by Paul Gilster | Feb 20, 2010 | Missions |
Every few weekends as we move toward the March 5 deadline for submission of abstracts to the next International Astronautical Congress, I’ll re-run this call for papers that I originally published in December. The Tau Zero Foundation hopes to energize discussion of FOCAL in the astronautical community and create a growing set of papers analyzing aspects of the mission from propulsion to communications, leading to a formal mission proposal. We hope anyone interested in furthering this work at the coming IAC in Prague will consider submitting a paper.
The Tau Zero Foundation is announcing a call for papers related to the FOCAL mission. The venue: The 61st International Astronautical Congress in Prague, which convenes on the 27th of September, 2010 and runs to October 1. Specifically, we are looking for papers for session D4.2, “Interstellar Precursor Missions,” whose focus is “…missions that significantly expand science — using existing and emerging power and propulsion technologies.”
Long-time Centauri Dreams readers are well aware of Claudio Maccone’s FOCAL concept, a mission to the Sun’s gravitational lens at 550 AU and beyond. FOCAL would make possible studies of astronomical objects at unprecedented magnifications. The electromagnetic radiation from an object occulted by the Sun at 550 AU (i.e., on the other side of the Sun from the spacecraft), would be amplified by 108. Moreover, whereas with an optical lens light diverges after the focus, light focused by the Sun’s gravitational lens stays fixed along the focal axis. Every point along the straight trajectory beyond 550 AU remains a focal point for any vehicle we put on this trajectory.
Imagine, then, two possible FOCAL mission targets. The first option would be to launch the probe toward the heliopause in the place where it is closest to the Sun, the direction of the incoming interstellar wind. This would allow useful studies of the heliosphere itself, but the deeper goal would be to reach 763 AU, the place where the cosmic microwave background will be focused by the Sun’s gravitational lens upon the spacecraft. As Maccone has shown, detecting lower frequencies pushes the focus further from the Sun — the focal distance, in other words, changes as a result of frequency.
We’ve learned how valuable information about the CMB is to cosmologists. Now imagine the result of examining the CMB with the vast magnifications possible through a FOCAL probe. But a second choice is also available. FOCAL could be optimized for close study of the Alpha Centauri stars, especially if current efforts pay off and we do find interesting planets around Centauri A or B. The flight path is problematic because the Centauri stars are so close, requiring ion propulsion to achieve the necessary spiral trajectory.
Addendum: So many readers have mentioned Dr. Maccone’s recent SETI Institute lecture that I want to go ahead and link to it now, although I was planning a separate piece on it next week. When I met with Claudio recently in Austin, he was getting ready to leave for the West Coast to make this presentation before concluding his US trip and heading back to Italy. What a pleasure it was to talk to him at leisure about FOCAL.
But all of these are matters that now need to be taken to the next step at the International Astronautical Congress, where they will gain further visibility in the scientific and industrial community. Papers are solicited on the propulsion problem — is a solar sail optimal? Nuclear-electric? Perhaps VASIMR? We also hope for submissions on the scientific return from a FOCAL mission, on telecommunications technologies, on computing requirements, and perhaps on the social and cultural value of a concept that would take human technologies further from the Sun than any previous missions.
Image: The FOCAL mission as currently envisioned by Claudio Maccone. The image is taken from the cover of his book Deep Space Flight and Communications: Exploiting the Sun as a Gravitational Lens (Springer/Praxis, 2009), and shows two 12-meter antennae operating through a tether which is gradually released, allowing a field of view much larger than that offered by a single antenna. Credit: Claudio Maccone/Springer.
The preliminary program for the Prague IAC has already been posted. The deadline for submitting abstracts to the Congress is 5 March 2010. Let me quote from the IAC documentation on what the criteria for selection will be:
Paper selection
Submitted abstracts will be evaluated by the Session Chairs on the basis of technical quality. Any relevance to the Congress main theme of ‘Space for human benefit and exploration’ will be considered as an advantage.
The criteria for the selection will be defined according to the following specifications:
* Abstracts should specify: purpose, methodology, results and conclusions.
* Abstracts should indicate that substantive technical and/or programmatic content is included
* Abstracts should clearly indicate that the material is new and original; explain why and how.
* Prospective authors should certify that the paper was not presented at a previous meeting and that financing and attendance of an author at the respective IAC at Prague to present the paper is assured.
Full information about the meeting and the submission process is available through the official Call for Papers & Registration of Interest.
by Paul Gilster | Feb 19, 2010 | Exotic Physics, Interstellar Medium |
Time dilation has long been understood, even if its effects are still mind-numbing. It was in 1963 that Carl Sagan laid out the idea of exploiting relativistic effects for reaching other civilizations. In a paper called “Direct Contact Among Galactic Civilizations by Relativistic Interstellar Flight,” Sagan speculated on how humans could travel vast distances, reaching beyond the Milky Way in a single lifetime by traveling close to the speed of light. At such speeds, time for the crew slows even as the millennia pass on Earth. No going home after a journey like this, unless you want to see what happened to your remote descendants in an unimaginable future.
Before Sagan’s paper appeared (Planetary and Space Science 11, pp. 485-98), he sent a copy to Soviet astronomer and astrophysicist Iosif Shklovskii, whose book Universe, Life, Mind had been published in Moscow the previous year. The two men found much common ground in their thinking, and went on to collaborate on a translation and extended revision of the Shklovskii book that appeared as Intelligent Life in the Universe (Holden-Day, 1966).
This one should be on the shelf of anyone tracking interstellar issues. My own battered copy is still right here by my desk, and I haven’t lost the sense of wonder I felt upon reading its chapters on matters like interstellar contact by automatic probes, the distribution of technical civilizations in the galaxy, and optical communications with extraterrestrial cultures.
Much has changed since 1966, of course, and we no longer speculate, as Shklovskii did in this book, that Phobos might be hollow and conceivably of artificial origin (the chapter is, nonetheless, fascinating). But for raw excitement, ponder this Sagan passage on what possibilities open up when you travel close to lightspeed:
If for some reason we were to desire a two-way communication with the inhabitants of some nearby galaxy, we might try the transmission of electromagnetic signals, or perhaps even the launching of an automatic probe vehicle. With either method, the elapsed transit time to the galaxy would be several millions of years at least. By that time in our future, there may be no civilization left on Earth to continue the dialogue. But if relativistic interstellar spaceflight were used for such a mission, the crew would arrive at the galaxy in question after about 30 years in transit, able not only to sing the songs of distant Earth, but to provide an opportunity for cosmic discourse with inhabitants of a certainly unique and possibly vanished civilization.
The songs of distant Earth indeed! An Earth distant not only in trillions of kilometers but in time. Memories of Poul Anderson’s Leonora Christine (from the classic novel Tau Zero) come to mind, and so do Alastair Reynolds’ ‘lighthuggers.’ Could you find a crew willing to leave everything they knew behind to embark on a journey into the future? Sagan had no doubts on the matter:
Despite the dangers of the passage and the length of the voyage, I have no doubt that qualified crew for such missions could be mustered. Shorter, round-trip journeys to destinations within our Galaxy might prove even more attractive. Not only would the crews voyage to a distant world, but they would return in the distant future of their own world, an adventure and a challenge certainly difficult to duplicate.
But while the physics of such a journey seem sound, the problems are obvious, not the least of which is what kind of propulsion system would get you to speeds crowding the speed of light. The Bussard ramjet once seemed a candidate (and indeed, this is essentially what Anderson used in Tau Zero), but we’ve since learned that issues of drag make the concept unworkable and better suited to interstellar braking than acceleration. And then there’s the slight issue of survival, which William Edelstein (Johns Hopkins) and Arthur Edelstein (UCSF) discussed at the recent conference of the American Physical Society (abstract here). The Edelsteins worry less about propulsion and more about what happens when a relativistic rocket encounters interstellar hydrogen.
Figure two hydrogen atoms on average per cubic centimeter of interstellar space, and that average can vary wildly depending on where you are. A relativistic spacecraft encounters this hydrogen in highly compressed form. Travel at 99.999998 percent of the speed of light and the kinetic energy you encounter from hydrogen atoms reaches levels attainable on Earth only within the Large Hadron Collider, once it’s fully ramped up for service. This New Scientist article comments on the Edelstein’s presentation, noting that the crew would be exposed to a radiation dose of 10,000 sieverts within a second at such speeds. Six sieverts is considered a fatal dose.
Traveling near lightspeed seems a poor choice indeed. The Edelsteins calculate that a 10-centimeter layer of aluminum shielding would absorb less than one percent of all this energy, and of course as you add layer upon layer of further shielding, you dramatically increase the mass of the vehicle you are hoping to propel to these fantastic velocities. The increased heat load would likewise demand huge expenditures of energy to cool the ship.
If travel between the stars within human lifetimes is possible, it most likely will happen at much lower speeds. Ten percent of lightspeed gets you to the Centauri stars in forty three years, a long but perhaps feasible mission for an extraordinary crew. If we eventually find shortcuts through space (wormholes) or warp drive a la Miguel Alcubierre, so much the better, but getting too close to lightspeed itself seems a dangerous and unlikely goal.
by Paul Gilster | Feb 18, 2010 | Deep Sky Astronomy & Telescopes |
We’re keeping a close eye on the WISE mission (Wide-field Infrared Survey Explorer) and the possibility of identifying brown dwarfs closer to our Sun than the Centauri stars. But WISE’s targets are numerous, and the early imagery coming back from the mission is promising indeed. To check out the capabilities of this space-based observatory, have a look at some of the new photos, which show M31, the Andromeda galaxy, at a variety of wavelengths. The first image was made with all four of WISE’s infrared detectors — the caption describes the color coding.
Image: The immense Andromeda galaxy, also known as Messier 31 or simply M31, is captured in full in this new image from NASA’s Wide-field Infrared Survey Explorer, or WISE. The mosaic covers an area equivalent to more than 100 full moons, or five degrees across the sky. WISE used all four of its infrared detectors to capture this picture (3.4- and 4.6-micron light is colored blue; 12-micron light is green; and 22-micron light is red). Blue highlights mature stars, while yellow and red show dust heated by newborn, massive stars. Credit: NASA/JPL-Caltech/WISE.
Notice the two satellite galaxies above Andromeda and to the left of center and also the blue M110 directly below the center of the spiral arms. Like our Milky Way with its Magellanic clouds, Andromeda has a number of these satellite galaxies gravitationally bound to it. Now compare this to Andromeda as seen by WISE’s shortest wavelength camera, where the disk takes on a modified, warped shape, doubtless the result of an ancient galactic interaction:
Image: This image from WISE highlights the Andromeda galaxy’s older stellar population in blue. It was taken by the shortest-wavelength camera on WISE, which detects infrared light of 3.4 microns. A pronounced warp in the disk of the galaxy, the aftermath of a collision with another galaxy, can be clearly seen in the spiral arm to the upper left side of the galaxy. Credit: NASA/JPL-Caltech/WISE.
And finally, note M31 as seen by the longest-wavelength detector aboard WISE. Here the spiral arms can be traced all the way to the center of the galaxy, outlined by the hot dust being heated by the formation of new stars, with star formation visible as well in the satellite galaxies M32 and M110.
Image: This image from WISE shows the dust that speckles the Andromeda galaxy’s spiral arms. It displays light seen by the longest-wavelength infrared detectors on WISE (12-micron light has been color coded orange, and 22-micron light, red). Credit: NASA/JPL-Caltech/WISE.
With capabilities like these, WISE’s discoveries not just far afield but in nearby space should be numerous, its targets ranging from asteroids and comets to the aforementioned brown dwarfs. Principal investigator Ned Wright (UCLA) calls the incoming imagery ‘a candy store of images coming down from space,’ and for good reason. Have a look below at the Siding Spring comet, which passed about 1.2 AU from the Earth and 2.25 AU from the Sun in October of last year. This interloper from the Oort Cloud, also called C/2007 Q3, is now leaving the inner regions of the system and heading back out to deep space.
Image: Astronomers will use these measurements to learn about the comet’s size, composition, reflectivity, and the size and makeup of the dust particles in its coma (the hazy cloud surrounding its nucleus) and its tail. WISE data on this and other comets will help unlock clues that lay within these icy time capsules, teaching us about our solar system’s evolution. In this image, 3.4-micron light is colored blue; 4.6-micron light is green; 12-micron light is orange; and 22-micron light is red. It was taken on Jan. 10, 2010. Credit: NASA/JPL-Caltech/UCLA.
That cometary tail, by the way, stretches about sixteen million kilometers. The mood around the WISE mission seems celebratory as these early images point to the spacecraft’s prowess. Says project scientist Peter Eisenhardt (JPL):
“All these pictures tell a story about our dusty origins and destiny. WISE sees dusty comets and rocky asteroids tracing the formation and evolution of our solar system. We can map thousands of forming and dying solar systems across our entire galaxy. We can see patterns of star formation across other galaxies, and waves of star-bursting galaxies in clusters millions of light years away.”
Tracking comets and asteroids is a secondary mission for WISE, but we’re learning that it’s a capable and sensitive observer of objects near and far. For those of us with a brown dwarf fixation, we should know much more about their numbers in nearby interstellar space by late this year, adding to our catalog of these objects as the whole-sky infrared sweep is concluded. WISE already has an asteroid under its belt, a near-Earth object discovered on January 12, and it will be scanning the sky one and a half times until October, when its coolant will run out.
by Paul Gilster | Feb 17, 2010 | Deep Sky Astronomy & Telescopes |
We know that new stars form out of cold gas and dust that are present in galaxies, but what accounts for the fact that star formation is slower than in earlier eras? Three to five billion years after the Big Bang, galaxies turned out stars at a much faster clip than they do today. The Milky Way seems to produce stars at a rate equaling about ten times the mass of our Sun each year, whereas similar galaxies earlier in their lives featured star formation rates that were up to ten times higher.
Michael Cooper (University of Arizona) and colleagues have gone to work on this question by studying data from the DEEP2 survey of 50,000 galaxies, picking a sample of one dozen massive galaxies to represent the average population. Working with the Hubble and Spitzer space telescopes as well as radio telescope arrays in France and California, the team then observed the selected galaxies in the infrared and measured their radio frequency emissions, making cold gas clouds visible.
Cooper and company were trying to discover whether older galaxies had a greater supply of gas and dust than similar galaxies today, or whether the efficiency of star formation somehow drops with time. This is tricky work because the dense, cool clouds of molecular gas in which star formation will occur are found at temperatures between 10 K and 100 K, emitting only tiny amounts of visible light. Having coalesced, a star’s radiation then dispels the gas and the star is visible.
Image: Viewed through the Hubble Space Telescope at visible light (left), a galaxy does not reveal its full secret underlying star formation. Only when observed using a combination of radio emission and infrared wavelengths, the galaxy reveals a massive, rotating disc measuring about 60,000 light years across (right). This disc consists of cold molecular gas and dust, the raw materials from which stars are born. Credit: University of Arizona.
The results seem straightforward, according to co-author Benjamin Weiner:
“What we found now is that galaxies like the ancestors of the Milky Way had a much greater supply of gas than the Milky Way does today. Thus, they have been making stars according to the same laws of physics, but more of them in a given time because they had a greater supply of material.”
Previous studies of this question had focused on bright objects, galaxies that were easier to study but not necessarily representative of the broader galactic population. Cooper’s team refined the methodology to select galaxies that could be considered ‘normal,’ firming up our picture of how galaxies make stars. They chose galaxies at redshifts of approximately 1.2 and 2.3, when the universe was 40% and 24% of its current age.
Adds Cooper:
“From our study, we now know that typical galaxies in the early universe contained three to ten times more molecular gas than today, a strong indication that the rate of star formation has slowed because those galaxies have less raw material available compared to when they were younger, and not because there was some change in efficiency with which they make new stars.”
The paper is Cooper et al., “High molecular gas fractions in normal massive star-forming galaxies in the young universe,” Nature 463 (11 February, 2010), pp. 781-784 (abstract).
by Paul Gilster | Feb 16, 2010 | Sail Concepts |
My interest in solar sail concepts goes back to the days of Cordwainer Smith’s “The Lady Who Sailed the Soul,” a science fiction tale (Galaxy, April 1960) whose evocative conjuring of a fantastic future has always stayed with me despite far more realistic sail concepts from the pen of Arthur C. Clarke and Poul Anderson, to name but a few. But magsails — craft that operate by creating a magnetic field that can interact with the solar wind — offer possibilities just as robust, provided we can tame the propulsive effects of that wind. And this may not be easy, given the changing speed and strength of this stream of charged particles outbound from the Sun.
Moving in some cases faster than 400 kilometers per second, the solar wind seems to offer a clear path to the outer system, but we know all too little about it. That’s why I always keep an eye on attempts to measure the solar wind, including the IBEX (Interstellar Boundary Explorer) mission that examines the interactions between the solar wind and the interstellar medium. Much closer to home is a new proposal by Mason Peck (Cornell University), whose work on modular spacecraft and self-assembly has been discussed previously in these pages. Now Peck is pondering tiny spacecraft to monitor the solar wind close to home.
We’re talking about craft no more than 1-centimeter square, a 25-micrometer thick object that weighs less than 7.5 milligrams. This New Scientist article writes up Peck’s idea of putting a swarm of such ‘smart dust’ spacecraft at the Lagrangian point between the Earth and the Sun, where it could monitor solar wind strength and alert us to blasts of charged particles that could disrupt communications and other electronic systems here on Earth. Think of these solar wind sensors as tiny solar panels with a radio antenna. They could give us an extra thirteen minutes warning of a storm compared to NASA’s Advanced Composition Explorer. And they should give us a richer picture of the solar wind itself.
Image: Initial prototype of Peck’s candidate MII spacecraft, shown next to a dime for scale. Credit: Mason Peck/Cornell University.
But ‘swarm’ spacecraft open up intriguing possibilities in other directions. They’re essentially micro solar sails that would be highly responsive to solar radiation. In fact, they’re much like dust, as Peck explains in a paper on the subject:
Dust in the solar system experiences a surprising lifecycle. Solar pressure and electrostatic forces can compete with gravity to give very small particles highly nontraditional orbits. Some dust finds a stable orbit; some dust gently lands on the surface of planets like our own, and some dust is energetically ejected from the solar system.
Dust particles vary in size from a few molecules to 100 ?m and have a mass smaller than a few ?g. At these mass scales, the acceleration due to what would be considered perturbation forces on larger bodies can no longer be neglected. In fact, we propose that they be harnessed and manipulated in order to enable new propulsion techniques and missions. Dust’s unique behavior motivates the present study of the orbital dynamics of extremely small bodies and the development of a spacecraft capable of exploiting on these physical principles.
Inspired by the orbital dynamics of dust, Peck and colleague Justin Atchison present their goal:
We propose to fabricate this dime-sized spacecraft on a single ultra-thin substrate of silicon. This choice reduces the total mass to fewer than 7.5 mg and makes the spacecraft bus itself a solar sail, yielding a lightness number ? of 0.0175. This architecture can provide passive solar sail formations and various passive methods of changing orbital energy. We also consider augmenting this architecture with traditional CP1 sail material (? of 0.1095) to reduce transfer times further.
Peck uses the term ‘Microscale Infinite-Impulse (MII) spacecraft’ to describe objects like these, and a prowl around his Web contributions at Cornell reveals possible applications from the kind of solar wind reporting discussed here to swarm spacecraft that can perform in-orbit inspections of larger craft, or even a chain of craft that can push into the interstellar medium, reporting by relaying data back to Earth through each subsequent craft. Peck’s paper “A Passive Microscale Solar Sail,” AIAA SPACE 2008 Conference & Exposition, San Diego, CA, Sep 9-11, 2008, is the place to begin. It’s available online. Thanks to John Freeman for the pointer to the New Scientist story.
