by Paul Gilster | Sep 6, 2008 | Research Tools |
I’m a great believer in the open courseware concept that MIT has done so much to promote. The idea is to do away with the password-protected gatekeeper function that so many university and college Web sites impose, opening access to those course materials an instructor chooses to put online. Some 1800 courses in 33 different disciplines have made their way to the Web via MIT’s gateway, their offerings ranging from audio of lectures, lecture notes and exams to PDFs and video files. It’s a pleasure to see that Bruce Irving is tracking MIT’s venture on his Music of the Spheres site, a post I’ve chosen to highlight from this week’s Carnival of Space collection.
Bruce notes one recent addition to the MIT catalog, a course called Space Systems Engineering that looks at design challenges in both ground and space-based telescopes, ultimately attempting to choose the top-rated architectures for a lunar telescope facility. But the MIT offerings are wide ranging. I’m seeing courses on aerospace engineering, structural mechanics, aerodynamics, space propulsion, satellite engineering, and one that caught Bruce’s eye as well, a course called Engineering Apollo: The Moon Project as a Complex System, with guest lectures by engineers who participated in Apollo.
What’s unusual to me isn’t MIT’s bold attempt to make university resources available throughout the globe, but the fact that open courseware hasn’t become more widespread. Against the objection that it diminishes the likelihood that a student will apply to a given school, its courses being available online, I can only state the obvious: No degree program flows from open courseware, nor does the online experience in any way equal the rich and interactive environment to be found on the actual campus. Open courseware does not try to replace traditional education, but to augment it by making the fruits of intellectual inquiry more widely available.
Making that point is simple once you’ve gone through some of MIT’s courses, seeing that some are far more complete than others in terms of materials. But what an exceptional resource for those trying to get a handle on how a subject is structured in a university environment, its basic premises and strategies, and the background materials by which to approach it. I think, too, of how many teachers in more remote environments may find stimulus here to revise and expand their current offerings through online suggestions. Bravo MIT, and here’s to open courseware spreading to other great universities.
by Paul Gilster | Sep 4, 2008 | Sail Concepts |
The vast laser-driven sails envisioned by Robert Forward have always fired my imagination. Hundreds of kilometers in diameter, they would rely upon a gigantic Fresnel lens in the outer Solar System to keep the critical laser beam tightly collimated over interstellar distances. Forward conceived of mission designs to stars as far away as Epsilon Eridani, journeys that could be achieved within a human lifetime. He even provided return capability through the use of a multi-part sail. You can read a fictional treatment of this in his novel Rocheworld.
But how do we get from here to there? As of today, we’re close enough to having an operational space sail that if we can talk SpaceX into lofting the NanoSail-D duplicate, we could be shaking out our first space sail within months. Assuming we do go operational before too many months (or years!) pass, the question then becomes, what kind of missions are possible between the laser-beamed lightsail of science fictional imagining and the practical workhorse sail that may well open up a space-based infrastructure for our use.
Such questions point to the pleasures of reading a new book on solar sails by three leading experts. Gregory Matloff has been examining the concept for the past thirty years, with seminal papers in the 1980s and continuing work on near-term concepts. His regular consulting at Marshall Space Flight Center keeps him in touch with co-author Les Johnson, NASA’s deputy manager of the Advanced Concepts office at Huntsville. The third author is Italian scientist Giovanni Vulpetti, who has spent most of his professional life on questions of interstellar propulsion ranging from antimatter annihilation to sail design, including the Aurora Project, a sail mission to the heliopause that grew out of earlier work at Italian aerospace firm Alenia Spazio and other European venues.
You would be hard pressed, in other words, to find a more knowledgable team to write a book titled Solar Sails: A Novel Approach to Interplanetary Travel (Copernicus, 2008), and it’s a pleasure to add that despite the sub-title, questions of interstellar significance receive solid treatment. Getting from here (a technology ready to fly for testing) to there (a genuine interstellar craft deploying sail technology) is a long haul, but near-term concepts for a Solar Polar Imager, putting a payload into a highly inclined orbit around the Sun to study its polar regions, are feasible. And so are missions like Heliostorm, which could use a sail to maintain a position between Earth and the Sun (at roughly 0.70 AU, but with a period of one year) to provide advance warning of solar storms.
We can add longer-term prospects that still fall well within our engineering capabilities, missions for comet rendezvous, Mars sample return and, via a Sundiver maneuver, a probe to the heliopause some 200 AU out. The latter, the authors note, could continue for a few decades more to study the environment at the Sun’s gravitational lens some 550 AU from Sol, providing a useful check on Einsteinian general relativity.
All of these concepts and more are discussed here, but it’s the longer-term missions (still using solar photons, not beamed lasers) that truly up the ante. I remember talking to Matloff about them during a Huntsville visit some years back, but the book lays them out sequentially, showing us the limits of the technology in terms of true interstellar missions, and pointing us toward the laser and other beaming options that may be necessary if we are to improve travel times significantly.
For in addition to the outer system work we’d like to perform, examining NEO deflection or developing mining strategies for interesting objects, we’d like to get all the way out to the Oort Cloud, where as many as a trillon comets may lurk. A specialized task indeed, as the authors note:
This is a task for the Oort cloud explorer, perhaps the ultimate sailcraft before a true starship. Imagine a sail 100 nanometers thick, perhaps a kilometer in radius, which is constructed of material capable of withstanding a perihelion pass of about 0.05 AU (about ten solar radii). Such a craft could perform a Sun dive and project its payload toward the stars at velocities in excess of 500 kilometers per second.
All of which gets us to a few thousand AU before the vehicle’s operational life ends, but we’re still — at 500 km/sec — talking about 2000 years to travel the 40 trillion kilometers to the Centauri stars. Can we do better? Matloff worked with Michael Mautner and Eugene Mallove in a series of papers in the Journal of the British Interplanetary Society back in the 1980s to examine such questions. An optimized interstellar sail would use a nanometers-thin monolayer of beryllium, aluminum or niobium, all highly reflective and temperature tolerant. It could be mounted partially unfurled behind an asteroid that would serve as a sunshade, then put into a parabolic solar orbit with a close pass by the Sun measured in the millions of kilometers.
The sail, of course, would be unfurled at the right moment to maximize acceleration. The results:
Analysis revealed that acceleration times measured in hours or days were possible. By the time the ship reaches the orbit of Jupiter, the sail could be furled, since acceleration has fallen to a negligible value. The sail could be used as cosmic ray shielding and later unfurled for deceleration. Flight times to Alpha Centauri, even for massive payloads that could carry human crews, could approximate a millennium. Of course the hyperthin sail sheets required to ‘tow’ such large, multimillion-kilogram payloads would be enormous — in the vicinity of 100 kilometers.
A thousand years to Centauri — this is the figure I recall from my Huntsville discussion with Matloff, and it stuck with me as a kind of insurance policy. If we were to learn, for example, that we had for reasons of survival to exit our Solar System, the prospect of getting at least a fraction of humanity to another, although demanding a lengthy, multi-generational voyage, would not be beyond the reach of a technology we could conceive of developing within this century, using the laws of physics as currently understood. All of which confirms what Robert Forward used to say, “Travel to the stars is difficult but not impossible,” a real reversal of many pre-Forward opinions.
There is more to be said about laser-beaming and other sail concepts (not to mention sail design, construction, deployment and trajectories), but we’ll look at the book’s treatment of these in a later post. Consider this simply a heads-up to alert you to a title that needs to be on your shelf if you’re seriously interested in the next step as we move beyond rocketry. And move beyond it we must, to explore the advantages of leaving the propellant behind to maximize payload for missions that may one day take us deep into the Solar System and beyond.
by Paul Gilster | Sep 3, 2008 | Outer Solar System |
With the Steins encounter looming, let’s keep the focus on the asteroid belt, in this case by examining a connection between that distant region and our own planet. Cosmic dust particles — tiny bits of pulverized rock up to a tenth of a millimeter in size — move continuously through the Solar System, a kind of micro-thin fog of micrometeorites that contributes hundreds of billions of particles to Earth’s atmosphere. New research into the makeup of some 600 of these particles now reveals their chemical and mineral content, allowing an overview that points to their origin. The suspected source: A group of asteroids between Mars and Jupiter.
You can see one of the Koronis asteroids in the image at left, which shows 243 Ida as photographed by the Galileo probe. What we now believe about the Koronis asteroids is that they were formed some two billion years ago by the breakup of a much larger asteroid. Within the Koronis family are the ninety or so Karin asteroids, which seem to be in a constant process of smashing into each other to create cosmic dust. Themselves the result of a far more recent collision (pegged at 5.7 million years), the Karin asteroids are useful because they are readily susceptible to spectroscopic analysis.
Image: This view of the Koronis asteroid 243 Ida is a mosaic of five image frames acquired by the Galileo spacecraft’s solid-state imaging system at ranges of 3,057 to 3,821 kilometers (1,900 to 2,375 miles) on August 28, 1993, about 3-1/2 minutes before the spacecraft made its closest approach to the asteroid. Galileo flew about 2,400 kilometers from Ida at a relative velocity of 12.4 km/sec. Credit: NASA/JPL.
Says Matthew Genge (Imperial College London), who studied the micro-meteorites in Antarctic dust:
“I’ve been studying these particles for quite a while and had all the pieces of the puzzle, but had been trying to figure out the particles one by one. It was only when I took a step back and looked at the minerals and properties of hundreds of particles that it was obvious where they came from. It was like turning over the envelope and finding the return address on the back.”
Genge’s micrometeorites contain information about the formation of asteroids and comets some four and a half billion years ago. As with Martian meteorites, the chance to study material from another celestial object without the expense of mounting an expensive mission is itself noteworthy. We’re synchronizing laboratory mineralogy with infrared observations of the Karin asteroids, work with implications for the origin of the Karin group in what seems to have been a rubble-pile asteroid.
The paper is Genge, “Koronis asteroid dust within Antarctic ice,” Geology Vol. 36, Issue 9 (September 2008), pp. 687-690 (abstract).
by Paul Gilster | Sep 2, 2008 | Outer Solar System
Get out to about 2.4 AU from the Sun (2.41 AU, to be precise) and your radio signals have a long travel time. It takes 20 minutes to cross the 360 million kilometers between Earth and the Rosetta spacecraft, and that, of course, is one-way. As we’ve learned from all our deep space missions, spacecraft are largely on their own for the brief and critical window of an encounter, like the one with asteroid Steins that is coming up for Rosetta.
Opportunities for possible trajectory correction maneuvers exist both on September 4 and 5th, but it’s on the 4th that Rosetta’s controllers will have their last chance to acquire optical images for navigation. Uplink commands for asteroid fly-by mode will be sent on the morning of the 5th and then we wait for results as the vehicle flips for observation and tracking. Rosetta will close to within 800 kilometers of the asteroid, passing it at a speed (relative to Steins) of 8.6 kilometers per second.
Image: The approach of Rosetta’s spacecraft to asteroid (2867) Steins on 5 September 2008. Steins is located in the main asteroid belt between the orbits of Mars and Jupiter. The encounter takes place during Rosetta’s first incursion into the main asteroid belt while on its way to Comet 67P/Churyumov-Gerasimenko. Credits: ESA, image by C.Carreau.
Closest approach is 18:58 UTC on the 5th, with the start of the science download at 2030. Long live the Internet — as we’re getting used to during and after these events, we’ll have imagery the next day, to be published on the ESA Rosetta site. Be aware of the newly reactivated Rosetta blog. Rosetta’s spacecraft operations site is available here.
by Paul Gilster | Sep 1, 2008 | Deep Sky Astronomy & Telescopes |
Although we often talk about the Magellanic Clouds as satellites of the Milky Way, recent research seems to point to a different conclusion. The dwarf galaxies may be moving too fast to be bound to our own, cities of stars simply flowing past us in the night. Be that as it may, the Milky Way still has over twenty other dwarf galaxies in orbit around it, eighteen of which have been the subject of recent work aimed at calculating their masses. The odd results have striking implications for dark matter.
For the dwarf galaxies around us vary greatly in brightness, from a thousand times the luminosity of the Sun to a billion times that amount. You would assume that the brightest dwarf galaxy would have the greatest mass, while the faintest would show the least. The surprise is that all the dwarf galaxies have roughly the same mass, some ten million times the mass of the Sun within their central 300 parsecs. Here’s Manoj Kaplinghat (University of California at Irvine) with a helpful comparison:
“Suppose you are an alien flying over Earth and identifying urban areas from the concentration of lights in the night. From the brightness of the lights, you may surmise, for example, that more humans live in Los Angeles than in Mumbai, but this is not the case. What we have discovered is more extreme and akin to saying that all metro areas, even those that are barely visible at night to the aliens, have a population of about 10 million.”
What’s going on? The dwarf galaxies, about half of them found within the past few years by the Sloan Digital Sky Survey, are primarily made up of dark matter, the ratio of dark to normal matter becoming as high as ten thousand to one (the latter are the most dark-matter dominated objects known). Even the faintest dwarf galaxies, all but invisible to us, contain huge amounts of dark matter. The researchers believe that clumps of dark matter can exist that contain no stars at all, and that a minimum dark matter mass exists — about ten million times the mass of the Sun — that allows stars within the dark matter to form into galaxies.
Image: Satellite galaxies studied by UCI researchers that are within 500,000 light-years of the Milky Way. Credit: J. Bullock/M. Geha/UCI.
Small, gravitationally bound clumps of dark matter are known as haloes, and there’s no reason why haloes can’t be smaller than the minimum size needed for galaxy formation. Indeed, the Milky Way may be teeming with them. Consider this, from the team’s letter to Nature:
The mass of the smallest dark matter halo is determined by the particle properties of dark matter. Dark matter candidates characterized as cold dark matter can form haloes that are many orders of magnitude smaller than the least luminous haloes that we infer from observations. Cosmological simulations of cold dark matter predict that galaxies like the Milky Way should be teeming with thousands of dark matter haloes with masses ? 106 M? , with a steadily increasing number as we go to the smallest masses. A large class of dark matter candidates characterized as “warm” would predict fewer of these small haloes.
Are we getting any closer to an understanding of the elusive particle that makes up dark matter? A key to understanding the stuff at the microscopic level may be operations at the Large Hadron Collider, scheduled to become operational this year. But even as we ponder the possibilities of actually creating dark matter in the lab, the astronomical outlook is all about building a larger dataset of low-luminosity galaxies. From the paper:
Future imaging surveys of stars in the Milky Way will provide a more complete census of low-luminosity Milky Way satellites, with the prospects of determining whether astrophysics or fundamental dark matter physics is responsible for setting the common mass scale. In particular, the masses for the faintest dwarf galaxies will become more strongly constrained with more line-of-sight velocity data. This will sharpen the observational picture of galaxy formation on these small-scales and provide data around which theories of galaxy formation may be built.
The paper is Strigari et al., “A common mass scale for satellite galaxies of the Milky Way,” Nature 454 (28 August 2008), pp. 1096-1097 (abstract).
