by Paul Gilster | Aug 17, 2010 | Interstellar Medium |
When the Project Daedalus team went to work to design a starship back in the 1970s, they contemplated using the atmosphere of Jupiter as their source for helium-3, an isotope needed in vast quantity for Daedalus’ fusion engines. More recently, though, attention has turned to the lunar surface as a possible source. Now the IBEX spacecraft, normally charged with studying the interactions between the heliosphere and what lies beyond, has been used to examine a useful recycling process as particles hit the Moon, pushed there by the Sun’s 450 kilometer per second solar wind.
A Glow of Energetic Neutral Atoms
The process is straightforward — lacking a magnetosphere, the Moon takes the full force of the solar wind, absorbing most of its particles into lunar dust. But the IBEX team, led by David McComas (Southwest Research Institute), has been able to show that about ten percent of the solar wind particles escape back to space in the form of energetic neutral atoms, or ENA’s, detectable by the spacecraft. IBEX, traveling in an eight-day orbit around the Earth, sees this ‘glow’ in its ENA detectors.
What IBEX detects are enough solar wind particles bouncing off the lunar surface as ENA’s to account for about 150 tons of hydrogen atoms per year, the rest remaining behind, some doubtless in the form of surface helium-3, whose measurement will one day help us calculate how useful a source it may become. But IBEX (Interstellar Boundary Explorer) is primarily focused on a much more distant venue. Its real mission is to see what happens to those same solar wind protons when they encounter interstellar atoms at the edge of the heliosphere.
ENA’s from Deep Space
Energetic neutral atoms are created at system’s edge when solar wind protons draw electrons from interstellar atoms, making them electrically neutral and thus no longer controlled by magnetic fields. Those ENAs that bounce back in the direction of the Earth can be recorded by IBEX, which studies a section of the sky about seven degrees across, scanning overlapping strips that complete a 360-degree map of the sky every six months.
The IBEX surprise, announced last October, was the discovery that the expected variations in emissions from the interstellar boundary were not evident. Instead, IBEX found what McComas at the time called “a very narrow ribbon that is two to three times brighter than anything else in the sky.” It’s chastening to remember that the Voyager spacecraft totally missed this feature because, unlike their point source measurements, IBEX can use its detectors to build up a complete map.
Charting Earth’s Magnetopause
The spacecraft has also been used to observe Earth’s magnetosphere from the outside, using the same ENA detection methods to see the interactions between the solar wind and the magnetic bubble surrounding our planet. Here the parallel between the heliosphere and the magnetosphere is interesting. The magnetosphere protects the Earth’s surface, causing the solar wind to pile up along its outer boundary (the magnetopause) before being diverted to the side. The heliosphere’s interstellar boundary, in a similar way, protects the Solar System from the worst effects of galactic cosmic ray radiation.
Image: IBEX found that Energetic Neutral Atoms, or ENAs, are coming from a region just outside Earth’s magnetopause where nearly stationary protons from the solar wind interact with the tenuous cloud of hydrogen atoms in Earth’s exosphere. Credit: NASA/Goddard Space Flight Center.
The IBEX team worked closely with the European Space Agency’s Cluster 3 spacecraft in observations made in March and April of last year. The new maps thus created show the teardrop shape of the magnetopause as solar wind protons pull electrons from hydrogen atoms in the Earth’s outer atmosphere. This region, the outer exosphere, is now shown to be tenuous indeed, with about eight hydrogen atoms per cubic centimeter. Thus ENAs helps us notch another needed measurement of a region that has been tricky to study.
The Wind and the Sail
You can see, too, that we’re gradually building up a picture of the solar wind that will help us analyze whether propulsion options like magsails have potential. Is the solar wind stable enough to allow accurate navigation by a magnetic sail-enabled spacecraft? Certainly the potential of hitching a 450-kilometer per second ride to the outer Solar System has appeal, and the deployment issues involved in large solar sails disappear with a magsail. But let’s see what IBEX and other missions can tell us about the solar wind as it reacts to the interstellar medium at system’s edge and plays against the magnetosphere closer to home.
You can read more about IBEX in this NASA mission page. For more on the solar wind’s interactions with the Earth’s magnetosphere, see Fuselier et al., “Energetic neutral atoms from the Earth’s subsolar magnetopause,” Geophysical Research Letters Vol. 37 (8 July 2010), L13101 (abstract). For the lunar observations, see McComas et al., “Lunar backscatter and neutralization of the solar wind: First observations of neutral atoms from the Moon,” Geophysical Research Letters Vol. 36 (2009), L12104 (abstract).
by Paul Gilster | Aug 16, 2010 | Exoplanetary Science |
Planetary systems around dim brown dwarfs are a fascinating thing to contemplate, and for a vivid imagining of future human activities on such planets, I’ll send you to Karl Schroeder’s Permanence. The 2002 novel posits ingenious engineering to sustain bases on such worlds, and even comes up with an interstellar propulsion method powered up by their energies that sustains an expanding starfaring culture. A brief sample of Schroeder’s universe (not enough to be a spoiler):
…the brown dwarfs each had their retinue of planets — the halo worlds, as they came to be called. And though they were not lit to the human eye, many of these planets were bathed in hot infrared radiation. Many were stretched and heated by tidal effects, like Io, a moon of Jupiter and the hottest place in the Solar System. And while Jupiter’s magnetic field was already strong enough to heat its moons through electrical induction, the magnetic field of a brown dwarf fifty times Jupiter’s mass radiated unimaginable power — power enough to heat worlds. Power enough to sustain a population of billions; enough to launch starships.
Speculative fiction has new wonders to mine as we learn more about brown dwarfs, and our discoveries are coming faster all the time. One reason is that we’re getting better at detecting them. In 2006, Katelyn Allers (University of Texas at Austin) and colleagues published a list of nineteen candidate brown dwarfs, all of them young and all but one now confirmed as either low-mass stars or sub-stellar objects. Allers’ team was able to use Spitzer data to search for infrared excesses, which is where the tale gets intriguing. The excess in the infrared is presumably due to circumstellar disks, leading us to wonder how small an object has to be to form with an accretion disk.
Current thinking has it that such disks probably don’t form around central objects smaller than a few Jupiter masses, but the fact that young brown dwarfs are more luminous, and hence easier to detect, than older ‘field’ brown dwarfs means we can find them by homing in on the places where stars are being born to study disk formation around these cool, low-mass objects.
This is what Paul Harvey, also at UT-Austin, has done, working with Allers and team to extend the early Spitzer results to luminosity levels that should allow the detection of objects as faint as two Jupiter masses. Their work used deeper Spitzer imaging of an area in the Ophiuchus star-forming region studied in the earlier work, where the youngest objects under investigation are evidently about one million years old.
The result: Eighteen new brown dwarf candidates with the near-infrared magnitudes and colors we would expect from such objects and the possible infrared excess that is consistent with a circum-object disk. From the paper:
Contamination by background field dwarfs in the brighter magnitude range of our sample and by extragalactic objects at the fainter magnitudes is likely to be significant. It is certainly possible that at least half of our candidates are such contaminants. Narrow-band filter photometry in progress, however, has shown that at least several of our candidates are likely to be low-mass BD’s with circum-object disks. It is likely that further candidates exist in our data set, though problems with diffuse 5.8 and 8μm emission in the region make it difficult to clearly confirm many more disk candidates.
If even the smallest brown dwarfs normally tend to form with accretion disks, the case for planets forming around these objects is strengthened, and given that brown dwarfs are found to be increasingly common (WISE will help us greatly in assessing their numbers), we may be looking at hosts of planetary systems of a kind we have only recently begun to imagine. The new work extends the earlier Allers study to luminosities a factor of ten below its limits, giving us new data about how smaller objects form with the accretion disks that can create companions.
The paper is Harvey et al., “A Spitzer Search for Planetary-Mass Brown Dwarfs with Circumstellar Disks: Candidate Selection,” available online. Thanks to Antonio Tavani for the tip on this paper.
by Paul Gilster | Aug 13, 2010 | Astrobiology and SETI |
Although it seems a long way from interstellar space, the early Earth is a fascinating laboratory for life’s development that should yield clues about how life takes hold elsewhere. Thus new work on the movements of the early continents catches the eye. In this case, the Gondwana supercontinent is found to have undergone a 60-degree rotation across Earth’s surface during a highly interesting period, the Early Cambrian. This is the fecund era when the major groups of complex animals appeared in relatively rapid succession.
Gondwana is what we can call the southern precursor supercontinent, a vast region that would eventually separate from Laurasia roughly 200 million years ago when the Pangaea supercontinent broke into two large areas. This Wikipedia article gives you the basics on Gondwana, noting that it included most of the landmass in today’s southern hemisphere, including Antarctica, South America, Africa, Madagascar, Australia, New Guinea and New Zealand, along with the Indian subcontinent and Arabia (although the latter two have, obviously, moved into the northern hemisphere).
Image: The paleomagnetic record from the Amadeus Basin in Australia (marked by the star) indicates a large shift in some parts of the Gondwana supercontinent relative to the South Pole. Credit: Ross Mitchell/Yale University.
The movement of the entire Gondwana landmass was relatively rapid, with some regions attaining a speed of at least 16 (+12/-8) centimeters per year about 525 million years ago. Compare that with the pace of today’s shifts, which are no higher than 4 centimeters per year. The intriguing question is whether the shift results from plate tectonics — the continental plates in motion with respect to each other — or ‘true polar wander,’ which involves the solid land mass down to the liquid outer core rotating together with respect to the planet’s rotational axis, changing the location of the geographic poles. More in this Yale University news release.
What study author Ross Mitchell (Yale University) found is that true polar wander is the most likely scenario, the rates of Gondwana’s motion exceeding those of plate tectonics of the past few hundred million years. But arguments about polar wander vs. plate tectonics are ongoing and have been for decades. Mitchell can only say “If true polar wander caused the shift, that makes sense. If the shift was due to plate tectonics, we’d have to come up with some pretty novel explanations.”
But back to the main issue, which is the effect such migration would have had on the environment and living creatures. In this model, Brazil shifted from close to the south pole toward the tropics, the kind of movement that would have affected carbon concentrations and ocean levels. Here’s what Mitchell has to say about the results:
“There were dramatic environmental changes taking place during the Early Cambrian, right at the same time as Gondwana was undergoing this massive shift. Apart from our understanding of plate tectonics and true polar wander, this could have had huge implications for the Cambrian explosion of animal life at that time.”
It will be fascinating to see how this work is followed up in other locales as we work to explain the shift’s effects. Mitchell and team did their work studying the magnetization of ancient rock in the Amadeus Basin of central Australia. The paper is Mitchell et al., “Rapid Early Cambrian Rotation of Gondwana,” in Geology Vol. 38, No. 8 (August, 2010), pp. 755-758 (abstract).
by Paul Gilster | Aug 12, 2010 | Outer Solar System |
It’s almost exhilarating to find that the volume of space studied in new work on the Trojan asteroids near Neptune includes an area through which New Horizons will pass on its way to Pluto/Charon. This used to seem like an all but unknowable region until Voyager 2 made its Neptune pass, and although it’s been a long time since we’ve had a spacecraft there, we’re learning much more about the outer system from Earth-based resources, as the discovery of objects like Eris and Sedna makes clear. We can surely look forward to more surprises as New Horizons moves toward its 2015 flyby and pushes on into the Kuiper Belt.
The latest find, based on data from the Subaru Telescope in Hawaii and the Magellan telescopes in Chile, is the first Trojan asteroid found at Neptune’s L5 Lagrangian point. Both the L4 and L5 Lagrangian points, 60 degrees ahead of and behind the planet, are stable, meaning that objects tend to collect there over time. Six Neptune Trojans are known in the L4 region, but until now the L5 point was hard to study because from our vantage on Earth, the line of sight is near the center of the galaxy. That called for a strategy using places where galactic dust clouds black out background light, revealing foreground asteroids. The result was the object called 2008 LC18.
Image: Discovery images of the L5 trailing Neptune Trojan 2008 LC18, taken at the Subaru telescope on June 7, 2008 Universal Time. The Neptune Trojan is seen moving from right to left near the center of the image. Each image is separated by about one hour in time. The background stars are stationary. This image only shows about 1 percent of the area of one image from the telescope. Credit: Scott Sheppard/Chad Trujillo.
Scott Sheppard (Gemini Observatory), explains the result:
“We estimate that the new Neptune Trojan has a diameter of about 100 kilometers and that there are about 150 Neptune Trojans of similar size at L5. It matches the population estimates for the L4 Neptune stability region. This makes 100-km-wide Neptune Trojans more numerous than similar-sized bodies in the main asteroid belt between Mars and Jupiter. There are fewer Neptune Trojans known simply because they are very faint since they are so far from the Earth and Sun.”
So now we’ve identified another Trojan population linked to Neptune to join the L4 asteroids there and the Trojans associated with Jupiter. The objects are useful adjuncts to planet formation theories. In this case, the fact that 2008 LC18 has an orbit that is highly tilted to the plane of the Solar System parallels the similar orbits of some L4 Trojans, and suggests the objects were captured during the early years of the Solar System, when Neptune itself was moving in a different orbit than today. We have much to learn about planetary migration as the giant planets refined their orbits and a chaotic system gradually settled into place.
The paper is Sheppard and Trujillo, “Detection of a Trailing (L5) Neptune Trojan,” published online in Science Express August 12, 2010 (abstract).
by Paul Gilster | Aug 11, 2010 | Sail Concepts |
When engineer Carl Wiley brought solar sails to a wide audience in 1951, he envisioned a particular kind of sail. Wiley, who wrote under the byline Russell Saunders, published “Clipper Ships of Space” in the May issue of Astounding Science Fiction that year, seven years before the first technical paper on sails, Richard Garwin’s “Solar Sailing: A Practical Method of Propulsion within the Solar System,” appeared in the journal Jet Propulsion. As you can see in the illustration, which ran with the original essay in Astounding, Wiley envisioned the sail as taking a parachute shape, with the payload attached to the sail circumference.
Varieties of Sail Design
But there are many ways of doing sails. Square or rectangular sail designs (think of those images of IKAROS shot by its detached cameras) have been the focus of recent work, with the result that many alternatives have not reached the same level of technological readiness. But along with parachute sails, spinning disk sails, ‘heliogyro’ designs and a variety of inflated sail concepts are all in the running for future development. Gregory Matloff (New York City College of Technology, CUNY) discussed the ‘hoop’ sail concept at the recent 2nd International Symposium on Solar Sailing, a study available in the proceedings.
A hoop sail is a disc-type sail attached to an inflatable rim, as in the diagram below. A payload could be distributed and hung from the hoop, with the advantage that no cables are necessary (contrast this with the extensive cabling of parachute-like designs), making the sail far easier to deploy. As opposed to other inflatable sail concepts, hoop sails are much less massive because they have only one surface, and present less surface area than other inflatable, hollow-body sails that could be endangered by micrometeorites — remember, the inflated rim is the only part of the sail that would use fill gas.
Hoop sails have made only a scant appearance in the technical literature, but going back through Matloff’s Deep Space Probes (Springer, 2000), I discovered in its 2nd edition that what work has been done has shown that a hoop sail structure is less massive than a square or rectangular boom supporting a sail’s cross members. Matloff quotes Colin McInnes from his 1999 book Solar Sailing (Springer-Praxis, 1999) on the growth of the overall sail concept:
Since Garwin’s paper initiated modern developments in solar sailing some forty years ago, the concept has inspired many individuals to devote their time and energy to advance the field. Countless technical papers have been written which demonstrate the potential advantages of solar sailing, many by graduate students who then move on to the more immediate problems of industry. Studies have been conducted which demonstrate the technical feasibility of solar sailing. However, for all these sometimes heroic efforts, an operational solar sail has yet to fly.
That’s a digression, but I had to include it to revel once again in the fact that the final sentence is no longer true, now that IKAROS is sending back data on sail performance every day. We can now look forward to trying out a variety of sail concepts as the idea is validated.
The Hoop Sail in Deep Space
But back to the hoop sail. Matloff’s continuing work on the concept has included an Oort Cloud Explorer (discussed in the appendix of Deep Space Probes), a sail 681 meters in diameter in which steering and attitude control would be provided by four smaller, five-meter hoop sails. Changing direction and attitude is an interesting issue, and it’s fascinating to consider how JAXA’s IKAROS team is experimenting with LCD units that allow the reflectivity of parts of the sail to be changed to achieve the same effects sought by the small adjunct sails that Matloff describes.
Image: Diagram of the hoop sail, of potential interest for deep space concepts. Credit: Gregory Matloff.
Massaging the numbers for a hoop sail to deep space, Matloff comes up with a ‘sundiver’ trajectory that allows a hoop sail spacecraft to exit the Solar System at about 635 kilometers per second (130 AU per year). This is sufficient to reach the inner fringe of the Oort Cloud in several decades, while a crossing to the Centauri system would involve 2,000 years. That’s about twice the time for an optimized mission with a space-manufactured sail.
Thus the limitation of the hoop sail, in the fact that it has perhaps 50 percent of the performance of the ultimate space-manufactured sails Matloff considers in the same volume. But as his presentation at ISSS 2010 shows, this configuration has undeniable advantages that are worth further study. Matloff goes into sail materials — beryllium seems the best candidate — and he considers a 40 nm beryllium sail to be near optimum. He assumes a rim and sail constructed of the same material, with the rim as an inflated torus with an internal fill gas. The payload is then suspended from the torus in equi-spaced sections so that pressure on the rim is uniform.
Sorting Out Known Issues
One problem, and a huge one, with sundiver missions is the temperatures involved. Assuming a perihelion of 0.07 AU, we can produce the 635 kilometers per second to reach Centauri A in 2000 years, but Matloff notes that in earlier work, 40 nm beryllium is found to become transparent at such temperatures. He juggles increased sail thickness and works with different perihelion distances to lower the sail temperature, finding decreased cruise velocity using such methods, with the Centauri journey extended to roughly 2900 years.
And then there’s this not inconsiderable problem:
Pushed by solar-radiation pressure, the sail membrane within the hoop, like the parachute sail, will in some ways resemble a soap bubble. It is not impossible that micrometeoroid impact punctures could rupture the sail. To prevent this, the sail could push against a network of fine carbon filaments that would serve as a ripstop.
Such a system seems to work and exacts no showstopping mass issues, leaving the hoop sail competitive with other interstellar sail configurations. We’re left with real advantages in scalability and ease of deployment even as the ripstop array increases total spacecraft mass by only a few percent. Thus a sail shaped like the rim of a bicycle wheel with the sail film inside the hoop emerges as an interesting design contender, one that could be used for ultra-thin film sails launched from Earth rather than being manufactured in space.
Matloff presents an interesting early application in Deep Space Probes, a lunar orbital or impact mission using multiple connected hoop sails, the central sail supporting the outer ones, which would be used for guidance and control, with payload suspended from the center of the central hoop. This one is low in mass, and relatively simple to stow and deploy. Such a mission could be a useful demonstrator of the concept, but we have a long way to go if we’re talking deep space. As the author notes in his ISSS 2010 presentation: “The work presented here should be viewed as a very preliminary analysis. Much additional research is required before interstellar hoop sails can be declared feasible.”
The presentation is “The Hoop Solar-Photon Sail and Extrasolar Travel,” available in the proceedings of the 2nd International Symposium on Solar Sailing. It’s interesting, too, to compare this work with a paper Matloff did on a different inflatable sail concept with sail experts Roman Kezerashvili, Claudio Maccone and Les Johnson back in 2008. It’s “The beryllium hollow-body solar sail: exploration of the Sun’s gravitational focus and the inner Oort Cloud” (preprint).
