by Paul Gilster | Sep 16, 2009 | Exoplanetary Science |
?I love to run into genuine enthusiasm when someone is doing cutting-edge science, and Didier Queloz (Observatoire de Geneve) has not let me down. Here the astronomer is discussing CoRoT-7b, which new studies have determined is a rocky world:
“This is science at its thrilling and amazing best. We did everything we could to learn what the object discovered by the CoRoT satellite looks like and we found a unique system.”
Amazing indeed. We knew from CoRoT’s transit measurements that the radius of this planet was about twice that of Earth. Queloz and team went to work with the HARPS spectroscope (High Accuracy Radial velocity Planet Searcher) at ESO’s La Silla site in Chile, gathering fully seventy hours of observations on the system. What emerged was the best mass measurement for an exoplanet yet.
Combining that revealed mass — five times that of the Earth — with CoRoT’s radius readings, we can deduce that CoRoT-7b is about as dense as Earth, and thus almost certainly a rocky world. This was tricky work, because stellar activity on the star’s surface can confuse the precise measurements needed for these radial velocity studies. Hence the need for the large number of observing hours.
The result, though, is clear. Team member Claire Moutou (Laboratoire d’Astrophysique de Marseille) sums up the method:
“Since the planet’s orbit is aligned so that we see it crossing the face of its parent star — it is said to be transiting — we can actually measure, and not simply infer, the mass of the exoplanet, which is the smallest that has been precisely measured for an exoplanet. Moreover, as we have both the radius and the mass, we can determine the density and get a better idea of the internal structure of this planet.”
The find, announced at the European Planetary Sciences Congress in Potsdam this week, marks the first time the density of an exoplanet this small has been measured. Interestingly, the small world Gliese 581e has a minimum mass about twice that of Earth, but because the exact shape of its orbit is unknown, its real mass is conjectural. CoRoT’s transit data, on the other hand, make the geometry of CoRoT-7b’s orbit definable and allow precise mass determination.
Image: The star CoRoT-7 is located towards the constellation of Monoceros (the Unicorn) at a distance of about 500 light-years. Slightly smaller and cooler than our Sun, CoRoT-7 is also thought to be younger, with an age about 1.5 billion years. It is now known to have two planets, one of them being the first to be found with a density similar to that of Earth. CoRoT-7 is in the centre of the image. Credit: ESO.
CoRoT-7b transits its star every 20.4 hours, orbiting a mere 2.5 million kilometers out, some 23 times closer than Mercury is to our Sun. Given that extreme proximity, this world is not a benign place to be, at least not for carbon-based lifeforms like ourselves. Says Queloz:
“CoRoT-7b is so close that the place may well look like Dante’s Inferno, with a probable temperature on its ‘day-face’ above 2000 degrees and minus 200 degrees on its night face. Theoretical models suggest that the planet may have lava or boiling oceans on its surface. With such extreme conditions this planet is definitively not a place for life to develop.”
But there is yet more news about this system. The HARPS dataset used in this work also reveals the presence of a second ‘super Earth’ orbiting the same star in a slightly wider orbit. CoRoT-7c orbits in three days and seventeen hours, with a mass about eight times that of Earth. Unfortunately, no transits are visible for this one, so we can make no measurements of planetary radius and density.
The paper on this work is Queloz et al., “The CoRoT-7 planetary system: two orbiting Super-Earths,” to appear in Astronomy & Astrophysics Volume 506-1 (22 October 2009). Check here for early access to this paper. An ESO news release is also available.
by Paul Gilster | Sep 15, 2009 | Advanced Rocketry |
?The last time we developed a new way of reaching orbit was back in the 1950s. How useful, then, to come up with one that allows huge weight reduction because it leaves propellant and energy source on the ground. Keeping the fuel at home or harvesting it along the way are key ways to conceptualize missions to the outer Solar System or beyond. But in more immediate terms, laser-beamed lightcraft can give us a relatively inexpensive way to low-Earth orbit as we begin to build a true space-based infrastructure.
Eric Davis (IASA) and Franklin Mead (formerly of the Propulsion Directorate, AFRL, now retired and pursuing independent research) envision a ground-based laser beam generator system made up of power supply, laser beam generator/transmitter and related tracking, hand-off and safety systems. As we saw yesterday, the system would power an air-breathing pulsed-detonation engine that feeds off ambient air turned into plasma by the laser from the ground, producing a ‘superheated plasma shock wave (with instantaneous pressures reaching tens of atmospheres) that generates thrust in the direction of the laser beam.’ And we’re off, handing the thrust duties off to on-board propellant only when we climb above the atmosphere and switch to laser-thermal rocket mode.
Image: A lightcraft test in 2000 at White Sands Missile Range in New Mexico. Credit: Rensselaer Polytechnic Institute.
The advantages of such a system are manifest. Moreover, they’ve been the subject of serious scrutiny for some time. The Strategic Defense Initiative Organization (SDIO) operated a Laser Propulsion Program that funded lightcraft studies in the 1980s, while in the following decade NASA MSFC and the Advanced Concepts Division of the AFRL Propulsion Directorate ran tests at White Sands. These early flights didn’t go very far (a lightcraft reached 43 meters in stabilized free flight), but don’t miss the significance, as Davis and Mead are quick to note:
This achievement can be compared to the first successful flights of Robert Goddard’s liquid propellant chemical rocket, which attained a height of 12.5 m after a 2.5 second burn in March 1926. In sharp contrast with Goddard’s rockets, there is absolutely no fuel on board the prototype Lightcraft, which has a diameter of 10 cm, mass of 20-40 g, and is machined from a solid block of 6061-T6 aluminum.
As to flight tests, they’ve encompassed diverse hardware:
Five different Lightcraft designs have been flight-tested using the pointing and tracking system on the PLVTS laser. Current Lightcraft designs are limited to about 60 g mass and 15 cm in diameter by the PLVTS laser. A MW-class laser will be necessary for a kilo-class Lightcraft to reach orbit and components for these lasers exist, which would demonstrate the feasibility of this technology for low cost access to space.
You may be wondering why Davis and Mead capitalize ‘lightcraft’ in the passages above. Eric tells me that the Air Force owns the lightcraft concept and capitalizes the word in its reports. In any case, the benefits of lightcraft are clear. We’re talking single stage to orbit (try that with chemical rockets!), little on-board propellant (using the ambient air as the working fluid), and payload mass fractions running as high as fifty to ninety-five percent. We’re also talking about reducing costs by two to three orders of magnitude. The required laser beam power is 0.1 to 1 MW per kilogram of vehicle mass, and as the authors point out, “The ground-based MW-class laser beam generator is state-of-the-art technology.” The cost of generating the needed electrical power is $0.10/kWh, or less than $2 per kilogram of payload.
These are powerful advantages over existing systems. Leik Myrabo, who has been at the heart of this research throughout the process, and who operates a company called Lightcraft Technologies, talks about propulsion concepts taking 25 years to mature. “That time is now,” he told Leonard David in this article, which notes that high-power laser experiments are in the works at the Henry T. Nagamatsu Laboratory of Hypersonics and Aerothermodynamics at the IEAv-CTA in Sao Jose dos Campos, Brazil, a joint effort betwen the US Air Force Office of Scientific Research and the Brazilian Air Force.
Image: New lightcraft experiments are being conducted at Henry T. Nagamatsu Laboratory of Hypersonics and Aerothermodynamics. Credit: A.C. Oliveira and I.I. Salvador/IEAv-CTA.
Myrabo holds the world altitude record for lightcraft in free flight, in an experiment funded by a grant to his company that attained an altitude of 71 meters. What we’re now seeing are encouraging developments in a technology that may soon achieve far greater heights, one that could by its economics drastically change our limited launch capabilites. As Myrabo told Leonard David, “It’s a matter of will and do we want to do it. This technology is now at the cusp of commercial reality.” For more, in addition to the Davis/Mead paper (citation in yesterday’s post), see Myrabo’s Lightcraft Flight Handbook, LTI-20: Hypersonic Flight Transport for an Era Beyond Oil (Apogee Books, 2009).
by Paul Gilster | Sep 14, 2009 | Advanced Rocketry |
Not the least of the objections against using laser propulsion to boost a lightsail to the stars is the engineering required to build the system. But theorists like Robert Forward, who originated the laser lightsail idea, never thought we would simply create such a system from scratch. We might ask, then, in the area of laser propulsion, what ideas are being experimented with right now, and might be capable of development into more advanced designs?
Enter the Lightcraft
Laser lightcraft command the attention here. Extensive work has been done on them at the Air Force Research Laboratory (AFRL), building upon earlier work at the AFRL Propulsion Directorate at Edwards Air Force Base. These early designs aim not at the stars, of course, but at a much more accessible target: Low Earth Orbit. A ground-based laser transmits power to the spacecraft, which collects the incoming energy and uses it to power its propulsion system. The beauty of this is that ambient air becomes the working fluid, allowing designers to leave the energy source on the ground.
Several lightcraft concepts have been considered, as Eric Davis (IASA) and Franklin Mead (Propulsion Directorate, AFRL, now retired — see addendum below) make clear in a recent paper. In the one just mentioned, what happens aboard the lightcraft is interesting indeed. The lower portion of the vehicle is a highly polished mirror (see illustration above — both images in this post are drawn from Davis and Mead’s paper). The craft itself looks something like a fat acorn. Kilojoule pulses from the ground installation are fed to the vehicle at the rate of 25 pulses per second. The system now turns ambient air into something more useful, as the paper explains:
The laser beam’s pulse interacts with the mirror, spreading out and focusing into an annular area inside the circumference of the craft. The intensity of the 18 microsecond pulsed laser is sufficiently high that atmospheric breakdown occurs in the annular area causing inlet air to momentarily burst into a highly luminous plasma (10,000 – 30,000 K), thereby producing a superheated plasma shock wave (with instantaneous pressures reaching tens of atmospheres) that generates thrust in the direction of the laser beam. A lip around the craft’s circumference, akin to a plug nozzle directs the expansion of the plasma, creating downward thrust expansion. Multiple laser pulses and an atmospheric refresh of breakdown air generate the flight.
Flight on a Beam of Light
A lightcraft, then, flies on a beam of laser light, turning its energy into thrust. Earlier designs examined the concept from various directions, including one that used a heat-exchanger aboard the rocket and transferred the beamed energy in such a way as to heat a working fluid like hydrogen or ammonia that would be carried onboard. That produces thrust through expansion through a nozzle, much like a chemical rocket.
Another possibility is to carry an onboard solid propellant. But the latest incarnation of the lightcraft operates in dual mode, using air as described above (turned into a plasma by the laser) and then switching to laser thermal rocket mode at higher altitudes (above about thirty kilometers).
The latter concept, of course, demands a small onboard fuel supply, but nothing like the massive fuel/payload ratios we see in today’s rockets. We’re talking about a spin-stabilized, single-stage transportation system to orbit. In its ‘airbreathing’ mode, the engine pulses at a variable rate to achieve what the authors call a ‘quasi-steady thrust,’ one that depends upon the Mach number and altitude along the craft’s flight trajectory.
Have a look at a diagram showing how the system would work. The lightcraft switches into laser thermal rocket mode as it climbs above the atmosphere, using the only fuel it needs to carry. And get this: The lightcraft system as envisioned by Davis and Mead is capable of mid-air hovering and powered descent and landing.
Crunching the Numbers
Cost? The ground-based laser installation, the AFRL study found, is the major expense of this transportation system, comprising about eighty percent of the total lightcraft system life-cycle cost. Launch costs as low as $74,141 per flight emerge from the studies, with estimated payload costs (using a 10 MW N2/CO2/H2 laser design) that should raise some eyebrows:
…a final total cost of $2,793 to launch a 5.25 kg payload to LEO. This new final result represents a cost of $532 per kg of payload (or $241 per pound) launched to LEO, which is 41 times lower than the space launch industry cost of $10,000 per pound for conventional chemical propulsion rockets.
The theoretical and experimental work already conducted by the AFRL Propulsion Directorate has demonstrated the practicality of the lightcraft concept. Tomorrow I want to run through some of the lightcraft’s other advantages, discuss the background of the idea (aerospace engineer Leik Myrabo has been working on this concept for thirty years), and consider where we are today. The paper is Davis and Mead, “Review of Laser Lightcraft Propulsion System,” CP997, Beamed Energy Propulsion, Fifth International Symposium (AIP, 2008), pp. 283-294.
Addendum: A note from a reader points out that Franklin Mead is now retired from the Air Force and pursuing research at his own company, Mead Science & Technology.
by Paul Gilster | Sep 11, 2009 | Culture and Society |
Learning how we connect with the universe is one of the most fruitful investigations of modern science. No matter how we approach the matter, we’re confronted with interesting possibilities. We study how gas giant planets may affect life on inner, terrestrial worlds by diverting asteroids from potential impacts. We look at issues like panspermia, wondering whether life’s building blocks (or even life itself) arrived from elsewhere in the cosmos.
In recent times, we’ve examined our Solar System’s movements through the galaxy to ask whether there may be clues to periodic mass extinctions on our planet. As we widen stellar habitable zones into galactic ones, our musings take us out into the universe. They also confront us with our own limitations — our eyes, notes astronomer James Kaler, see wavelengths between 0.00004 and 0.00008 of a centimeter. Kaler calls our visual spectrum “…but one octave on an imaginary electromagnetic piano with a keyboard hundreds of kilometers long.”
That imaginative expansion of scale is what Kaler (University of Illinois, Urbana-Champaign) sets about exploring in his new book Heaven’s Touch: From Killer Stars to the Seeds of Life, How We Are Connected to the Universe (Princeton, 2009). He’s interested in everything from lunar and solar tides to solar storms that can knock out power grids on Earth, and he moves readily through the effects of a nearby supernova to hot-button issues like climate change. He’s a clear writer who consolidates rather than surprises, attempting to pull together our current theories on what we see, and what we don’t.
Here, for example, is a brief take on dark matter:
Our Galaxy, its stars revolving around the center under the influence of their combined gravity, is spinning too fast for what we see. Galaxies in clusters orbit around the cluster’s centers under the influence of their mutual gravities, but again, they move faster than expected. There must be something out there with enough of a gravitational hold to do the job, to speed things up, but it is completely unseen. Dark matter. It surrounds galaxies, pervades their clusters. We have no idea what constitutes it. Rather, there are many ideas, but none that can be proven. Add it all up based on the amount of mass needed to yield dark matter’s gravity, and lo, one finds another 20 percent, getting us up (once the numbers are rounded off) to a quarter of that required to unfold the Universe, but still not enough.
The passage moves on to an examination of dark energy. I quote this paragraph on dark matter because we’ve been discussing the possibilities in recent comments here on Centauri Dreams, but as Kaler notes, we’re a long way from knowing what makes up dark matter and some theories still reject it altogether. Any consistent theory must square both those anomalous galactic rotations and the evidence from gravitational lensing around galactic clusters, a challenge laid out in more fine-grained detail in Evalyn Gates’ superb Einstein’s Telescope (W.W. Norton, 2009).
In any case, I haven’t finished the book, so I’ll post more on Heaven’s Touch as I push farther into it. I’m not finding anything that would startle Centauri Dreams readers but because I admire Kaler’s clarity, I’m hoping this volume will strike a chord with the broader public, one for whom otherwise esoteric topics like the shielding effects of the solar heliopause on our system will awaken an interest in our planet as a celestial wanderer, a member of a system that is itself moving through a medium we only partially understand, one that has affected everything that we are and do.
by Paul Gilster | Sep 10, 2009 | Culture and Society |
The other day I made a crack about a particular piece of artwork not being up to snuff, said item being an illustration accompanying a news release about a recent astronomical find. Maybe I was just out of sorts that day. In any case, what’s significant to me about much of the artwork floating around to illustrate news stories is that it’s generally quite good. Sure, we’re talking about ‘artist’s concepts’ of things like exoplanets and other distant objects, but they’re usually concepts informed by current data and they’re well executed.
Then I ran across Jeff Foust’s essay on art and space in the Space Review and got to thinking about what had propelled me as a kid into this kind of work. We had a fabulous network of community libraries in St. Louis back in the 1950s and ’60s, and I made good use of three of them in particular. I’d stock up on science books and more or less read the astronomy sections straight through, starting at one end and working across. The photographs of astronomical objects were helpful, but the artwork was often what seized my mind.
Image: An artist’s conception of the Milky Way as seen from outside. Credit: NASA, ESA and The Hubble Heritage Team (AURA/STScI).
Naturally, Chesley Bonestell comes to mind, the man responsible for blowing more minds in that era than any other space artist (I still see Titan in Bonestell’s terms, despite everything we’ve learned about it since). Be sure to read Gregory Benford’s reprint of an early essay he wrote on a visit to Bonestell’s home. A snippet:
Does he ever read the things he has illustrated? No, he doesn’t like science fiction very much. Not enough solidity, perhaps. He rarely if ever willingly puts a human artifact into his work, a spaceship or a pressure dome, or a space-suited figure. He doesn’t have any idea of what the future will bring and feels awkward trying to visualize it. But stars and planets, yes, the astronomer friends he has can give him descriptions of how things must be there and he can see it, too, in some closed mind’s eye, so that it comes out right. Most science fiction is quickly outdated, anyway. Look at all the fins on space ships, and the cloudless Earths. Better to stay away from it.
Fascinating. But Foust reminds me that so doughty a figure as Alan Bean is a well-known artist in his own right, having recently opened an exhibition at the National Air and Space Museum in Washington. It’s amusing to see him quoted as saying people used to chide him about painting Earthly scenes. After all, he was the first artist in history to go some place besides the Earth. Why wasn’t he painting scenes from his Apollo 12 experience in 1969?
Bean went on to become a full-time artist, and I can only imagine the reaction of some of his associates when he made that move. Now he’s working in intriguing mixed media, using textures he creates from objects like lunar spacesuit boots and working lunar dust particles into his work, extracted from the patches of his suit. If the first career was satisfying, Bean now says he has developed “the heart of an artist,” a changeover that took 28 years to accomplish.
Image: Alan Bean surrounded by his paintings. Credit: Alan Bean.
Foust also looks at a panel on space art at the recent Mars Society Convention at the University of Maryland, where author Andrew Chaikin reminisced:
“I can say that I’m probably sitting here today because of space art. That’s what hooked me when I was five years old. The illustrations in my astronomy books when I was a kid were, as I have written, like ‘magic portals’ that transported me from my parents’ house to other worlds.”
Chaikin and I seem to have shared a common childhood experience. And I also think Emil de Cou, associate conductor of the National Symphony Orchestra in Washington, is on to something important:
“There used to be a much closer overlap between the imaginative source of science and art that was shared in the years before that caught so much of our imagination. That’s why everybody is in this room today, not so much because of some hard scientific fact that you read as a child, but it was from reading Amazing Stories magazine, or watching Star Trek or Lost in Space.”
In my case, it was indeed Amazing Stories, along with the Astounding/Analog of the later Campbell era and H.L. Gold’s Galaxy, that provided much of the imagery. Those wonderful magazine covers worked their magic as much, if not more so, than blurry views from Palomar, keeping me wondering what it would be like to actually see some of these objects up close, if not walk upon their surfaces. Can art reawaken the spirit of exploration that seems so much on the wane?
Let’s hope so. We have no shortage of talented artists working this turf, and the Net is spreading their work more widely than ever. Have a look at the members’ page for the International Association of Astronomical Artists, where you’ll find Web sites listed for Lynette Cook, Don Dixon, Dan Durda, David Hardy, Rich Sternbach and many more. A few hours exploring these scenes may remind you of that early thrill of discovery that propelled so many into space sciences.
