by Paul Gilster | Apr 24, 2009 | Sail Concepts |
A note from Eric Davis (Institute of Advanced Studies at Austin) fills me in on the details of the upcoming 6th International Symposium on Beamed Energy Propulsion, to be held in Scottsdale, AZ during the first week of November. Much of the program is of interest to interstellar studies, ranging from the basic science and technology of laser, microwave and particle beam propulsion to specifics relating the topic to beamed interstellar missions.
The latter subject will always be associated with Robert Forward, whose studies of beaming technology and sails made us understand that reaching the stars was not necessarily impossible. Lasers were the key, as Forward learned through his work with Ted Maiman at Hughes Research Laboratory. Years later he recalled his ‘eureka’ moment:
“I knew a lot about solar sails, and how, if you shine sunlight on them, the sunlight will push on the sail and make it go faster. Normal sunlight spreads out with distance, so after the solar sail has reached Jupiter, the sunlight is too weak to push well anymore. But if you can turn the sunlight into laser light, the laser beam will not spread. You can send on the laser light, and ride the laser beam all the way to the stars!”
That exuberant statement is pure Forward, written in the poignant memoir he knew he would never finish after the discovery of his terminal cancer. I can only imagine with what gusto he would have approached the coming ISBEP conference, whose preliminary schedule is available at the meeting site. Like the other players in this field, Forward knew that beaming energy to a spacecraft gets around the sheer inefficiency of carrying fuel aboard a rocket and allows missions — and payload sizes — that would otherwise be impossible.
Image: A near-term sail experiment under microwave beam. Courtesy of James Benford, Microwave Sciences.
The idea goes back to Tsiolkovsky in the 1920s, and received a powerful boost from Arthur Kantrowitz, whose 1972 paper “Propulsion to Orbit by Ground Based Lasers” examined ways to lift small satellites into Earth orbit. Forward and later theorists like Geoffrey Landis have pursued laser beaming to solar sails that can, in some designs, be hundreds of kilometers to the side, although tiny micro-sail designs are also in play (and I notice that their proponent, Jordin Kare, is on the program committee for ISBEP).
So is Leik Myrabo (Rensselaer Polytechnic Institute), who has demonstrated the efficacy of laser beaming in laboratory experiments and continues to study ‘lightcraft’ technology for launch from the ground. Lasers, of course, are only one option, for microwave beaming makes possible a sail that can be ultra-light, composed of a non-continuous surface. James and Gregory Benford’s lab work on microwave sails shows how powerful the concept is. For that matter, particle beaming to a magnetic sail falls under the same rubric considered at the conference, although all such concepts demand major investments in space infrastructure before interstellar use.
The American Institute of Beamed Energy Propulsion, which is organizing ISBEP 6, recently conducted a survey among specialists on topics of interest for the conference. The top vote getter was “BEP for Interplanetary/stellar propulsion,” which has resulted in Eric Davis chairing the upcoming session on that topic. And here I’ll go on to quote program chair Andrew V. Pakhomov (UAH):
“…it encompasses any type of mission and any type of vehicles, systems, or even networks as long as they go beyond the lunar orbit and as far as energy can be supplied remotely. Essentially, we have to continue the discussion of far frontiers of advanced propulsion opened by Eugene Sänger, Sebastian von Hoerner, George Marx, Wolfgang E. Moeckel, and Bob Forward, where the challenge is always the same: scientifically sound assessment of our technical abilities and limitations.”
Backing out to the broader perspective, I would argue that the necessary infrastructure for the kind of interstellar missions that will be discussed in Scottsdale may eventually become available to us, perhaps even at the huge scales envisioned by Robert Forward. But my guess is that they’ll evolve naturally as we move into nearby space for protection (asteroid deflection), energy and resources. And it’s fascinating to speculate on futures involving the construction of large structures via nanotechnology.
Dr. Pakhomov notes that he is hoping to attract a broad range of participants, particularly students, to the Scottsdale gathering. Full information on schedule and registration can be found at the ISBEP 6 site, along with contact information. Submissions for the interstellar session are due no later than September 15.
by Paul Gilster | Apr 23, 2009 | Asteroid and Comet Deflection |
Diverting incoming asteroids is a high priority item, and so is a mission to a nearby asteroid for a close-up study of its composition and a shakeout of operating technologies. Think about the movie Deep Impact. Nukes are used to break up an incoming object, in this case a comet, but the resultant deadly chunks are still headed toward Earth. The planet suffers one disastrous collision, but it turns out to be survivable due to quick thinking and the willingness of a spacecraft crew to sacrifice themselves by blowing up the remaining impactor.
Get past the Hollywood cliffhanger elements and Deep Impact had its moments (in any case, I’ll sign off on any movie with Robert Duvall in it). The use of nuclear weapons in the movie does raise a legitimate question — do we know enough about what might hit us to predict what would happen if we did try to destroy it this way? That’s one reason we need early missions to study Earth-crossing asteroids, and it’s also a reminder that keeping our deflection options open means looking at entirely new solutions.
Tethers for Deflection
Enter David French, an aerospace engineering doctoral student at North Carolina State University. French has come up with a technique for deflecting an Earth-crosser that is, in its scale, a reminder of the magnitude of the danger. It involves attaching a long tether and ballast to the incoming object. “You change the object’s center of mass,” says French, “effectively changing the object’s orbit and allowing it to pass by the Earth, rather than impacting it.”
We’re talking about a tether between 1,000 and 100,000 kilometers in length, the latter being long enough to wrap around the earth two and a half times. An extreme solution? Perhaps, but considering the political obstacles we face in deploying any sort of nuclear technology, maybe it’s best to keep even unlikely options open. In any case, tethers (especially electrodynamic ones) have long interested NASA and include uses that could morph advanced tethers into payload delivery systems as the necessary background work is accomplished.
Electrodynamic Tethers for Propulsion
Electrodynamic tethers can provide propulsion because of the force a magnetic field exerts on the wire when an electrical current is passing through it. Do this with a tether in Earth orbit and the Earth’s magnetic field can do the accelerating, launching a payload connected to the wire without the need for fuel. Thus the MXER (Momentum Exchange Electrodynamic Reboost) tether, which throws the payload and then replaces lost kinetic energy by providing power to a conducting section of tether that allows the MXER station to regain altitude, leaving it recharged for another spacecraft launch.
Image: The Momentum-Exchange/Electrodynamic-Reboost Tether Concept. Credit: Tethers Unlimited.
The point being, tethers of various kinds have an interesting future (imagine what we might do with a tether system adapted for the magnetic fields in Jupiter space), and one that may adapt to this new use. The best space work doubles up on resources and extends existing ideas into new directions, so we’ll see what may come of the asteroid tether concept. French will present it at the AIAA SPACE 2009 Conference this fall.
In the meantime, the larger picture is that even those who disparage the need for space exploration can see the necessity of protecting the planet from danger, a fact that may ultimately be our best insurance for overcoming short-sighted opposition and developing a robust deep space infrastructure. “The prospect of hanging,” said Samuel Johnson, “concentrates the mind wonderfully.” So too will the prospect of future impacts as we continue to discover and catalog near-Earth objects.
The Tethers Unlimited site has descriptions of numerous types of tethers (this is Robert Forward’s old company). For MXER, one interesting take is Sorensen, K.F., “Conceptual Design and Analysis of an MXER Tether Boost Station”, AIAA Paper 2001-3915, available here.
by Paul Gilster | Apr 22, 2009 | Deep Sky Astronomy & Telescopes |
Three brown dwarfs with masses that push up against the boundary between star and planet have been identified in IC 348, a star-forming region some 1000 light years from Earth in the direction of the constellation Perseus. The dwarfs do not appear to be gravitationally bound to a star although they are bound by the cluster, and they’re useful as we try to broaden our understanding of the mass distribution in newly formed stellar populations.
Andrew Burgess (Observatoire de Grenoble) has this to say about the find:
“There has been some controversy about identifying young, low mass brown dwarfs in this region. An object of a similar mass was discovered in 2002, but some groups have argued that it is an older, cooler brown dwarf in the foreground coinciding with the line of sight. The fact that we have detected three candidate low-mass dwarfs towards IC 348 supports the finding that these really are very young objects.”
Image: IC 348, the star-forming region where the brown dwarfs were discovered. Credit: Adam Block and Tim Puckett.
Bear in mind that brown dwarfs cool with age. This news release indicates that current models show temperatures of between 600-700 degrees Celsius for the objects, extremely cool for newly formed brown dwarfs, with the implication of low mass. Again we push into areas of brown dwarf and sub-stellar object discovery that show how much work we have ahead as we catalog the local neighborhood.
So are brown dwarfs ubiquitous, occurring in sufficient numbers to account for an appreciable amount of the ‘dark matter’ in the galaxy? Studies are ongoing, but the three major ones that have already been completed indicate that the answer is no. The MACHO Project (Massive Astrophysical Compact Halo Object, a term for large astronomical bodies that can explain what seems to be dark matter in galactic haloes), along with the EROS and OGLE collaborations, all involved studies of the Small and Large Magellanic Clouds, satellite galaxies to our own Milky Way.
Using stars in the LMC and SMC as light sources, the teams observed the stars for several years and looked for the kind of lensing events that would indicate the presence of a dark object between the star and the observer. The MACHO and EROS teams announced the detection of three MACHOs as far back as September of 1993. That was after looking at 1.8 million stars for one year (MACHO) and 3 million stars for three years (EROS). By the end of the decade, the teams had a combined score of about twenty microlensing events.
Here’s a quote from Evalyn Gates’s new book Einstein’s Telescope: The Hunt for Dark Matter and Dark Energy in the Universe (W.W. Norton, 2009), a survey of where we stand on dark energy and dark matter studies with a focus on gravitational lensing:
The final paper of the MACHO collaboration, published in 2000, concluded that a Galactic halo consisting entirely of MACHOs was now ruled out, and estimated that about 20% of the Galactic halo was in the form of MACHOs. The EROS team preferred to present its results as an upper limit on the number of MACHOs in the halo, with no more than about 8% of the halo in MACHOs having masses of about one-tenth to one times the mass of the Sun. A combined analysis of the two experiments showed that, within the uncertainties of each experiment, they are consistent with each other and that less than 20% of the halo is in the form of MACHOs.
Gates goes on to say:
MACHOs, the least exotic candidates for dark matter, have now been effectively ruled out as the main component of the dark matter, leaving WIMPs to dominate the Galaxy…. Nevertheless, there seems to be evidence for some MACHOs in the Galactic halo, even if not enough to be interesting from a dark matter point of view.
WIMPs are Weakly Interacting Massive Particles, hypothetical particles that cannot be seen directly and do not interact strongly with atomic nuclei. That makes them far more exotic than MACHOs as a dark matter solution, but currently they seem to have taken the lead over the MACHO theory. Given the progress of our studies in these matters, though, it would be premature to call this investigation over, and further dark matter surprises seem likely.
by Paul Gilster | Apr 21, 2009 | Exoplanetary Science |
Gliese 581, the star that teased us a few years back with reports of a ‘super-Earth’ planet in the habitable zone, is back in the news. Michel Mayor’s Geneva team has located a fourth planet in the system, Gliese 581 e, which weighs in at a mere 1.9 Earth masses, making it the least massive exoplanet ever detected. Orbiting its primary in 3.15 days, the newly found world is too close to the star to be in the habitable zone, but the other shoe that drops here is that Gl 581 d may itself be.
Image: By refining the orbit of the planet Gliese 581 d, first discovered in 2007, a team of astronomers has shown that it lies well within the habitable zone, where liquid water oceans could exist. This diagram shows the distances of the planets in the Solar System (upper row) and in the Gliese 581 system (lower row), from their respective stars (left). The habitable zone is indicated as the blue area, showing that Gliese 581 d is located inside the habitable zone around its low-mass red star. Credit: Based on a diagram by Franck Selsis (University of Bordeaux).
Watching this story unfold has been instructive. Early indications that Gl 581 c was in the habitable zone were quickly challenged, with a consensus developing that the planet was too hot for liquid water to exist on its surface. Indeed, it might well resemble Venus more than Earth, a notion that was rarely picked up by the media. The belief that Gl 581 c is habitable has become more or less codified in some quarters — a National Geographic special not long ago depicted it as a green and blue living world — long before the scientific process could come to a resolution on the matter.
At the same time, Gl 581 d, a planet seven times as massive as the Earth, was thought to be on the very outer edge of the habitable zone, a longshot candidate for liquid surface water. The Geneva team, in the process of discovering the new planet, has now refined the orbit of Gl 581 d, the updated work indicating that it could be an interesting place indeed:
“Gliese 581 d is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star,” says team member Stephane Udry. The new observations have revealed that this planet is in the habitable zone, where liquid water could exist. “‘d’ could even be covered by a large and deep ocean — it is the first serious ‘water world’ candidate.”
So now we have firmer indications of a planet in the habitable zone. And as this news release points out, the progress of our work in the Gliese system is nothing short of phenomenal. Michel Mayor, the man who, along with Didier Queloz, was behind the planetary discovery at 51 Pegasi just a decade ago, notes that the mass of Gl 581 e is fully eighty times less than that of 51 Pegasi b. That’s also a reminder that low-mass red dwarfs are good places to look for small worlds, since the gravitational tug of close-in planets is more pronounced.
Look for the paper on this find to run in Astronomy & Astrophysics. It’s Mayor et al., “The HARPS search for southern extra-solar planets: XVIII. An Earth-mass planet in the GJ 581 planetary system,” available here.
by Paul Gilster | Apr 20, 2009 | Exoplanetary Science |
News about a nearby brown dwarf occupies us this morning, but first, a quick site update. The recent server problems did not, fortunately, result in the loss of any data, but I’ve had to make some temporary software changes to get Centauri Dreams back up. Expect more changes in coming weeks as I replace these fixes, so you may see things in transition for a time, but the server switchover is complete. One remaining problem is a snafu in image uploads that I hope to fix soon.
Now, to brown dwarfs. Seeing them is tricky business. Too small to be stars (although they do fuse deuterium), too massive to be planets, they’re hard to pick out in visible light and are generally detected at infrared wavelengths. Now a faint brown dwarf orbiting the nearby star Wolf 940 has been discovered. The primary is a red dwarf some 40 light years from Earth, orbited by its dim neighbor at a distance of some 440 AU.
This may bring to mind our recent discussion of Lorenzo Iorio’s work, which settled on a figure of 3,736-3,817 AU from the Sun as the nearest distance an undetected brown dwarf could exist near us. Wolf 940 B is obviously in a much tighter relationship than that with its primary. And astronomers studying the object hope that it may prove useful in telling us about the age and composition of brown dwarfs.
Ben Burningham (University of Hertfordshire) notes the dwarf’s cool temperature (by stellar standards) of 300 degrees Celsius, and says that its proximity to the red dwarf may come in handy:
“What’s so exciting in this case, is that we can use what we know about the primary star to find out about the properties of the brown dwarf, and that makes it an extremely useful find. You can think of it as a Rosetta Stone for decrypting what the light from such cool objects is telling us.”
The UKIRT Infrared Deep Sky Survey, based on Mauna Kea, is carrying out the work that resulted in this detection, flagging the brown dwarf as a companion to Wolf 940 after studying their common motion. We’re in an era of large scale surveys, and the question that now arises is whether brown dwarf/red dwarf binaries are unusual or whether we’re going to learn that red dwarfs often have such companions. In any case, Wolf 940 B should be helpful in finding out more about brown dwarfs and the nature of warm planetary atmospheres. The paper will run soon in Monthly Notices of the Royal Astronomical Society.
Speaking of nearby objects, Max Wolf, who discovered Wolf 940 some ninety years ago, seems to have had a penchant for finding them. He’s also the discoverer of Wolf 359, located 7.7 light years away and, like Wolf 940, a red dwarf (one with a relatively high flare rate, at that). Max Wolf was also a prolific asteroid finder, a pioneer in astrophotographic techniques, and the discoverer of several comets. He is not, however, associated with the so-called Wolf-Rayet stars. That would be the French astronomer Charles Wolf.
