by Paul Gilster | Apr 14, 2009 | Outer Solar System |
It was Percival Lowell who suggested that anomalies in the orbit of Uranus might point to the existence of the body he called ‘Planet X.’ The discovery of Pluto in 1930 gave us confirmation of a planet beyond Neptune (since downgraded, of course), but the idea of other large bodies in the outer Solar System still has its appeal, and although we’ve found such interesting objects as Eris and Sedna, questions remain about what else might be found lurking at the fringes of the system.
Theories of the Outer System
Thus the active theorizing, which includes one study speculating on an Earth-sized planet at 100 to 170 AU, a body that would help to explain what we know about the architecture of the Kuiper Belt. Another investigation looked at a possible Mars-sized body at 60 AU, which would help us understand the distribution of various Trans-Neptunian Objects, a term that basically covers any object orbiting the Sun beyond Neptune.
Other theories abound, one of which sees a giant planet (roughly 1.5 times as massive as Jupiter) at 25,000 AU, causing perturbations in particular cometary populations in the Oort Cloud. References to these and other work can be found in a new paper by Lorenzo Iorio (INFN, Pisa), who goes on to look at what may be the most seductive of all these hypotheses, that a small star thousands of AU from the Sun may cause perturbations that affect the inner planetary regions.
Image: An artist’s concept of the view from Eris with Dysnomia in the background, looking back towards the distant sun. What other objects, perhaps larger still, might lurk in the outer Solar System? Credit: Robert Hurt (IPAC).
A Companion for the Sun
It’s fascinating to speculate, as we have here often, about the existence of a star closer to us than the Centauri trio. Thus the ‘Nemesis’ theory, that the Sun is actually a binary star whose companion may well account for cometary impact events on Earth, a signature perhaps revealed by the periodicity of such events. Iorio weighs the evidence for a brown or red dwarf in such a configuration, looking at its gravitational effects on the inner rocky planets.
[Note the addendum below]. The result is a set of interesting constraints on what may lurk beyond our currently understood system. A brown dwarf of 75 to 80 Jupiter masses, for example, cannot orbit closer than roughly 1.8 to 1.9 light years from the Sun, while the minimum distance for a red dwarf ranges from 2.1 to 5.6 light years depending on its mass. If there’s a Jupiter-sized body out there, it should be no closer than 13,500 AU (recall that Proxima is 10,000 AU from the primary Centauri stars). An Earth-sized object might be found beyond about 750 AU but no closer.
Addendum: Within the last few days, the author uploaded a second version of this work which I didn’t become aware of until this PM (thanks, Bynaus!). In the second version, these numbers change significantly. A brown dwarf is now constrained to 3,736-3,817 AU from the Sun, while an M-dwarf must be no closer than 3,793 AU to 7,139 AU. Quite different results, and obviously even more interesting in terms of interstellar targets, if indeed it turns out that any such objects do exist. Dr. Iorio’s comment re the new version: “Rewritten version amending the previous one which contained a serious error. Results changed.” Indeed!
Filling Out the Census
Using Dr. Iorio’s computations, then, we can’t say that any of these objects exist, but only that if any of them do, they are not to be found closer than these figures suggest. The closest star could be far closer than the Centauri stars, although we do not, obviously, have confirmation that such an object is there. Finding a brown dwarf or an M-dwarf within several thousand AU would change the prospects for our first interstellar probe considerably, and would surely serve as an inducement to propulsion research if planets were to be found there. [This section re-written due to the changes to the paper noted above].
All of which is a useful reminder that even as we look into the remotest parts of the universe in search of cosmic history, we still have much to consider in regions not far from our own Sun. We have plenty of work ahead in building the local census. I also think of deep space explorers like Caltech’s extraordinary Mike Brown, whose investigations keep filling in the blanks in our outer system sketchbook. What challenging projects await the students Mike Brown’s generation is now training!
The paper is Iorio, “Constraints on Planet X and Nemesis from Solar System’s inner dynamics,” available online.
by Paul Gilster | Apr 13, 2009 | Deep Sky Astronomy & Telescopes |
The idea that the Moon was formed through the impact of a Mars-sized object with the early Earth (the ‘big whack’) has gained credibility over the years. Call it ‘Theia,’ a hypothetical planet that may have formed in our system’s earliest era. And place it for argument’s sake at either the L4 or L5 Lagrangian point, where the gravitational influences of other developing planets like Venus may have destabilized its orbit, accounting for the subsequent impact.
It’s an interesting notion that helps us to understand why the Moon has such a small iron core. In the early Solar System, both Theia and Earth would still have been molten, so that heavier elements like iron sank into their cores. The effect of the impact, say Princeton University’s Edward Belbruno and Richard Gott, would have been to strip away primarily the lighter elements on the outer layers of the two planets, providing the building blocks for the Moon.
Image: The Lagrangian points of the Earth-Sun system (note the James Webb Space Telescope at L2). Credit: NASA.
It’s helpful that where an object forms tells us much about its characteristics. Consider water, for example, The gas and dust that gave birth to our Solar System changed as temperature varied, with the Earth at a warm 1 AU forming largely out of silicon, magnesium and iron, with enough oxygen to oxidize much of the former. Proximity to the Sun meant that the infant Earth had little water, especially compared to bodies that formed further out in the system, and volatiles like ice may have been delivered to our world by incoming objects from the outer regions of the cloud — beyond the so-called ‘snow line’ — where water is more common.
In the case of the Theia investigation, we can assume that any L4 or L5 asteroids with the same composition as the Moon and the Earth would therefore have formed in roughly the same orbital location. And evidently it is precisely in these L4 and L5 regions where Belbruno and Gott’s calculations show Theia could have grown.
How to learn more? As we’ve noted earlier in these pages, NASA’s twin Solar Terrestrial Relations Observatory (STEREO) spacecraft are about to enter the L4 and L5 regions, each 93 million miles away along Earth’s orbit. “These places may hold small asteroids, which could be leftovers from a Mars-sized planet that formed billions of years ago,” says Michael Kaiser, project scientist for STEREO at NASA GSFC. If so, STEREO’s imaging instruments may be able to spot one or more of them.
With STEREO now nearing both regions, and with both remaining in the field of view of the two craft after passage through these huge swathes of space, we have an opportunity to put the Theia theory to a useful test. Want to get involved? Anyone with an Internet connection can help in the painstaking work of spotting an asteroid as it changes position against background stars. The data can be viewed at the Sungrazing Comets site, where both STEREO and SOHO (Solar and Heliospheric Observatory) data are available for access.
by Paul Gilster | Apr 10, 2009 | Culture and Society |
1951’s The Man from Planet X is a creepy Edgar G. Ulmer film involving an inscrutable alien whose small craft falls to earth in the moors of Scotland. There he is attacked, exploited and ends up being killed in spite of the fact that his real mission was apparently peaceable. The film is noir-like, the sets foggy and surreal, and although the dialog positively creaks, the moody atmosphere still puts a chill up my spine.
I mention this personal favorite because my copy of The Man from Planet X has a glitch, a defect in the aging tape that causes the image to jitter for a ten second period just as actress Margaret Field is getting progressively spooked by the strange alien craft. You would think that an upgrade to DVD is in order, and indeed, that’s my only real choice. But the other night, watching a DVD of Alec Guinness in the delightful Our Man in Havana (1959), I saw the image lock up and freeze, decomposing into pixels that reconfigured themselves only after a couple of minutes had passed. This, mind you, was on a relatively new DVD.
The Pleasures of an Older Medium
There are times when the older medium, VCR tape, is actually preferable in one sense — it recovers better from the kind of errors that can lock up a DVD. I think about these things because my obsession with the past and the recovery of seemingly lost history is very much involved with technology. Consider how image processing is now used to restore what was written on an ancient papyrus, or the way the Beowulf manuscript has been enhanced and re-examined through various electronic filters to tease out new information. See The Electronic Beowulf for more.
We are now able to extract information from palimpsests — vellum manuscripts once scraped and used again for new material written over the old — by imaging what was written in the earlier layer. Parchment was scarce in the Middle Ages, which is why a manuscript might be scraped with powdered pumice. The seemingly destructive practice yields at least some of its secrets to so-called multispectral filming, which recently recovered four-fifths of a priceless text by Archimedes. Various forms of x-ray imaging are also in play.
Lunar Images Back from the Dead
But back to space. The Lunar Orbiter Image Recovery Project shows us how heedless we’ve been in our rush to digitize. Located at NASA Ames, LOIRP had to acquire one of the last surviving Ampex FR-900 machines to play back analog data from our early Moon probes, information now being digitized to tease out new science. I think about that when I look at tape cassettes I recorded thirty years ago. I need a tape deck to play them and wonder how long cassette options, still built into many multimedia centers, will be available. Think of 8-track players.
Image: Restored Lunar Orbiter view of Earthrise over the Moon. The FR-900 tape drives that held this and other images sat in a Sun Valley, CA barn for several decades before this restoration was made.
If you have a collection of old data, as I do on my hundreds of old movies recorded over two decades, you’re faced with the need to move to new formats to keep it viable. Here again the oddity: We have written manuscripts that are readable from the last two millennia, physical objects whose one great characteristic is the preservation of the content they carry. The photos we stuffed in our living room drawers are still there, but how long will the online service that hosts our new digital snaps stay healthy enough to host them?
David Pogue recently talked to Dag Spicer, curator of the Computer History Museum in Silicon Valley, about the issue. Spicer had this to say:
One of the technologies for really long-term preservation was developed at Lawrence Livermore National Laboratory. It was, I think, a titanium disk about the size of a long-playing record, and it was supposed to last 10,000 years. But then they realized that there were some assumptions that weren’t right, and that it would not last 1,000 years, it might only last 20.
Otherwise, as far as I know, no one is working on this problem. It’s really in no one’s interest, no manufacturer’s interest; they want to keep selling you more hard drives every two to five years, or more blank CDs, and what have you.
Recovering Old Machines, Building Better Ones
Much of our early space data exists under the same cloud. We need specialized machines to read the information, and the machines are increasingly scarce. We’re in the data gap between analog and digital, a place in which a few fumbles can make key aspects of who we were in the 20th Century unrecoverable. Meanwhile, the Long Now Foundation keeps pushing the value of planning for the long term, a set of values that should eventually help us create data formats and methods that don’t, like too many old films, let our images and stories gradually deteriorate inside cannisters nobody ever opens.
We’ll get there, of course, but the experience should serve as an object lesson in how to place change in context. We are pushing inexorably toward a future that, some of us believe, involves movement into the outer Solar System and beyond. We are taking data at unprecedented rates through a wide variety of experiments and observations. The challenge will be to preserve what we have while going forward, a task that should be hard-wired into the design of future technology.
by Paul Gilster | Apr 9, 2009 | Missions |
Imagine frozen pellets of deuterium and helium-3 being ignited by electron beams to produce fusion, all this occurring in a combustion chamber fully 330 feet in diameter. Such was one early concept for Project Daedalus, the British Interplanetary Society’s starship design that would evolve into a two-stage mission with an engine burn — for each stage — of two years, driving an instrumented payload to Barnard’s Star at twelve percent of the speed of light.
We’ve been kicking the Daedalus concept around here recently because the BIS is developing, in conjunction with the Tau Zero Foundation, Project Icarus, a revisiting of the original Daedalus concept. The Daedalus propulsion system required fifty billion fuel pellets, thirty thousand tons of helium-3 and 20,000 tons of deuterium, as massive an undertaking as our species has ever attempted, given that the helium-3 would have to come from the atmosphere of a gas giant like Jupiter. Icarus will study what Daedalus might look like with newer technologies.
Image: A diagram of the Daedalus starship. Credit: Adrian Mann.
For propulsion inspiration, the original Daedalus team turned to Friedwardt Winterberg, who had studied fusion initiation through electron beams, and it was because of that involvement that Project Icarus team leader Kelvin Long contacted Dr. Winterberg again with news of the Icarus study. Amongst their e-mail exchanges were some comments I found interesting, and because Dr. Winterberg has given permission to use parts of these, I want to run one of them now. In this first excerpt, he speaks of the background of propulsion studies and what may be feasible as we expand into our own Solar System:
When I first had thought of the fusion-micro-explosion propulsion system almost 40 years ago, I never thought about interstellar spaceflight. I rather thought about a high specific impulse – high thrust propulsion system for manned spaceflight within the solar system. Instead of an interstellar probe, one could build in space very large interference “telescopes” with separation distances between the mirrors of 100,000 km, for example, in the hope to get surface details of other earthlike planets. And by going to 500 AU at the location of Einstein gravitational lens -focus, one could use the sun as a telescopic lens with an enormous magnification.
Yes, and it’s fascinating to speculate on how dramatically our observations of other solar systems may change our mission concepts. After all, when Daedalus was envisioned, no one knew whether there were planets around other stars, and Barnard’s Star was chosen specifically because there was at least some evidence of one or more planets there. A flyby probe would be a way to find and characterize these planets, but in fifty years or less, we may be able to see distant exoplanets clearly enough to limit actual missions to specific, high-value targets. Dr. Winterberg continues:
All this needs a very powerful propulsion system. Before going to Alpha Centauri (or Epsilon Eridani), one should aim at comets in the Oort cloud. Since there is water abundantly available, [this] invites the use of deuterium as rocket fuel. Unlike a DT micro-explosion where 80% of the energy goes into neutrons, unsuitable for propulsion, it is not much more than 25% for deuterium. A deuterium mini-detonation though requires at least 100 MJ for ignition, but this can be provided with a magnetically insulated Gigavolt capacitor, driving a 100 MJ proton beam for the ignition of a cylindrical deuterium target…
We’ve looked before at how this might be done, specifically in one of Dr. Winterberg’s papers, as examined by Adam Crowl in this essay. But how do we proceed to put this power to work? A primary destination emerges:
In reaching the Oort cloud, and there establishing human colonies, one may by “hopping” from comet to comet ultimately reach a “new” earth.
Recent research I believe, suggests that in already 100 million years the earth may become inhabitable through the loss of oxygen.
But 100 million years gives us still plenty of time.
Plenty of time indeed. The notion of moving from comet to comet is appealing, invoking as it does the possibility of a gradual expansion deeper and deeper into the Oort, with the prospect of eventually encountering comets in a similar cloud around the Centauri stars. Moving a step at a time may obviate the need for a single interstellar crossing, breaking it into stages. In any case, such a crossing would be an enormous undertaking, as Dr. Winterberg goes on to note:
I cannot see how except with gargantuan space craft interstellar space flight will ever be possible. And I have little taste to speculate about the surprises the physics of tomorrow might bring. It is possible that big surprises still wait for us, but the opposite is possible as well. Discovering the laws of nature is like discovering America. It may happen just once. I therefore like to think what is possible with what we know now.
A sound approach, and one with the added benefit of not requiring new physics to work. The Tau Zero Foundation hopes to encourage both approaches, missions based on known physics and rigorous examination of possible ‘breakthrough’ concepts. We don’t know whether the latter will turn up or not, but we learn valuable lessons about the universe from the attempt to find them even if breakthroughs don’t emerge.
As to the size of a starship, my own guess is that gargantuan craft are not the future for our early interstellar probes, assuming we build such. It may well be that we can couple propulsion advances (possibly via beamed microwave or laser designs) with nanotechnology to produce robotic probes of extremely small size. But assuming we do follow the fusion route, here are Dr. Winterberg’s further thoughts:
And there I think fusion propulsion with deuterium appears quite obvious, with water (in the comets of the Oort cloud) in interstellar space widely available. But as we have known since the paper by Trubnikov (2nd UN Conference in Geneva in 1958 on the peaceful use of atomic energy), deuterium fusion with magnetic confinement is unlikely possible for a fusion reactor of reasonable dimensions. The situation is quite different with a deuterium detonation. There, the DD reaction produces T and He3, which in a secondary reaction burn with D. This was “nicely” demonstrated by the 15 Megaton fission triggered deuterium bomb test in 1952.
For propulsion, the pure fusion fire ball can with much higher efficiency (if compared to a pusher plate) be deflected by a magnetic mirror, also avoiding the ablation of a pusher plate. The ignition, requiring more than 100 MJ, can be done with some kind of particle accelerator. The LHC at CERN can store several 100 MJ energy in a particle beam moving with almost the velocity of light. No laser can that do yet. Unlike lasers, particle accelerators are very efficient. And to get a high fusion yield of say about 1kt, cylindrical targets with axial detonation should be used, where a mega-gauss magnetic field entraps the charged fusion products, as it is required for detonation.
The paper on this work is “Deuterium microbomb rocket propulsion,” which Dr. Winterberg presented at the 2008 Advanced Propulsion Workshop in Pasadena (abstract). Centauri Dreams thanks Dr. Winterberg for being willing to share this correspondence with its audience.
by Paul Gilster | Apr 8, 2009 | Exoplanetary Science |
Kepler’s dust cover has now been jettisoned, meaning the search for extrasolar ‘Earths’ is not long from commencing. The cover stayed in place for so long because the spacecraft’s photometer had to make measurements of electronic noise that will later have to be removed from the science data. Mission engineers will now continue with the calibration process for several weeks using images of actual stars.
Our debates over the ‘rare Earth’ hypothesis will be getting firm data in short order because of Kepler. Three years from now, having had time to detect terrestrial-class planets in the habitable zone of their stars, confirm the detections and further examine the results, we should have at least a sense of how common such planets are. Finally we can move beyond informed speculation with the sort of hard data we need. And as far as the first terrestrial planet detection in the habitable zone, CoRoT may just beat Kepler to the punch.
Meanwhile, the astrobiological side of the ‘rare Earth’ debate gets more and more interesting with news that at least one important prebiotic chemical is in short supply around small M-dwarfs and their brown dwarf cousins. Have a look at the graph below, which clearly shows the gap in hydrogen cyanide (HCN) for small, cool stars as composed to Sun-like stars, whereas the baseline acetylene figures are roughly similar (and demonstrate that the method works).
Image: NASA’s Spitzer Space Telescope detected a prebiotic, or potentially life-forming, molecule called hydrogen cyanide (HCN) in the planet-forming disks around yellow stars like our sun, but not in the disks around cooler, reddish stars. The observations are plotted in this graph. Light wavelengths are shown on the X-axis, and the relative brightness of disk emission is shown on the Y-axis. The signature of a baseline molecule, called acetylene (C2H2), was seen for both types of stars, but hydrogen cyanide was seen only around stars like our sun. Credit: NASA/JPL-Caltech/JHU.
Ilaria Pascucci (Johns Hopkins) is lead author on the paper on this (slated for the Astrophysical Journal), describing her team’s investigation of seventeen cool and forty-four Sun-like stars with the Spitzer telescope’s infrared spectrograph. These are young stars presumably in the planet formation process, and none of the M-dwarfs and brown dwarfs in the mix showed a notable hydrogen cyanide signature.
Thus we add substance to the problematic nature of life around red dwarfs. Stellar flares have always been an issue, although some believe they could serve as a spur to evolution under the right circumstances. But a deficiency in hydrogen cyanide is more troublesome still, for HCN is a component of adenine, a basic element of DNA. Says Spitzer program scientist Douglas Hudgins:
“Although scientists have long been aware that the tumultuous nature of many cool stars might present a significant challenge for the development of life, this result begs an even more fundamental question: Do cool star systems even contain the necessary ingredients for the formation of life? If the answer is no then questions about life around cool stars become moot.”
We still haven’t found a planet in the habitable zone of an M-dwarf despite the Gliese 581 c finding — the planet is now believed to be far too hot for liquid water to exist on its surface. If life on Earth got its impetus from prebiotic molecules in the early protoplanetary disk, then a lack of same under these conditions makes M-dwarfs look less and less hospitable. That’s a downer for those of us fascinated with potential life around these dim stars, but of course the investigation of these matters is in its early stages.
