Centauri Dreams readers know that I'm a great supporter of solar sailing as a technology that has interstellar ramifications as well as immediate practical value right here in the Solar System. What's particularly appealing about the solar sail is that we've already shaken out many of the problems and are ready to begin testing sails in space, which is why it's so frustrating to see NASA and ESA locked in to budgetary constraints that keep that vital next step from happening. NanoSail-D is one cheap way we might fly a sail soon, and so is The Planetary Society's LightSail project, but as with so many aspects of the space program, we seem to be well behind earlier optimistic schedules. In that environment, though, it's important to keep the goal in front of us and to continue the work on solar sail theory. In June of 2007, the 1st International Symposium on Solar Sailing took place at Herrsching at Lake Ammersee, Bavaria. The 2nd in this symposium series is now scheduled for July...

As we await results from ongoing observations of the Alpha Centauri stars, let's summarize for a moment what we currently know. While the subject is still up for debate, a number of studies have suggested that terrestrial planets can form around either Centauri A or B, with planetary systems extending as far out as 2.5 AU. And while planets have been discovered in binary systems not dissimilar to the Centauri stars, current estimates are that Centauri B has the greater chance of having a planet within the habitable zone. A warm blue and green world with oceans and continents, not so different from Earth, perhaps, could yet be found around Centauri B. Supposing this scenario is proven correct, Greg Matloff (CUNY) has gone to work on how we might use Centauri A, even if it turns out to be without planets, to help us explore Centauri B. He's thinking, of course, in terms of solar sails and the need to decelerate upon arrival in the destination system. Centauri A, a G2V star, is larger...

What happens to a solar sail as it flies through space? Made of the most diaphanous materials possible, the sail gradually begins to degrade. Roman Kezerashvili and Gregory Matloff (CUNY) have looked closely at problems like these and have considered everything from sail thickness and performance to the merits of different metallic films. The sail material of choice seems to be beryllium, three times lighter than aluminum, and with a usefully high melting point. One interesting configuration is a twin-walled, hydrogen-inflated sail with walls ten to twenty nanometers thick. But build such a craft carefully. If solar radiation causes the constituent beryllium to become degraded, the structural integrity of the sail is at risk and hydrogen begins to escape. Solar sails need flight testing (and The Planetary Society's plans for a solar sail seem to be developing nicely), but we'll doubtless learn huge lessons from those early tests that will substantially revise our thinking. What...

From the fusion-powered Project Icarus, designed to handle the interstellar long-haul, to the first tentative solar sail experiments in near-Earth space seems like quite a jump. But we needed to be reminded of the need for research on both ends of the spectrum, the things that are doable today and the concepts we want to shape for tomorrow. How heartening, then, to see the Planetary Society's new commitment to the solar sail in the form of a project called LightSail, which will include several missions designed to demonstrate the potential of photons for propulsion. Make no mistake about it, solar sail technology is a practical solution for leaving the fuel at home. We know the principle works. Photons carry no mass but they do carry momentum. Missions designed to operate close to the Sun already have to factor in the pressure exerted by the photon stream, and in a famous case, controllers used the solar panels aboard the Mariner 10 spacecraft (launched in 1973 to explore Mercury) to...

It's a long way from the back of an envelope to a deployed spacecraft, which is one reason why scientists write papers in journals and gather at conferences. Such venues are where ideas get shaken out, problems identified and solutions proposed. We sometimes talk about realistic technologies like solar sails as if all that remained were to build them, but as Roman Kezerashvili demonstrated at the recent conference in Aosta, there is a range of problems that we are only beginning to consider. Kezerashvili (City University of New York), working with colleague Justin Vazquez-Poritz, has identified a significant area of concern for mission concepts that pass close by the Sun. Often called 'Sundiver' missions (a Gregory Benford coinage, if memory serves), these sails would be so constructed as to survive an extremely close solar pass. Perhaps protected behind an occulter until periherlion, the sail would then be unfurled to receive the full force of the Sun's photons at extremely close...

I was startled to see the Breakthrough Propulsion Physics project make the pages of The Atlantic in its current issue. Novelist Thomas Mallon, in an essay largely devoted to solar sailing and The Planetary Society's efforts in that direction, gives vent to some of the frustration, if not exasperation, many of us feel as we see basic research losing out to short-term missions whose purpose is by no means clear. "American politicians now mostly avoid the old conditional trope 'If we can put a man on the moon' — because we can't, not anymore," writes Mallon, who goes on to lament the passage of the BPP project and, five years later, NASA's Institute for Advanced Concepts. Questioning Why We Explore In Mallon's view, the sense of exploration is itself under attack: Even the most spectacular unmanned successes of the American space program — from the Voyager probes of the '70s to the Galileo and Cassini missions of the '90s — seem to belong to a fading worldview. A...

A note from Eric Davis (Institute of Advanced Studies at Austin) fills me in on the details of the upcoming 6th International Symposium on Beamed Energy Propulsion, to be held in Scottsdale, AZ during the first week of November. Much of the program is of interest to interstellar studies, ranging from the basic science and technology of laser, microwave and particle beam propulsion to specifics relating the topic to beamed interstellar missions. The latter subject will always be associated with Robert Forward, whose studies of beaming technology and sails made us understand that reaching the stars was not necessarily impossible. Lasers were the key, as Forward learned through his work with Ted Maiman at Hughes Research Laboratory. Years later he recalled his 'eureka' moment: "I knew a lot about solar sails, and how, if you shine sunlight on them, the sunlight will push on the sail and make it go faster. Normal sunlight spreads out with distance, so after the solar sail has reached...

We need to find a way to get NanoSail-D into space. You'll recall that the original NanoSail-D perished in the explosion of a SpaceX Falcon rocket. But the opportunistic mission, a sail whose central components are three inexpensive Cubesats, two of which house a small, deployable sail, may yet get into the black. As we noted in this story from August of last year, a duplicate NanoSail-D is available. The trick is to find the funding and the booster. A joint project featuring The Planetary Society, NASA and the Russian Space Research Institute is attempting to do just that, looking at a solar sail experiment that may or may not involve NanoSail-D. The question is whether the 7 by 7-meter sail is the payload the mission planners will choose, the other option being a Russian-designed sail experiment of equally small size. You can read more about the design choices at The Planetary Society site. Image: The Huntsville-based NanoSail-D team stands with the fully deployed sail at ManTech...

New propulsion technologies are under study in the laboratory, even if finding the funding for such work is always a problem. James and Gregory Benford have demonstrated that a powerful microwave beam can push an ultra-light carbon sail even to the point of liftoff under lab conditions at 1 gravity. That's useful information, for if we can leave the propellant at home, we can contemplate deep space missions driven by beamed microwaves, a technology that not only can pack a wallop, but is also less destructive to sail materials than a laser, meaning the sail can be brought to high temperatures more efficiently. Unusual Acceleration Yesterday we talked about a possible 'Sundiver' mission built around the microwave beaming idea. The Benfords' version of this mission depends upon a second effect they observed in the lab. The photon pressure applied to the small sail they used could not account for the observed acceleration. Something was clearly coming out of the carbon lattice, but what...

A 'Sundiver' mission may offer the best acceleration we can muster given the current state of our technology. New Horizons is currently moving toward Pluto/Charon at roughly 19 kilometers per second, but back of the envelope calculations can pull out 500 kilometers per second for a solar sail that makes the optimum close approach to our star and then unfurls to full diameter, riding the photon storm outward to the edge of the Solar System and beyond in record time. But Sundivers are tricky missions even on paper (we have yet to attempt one). Gregory Benford (UC-Irvine), who coined the 'Sundiver' term, and brother James (Microwave Sciences) have studied the matter in depth, and bring a unique perspective. They've not only theorized about sails and acceleration, but have actually tested the concept in the laboratory. Specifically, they've used an intense beam of microwaves to lift a carbon sail vertically in a vacuum chamber, and have studied how to spin and control it. A Sail Takes...

Want to get to the outer Solar System quickly? Try this on for size: Two and a half years to reach the heliopause, six and a half years to get to the Sun's inner gravitational focus (550 AU), with arrival at the inner Oort Cloud in no more than thirty years. A spacecraft meeting those targets is moving at 403 kilometers per second, roughly twenty times as fast as anything we've put into space before. Such a mission could perform useful astrophysical observations enroute, explore gravitational focusing techniques, and image Oort Cloud objects while exploring particles and fields in that region that are of galactic rather than solar origin. The combined Oort Cloud explorer/gravity focus probe grows out of work by Gregory Matloff and Roman Kezerashvili (CUNY), Italian physicist Claudio Maccone and Les Johnson (NASA MSFC). Matloff, of course, has been studying solar sail technologies for decades, looking at missions that could reach velocities in the range of 0.003c-0.004c, with metallic...

Mention beamed propulsion and people invariably think you're talking about lasers. The idea seems obvious once you've gotten used to solar sail principles -- if photons from the Sun can impart momentum to push a sail, then why not use a laser beam to push a sail much farther, into the outer Solar System and beyond? These are regions where sunlight is no longer effective, but a laser infrastructure of the kind envisioned by Robert Forward could produce a tightly collimated beam that could drive the sail to an appreciable fraction of the speed of light. But are lasers the best way to proceed? Although he would sketch out a range of missions with targets like Alpha Centauri and, the most audacious of all, Epsilon Eridani (this for a manned crew, with return capability!), Forward himself quickly turned away from lasers and began exploring microwave propulsion. I'm fairly certain the turn to microwaves came at Freeman Dyson's suggestion, and when I asked Dyson about it in an interview...

For all their attraction as a way to leave weighty propellant behind, solar sails have a fundamental limitation. Their power source is the Sun. As you move away from the Sun, the amount of available light drops according to the inverse square law -- a spacecraft that doubles its distance from the Sun encounters only a fourth of the sunlight previously available. Quadruple the distance and the sunlight drops to a sixteenth of what it was, making sail operations problematic in the outer Solar System. And what of the stars? Solar sail specialist Greg Matloff has been juggling the numbers on interstellar travel via solar sail for decades now, and even with the best case scenario involving an extremely close solar pass, a thousand years to Centauri is about as good as it gets. And that's quite a stretch in itself. Epsilon Eridani would actually make an easier mission as it's much closer to the ecliptic, so you get 30 kilometers per second (Earth's orbital velocity around the Sun) from...

All of nature is a kind of laboratory, which is why good propulsion ideas can flow from astronomical observations that show us how things work. Recent news about the solar wind is a case in point. An analysis of data from the Ulysses spacecraft shows that the solar wind is now lower than at any time previously measured. That has implications for the heliopause, that region where the solar wind encounters true interstellar space, for this region plays a role in shielding the Solar System from the effects of galactic cosmic rays. "Galactic cosmic rays carry with them radiation from other parts of our galaxy," says Ed Smith, NASA's Ulysses project scientist at the Jet Propulsion Laboratory in Pasadena, Calif. "With the solar wind at an all-time low, there is an excellent chance the heliosphere will diminish in size and strength. If that occurs, more galactic cosmic rays will make it into the inner part of our solar system." Image: The Ulysses spacecraft. Credit: Jet Propulsion...

Who would have thought the planet Mercury would prove so useful in explaining how solar sails work? The Messenger spacecraft's recent course adjustment maneuvers have proven unnecessary because controllers have been able to use its solar panels creatively, harnessing solar radiation pressure (SRP). And what better place to shake out such methods but on your way to a Sun-drenched planet that moves in an environment where SRP can be eleven times higher than that near Earth? It may come as a surprise that we are already using solar sailing techniques on operational missions, but Messenger is not the first. In fact, we can go back to another Mercury mission, Mariner 10, which took advantage of the effect of solar photons on its twin solar panels, each about nine feet in length and three feet in width, a highly usable 55 square feet that not only generated power but got the spacecraft out of serious trouble. Launched in 1973, Mariner 10 ran into problems with its stabilizing gyroscopes...

The vast laser-driven sails envisioned by Robert Forward have always fired my imagination. Hundreds of kilometers in diameter, they would rely upon a gigantic Fresnel lens in the outer Solar System to keep the critical laser beam tightly collimated over interstellar distances. Forward conceived of mission designs to stars as far away as Epsilon Eridani, journeys that could be achieved within a human lifetime. He even provided return capability through the use of a multi-part sail. You can read a fictional treatment of this in his novel Rocheworld. But how do we get from here to there? As of today, we're close enough to having an operational space sail that if we can talk SpaceX into lofting the NanoSail-D duplicate, we could be shaking out our first space sail within months. Assuming we do go operational before too many months (or years!) pass, the question then becomes, what kind of missions are possible between the laser-beamed lightsail of science fictional imagining and the...

Remember the great scene in Contact, when the fabulously rich S. R. Hadden (John Hurt), who funded the stargate device that has been destroyed by sabotage, says "Why build one when you can build two for twice the price?" He then reveals the existence of a second facility off the coast of Japan, which is what Ellie Arroway uses on her interstellar trip. So is solar sail expert Greg Matloff a ringer for S. R. Hadden? Read on. Greg's recent phone call may not have been as dramatic as that scene in Contact, but he was able to tell me that although NanoSail-D did perish in the SpaceX Falcon explosion, there is a second sail. Marshall Space Flight Center built two. So now we're in the energizing position of having a second chance at a sail deployment in space, and it could be done soon via the next Falcon launch, if SpaceX will cooperate in the enterprise. And here's why they should: Launching a payload on the Space Shuttle costs approximately $10,000 per pound. That's pricey, and the...