The thread on SETI’s Paradox and the Great Silence has continued with considerable gusto, enough so that we’re pushing the database limits on comments there. So I’m starting an overflow topic for those who want to keep the debate going. Please post any further responses to the SETI thread here, where we’ll have plenty of room.
It’s been some time since Centauri Dreams looked at the work Gregory Benford (University of California at Irvine) and his brother James (Microwave Sciences) are doing with solar sail concepts. But I just noted, in paging through a back issue of the Journal of the British Interplanetary Society, that their proposal for a microwave-beamed sail was written up there, based on a talk at the 2005 IAA symposium in Aosta, Italy. And because I want to keep sail concepts visible in a time when funding constraints have all but driven them from the news, let’s revisit that work.
What got the Benfords headlines not so long ago was the speeds they were proposing. Five years or less to Pluto? That’s almost a halving of New Horizons’ travel time, and it makes for some intriguing conjecture indeed. The Benfords learned from earlier laboratory experiments that heating up ultralight carbon sail materials causes accelerations greater than would be expected from the pressure of photons alone. Apparently causing the effect are molecules evaporating from the hot side of the sail.
So here’s a solar sail scenario that a science fiction writer (especially one named Benford) could put to good use: ‘Paint’ various compounds onto a sail to take advantage of these effects, which vary depending on how they’re managed. The duo envisage a ‘Sundiver’ mission that uses microwave beaming, a close Solar pass and a second boost from the Earth-orbiting microwave transmitter to achieve maximum speed.
The solar sail could be deployed in low Earth orbit by conventional rocket. It would then be launched by microwave beam, the heat of which would cause a polymer layer to desorp from the sail (think of desorption as the opposite of absorption — some of the ‘paint’ material is released to provide propulsion).
The microwave beaming cancels most of the sail’s orbital velocity around the Sun, causing it to fall toward it. The craft approaches edge-on but at perihelion, a few solar radii out, it rotates to face the Sun. Now a second layer of polymers desorps away under the intense sunlight, and the craft gets a 50 kilometer per second boost, departing the area as a conventional reflecting solar sail with its final layer of aluminum now exposed. Mission speed is approximately 10 AU per year. A parting boost from a microwave transmitter in Earth orbit could add still more delta-V.
The numbers in this paper are quite encouraging. The authors explore, for example, Mars travel times of roughly one month (New Scientist liked that headline) though they pass on the question of how to slow down once the vehicle arrives. And it’s useful indeed to know that beamed power from Earth can heat sail temperatures enough to simulate the conditions of a near Solar pass, allowing a nearby ‘laboratory’ to pursue materials research on a space-based sail.
The paper is Gregory Benford and James Benford, “Power-Beaming Concepts for Future Deep Space Exploration,” in the Journal of the British Interplanetary Society Vol. 59 No. 3/4 (March/April 2006), pp. 104-107. It outlines a hybrid concept that draws on the best features of solar and laser-driven sails, adapting the beaming strategy to the microwave region for maximum efficiency. And it’s the kind of thinking that could push sails out past the heliopause.
The news about possible surface water on today’s Mars points out how far we are from characterizing life’s possibilities even in our own Solar System, much less around other stars. It may take boots on the ground on Mars to solve the question once and for all, but life in underground aquifers certainly is a plausible proposition, and the sooner we have proof (and samples to study), the better for astrobiology in general.
Meanwhile, we push on with the very early wave of exoplanet studies, remembering that it’s just over a decade since 51 Peg gave us the first confirmed detection around a main sequence star. I can’t imagine a more fruitful field for a young astronomer to head for, with so many possibilities for study that you begin to wonder whether we’ll have the human resources to keep up with the vast data inflow that’s coming.
Some of the more intriguing recent work concerns binaries and the planets around them. If we’re getting fairly sanguine about the possibility of planets around close binaries like Centauri A and B, the question of what happens when the stars are farther apart stands open. A new paper by Daniel Malmberg (Lund Observatory, Sweden) and colleagues probes this question for two kinds of binary systems. Co-planar systems are those in which the two stars evolved as a binary from the start, drawing on the same primordial materials. Planets around these stars are likely in the same orbital plane. But the paper’s real focus is on stars that acquire a binary partner later in their evolution in a young stellar cluster.
When that happens, the result is a planetary system that’s randomly oriented with respect to the companion star. The authors go to work on such systems using the Kozai mechanism, first applied to asteroids in inclined orbits in our own Solar System as they are affected by Jupiter. The Kozai mechanism causes planets in wide binaries (the authors study 1000 AU separations, for example, as opposed to the 23 AU mean separation of Centauri A and B) to vary their orbital eccentricity. This is one way to explain the high eccentricity of the planet around 16 Cygni B.
Interesting things follow from all this. If you start with our Solar System and introduce a second, widely separated star, Neptune and Uranus will cross in their orbits if the inclination between companion star and the plane of the planets is greater than 43 degrees. Planetary ejections can follow. The Kozai numbers indicate this scenario will be the case in 73 percent of the binary systems formed by stellar encounters. And it could be that some of the observed extrasolar systems we see are the result of this kind of planet-scattering in a system that was once much like our own, with planets orbiting around a single original star.
Intriguing stuff, and it makes good sense, given that stars forming in groups are going to be in a crowded environment during the first 100 million years after their birth. That makes single stars into binaries easily enough, with obvious implications for stripping the system of one or more planets, and greatly affecting the orbits of those that remain.
The paper is Malmberg et al., “The instability of planetary systems in binaries: how the Kozai mechanism leads to strong planet-planet interactions,” submitted to Monthly Notices of the Royal Astronomical Society and available online as a preprint.
A note from Ian Jordan (Space Telescope Science Institute) passes along the welcome news that presentations and webcasts from last week’s Astrophysics Enabled by the Return to the Moon 2006 workshop at STScI have been posted online (available here). There’s plenty to dig into here, but of specific note for exoplanet research are the presentations by Webster Cash, Maggie Turnbull, Sara Seager and Peter McCullough.
Centauri Dreams readers have read about all four of these scientists in the past year or so. Maggie Turnbull (Carnegie Institution of Washington) specializes in identifying stars that may have terrestrial planets around them. In an earlier post, we looked at some of her picks. Sara Seager (also at Carnegie) is particularly known for her work on HD 209458B, a hot Jupiter that transits its star and thus offers up much useful data. And Peter McCullough (Space Telescope Science Institute) is getting remarkable results from the XO telescope in Hawaii, collaborating with amateur astronomers looking for transits. They’ve already found a Jupiter-class planet around a Sun-like star in Corona Borealis.
As for Cash (University of Colorado at Boulder), he’s well known here for his work on the New Worlds occulter that holds so much promise for direct imaging of distant planets. Although New Worlds was not chosen in the latest round of Discovery mission studies, Northrop Grumman continues to pursue New Worlds as avidly as Cash himself, having backed the original concept of flying the mission in tandem with the James Webb Space Telescope. That idea may now be on hold, but Northrop Grumman’s Amy Lo is working to prove that New Worlds will work just as well with a smaller telescope (interestingly, Hubble is not up to the job for orbital reasons, as this article explains).
A starshade in the form of a flower-petal has a certain aesthetic appeal even beyond the numerous studies that show it to be good science. Odds are strong that New Worlds will work, and at a fraction of the cost of what Terrestrial Planet Finder had been evolving toward. Tens of meters across and made of kapton (similar to mylar), the shade may represent our best bet for direct detection of Earth-like worlds, which makes Cash’s comments at the STScI workshop well worth your time.
The COROT satellite, slated for transit studies of nearby stars in search of exoplanets, has completed fueling up operations. Launch is scheduled for December 21 at the Baikonur cosmodrome in Kazakhstan. Nine days were required to top the satellite’s tanks even though it is only carrying 40 litres of hydrazine, due to the highly poisonous nature of the fuel. A French project with ESA participation, COROT will be the first space misson specifically dedicated to finding extrasolar planets, and it may give us our first detection of rocky worlds only a few times larger than Earth.
