Solar Sailing’s ‘Gossamer Road’

With more attention now being focused on possible missions to an asteroid, we should keep in mind that DLR, the German Aerospace Center, has been looking into an asteroid mission via solar sail for some time now. One 2006 paper from DLR’s Institute of Space Simulation pondered a 70-meter sail for use in a projected mission to the Near-Earth Object 1996FG3 within ten years of launch. It’s an interesting notion, one that would involve the sail hovering over the NEA hemisphere opposite to the Sun, deploying a lander and return capsule.

DLR has been into serious sail studies for some time now, as the photo below attests. It’s a 1999 shot of the ground deployment of a square solar sail 20 meters to the side. As you can see, this is a square sail made up of four triangular sail segments, an exercise that could readily lead to a sail deployment in space if the European Space Agency opts for funding such a mission. Just what ESA has in mind for such technology was the subject of a presentation at the just concluded Second International Symposium on Solar Sailing in Brooklyn.

Image: DLR’s deployed solar sail, seen at the Center’s facility in Cologne. Credit: DLR.

I’m looking at the paper on “The 3-Step DLR-ESA Gossamer Road to Solar Sailing,” available in the proceedings of the conference, and enjoying the ‘gossamer road’ metaphor that is so reminiscent of the fabled Silk Road, that network of trade routes that took its name from the Chinese silk trade and reached across Asia to the Mediterannean, Europe and north Africa. Maybe the ‘gossamer road’ is an indicator of enthusiasm at DLR and ESA for renewing sail work, for late last year the two agreed on the road map to solar sailing presented here.

Three consecutive steps define the roadmap:

  • Gossamer-1: A 5-meter square solar sail launched as a deployment demonstrator to a 320 kilometer Earth orbit. Documentation of the deployment is to be handled by two onboard cameras (which inevitably calls up the images of the IKAROS sail deployment, similarly tracked). This demonstrator mission would be launched in 2013.
  • Gossamer-2: A 20-meter square sail launched to a 500 kilometer Earth orbit. Here the idea is to test orbit and attitude control of a sail built out of thinner materials than the 7.5 µm Kapton used in Gossamer-1. Launch in 2014.
  • Gossamer-3: A 50m x 50m solar sail launched to a 10,000 kilometer Earth orbit, with testing of orbit and attitude control and, as with the earlier missions, documentation by onboard cameras. An acceleration > 0.1 mm/s2 is sufficient for the sailcraft to leave the Earth’s gravitational field after a period of about 100 days.

As you see, the gossamer missions build into growing layers of complexity and, because of limited budgets and a tight time schedule, tap the technologies and materials already developed in earlier DLR and ESA sail work. The paper notes that DLR has already done extensive work not only on sail materials but on the boom technology that supports the sail.

Also supporting the Gossamer project is a Light Pressure Measurement Facility (LPMF) set up by the DLR Institute for Space Systems in Bremen and Berlin. This is a key issue, because the reflectivity of the sail materials determines the efficiency of the propulsion achieved, and a variety of processes during a mission can cause that reflectivity to degrade. DLR is also setting up a Complex Irradiation Facility, now being commissioned, to examine the effects of the solar wind and electromagnetic radiation on sail materials. The trick here is to extrapolate from short-period degradation caused by high intensity bombardment in the facility to the longer, slower processes that a sail will experience in the space environment.

It’s interesting to see that so much recent sail technology has revolved around CubeSats, miniaturized satellites weighing no more than one kilogram that typically work with off-the-shelf electronics. CubeSats were developed as a way for universities to become involved in space exploration, but their small size and inexpensive components make them ideal for experimentation of all kinds. These ‘nano-satellites’ play a role in the NanoSail-D and the Planetary Society’s Lightsail-1 projects as well as DLR’s Gossamer program, allowing early risks to be spread over a number of low cost missions. It’s satisfying to think that IKAROS will soon be joined by other experiments shaking out a future workhorse propulsion system.

The asteroid mission referenced above is discussed in Dachwald et al., “Multiple rendezvous and sample return missions to near-Earth objects using solar sailcraft,” Acta Astronautica 59 (2006), pp. 768-776.


Sasselov: Planets ‘Like Earth’ in Kepler Data

Dimitar Sasselov, a co-investigator on the Kepler mission, said in a TED Talk just posted that Kepler had uncovered numerous terrestrial planet candidates in its early data. Have a look at the video below (around the 8-minute mark). “Small planets dominate the picture,” says Sasselov, showing a chart of planet candidates. A great deal of work has to go into confirming these results, but Sasselov goes on to say “The statistical result is loud and clear, and the statistical result is that planets like our own Earth are out there. Our Milky Way galaxy is rich in these kinds of planets.” How many will be confirmed, and how many shown to be habitable? Much work ahead.


The Solar Sail in Context

The final day of the Second International Symposium on Solar Sailing (ISSS 2010) kicks off this morning with Roman Kezerashvili (City University of New York) discussing solar sail missions as a way of testing fundamental physics. Last year in Aosta I listened with fascination as Kezerashvili discussed close solar passes (‘Sundiver’ missions) that could approach as close as 0.05 to 0.1 AU to the Sun, depending on the development of materials technology. The remarkable feature of his talk, though, was the consideration of General Relativity’s effects in such close proximity to the Sun, which could create huge navigation issues.

The ‘Sundiver’ as an Exercise in Physics

Fail to account precisely for spacetime curvature and frame dragging in this environment and such a mission could find itself with a million-kilometer deflection enroute to its target. Even more exotically, time slows in close proximity to the Sun due to relativistic effects, so that the observer on Earth measures about 31 more seconds per year than the observer at 0.01 AU. It will be interesting to see how Kezerashvili follows up his earlier work with colleague Justin Vázquez-Poritz in Physics Letters B on these issues. Today’s talk, based on a paper with Vázquez-Poritz, looks at the Poynting-Robertson effect on solar sail trajectories, and is available in the proceedings.

The effect now partially named after him was first examined by John Henry Poynting in 1904 and later analyzed as an effect of special relativity by Howard Robertson. It has been analyzed in terms of drag on dust grains in the Solar System, which are found to spiral inward as the result of the tangential component of the Sun’s radiation pressure. In solar sail terms, this drag force can influence long-range missions, for the fraction of the Sun’s radiation absorbed by the sail will produce a drag force sufficient to slow the orbital speed of a sail in a solar orbit, and to decrease the cruising velocity and heliocentric distance of a sail on an escape trajectory.

The Poynting-Robertson effect may sound like a small issue, but weigh it against lengthy mission times of the sort we need to contemplate a journey to the heliopause and beyond. A ‘Sundiver’ sail deployed at 0.02 AU would find its cruising velocity decreased by 20 meters per second, a cumulative effect that would decrease its distance from the Sun by more than 20 million kilometers after a 30-year voyage. By exploring these effects, we learn what adjustments to incorporate in future mission planning and are able to examine exotic physical effects in an environment we can produce in no Earth-bound laboratory.

NASA and the Sail

The ISSS 2010 proceedings are stuffed with good material and it will take a while for me to go through all these papers with care. With IKAROS thus far a triumph, where is NASA in current solar sail research? Les Johnson (MSFC) discussed the agency’s progress in the production and testing of two different 20-meter solar sail systems, one developed by ATK Space Systems and the other by L’Garde. Both successfully underwent vacuum testing in NASA Glenn Research Center’s Space Power Facility at Plum Brook Station, Ohio. The ATK sail is shown at left, the L’Garde sail below, as deployed at the Plum Brook Facility (photo credit: NASA GRC).

In his paper in the proceedings, Johnson goes through the software tools, computational methods and optical diagnostic system developed in support of these sails, along with their structural analysis and attitude control systems. He also discussed the development of NanoSail-D, a small sailcraft system developed around a CubeSat. You may recall the failure of the Falcon-1 rocket in 2008 that destroyed the first of these sails, but the second is to be flown this fall in a test of sail deployment and deorbiting using atmospheric drag (the ‘D’ in NanoSail-D stands for ‘De-orbit,’ growing out of NASA Ames’ interest in developing a way of using atmospheric drag to deorbit a small satellite).

So we have the NanoSail-D flight spare scheduled for a fall launch, but what is the future direction of the already extensive NASA work on solar sails? The sad fact is that NASA is not currently funding solar sail technology. Les Johnson goes on to report the upside:

However, NASA is now preparing for a dramatic change in focus toward the development of advanced space technology that will enable new human and robotic exploration of the solar system. Solar sails are a technology that can support this aim, and it is likely that within the next few years NASA will again be aggressively advancing the technology toward mission implementation.

Maybe the success of IKAROS will be a spur to the other major space agencies to increase their interest and funding in sail technologies. Let’s hope so.

Sail/Fusion Hybrids?

And what of deep space concepts? Tau Zero practitioner Pat Galea, whose photos from ISSS 2010 are becoming available on Flickr, today looks at the interesting question of whether there is any synergy between sail and fusion concepts for Project Icarus, the ongoing re-thinking of the Project Daedalus starship design of the 1970s. We’re in the early days of Icarus, with major issues of configuration still unresolved, but Galea uses Daedalus as a starting point and looks to establish boundary conditions, assuming Alpha Centauri A as the probe’s destination.

Icarus has the ambitious goal of reaching speeds between ten and twenty percent of lightspeed, and if at all possible, the designers would like to allow deceleration, either slowing the craft enough to increase encounter time at the target or, in the best case, allowing it to enter an orbit around the star. Remember, Icarus (based on Daedalus) may mass a whopping 50,000 kg after fuel depletion. Galea finds that even assuming an ideal sail, deceleration to orbital capture would involve a sail 944 kilometers in diameter. It would be interesting to compare this number with the specs on a magsail, and I imagine the Icarus team will be running those numbers in the future.

A 944-kilometer sail seems out, but sail technologies can be of use in other aspects of this fusion-centric mission. Galea goes on to ponder using gravitational lensing for communications, noting that solar sail missions to 550 AU and beyond are not all that different from concepts already under discussion for interstellar precursor missions. Claudio Maccone’s FOCAL mission stretches the technology but comes up with realistic methods to reach these distances and deploy tethered-antennae lensing equipment. Maccone has also shown the viability of gravitational lensing for communications at interstellar distances. Thus a mission to 550 AU to establish a communications relay to support Icarus remains a possibility.

Another potential use of sails would be the deployment of sub-probes once Icarus passes through the destination system. The original Daedalus design included up to eighteen sub-probes that would investigate planets in the Barnard’s Star system. Deploying sail-based sub-probes would work if Icarus can decelerate (presumably using its fusion engines) into a stellar orbit. Remaining to be considered is whether solar sails could be of value for these sub-probes in a decelerated flyby, which is the more likely scenario given the huge difficulty in decelerating such a large payload to an orbit in the target system. Galea’s conclusions follow:

The likely large mass of the Icarus craft at launch and arrival in the target system, together with the high interstellar speed renders the use of solar sails implausible for useful acceleration or deceleration of the craft as a whole. However, the two aspects of the mission that could usefully use sails are the deployment of sub-probes in the target system, and the deployment of the gravitational lens communications receiver… Both of these types of craft have similar requirements to solar sails that are traditionally discussed for interplanetary missions.

Finding the Baseline

Figuring out the limits on things is how we proceed with developing new technologies. Not long ago I discussed Ralph McNutt’s recent work at JHU/APL on realistic manned missions to the outer planets. The huge price tag had a number of correspondents baffled. How could we justify the outlay of trillions of dollars on a handful of missions to explore these planets? But nobody was arguing that we should. The point of such studies is to develop the baseline, to tell us where we are today and where we will be in the near future in terms of costs and capabilities. By doing such studies, we learn where we need to improve our methods and revise our thinking.

There is a huge gap between the first successfully deployed sail in space (IKAROS) and the deep space concepts that are kicked around in the literature. But it is only by analyzing those concepts and pushing the limits of our current science that we get a realistic view of the goal. Conferences like ISSS 2010 go from present and near-term all the way to remote future uses of technologies we can’t yet build today. They’re necessary, mind-bending exercises in the art of the possible based on the achievements we’ve already produced. And this year, IKAROS gave every solar sail-minded scientist cause for celebration and renewed effort.

Tomorrow: A look at DLR and ESA’s solar sail work.


A New View of Ontario Lacus

Before I move into today’s story on Titan, I want to mention that those of us who weren’t able to attend the ongoing Second International Symposium on Solar Sailing (ISSS 2010) can take heart in the fact that selected papers from the proceedings have been quickly published online. Conferences vary tremendously in the resources they make available during and after the event, but the ISSS organizers are obviously intent on wide distribution of these interesting talks. Let’s hope those papers not yet included will find their way online in coming days.

TZF’s Pat Galea has posted a number of photos from day one of the event on Flickr, including this shot of JAXA’s Osamu Mori delivering an early talk on the IKAROS mission. Project leader for IKAROS, this man is a solar sail pioneer.

For those of you who’ve asked, the focus of ISSS 2010 is indeed near-term, although several longer-range papers will be presented. With our first operational solar sail only recently launched, this is a time to evaluate where we are and what the next steps will be. The conference is described in the proceedings as being:

…focused on recent advances in solar sailing technologies, near-term solar sailing missions and the physics of solar sailing. Areas of particular interest included dynamics analysis and testing of solar sails, advanced materials and structural concepts of solar sails, hardware and enabling technologies, mission architectures and programs, navigation, control, and modeling.

A Receding Shoreline on Titan

But on to Titan, where the lake levels of Ontario Lacus have continued to spur interest. Last week we learned that this, the largest lake in Titan’s southern hemisphere, is showing clear signs of liquid methane evaporation. It took an examination of four years of Cassini data to show a 1-meter drop in the lake level per year, evidently the result of seasonal evaporation of liquid methane from the mixture of methane, liquid ethane and propane that fill the lake.

The researchers used data from Cassini’s Synthetic Aperture Radar (SAR), studying the intensity of the radar backscatter to derive information about the composition of surface features. They were also able to tap radar altimetry data collected across part of Ontario Lacus from a December 2008 flyby. Oded Aharonson (Caltech) notes the effectiveness of the instruments and the implications of their findings:

“The combination of SAR and altimetry measurements across the transect gave information about the absorptive properties of the liquid, and argues that the liquids are relatively pure hydrocarbons made up of methane and ethane and not a gunky tar.”

Image: This image of Ontario Lacus, the largest lake on the southern hemisphere of Saturn’s moon Titan, was obtained by NASA’s Cassini spacecraft on Jan. 12, 2010. North is up in this image. Objects appear bright in the radar image when they are tilted toward the spacecraft or have rough surfaces. The lake surface appears dark because it is smooth. The northern shoreline features flooded river valleys and hills as high as 1 kilometer (3,000 feet) in altitude. Credit: NASA/JPL-Caltech.

Cassini’s radar can see through the liquid down to a depth of several meters. The radar will then bounce off the lake floor or, in deeper areas, will be completely absorbed, so that the signature is black. Alexander Hayes, a Caltech graduate student who worked with Aharonson on the project, notes that the lake liquid’s optical properties have been characterized enough to allow the local slope of the lakebed (bathymetry) to be detected. The team was thus able to calculate the slope of the lakebed around the entirety of Ontario Lacus.

The slope turns out to be fairly steep along the lake’s northern boundary as it runs up against a range of mountains, while the lake is at its most shallow and gently sloped along the southern edge, and it is here that sediment is accumulating. Along its eastern shore, the slope of the lake is somewhat steeper. “This is what we are calling the ‘beachhead,'” Hayes says.

The Approach of Autumn

What the researchers have found at Ontario Lacus parallels what Cassini shows about the evaporation of methane from nearby lakes, comparing 2007 data with data from May of 2009. The radar-attenuating liquid decreased or disappeared entirely in these, indicating a reduction of the liquid levels. The same one meter per year loss rate emerges in these results. If you haven’t already seen it, the video tour of Ontario Lacus based on radar data from Cassini’s flybys of Titan is well worth a look.

Cassini’s continued presence in the Saturnian system is paying major dividends. The spacecraft arrived in 2004, when the southern hemisphere of Saturn and its moons was experiencing summer. Now we’re seeing autumn approaching on Titan, a place whose year is the equivalent of about 29 Earth years. As for Ontario Lacus itself, the evaporation now seen from the lake would most likely reverse during winter in the southern hemisphere. It’s breathtaking to consider that we’re dealing with a lake whose surface area (about 15,000 square kilometers) is only slightly smaller than Lake Ontario here on Earth.


Musings on Sails and Stars

Solar Sails in Brooklyn

I should probably clean out my office, and would, if I could find the time, but things keep happening in the deep space community and I keep writing about them. I had the program for ISSS 2010 (the Second International Symposium on Solar Sailing) right beside me when I started to write yesterday’s entry, and by the time I got to the part on the conference, the program had disappeared into the wilderness of printouts, notebooks and letters. Thus I missed the fact that Colin McInnes would be in attendance at the sessions, a major addition to the already stellar lineup. McInnes could be said to have written ‘the’ book on solar sailing, a densely packed tome that lays out the principles and speculates on future missions.

Meanwhile, it’s heartening to see how international the solar sail effort has been from the outset, even if all the space agencies have continued to wrestle with their own funding demons. Much good work has gone on at Germany’s DLR, for example, to be reported on in the context of a DLR-ESA roadmap this morning (Tuesday). JAXA is, of course, present in a big way, with performance analyses of the IKAROS sail and discussions of the new technologies, especially in hybrid propulsion, that it represents. Les Johnson will bring the audience up to speed on NASA’s solar sail history and new mission ideas will be represented from each agency.

At left is McInnes’ Solar Sailing: Technology, Dynamics and Mission Applications (Springer/Praxis, 1999), an unfortunately expensive but critical addition to the library on deep space propulsion. I love the quote from Voltaire’s Micromegas (1752) that introduces the volume:

Our traveler knew marvelously the laws of gravitation, and all the attractive and repulsive forces. He used them in such a timely way that, once with the help of a ray of sunshine, another time thanks to a co-operative comet, he went from globe to globe, he and his kin, as a bird flutters from branch to branch.

Hearing those lines and thinking of that fluttering bird inevitably brings IKAROS to mind and those first, thrilling images of a solar sail deployment in space. McInnes will be discussing an idea he also kicked around in the book in a session at ISSS 2010, a near-term ‘pole sitter’ mission using solar sail methods (his title in the conference program indicates ‘hybrid propulsion,’ so I’m wondering if he’s thinking, like JAXA, about onboard solar cells).

Pondering Stellar Destinations

As long as we’re talking about solar sails, we can dream about future interstellar targets that may one day be visited by hybrid versions of this technology, perhaps using beamed propulsion. A new paper about the incidence of binary and multiple star systems is just out (thanks to Antonio Tavani for the pointer on this). It involves systems in which one member is a star like our Sun. The idea that most systems involving a star of our Sun’s mass are binary or multiple is problematic. It implies a lack of planetary stability that could compromise the possibilities for life. Now Deepak Raghavan (Georgia State) and colleagues are reporting the results of their own survey, one that tunes up and extends these earlier assessments.

Raghavan’s team worked with a sample of 454 stars chosen from the Hipparcos catalog, all like the Sun and within a range of 25 parsecs from Sol. What emerges is that 54 percent (plus or minus 2 percent) of Solar-type stars in our neighborhood are single (the earlier best estimate was 43 percent). The authors believe that earlier studies were off-target because they lacked the more accurate astrometry data available from the Hipparcos catalog, and in the case of a major study in 1991, were beset with parallax errors that skewed the results. In addition, the new research works with a much larger sample (454 vs. 164 Solar-type stars).

If we’ve overestimated the number of Sun-like stars with stellar companions, does this change our outlook on life elsewhere? We’re really asking whether planets are as likely to form and be stable in multiple star systems as they are around single stars. We’re accumulating enough data from the ongoing exoplanet hunt to start to make reasonable projections about at least part of this. Let’s wade into the paper to address the question, which resonates in light of that binary system with a distant companion we’ve got a scant 4.3 light years away from Earth. The first point is to examine this study’s conclusions as compared to earlier work:

Contrary to earlier expectations, recent studies… have shown that planetary systems are quite common among binaries and multiple systems. In a comprehensive search for stellar companions to the then known exoplanet hosts, Raghavan et al. (2009) concluded that even against selection effects, as many as 23% (30 of 131) of the exoplanet systems also had stellar companions. A recent report… showed that 17% (43 of 250) of exoplanet hosts were members of binary or multiple systems. In comparison, 30% (11 of 36) exoplanet systems of this study have stellar companions. This represents the largest percentage of stellar companions in any sample yet of exoplanet systems, likely due to the thoroughness of companion detection in this sample of nearby Sun-like stars. This is however still smaller than the 46% of stars having companions in the overall sample, presumably because all the exoplanets discovered to-date are from surveys that avoid known spectroscopic binaries.

So far, so good. The case for planets in binary or multiple systems is strong, although this will obviously depend upon the specifics of the system in question. A strong case can be made for stable planetary orbits out to 3 AU and perhaps a bit farther around Centauri A and B, for example, but whether or not planetesimals have been able to form around these stars that could produce rocky planets is still an open question, one we’ve discussed here on numerous occasions. But it’s clear that ruling out planets because of multiple star systems is a mistake.

Raghavan and colleagues then put numbers to our expectations in such systems (the italics in the quotation below are mine):

One key question is whether binary and multiple systems are equally likely to form planets as are single stars. Our results show that 9% ± 2% of the single stars have planets, compared to 7% ± 2% of binaries and 3% ± 3% of triples. These fractions are statistically equivalent, suggesting that single stars and stars with companions are equally likely to harbor planets. Moreover, while sufficiently short-period binaries will disrupt protoplanetary disks, hampering planet formation around either star… many binaries in this study have sufficiently long periods to foster planet formation around each stellar component.

I’ve left off some of the internal references for clarity as well as one of the supporting figures, but the conclusion seems robust. It remains for us to learn more about long-term planetary stability in the varying multiple systems we’re uncovering.

We now have three ongoing planet searches of Centauri A and B and may learn in coming months whether the average separation of these stars (about 24 AU) is sufficient to allow rocky planets to form (our earlier discussion of Ji-Wei Xie’s paper on the matter, which argues that a habitable planet is feasible around Centauri B, is here). It will be fascinating to learn how close stars can be to each other to allow planetary systems to form around each, but it’s clear the search for life will take in many multiple star systems as we learn more about planetary lifetimes.

The paper is Raghavan et al., “A Survey of Stellar Families: Multiplicity of Solar-Type Stars,” accepted for publication by The Astrophysical Journal and available as a preprint.