by Paul Gilster | Oct 24, 2008 | Deep Sky Astronomy & Telescopes |
Asteroseismology is the science of looking inside a star by studying the oscillations made by sound waves as they move throughout its interior. A recent news release from the COROT team calls these ‘Sun-quakes’ when they occur on our own star, and points out that the effect can be compared to seismic waves on Earth, whose examination can tell us much about what is happening below the surface. The Solar and Heliospheric Observatory (SOHO) mission, launched in 1995, studies our Sun’s oscillations, but COROT is now extending the science to other stars.
All three of the stars the mission has studied for this purpose — HD49933, HD181420 and HD181906 — are main sequence stars hotter than the Sun. And while stellar oscillations can be studied by ground observatories, moving to space offers serious advantages. So says Malcolm Fridlund, ESA project scientist for COROT, who notes the limitations of such observations when made from Earth:
“Adverse weather conditions, plus the fact that you cannot observe stars during daytime, oblige ground astronomers to interrupt their observations. Now, the key to detecting such small stellar oscillations from big distances is not only the sensitivity of an instrument, but also the opportunity of observing the star without interruption: any interruption produces noise in the data that can cover a signal completely. Therefore, to be certain, we must approach the question with the right instruments and from space.”
The method studies variations in the light caused by changes on the stars’ surfaces, revealing internal structure and helping to explain how energy is transported from the core to the surface. COROT’s 27-centimeter telescope is designed to look for tiny changes in brightness like this, and while we normally think of COROT as a planet-hunter, we don’t want to overlook this valuable and no less significant objective. After all, this observatory’s name stands for Convection Rotation and Planetary Transits. COROT is designed to create observations that will give us a deeper understanding of a star’s mass, age and chemical composition while comparing the Sun with other stars.
Image: When looking at stars, COROT is able to detect ‘starquakes’, acoustical waves generated deep inside a star that send ripples across a star’s surface, altering its brightness. The exact nature of the ripples can allow astronomers to calculate the star’s mass, age and even chemical composition. Credit: CNES.
Early results show that the oscillations of the three stars studied are 1.5 times as large as the Sun’s. And these oscillations, which create the variations in brightness COROT is now studying, are the only thing other than neutrinos that produce information about the inside of such stars. We’d better make the most of the data. For more results from the early COROT work on asteroseismology, see Michel et al., “CoRoT Measures Solar-Like Oscillations and Granulation in Stars Hotter Than the Sun,” Science Vol. 322. no. 5901 (24 October 2008), pp. 558-560 (abstract).
by Paul Gilster | Oct 23, 2008 | Exoplanetary Science |
We’re all interested in transiting planets smaller than the Neptune-sized Gliese 436b, and sure to find many of them as our methods improve. One day soon, via missions like COROT or the upcoming Kepler, we’ll be studying planets close to Earth mass and speculating on conditions there. But here’s a scenario for you: Suppose the first Earth-mass detection isn’t of a planet at all, but a moon orbiting a much larger planet? That challenging scenario comes from David Kipping (University College London) in a new paper on the detection of such moons.
I should be calling them ‘exomoons,’ the satellites of planets around other stars. It’s reasonable enough to assume they’re out there in the billions given the nature of our own Solar System. And compared to the multitude of giant planets found thus far, an Earth-mass exomoon in the habitable zone would seem to offer a far more benign environment for life. The trick, of course, is to pull off a detection, for most exomoons are going to be smaller than the Earth. Varying orbital distances will make the moon hard to spot during a transit, at times hiding the moon behind or in front of the planet. But Kipping notes that variations in the time a planet takes to transit its star could be one clue to the presence of such a moon.
It’s an interesting thought, but does it tell us enough? The transit timing variation (TTV) signal varies according to both the mass of the exomoon and its orbital separation from the planet it circles. It becomes impossible using transit timing variations alone to determine the mass of the exomoon without plugging in some value for its orbital separation. It’s a conundrum unless a secondary method can be found that works in conjunction with transit timing variations to tease out the exomoon’s parameters. Kipping finds that method in transit duration variation (TDV), which offers a signal of the same magnitude, and one that can be larger than TTV itself.
Measure multiple transits over a period of time and periodic changes in its duration are what make up the TDV value. In a recent email, Kipping said this about the relationship between TTV and TDV:
The moon and planet both orbit a common centre of mass, albeit a position very close to the planet’s centre. The effect of this is that the planet seems to wobble… As you can see, not only the position, but the velocity of the planet is shifting constantly. The spatial wobble causes TTV and the velocity wobble causes TDV. Hence, you will see why they must be 90 degrees out of phase!
Here is an animation of the process (all effects greatly exaggerated for clarity):
[kml_flashembed movie=”https://centauri-dreams.org/wp-content/images/wobble.swf” height=”250″ width=”400″ /]
I send you to the paper for the relevant equations, but using them Kipping is able to show that transit duration variation allows us to measure the moon’s mass without making assumptions about its orbital separation. It then becomes possible to derive the orbital period itself. A hypothetical planet identical to GJ 436b, for example, but with a 35.7 day period in a circular orbit would be in the habitable zone of the star it circles. An Earth-mass exomoon around such a world would be an achievable target. Studies of GJ 436b show that such a transit timing variation signal would be well within reach of existing instruments. From the paper:
This suggests that the detection of the exomoon should be presently possible through TTV from the ground and feasible with TDV in the near future. This illustrates that even ground-based instruments could detect an Earth-like body in the habitable zone using timing effects.
All of which points to data future observers should be gathering:
We also find that current ground-based telescopes could detect a 1 [Earth mass] exomoon in the habitable zone around a Neptune-like exoplanet. The author would therefore encourage observers to produce not only their mid-transit times, but also transit durations for each transit, rather than composite lightcurve durations. This will allow constraints to be placed on the presence of exomoons around such planets.
The science of exomoons takes us yet deeper into understanding exoplanetary systems. Not only am I jazzed about the scientific implications here, but I’m reminded to ask readers for recommendations on science fiction treatments of habitable moons around gas giants. Who knows what settings may become imaginable as we begin the detection of such moons through planetary transits? The paper is Kipping, “Transit Timing Effects due to an Exomoon,” accepted by Monthly Notices of the Royal Astronomical Society and available online.
Addendum: The original paper on using transit timing variations to detect exomoons is Sartoretti and Schneider, “On the detection of satellites of extrasolar planets with the method of transits,” Astronomy and Astrophysics Supplementary Series 134 (1999), pp. 553-560 (abstract). In an email, Dr. Schneider notes that the COROT team had already begun searching for TTV signatures before the appearance of Dr. Kipping’s paper. It will be interesting to see how TTV and TDV play out in the analysis of any resulting data.
by Paul Gilster | Oct 22, 2008 | Astrobiology and SETI |
A new look at Stanley Miller’s experiments at the University of Chicago in the early 1950s offers up an intriguing speculation: Volcanic eruptions on the early Earth may have been crucial for the development of life. Miller used hydrogen, methane and ammonia to re-create what was then believed to be the the primordial atmosphere on our planet, operating with closed flasks containing water in addition to the gases. An electric spark was then used to simulate lightning, and as anyone who has ever cracked a textbook knows, the water became laden with amino acids after a few weeks.
Image A: The apparatus used for Miller’s original experiment. Boiled water (1) creates airflow, driving steam and gases through a spark (2). A cooling condenser (3) turns some steam back into liquid water, which drips down into the trap (4), where chemical products also settle. Credit: Ned Shaw, Indiana University.
It never occurred to me that samples from the original experiments might have survived after all these years, but fortunately Jeffrey Bada (University of California at San Diego) discovered them after Miller’s death in 2007. And given the increasing sophistication of our tools for chemical analysis, it was a natural move to look for chemicals within those samples that might have eluded detection fifty years ago. Working with and re-interpreting old data is fascinating enough and is going to become more and more common in all the sciences, now that computers have given us the ability to generate and store such vast quantities of information. But re-analyzing samples from experiments as historic as these to pull out new insights puts a bit of a chill down the spine. If only Miller could have known about this work!
Miller’s three different experiments included one that injected steam into the gas to simulate a volcanic cloud, and it turns out that it is that experiment that produced the widest variety of compounds. The work, performed at NASA GSFC, turned up fruitful results indeed, some 22 amino acids, ten of which were new to this kind of experiment. A key factor is the change in our thinking about the ancient atmosphere, which is now believed to have been composed mostly of carbon dioxide, carbon monoxide and nitrogen rather than the mix Miller originally used.
Image B: The apparatus used for Miller’s “second,” initially unpublished experiment. Boiled water (1) creates airflow, driving steam and gases through a spark (2). A tapering of the glass apparatus (inlay) creates a spigot effect, increasing air flow. A cooling condenser (3) turns some steam back into liquid water, which drips down into the trap (4), where chemical products also settle. Credit: Ned Shaw, Indiana University.
How do changing views of the atmosphere affect the outcome? Daniel Glavin (GSFC), who analyzed the samples at Goddard, has this to say:
“At first glance, if Earth’s early atmosphere had little of the molecules used in Miller’s classic experiment, it becomes difficult to see how life could begin using a similar process. However, in addition to water and carbon dioxide, volcanic eruptions also release hydrogen and methane gases. Volcanic clouds are also filled with lightning, since collisions between volcanic ash and ice particles generate electric charge. Since the young Earth was still hot from its formation, volcanoes were probably quite common then. The organic precursors for life could have been produced locally in tidal pools around volcanic islands, even if hydrogen, methane, and ammonia were scarce in the global atmosphere. As the tidal pools evaporated, they would concentrate the amino acids and other molecules, making it more likely that right sequence of chemical reactions to start life could occur. In fact, volcanic eruptions could assist the origin of life in another way as well – they produce carbonyl sulfide gas, which helps link amino acids into chains called peptides.”
Jeffrey Bada, a co-author of the paper on this work, was Miller’s graduate student between 1965 and 1968, and continued working with the scientist in the intervening years. In addition to its provocative insight into the potential role of volcanic activity, the new work is a reminder that the things we do sometimes survive us in the most unexpected ways. I’m thinking of a particular researcher I once worked with, now gone, who would have found great pleasure in that notion. The paper is Johnson et al., “The Miller Volcanic Spark Discharge Experiment,” Science 322, No. 5900 (17 October 2008), p. 404 (abstract).
by Paul Gilster | Oct 21, 2008 | Sail Concepts |
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 some years back, his response all but confirmed the fact. “It doesn’t matter who came up with it,” Dyson said, “the question is whether it would work. It’s problematic but a good system to look at.”
Problematic indeed. But also a system with serious advantages over lasers. Forward wanted to reduce the weight of his unmanned probe as much as possible, so he conceived of making it out of nothing more than a wire mesh a solid kilometer in diameter, one that weighed a mere sixteen grams and included microchips at each intersection in the mesh. The name Starwisp seemed a natural for this spider’s web of a starprobe, a mission so lightweight that it would actually be invisible to the eye.
Forward intended to accelerate Starwisp at 115 g’s using a 10 billion watt microwave beam that would take it to one-fifth of the speed of light within days. It’s probably the speed of Starwisp as much as anything else that catches the imagination. In a time when we speak of thousand year sail missions to the Centauri stars as the fastest conceivable using near-term technologies (and even that is quite a stretch), Forward was talking about putting a probe with data return capability into Centauri space within twenty one years. It would be a fast flyby, to be sure, but all those microchips embedded in the vehicle would use microwave power to return imagery as Starwisp ripped through the Centauri system.
And here’s where the advantages come in. A laser beam mission requires a sail, but if you want to reduce the size and weight of your vehicle, microwaves can operate with something much more like a grid. It’s a function of the wavelengths involved. Recently I discussed these matters with microwave specialist James Benford, president of Microwave Sciences in Lafayette, CA. We’ll be talking about the beamed propulsion experiments that Benford and his brother Gregory performed at the Jet Propulsion Laboratory in coming weeks. But for now, Benford was musing about Forward’s thinking as it moved into the microwave realm:
Bob could see that one of the advantages of microwaves is that the wavelength is comparable to the human hand. These are dimensions you can see as opposed to lasers, which operate at invisible, minute wavelengths. With microwaves, you could push a grid that you could see right through. It would therefore be much lighter in mass and yet still rigid, because the grid has only to be spaced more than some fraction of a wavelength. Ordinary window screen would be just fine; in fact, it would be more than you need. The whole point is that microwaves are stopped completely by a conducting surface as long as the gaps are smaller than a wavelength or so.
All of which makes for powerful advantages. Then we can throw in the cost factor. Benford again:
Microwaves are a whole lot cheaper than lasers, typically by two orders of magnitude in terms of the cost of the optic that you use to broadcast, or the power efficiency of the laser. Optical surfaces for good telescopes that are used in lasers cost on the order of a million dollars per square meter. Whereas a good microwave surface is somewhere south of ten thousand dollars per square meter and in fact, at the kind of wavelengths Bob was talking about, which were down in the lower microwave region, that’s where commercial satellite antennas are available on the order of ten dollars a square meter. So the differences in economy are enormous.
Why isn’t Starwisp at the forefront of interstellar mission thinking? Alas, Geoffrey Landis went to work on the concept and discovered that the effect of the intense microwave beam on the materials Forward was working with would be disastrous. The lighter the wires the better for propulsion purposes, but wires as light as Starwisp’s would absorb rather than reflect the microwaves, destroying the craft within microseconds. Moreover, those long microwave wavelengths (compared to visible light) make for enormous beaming systems — Forward wrote about a lens 50,000 kilometers in diameter in his original Starwisp paper.
A lens considerably larger than the diameter of Earth? Clearly, something has to give. But the advantages of microwaves are unmistakable both within the Solar System and beyond. We’ll be talking more about microwaves soon, with more from my interview with Jim Benford and thoughts on how the microwave concept, already well established in the laboratory, can be applied to practical space technologies.
Until then, if you’re interested in the original Starwisp paper, it’s Forward, “Starwisp: An Ultra-Light Interstellar Probe,” Journal of Spacecraft 22 (1985b), pp. 345-50. And see Geoffrey Landis’ significant follow up, “Advanced Solar- and Laser-Pushed Lightsail Concepts,” Final Report for NASA Institute for Advanced Concepts, May 31, 1999 (downloadable from the still available archives at the NIAC site).
by Paul Gilster | Oct 20, 2008 | Research Tools |
Few places on Earth please me more than the Scottish highlands, to the point that I used to daydream about moving to Inverness (this was before that city’s population explosion, back when it weighed in at a sedate 50,000 inhabitants). But I’ll take anywhere in Scotland, and when I realized I wouldn’t be able to make the International Astronautical Congress in Glasgow this time around, I found myself sinking into a multi-day funk. Fortunately all is not lost, as the IAC, organized this year by the British Interplanetary Society, has left a digital record behind.
The Web is second best to being there, to be sure, but it helps to be able to listen in on key talks. I’ll leave you to page through the images and video from the event, pleased to note that Kelvin Long’s highlight lecture Fusion, Antimatter & The Space Drive is available in its entirety. Interstellar advocate Long is a member of the BIS as well as an active player in the Tau Zero Foundation. If you can set aside 45 minutes or so, you’ll find him ranging through interstellar issues from the magnitude of the distances involved to the basic technologies that could eventually bridge them, with nods to futuristic concepts like antimatter rockets and space drives. Given the BIS’ involvement in the now legendary Project Daedalus, the first serious engineering study of a starship, Kelvin’s knowledgeable comments on that proposal are well worth hearing.
We’re clearly building toward a future in which all major conferences become available through streaming and archival video, even if at present such coverage can be spotty. The recent Division of Planetary Sciences meeting in Ithaca, NY is a case in point, with all talks made available by the American Astronomical Society via webstreaming. Those of us with limited travel budgets have never had a better opportunity to participate in distant conferences than we have through Web-enabled sessions like these. With DPS 2008 now ended, the presentations are being assembled in a permanent video archive to be hosted by the AAS — I’ll pass that link along as soon as it becomes available.
Because I hadn’t checked the DPS site recently, I had to be reminded of the AAS contribution by last week’s Carnival of Space, which offered pointers to the Martian Chronicles blog. Ryan Anderson, Briony Horgan and Melissa Rice, the writers behind Martian Chronicles, are graduate students at Cornell with a passion for Mars. Last week Ryan devoted his attention to sessions he attended at the DPS conference, walking readers through discussions ranging from exoplanets to the mysteries of Titan.
As you wait for the archived DPS sessions, you can page through the Martian Chronicles entries, starting with this one, to get an idea of the range of studies presented last week. This is handy stuff for deciding which presentations you might want to view when the webstream archives become available. Controversial points are sprinkled throughout:
Another talk suggested that the methane in Titan’s atmosphere could be created by reaction of heavier organic molecules with hydrogen, but it was shot down in the questions session by people pointing out that the heavier organic compounds form from methane in the first place, and that when the heavy compounds lose their hydrogen, it escapes to space, making it a decidedly one-way sort of reaction.
A third Titan talk took a look at the absorption of infrared light when it goes through liquid methane and suggested rather controversially that some of the “lakes”that people are seeing may only be a few millimeters deep! This spurred a discussion of how well one can tell the depth of a body of liquid just by looking at it. One of the audience members said that “I wouldn’t gauge the depth of Cayuga lake by how deep I can see” but the counterargument was that, in the infrared, lakes are much clearer than they are at visible wavelengths.
Ryan is quick to note the places of unusual interest, as for example David Charbonneau’s discussion of an exoplanet 1.7 times as large as Jupiter but with roughly the same mass, a planet whose density is something like balsa wood. I’ll queue that one for playing, along with other presentations covering the discoveries now being extracted from transiting planet observations. Both the IAC and DPS sessions as preserved by the Net take a bit of the sting out of not being able to attend in person and should serve as a reminder to all conference organizers of the need to build and maintain permanent archives.
