by Paul Gilster | Feb 7, 2008 | Outer Solar System |
Saturn’s rings turn out to be more dynamic than expected, and it’s clear that what Cassini has to tell us about them — and about the rest of the Saturnian system — is only beginning. Throw Enceladus into the picture and things get even more complicated, and interesting. Geysers on the moon have already been found to supply content to the so-called E-ring, while material flowing from it in the form of the gas of electrically charged particles called plasma is now known to influence Saturn’s magnetosphere. The latest discovery is that this plasma is, in turn, being drawn into Saturn’s A-ring, where it is being absorbed.
Image: Enceladus seen across the un-illuminated side of Saturn’s rings. A hint of the moon’s active south polar region can be seen as a just slightly dark area at bottom. This view was obtained from about 1 degree above the ringplane. Enceladus is 505 kilometers (314 miles) across. Credit: NASA/JPL/Space Science Institute.
Unlike Jupiter, then, Saturn seems to have acquired a way to soak up low and high-energy particles from the plasma cloud that surrounds it. How to detect the phenomenon? Cassini’s arrival at Saturn in 2004 included a close flyby over the A-ring. The spacecraft was able to determine that radio signals were being emitted from that part of the A-ring in collision with the plasma. From the signals, the density of the plasma could be inferred, as could the change in density over time. William Farrell (NASA GSFC) describes what is happening:
“As we approached the A-ring, the frequency dropped, implying that the plasma density was going down because it was being absorbed by the ring,” said Farrell. “What really drove this home was what happened to the signal when we passed over a gap in the rings, called the Cassini division. There, the frequency went higher, implying that the plasma density was going up because plasma was leaking through the gap.”
It’s an interesting result given the distance between the A-ring and Enceladus: about 100,000 kilometers. Now we’re seeing that a portion of Enceladus’ mass is being delivered to the outer edge of the ring. Good work by Cassini’s Radio and Plasma Wave instrument, whose data is studied by Farrell et al. in “Mass unloading along the inner edge of the Enceladus plasma torus,” Geophysical Research Letters Vol. 35 (January 23, 2008), L02203 (abstract). A NASA news release is here.
by Paul Gilster | Feb 6, 2008 | Missions |
Project Daedalus, discussed frequently in these pages, was the first in-depth design study of an interstellar probe. Its projected fifty-year flyby mission to Barnard’s Star at 12 percent of the speed of light was beyond contemporary technology (and certainly engineering!), but not so far beyond as to render the design purely an intellectual exercise. I bring up Daedalus again because I keep getting asked about Project Longshot, which some have mistakenly seen as a successor to Daedalus with a NASA pedigree. And wasn’t Longshot a far more advanced design?
Actually, no. But the other day I again ran into Longshot in the form of an online post describing it as a hundred-year mission to Alpha Centauri (true enough), evidence that NASA had the technology right now (not true) to get us to the nearest stellar system in a century, which would be faster by far than the thousand years I’ve always used as an absolute minimum for getting there with the technology we have today. Even that 1000 years is deeply problematic. I mentioned it several years back to Les Johnson at NASA’s Marshall Space Flight Center and he said, “If we can get to Alpha Centauri in a thousand years, I’ll take it!” Meaning we were, in his view, not even that far along.
So what is this Project Longshot? The first thing to do is to untangle its origins. This design for an unmanned interstellar probe grew out of the NASA/USRA University Advanced Design Program, which ran between 1984 and 1994. The idea of the program was to help integrate future NASA design projects into university curricula. Engineers from the agency would work with students and faculty from US engineering schools, thus fostering engineering design education and adding synergies to NASA’s own efforts in the area of advanced space design. Project Longshot was a concept that grew out of this program’s involvement with the U.S. Naval Academy, including seven first class midshipmen, faculty advisors and two visiting professors, one of whom was NASA representative Stephen Paddack, a visiting professor based at Goddard Space Flight Center.
As for using current technologies, the Longshot team made no such claim. This is what they had to say:
Our probe will be a completely autonomous design based upon a combination of current technology and technological advances which can be reasonably expected to be developed over the next 20 to 30 years. The expected launch date is in the beginning of the next century with a transit time of 100 years.
The expected launch date, in other words, would have been about now, but the technologies anticipated for it to occur still have a long way to go. Longshot was conceived as being built with modular components on the ground and then launched to low-Earth orbit for assembly at the space station presumed to be operational there. The enabling technologies included a “pulsed fusion micro-explosion drive” (I’m quoting from the Project Longshot report) with a specific impulse of 1 million seconds, along with a long-life fission reactor with 300 kilowatts power output.
The differences between this concept and Project Daedalus are profound in both emphasis and execution. Daedalus was to be a fast flyby of Barnard’s Star, scattering smaller probes as it entered the system to explore any planets found there. Longshot, audaciously enough, was intended to carry enough fuel to actually brake as it entered the Centauri system and go into orbit around Centauri B, which the report erroneously calls Beta Centauri (Beta Centauri is another star altogether, the components of Alpha Centauri being Centauri A, B and Proxima Centauri). That last just reminds us that the pooled light of the three Alpha Centauri stars makes it appear to be a single star, so that the second brightest object in Centaurus came to be known as Beta Centauri.
Needless to say, including enough fusion fuel to slow an object traveling at these speeds to brake into orbit around Centauri B would require an engine far more efficient and powerful than anything envisioned for Daedalus. That’s because you’re carrying, when you begin the journey, not just the fuel to get you up to cruising speed, but all the fuel needed for the deceleration. The numbers quickly start running away with you here — while Daedalus offered a first-step flyby that strained every technological resource we would possess in the near future (including the need to mine for helium-3 in places like the atmosphere of Jupiter), Longshot pushed credibility to the max by insisting that a similar design could stay in the Centauri system and do useful science, reporting the results via a laser beam communications system that seems workable.
Where Longshot was perhaps closest to technological realization was in the area of autonomy. Here’s what the report says on this subject:
Due to the great distance at which the probe will operate, positive control from earth will be impossible due to the great time delays involved. This fact necessitates that the probe be able to think for itself. In order to accomplish this, advances will be required in two related but separate fields, artificial intelligence and computer hardware. AI research is advancing at a tremendous rate. Progress during the last decade has been phenomenal and there is no reason to expect it to slow any time soon. Therefore, it should possible to design a system with the required intelligence by the time that this mission is expected to be 1aunched.
All of which seems reasonable enough. The Longshot report was compiled between 1987 and 1988, and we have certainly seen our share of computer advances in the time since. Indeed, I am now and again told by its partisans that the ‘Singularity’ event could happen any time now, but certainly within the next few decades, in which case AI systems to run such a probe would be plentiful, one assumes, although whether intelligent hardware would not want to re-design the whole spacecraft remains an unanswered question.
I, for one, appreciate the report’s attention to long-term thinking. In discussing the “human side of the infrastructure” supporting Longshot, the authors note that given the time for design, procurement, in-orbit assembly and transit, the likely time before return of data would be more on the order of two centuries than one. And they go on to say this:
…the greatest challenge comes with the caretaker portion of the mission — the century of travel time when very little data will be transmitted. The problem here is not the number of people required, since it will be small, but rather the time involved. There has never been a similar project in modern history carried out over such a long time scale. However, there have been organizations which have lasted for such a time. In fact, some have lasted longer! Some examples include: the militaries of nations such as the U.S. and the U.K., various research institutions like the National Geographic Society and the Smithsonian Institution, and private organizations such as the Red Cross and the Explorer’s Club.
Robert Forward used to worry about precisely this point. In considering one of his mind-boggling lightsail designs, he wondered what political will might be needed to keep the power supplied to the huge lasers that drove the lightsail over spans of a century or more. You can see the subject entertainingly explored in his novel Rocheworld (1990), expanded from his 1984 work The Flight of the Dragonfly. We’ve clearly got the patience to work with probes that are thirty years old and more, as witness our Voyagers and Pioneers, but a century or longer imposes more challenges, especially given the political changes that might take place in the interim.
The Longshot team pondered the possibility of laser lightsails for its work as well, but ended up with pulsed fusion. And again, the report points out that such a drive “…is not a current, but rather an enabling technology.” The concept is to fire high energy particle beams at small, fusion-able pellets whose implosion and subsequent channeling out the nozzle would drive the vehicle. Helium-3 is deemed necessary, as with Daedalus, with atmospheric mining of Jupiter being just one of the methods discussed for gathering sufficient quantities. “…[T]he collection of fuel will be the most difficult and time consuming portion of the building,” says the report, and that’s something of an understatement.
Project Longshot, then, should be seen as a gutsy academic exercise that never proceeded to the intricate analysis given to Daedalus, lacking the resources of time and expertise that the British Interplanetary Society was able to deliver to the latter. Even so, the Longshot report is a fun read that places many of our current interstellar concepts in context. The rough sketch of an interstellar probe called “Project Longshot: An Unmanned Probe to Alpha Centauri” can be downloaded here.
by Paul Gilster | Feb 5, 2008 | Exoplanetary Science |
Watching two suns over Tatooine’s sky in the original Star Wars movie was a breathtaking experience, particularly given where most science fiction films were at the time. Here was an attempt to convey a truly alien landscape. But a second thought quickly came unbidden. Was this planet not in an extremely unstable orbit, moving around both stars simultaneously in an obvious habitable zone? The suspicion was that a planet could orbit one or the other members of a binary system, but surely not both unless its orbit were extended so far out into the planetary nether regions as to make life doubtful.
Image: The twin suns of Tatooine. Are planetary orbits like this possible? Credit: © Lucasfilm Ltd. & TM. All Rights Reserved.
That was back in the 1970s, of course, but take a look at the situation today. The ‘hot Jupiter’ in the triple system HD 188753 is interesting, but the planet in question orbits but one of the stars. The early discussion of HD 188753 Ab was quick to raise the Tatooine parallel, which was first suggested by Caltech scientist Maciej Konacki. But at 0.04 AU, this gas giant hugs its G-class star, with the other two stars orbiting each other and also — at Saturn-like distance — the same star orbited by the planet.
We do have one known exoplanet that orbits twin stars, but PSR B1620-26 is not your average stellar system, consisting of a pulsar and a white dwarf. Even so, twenty percent of known exoplanets are in multiple systems, and it’s interesting to speculate on whether a planet with a double sunrise like that of Tatooine might exist. Radial velocity techniques avoid short-period binaries of the sort that might make this possible, and the wide orbits of such planets would likely make transit detections quite difficult. But Cheongho Han (Chungbuk National University, Korea) is now arguing that microlensing techniques might work here.
From the paper:
The general geometry of a planet revolving around the stars of a close binary is such that the separation between the stars is much smaller than the star-planet separation… Under this geometry, the lensing behavior of the triple lens system can be greatly simplified because the close stellar binary pair and the planet can be separately treated.
Cheongho Han argues that a planet like this would be involved in perturbations at a common region around the center of mass of the binary stars, creating a detectible microlensing signature. Despite the extreme difficulty of the task, such analysis of microlensing data has already begun. Multiple telescopes at different locations, allowing continuous coverage of microlensing events, should be able to detect a planet that fits this description. Given the difficulty of radial velocity and transit methods for this work, it may be that microlensing will be the method of choice to find a planet that, like Tatooine, would experience multiple sunrises. It would be a view to be savored, though not, one suspects, one that living beings would be around to observe.
The paper is Cheongho Han, “Microlensing Search for Planets with Two Simultaneously Rising Suns,” a draft of which is available here.
by Paul Gilster | Feb 4, 2008 | Exoplanetary Science |
We’re still arguing about how giant planets form around Sun-like stars, but terrestrial planets seem to be less controversial. Assuming the model is right, we start with a swarm of planetesimals in the range of one kilometer in size. As these objects grow, out to a range of at least 2 AU, the largest bodies at some point go through a runaway period of chaotic growth marked by collisions. Emerging from the debris should be terrestrial worlds, some in Earth-like orbits. Add to this the fact that gas and dust disks seem to be relatively routine outcomes of star formation and you have an indication that small rocky planets may be widespread.
The problem with all this is that theory has to be matched with observation. On that score, new work by Mike Meyer (University of Arizona) and colleagues Lynne Hillenbrand and John Carpenter (California Institute of Technology) is instructive. The researchers chose to look at mid-range infrared emissions at the 24 micron level, a range chosen because it originates between 1 and 10 AU from the parent star. As targets, they chose 328 Sun-like stars in spectral types F5-K3, with masses generally not dissimilar from the Sun (though in some cases ranging as high as 2.2 solar masses). A finding of excess emissions at 24 µm was taken to be evidence of dust debris from planetesimal collisions.
The conclusions from this work are absorbing indeed. From the paper (internal references omitted for brevity):
We suggest that SST observations at 24 µm can be interpreted as evidence for terrestrial planet formation occurring around many (19–32 %), if not most (62 %), sun–like stars. This range is higher than the observed frequency of gas giant planets (6.6–12 %) within 5–20 AU…but comparable to the inference that cool dust debris beyond 10 AU might be very common…Radial velocity monitoring of low mass stars, micro-lensing surveys, as well as transit surveys such as COROT and Kepler, will provide critical tests of our interpretation.
And so we test theory with observation (and note the reference to the doughty COROT mission, gathering key data at unprecedented rates as its work continues, and doubtless setting us up for its share of surprises). But the broader picture growing out of the work of Meyer’s team is that terrestrial planets may be common around Sun-like stars, an assumption most everyone connected with the exoplanet hunt would be delighted to see confirmed. Not only would it be striking evidence that the formation mechanisms for Earth-like planets are becoming better understood, but it would strengthen the hope for living worlds around stars for which the conditions of life may not be so rare after all.
The paper is Meyer et al., “Evolution of Mid?Infrared Excess around Sun?like Stars: Constraints on Models of Terrestrial Planet Formation,” Astrophysical Journal Letters 673 (February 1, 2008), pp. L-181-L184 (abstract); also available in full text here. I notice that Science News has picked up on this team’s work as well, with a story quoting Caltech astronomer Charles Beichman (not a member of Meyer’s group):
“Meyer’s result is exciting confirmation that around many other stars like our sun, the region analogous to our own asteroid belt is full of solid material, possibly related to past or present planet formation.”
Note the ‘possibly’ in that sentence, a reminder of how much work remains to be done. Beichman goes on to call the work “…a good sign that the basic stuff of planetary systems is widespread.” All of which gibes with current thinking, but there is no substitute for getting the right hardware into space (think Kepler and beyond) to verify the existence of those tantalizing worlds. Kepler is scheduled for launch next February.
by Paul Gilster | Feb 2, 2008 | Culture and Society |
Sending data-rich broadband signals between the stars is no easy matter. Interstellar gas has the effect of disrupting such signals, the result varying depending upon the frequency. Narrow-band signals are easy, broadband hard. But Seth Shostak reports on galactic Wi-Fi, looking at Swedish work that exploits orbital angular momentum, a ‘twisting of the wave’s electric and magnetic fields,’ that may allow much more information to be encoded in the same signal without the disruption that distances in the hundreds of light years invariably impose. One signal becomes a cipher for another, with obvious SETI implications.
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New Scientist (behind its firewall, alas) looks at the work of Alexander Shatskiy (Lebedev Physical Institute, Moscow) on how to detect a wormhole. Shatskiy’s paper “Passage of Photons Through Wormholes and the Influence of Rotation on the Amount of Phantom Matter around Them” (abstract) makes the pitch that something called ‘phantom matter’ could hold the mouth of a wormhole open. Possessed of negative energy and negative mass, such matter might be detectable because it would create optical effects opposite to those of gravitational lensing.
A wormhole signature? Light moving through the wormhole from where/whenever should emerge as a bright ring, while stars behind the wormhole would shine through the middle. All of which reminds me of the classic paper by John Cramer et al. (“Natural Wormholes as Gravitational Lenses,” Physical Review D March 15, 1995. pp. 3124-27), which likewise speculates on wormhole signatures, though with a different result. The paper argues that when the wormhole moves directly in front of a light source, a halo would form around it. What you get when a wormhole occults a star is first the spike, then the halo, then a second spike, a characteristic signature indeed for astronomers lucky enough to spot it.
The difference between the two descriptions is a reminder that we have no idea whether ‘phantom matter’ or a negative mass cosmic string of the sort the Cramer paper discusses even exist. Astrophysicist Geoffrey Landis, a co-author of the Cramer paper, told me several years back that the attempt to identify a wormhole is purely speculative, but surely worthwhile:
“We published our paper on this because people are actively hunting for gravitational lenses with spectrophotometers that can track these effects. We wanted to say, keep your eyes open for this particular signature. You probably won’t find it, but if you do, it would be our first evidence that wormholes actually exist.”
And if they do exist, wormholes open startling possibilities for moving through space and, conceivably, time. Once the excitement of such a detection wore off, the tricky realities for those thinking in terms of fast transit would emerge: If we were to prove a wormhole existed, how would we get to it in the first place? How would we know where or when it led? Even so, a demonstration that at least a few wormholes (created, presumably, in the Big Bang) had somehow managed to stabilize themselves through some exotic mechanism and might still offer gateways to elsewhere would be one of the great results of science.
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The 39th Carnival of Space is available at Visual Astronomy, where Sean Welton has harvested recent space-related materials. Of particular interest to Centauri Dreams readers is Phil Plait’s entry on asteroid 2007 TU24, with a useful and obviously necessary (judging from the bogus information that has floated around the Internet about this object) video explanation of why it poses no current threat. The frustration for those of us who worry about long-term dangers from Earth-crossing objects is that the public reaction is all too easily polarized between disinterest and panic. Isn’t prudent planning a better response?
