory that will pass through a defined target located at the first Sun-Earth Lagrange point (L1). To win Prize Two and the $2,500,000 purse, a Team must enter a defined region above or below the ecliptic plane at L1 and remain there for 90 consecutive days.
We’ve all had an education in what prize challenges can do for technology through the success of the Ansari X Prize competition and earlier, oft invoked challenges like the Orteig Prize that Lindbergh clinched by flying the Atlantic. I also like the wonderful science fiction association with Arthur C. Clarke’s “The Wind from the Sun,” originally published in 1964 under the title “Sunjammer.” Using yacht racing as the metaphor, Clarke told a bold tale of a race to the Moon using solar sails and largely introduced the sail concept to the general public (although, to be sure, Jack Vance’s “Gateway to Strangeness” and Cordwainer Smith’s haunting “The Lady Who Sailed the Soul” had appeared several years earlier in Amazing Stories and Galaxy respectively).
We must hope for keen interest in a sail competition as one way to keep the technology developing in a time of steep budget cuts. Getting private industry and academia reenergized over solar sail work (especially after the failure of the Planetary Society’s Cosmos 1) cannot help but advance the state of the art, and it is becoming increasingly clear that solar sails are one area where the private sector’s contribution can be immense.
by Paul Gilster | Feb 21, 2006 | Sail Concepts |
Solar sails are ideal for long missions within the Solar System, but their manifest advantages (no fuel onboard!) are not unalloyed. A major issue is the time needed to escape Earth orbit. Working the numbers on this, Gregory Benford noted that if a sail used the momentum from solar photons alone, unassisted by any other propulsive force, it would require time scales on the order of years to escape from Earth’s gravity. And that’s with sail deployment from an altitude of 800 kilometers, beyond the reach of decelerating air drag.
What we can do to get that sail on its way faster is the subject of Benford’s new paper in Acta Astronautica, written in collaboration with Paul Nissenson. One possibility is to coat the sail with a material that sublimes; when heated, the material vaporizes and is ejected, adding to the momentum transfer of photons (Benford and his brother James at Microwave Sciences have done groundbreaking work on the nature of such sublimation, also called desorption).
But the paper goes further; its authors examine the idea of decreasing sail escape time by using a high-power ground or space-based photon generator — a beamer — that would increase the photon density well beyond solar power alone. Their work on the orbital dynamics of such a beamer/sail combination assumes that an orbiting beamer (ORB) would be deployed behind the sail in the same initial circular orbit. When the beamer illuminates the sail, a resonance between the two is established, with sail and beamer returning to their original positions after a certain number of orbits, where the sail is boosted once again.
The sail trajectories thus created are shown in the image below. From the paper:
“Once the beamer illuminates the sail, the two bodies are no longer in synchronous orbits. After a certain number of orbits, the beamer and solar sail ‘resonate:’ the two bodies meet nearby in space, with the beamer trailing the sail allowing the photon beam to focus well on the sail. A specific energy is given to the sail so that resonance occurs in a relatively small number of orbital periods.”
More economical, perhaps, and easier to maintain would be a ground-based beamer, and the paper offers equivalent calculations for this scenario. In both cases, the effects are startling: using the orbiting beamer method, the sail’s escape time is reduced by over two orders of magnitude if deployed at 800 kilometers. Ground-based beamers also reduce escape time over sunlight alone, with a coating of subliming material capable of producing even higher momentum transfers (the relationship of beamer power and sail temperature gets a close look here). “Engineering optimal desorbed or sublimed fuels in sails made of carbon fiber,” write the authors, “is a promising goal.”
The beauty of beamer technology is that such an installation could drive numerous sails, lowering the cost for each mission. “In this way, it somewhat resembles the railroad, which gives no benefit until the last length of track is laid. Still, a small low power beamer could assist the launch of sails from orbit as a first demonstration of the principles we have examined here, eventually building a utility in orbit, leading to fast deep space missions.”
The paper is “Reducing solar sail escape times from Earth orbit using beamed energy,” in Acta Astronautica Vol. 58, Issue 4 (February 2006), pp. 175-184.
Centauri Dreams‘ take: Solar sails won’t take us to the stars, but sails pushed by lasers — lightsails — just might, as Robert Forward showed convincingly in a series of papers in the 1980s. What this early work on laser-assisted solar sails is doing is building a groundwork for a space-based infrastructure that one day may see large laser installations capable of propelling sails to speeds that could reach the nearest stars in a human lifetime. Forward’s concepts were mind-boggling in their engineering, but I can imagine his pleasure as the far more practical details of interplanetary mission planning are worked out. Beaming technologies have a major role to play as we push outward to the edges of the Solar System and beyond.
by Paul Gilster | Feb 20, 2006 | Exoplanetary Science |
Margaret Turnbull (Carnegie Institution of Washington) has a job Centauri Dreams can’t help but envy. The astronomer is a specialist in identifying stars that have habitable zones — stars, in other words, where life is possible. Back in 2003, Turnbull and colleagues published a list of 17,129 such stars, based on factors such as age (how long does it take life to develop?), stellar mass (larger stars may not live long enough to produce productive habitable zones) and metallicity (a measure of the heavy metals needed for planetary formation).
Narrowing a galaxy of between one and two hundred billion stars down to 17,129 candidates is no small feat, but Turnbull has now gone one better, choosing the top five candidate stars for those engaged in SETI, the search for extraterrestrial intelligence. That list involves choosing stars where technological civilizations are most likely to have developed, but Turnbull complements it with a second list of six stars likely to have Earth-like planets in orbit about them. For the latter, we wait, of course, for the again delayed Terrestrial Planet Finder and similar programs. As Turnbull says, “It’s impossible to know the true nature of those planets until we can directly image them.”
Terrestrial Planet Finder or no, these lists are fascinating. For the SETI search, Turnbull likes Beta CVn, a Sol-like star in Canes Venatici that is roughly 26 light years away (around which, it must be added, no planets have yet been found). The other SETI candidates are HD 10307 (42 light years away, a Sun-like member of a binary system); HD 211415 (cooler than the Sun, and with half its metal content); 18 Sco (a twin of the Sun in Scorpio) and 51 Pegasus (immortalized in the first detection of an exoplanet around a main sequence star).
But the list of top candidate stars for Terrestrial Planet Finder is, in my judgment, more interesting. Here Turnbull chooses K-class stars a bit dimmer than the Sun, reasoning that their inherent brightness is not high enough to complicate the planet hunt. The choices for TPF are these:
Epsilon Indi A: almost 12 light years away, it’s about a tenth as bright as the Sun
Epsilon Eridani: about 10.7 light years from Earth and somewhat smaller than Sol
Omicron2 Eridani: the same age as the Sun, some 16 light years out
Alpha Centauri B: the closest target, with the added virtue that Alpha Centauri A is close enough that the planet hunt around B may also tell us much about Centauri A’s potential planets
Tau Ceti: metal-poor but old enough for complex life to have evolved
Turnbull’s work on candidate stars was presented last Saturday at the 2006 annual meeting of the American Association for the Advancement of Science in St. Louis.
Centauri Dreams‘ take: In the list above, two stars stand out. Tau Ceti seems somewhat less likely as a home for life given studies indicating heavy cometary bombardment would be likely in this system. It is, nonetheless, a G-class star like the Sun that has elicited intense interest ever since Frank Drake turned the telescope at Green Bank (West Virginia) towards it in the first SETI attempt.
As to Centauri B, this seems a likely and outstanding choice. Paul Wiegert and Matt Holman showed in 1997 that stable orbits can exist within 3 AU of Alpha Centauri A or B, while the habitable zone around Centauri A should extend from 1.2 to 1.3 AU, with a zone around Centauri B of 0.73 to 0.74 AU. These findings, along with the proximity of the Centauri system to our own, make it an obvious candidate for close scrutiny and, one day, an interstellar probe. Not mentioned here is Proxima Centauri, but this M-class red dwarf is intriguing on its own, and as more and more work suggests that red dwarfs could offer stable habitable zones, we may see its fortunes rise. Turnbull didn’t choose it, among other reasons, because she wanted to work with brighter stars.
On Terrestrial Planet Finder itself, I’m now seeing launch dates as late as 2020, though these are only guesses given the current budgetary limbo. I will be talking to Webster Cash (University of Colorado at Boulder) about his New Worlds Imager technology later this week, and will post the interview here. Cash’s designs seem far and away the most effective (and affordable) for the task of ferreting out Earth-like worlds. The TPF budgetary delays may be a blessing in disguise in at least one sense — they may offer Cash an unexpected opportunity to convince NASA of the advantages of his breakthrough imaging system.
by Paul Gilster | Feb 18, 2006 | Research Tools |
Ernst Rutherford once said that a good scientist should be able to explain his work to a barmaid. Rutherford’s point was well-taken. He did not mean to say that every layman could or should be brought to understand the details of every scientist’s experiments. But he did believe that scientists have an obligation to communicate their findings and to keep in touch with the community around them.
Which inspires a reminiscence on the same subject. Back in 1972, I was a graduate student taking a course in Indo-European linguistics, feeling overwhelmed with the details of sound changes as they moved through evolving languages and fascinated with their derivations in the modern world. One day in our campus cafe, I overheard two fellow students from the class discussing their work. Christmas break approached, and one of them observed, “My parents will want to know what I’m studying. How can I possibly explain Indo-European to them?”
And my thought was, if you can’t explain what you’re doing at this stage of your career, why on Earth do you think you can teach this subject later on? Let me add Erwin Schrödinger’s thought on the same subject to Rutherford’s: “If, in the end, you cannot explain your doings to the average person, your doing has been for naught.”
Of course, researchers aren’t necessarily teachers — at least, not happily so — and not everyone finds the time to bring complicated findings into the public arena, leaving that job to public relations departments and science writers. But physicists and astronomers who would like to tune up their communications skills have a wonderful tool at hand. The complete proceedings from the 2005 conference called “Communicating Astronomy with the Public” is available for free download online. Or if the size of this daunting PDF file puts you off, individual papers can be downloaded at this Web repository.
The conference, held at the European Southern Observatory’s headquarters, was a four-day affair attended by over one hundred astronomers, public information officers, science writers and other professionals with a stake in getting science across to the public. Topics ranged from broad issues like “Closing the Culture Gap between Scientists and Science Communicators” to highly specific case studies, such as “Communicating Chandra’s X-ray Astronomy to the Press and Public.” Tucked within these presentations are good ideas for any scientists hoping to explain someone else’s work or refine the public face of their own.
Centauri Dreams‘ note: Conference organizers should note how readily this ESO/ESA/IAU conference has been made available over the Internet. We need to consider how more scientific conferences can open up access in a similar way, not only through Web repositories of the relevant papers, but podcasts of their presentation. The tools for doing these things are becoming trivially simple to use.
by Paul Gilster | Feb 17, 2006 | Deep Sky Astronomy & Telescopes |
As if we needed another reminder of how much we have to learn about the galaxy, now comes word that an entirely new kind of cosmic object has been identified. Working with the Parkes radio telescope in eastern Australia, a multi-national team has found a type of neutron star that is all but undetectable most of the time, while occasionally releasing a single burst of radio waves. The time interval between bursts has thus far been observed to vary between 4 minutes to 3 hours.
Detection of these objects — called Rotating Radio Transients — is a formidable challenge due to the sporadic nature of their emissions. “These things were very difficult to pin down,” says Dr Dick Manchester, a member of the research team and a veteran pulsar hunter who works for CSIRO, Australia’s Commonwealth Scientific and Industrial Research Organisation. “For each object we’ve been detecting radio emission for less than one second a day. And because these are single bursts, we’ve had to take great care to distinguish them from terrestrial radio interference.”
That’s no easy challenge, and it leads the team to conclude that the objects, eleven of which have been identified so far, are indicators of a much larger population, perhaps a few hundred thousand in the Milky Way. The more studied and better understood form of pulsars, emitting regular radio pulses up to hundreds of times per second, are probably far outnumbered by the new objects.
Centauri Dreams note: It’s hard to use the term ‘conventional’ when describing pulsars, which is why I avoided it above. But the kind of pulsar we’re familiar with results from the supernova explosion of a massive star, leaving a collapsed core perhaps ten miles in diameter that is largely made up of neutrons. Such pulsars spin at enormous rates; a pulsar, known as PSR J0205+6449, for example, presently rotates 15 times every second. But the rotational rate slows as the pulsar ages, and after a few million years, such a pulsar will lose the energy needed to generate its radio and x-ray emissions.
How these newly identified objects fit in with the standard pulsar model will be interesting to follow. The paper is McLaughlin, Lyne, Lorimer et al., “Transient radio bursts from rotating neutron stars,” Nature 439 (16 February 2006), pp. 817-820. An abstract is here, and Nature.com also offers a helpful background article.
