by Paul Gilster | Oct 20, 2006 | Antimatter, Exotic Physics |
We’re a long way from knowing how to put antimatter to work in starship engines, but developments in this field are well worth following. Even in the short term, designs like Steven Howe’s antimatter sail hold rich promise for shortening travel times to the outer Solar System and for interstellar precursor missions. Howe’s sail would embed uranium-235 in the sail and let antihydrogen released from the spacecraft initiate a powerful fission reaction.
A major obstacle in building such designs is figuring out how to ramp up production of antimatter. But as we work such issues out, the Alice in Wonderland world of antimatter research continues to prove fascinating in its own right. Thus the word out of CERN that physicists have found a way to make matter and antimatter combine — briefly, to be sure — into a extremely unstable substance called protonium.
Call it ‘anti-chemistry.’ The work at CERN had been dedicated to producing antihydrogen. Just as hydrogen is made up of protons and electrons, anti-hydrogen is formed from antiprotons and anti-electrons, or positrons. But a bit of the protonium hybrid was created at the same time. The hope is to produce larger quantities of the stuff, providing a unique testbed for particle physics.
And exactly what is protonium? According to this story in Chemistry World, it is antiprotonic hydrogen, a composite of a negatively charged antiproton paired with a positively charged proton. To produce it, CERN’s Athena collaboration cools antiprotons down to the point that they can be caught in an electrostatic trap. The reaction occurs between the antiprotons and H2+ ions, which consist of two hydrogen atoms missing one electron. The result is protonium, with a neutral hydrogen atom left over.
Learning more about the interactions between particles and antiparticles should reinforce our thoughts on some of the fundamental theories of particle physics. And perhaps those evanescent reactions will also help us learn how to produce antimatter in larger quantities, not to mention tutoring us on the best methods of storage. Steve Howe’s antimatter sail contains some exotic storage methods indeed, using anti-hydrogen pellets in micro-traps. See his Phase II study “Antimatter Driven Sail for Deep Space Missions.” at NASA’s Institute for Advanced Concepts (NIAC) for an analysis of this ingenious method.
by Paul Gilster | Oct 19, 2006 | Communications and Navigation, Culture and Society |
It won’t get us to the stars, but the navigation practiced by ancient Polynesians — sailing by the stars — continues to fascinate a new generation. And since Centauri Dreams often cites the remarkable voyages of these people as they populated the Pacific, it seems appropriate to focus today on an Australian Broadcasting Company story about an art that has been all but lost. A man named Hoturoa Kerr, who is a lecturer at the University of Waikato (Auckland, NZ), is teaching celestial navigation in an oceanic context to his students.
Finding your way over ocean swells on a body of water as big as the Pacific sounds all but impossible, particularly if your vessel is a small, double-hulled canoe. But Kerr took a GPS with him on a canoe journey from New Zealand to the Cook Islands in a vessel called the “Te Aurere”, checking the work of a navigator aboard the craft who used the old methods. At the end of the journey, he found that at any time, the navigator was no more than twelve miles off the GPS reading.
The Polynesians call it ‘way-finding,’ and it’s a method that relies on more than stars. As the night ends, the navigator takes a final bearing based on star positions, then checks the motion of the canoe as it travels over the ocean swells. During the daylight hours, he will keep the canoe in the same position with regard to the swells, which will usually change little before the Sun sets and the stars again emerge.
Way-finding was good enough to direct the diaspora that began 5000 years ago as the ancestors of the Polynesian peoples pushed eastward into the Pacific. 3,500 years ago they occupied in less than a half-dozen generations the island chains of Fiji, Tonga, and Samoa. The next wave took them, now using larger double canoes, to Tahiti and the Marquesas, then across thousands of miles of open water to Hawaii, Easter Island, and New Zealand, navigating just by the stars, the wind, the ocean swells, and the flight of birds.
Te Aurere has thus far journeyed over 30,000 nautical miles using way-finding alone. “Spiritually it’s a canoe that carries us in terms of our mind and our thinking and everything else,” says Kerr. “And when you sail a canoe, you sail for distant horizons. So what I’m hoping is that with these young people it makes them look towards distant horizons as goals for them in their life.” Such horizons are always worthwhile. And perhaps they’re not so different from the far more distant horizons we may one day embark for out in the Orion Arm.
by Paul Gilster | Oct 18, 2006 | Astrobiology and SETI |
Many years back I wrote an article for Glenn Hauser’s Review of International Broadcasting called “Where the Real DX Is.” DX is the shortwave radio term for seeking out distant signals, a sport in which the smaller and fainter the station, the more interesting the catch. I was laboring with an old FRG-7 receiver to attempt impossible receptions like the Falkland Islands and Tristan da Cunha (neither of which I ever heard), but in the back of my mind were the nearby stars. What about receiving a signal from one of them?
And while I wrote about the emerging SETI scene, my real thinking was that an extraterrestrial reception wouldn’t be from a beacon — I still doubt these exist — but from accidental leakage from a technological society. Now a new paper by Harvard’s Abraham Loeb and Matias Zaldarriaga suggests an interesting strategy for finding such leakage, via a a low-frequency radio telescope study that will look at highly redshifted 21 centimeter emissions from hydrogen. The Mileura Wide-Field Array (MWA) is designed to tell us about the state of neutral hydrogen in the early universe, before it became ionized by the first galaxies.
Which is a fascinating project in itself, because it tells us about the universe only a few hundreds of millions of years after the Big Bang, helping us learn about early star and black hole formation. But given the paucity of SETI funding (most of it today comes from non-profits), Loeb sees a wonderful synergy between this work and a SETI hunt in the frequencies the MWA will probe. Why not use MWA and other radio observations of the redshifted hydrogen to look for extraterrestrial DX?
This strategy is novel because by the time you’ve redshifted the 21 centimeter hydrogen (1420 MHz), you wind up in the frequency range between 80 and 300 MHz. That’s not where the typical SETI search is, but it’s in the middle of the band in which our own civilization would be the most luminous from another star. Indeed, where we’re putting out the most significant signals is in radio emissions from military radars, FM radio broadcasts, and TV, all of which can be found in a wide band from roughly 40 to 800 Mhz.
Thus the Mileura Wide-Field Array gives us the chance to piggyback SETI on the back of a well-funded area in cosmology, looking at a significantly different part of the spectrum than existing SETI searches. Again, we’re not looking for beacons here (conventional thinking says these might be best searched for in the 1420 to 1660 MHz water hole band) but incidental emissions from a technology at work. And yes, the frequency range Loeb and Zaldarriaga are talking about is tricky. From the paper:
Of course, our own radio broadcasting is far greater at lower frequencies, so at least for the purpose of “eavesdropping” on another civilization, lower frequencies might be more interesting. The fact that our civilization makes much use of the lower frequency spectrum presents severe technical di?culties for SETI programs trying to operate in this frequency range as they have to ?lter-out our own radio-frequency interference (RFI). Thus, 21cm cosmology is a case in which an unrelated science driver will open a new and potentially more suitable window for SETI programs. The interest in high-redshift 21 cm surveys means that there will be signi?cant efforts to control RFI by, for example, placing the observatories in remote locations with the lowest RFI record (such as China, Australia, Africa, or even the moon), as well as developing new ?ltering techniques for RFI and ionospheric noise. Obviously SETI programs could bene?t signi?cantly from these technological developments.
Can the leakage of electromagnetic radiation be detected from planets around nearby stars like Centauri A and B? Loeb believes the Mileura Wide-Array may be up to the challenge. The Foundational Questions Institute agrees and has given him a grant to initiate such a search as the MWA work proceeds. You can read more about optimizing the software for detecting such signals in Loeb and Zaldarriaga’s paper “Eavesdropping on Radio Broadcasts from Galactic Civilizations with Upcoming Observatories for Redshifted 21cm Radiation,” available online at the arXiv site.
by Paul Gilster | Oct 17, 2006 | Exoplanetary Science |
How could you possibly study the interior of a giant planet orbiting another star? Especially when that planet is so drowned in its star’s light that we can’t even see it? Various methods suggest themselves, including transits, those cases wherein the exoplanet happens to pass between us and the star it circles. A transit gives you the chance to measure both mass and size. Throw in inferences based on slowly evolving planetary models and you can draw some tentative conclusions. You also wind up with even more questions.
And as Tristan Guillot would probably point out, we now have twenty gas giants whose mass and size can be determined, including those within our own Solar System. Guillot, who works in one of the most celestially beautiful places on Earth (he’s at the Observatoire de la Cote d’Azur in Nice), makes it his business to compare and contrast what we see around Sol with the rising number of giant planets we’re finding around other stars using the transit method.
Several interesting things emerge. All gas giants seem to be made of hydrogen and helium wrapped around a dense core that is presumed to be made of compressed water and rocks. That’s from standard models of planet formation, and when you run the numbers, you expect to see a core of about ten Earth masses. And indeed, this accounts for Uranus and Neptune, but Jupiter and Saturn remain enigmatic: Jupiter’s core seems to be only a few times the mass of Earth, while Saturn’s may be as much as 25 times Earth mass.
So we have much to learn here in our own system. And exoplanets present an even greater discrepancy. The hydrogen/helium constituents are what we expect, but Guillot and colleagues are finding much larger cores — up to one hundred times that of Earth’s mass. Not surprisingly, the metallicity of the star (its relative richness in heavier elements) seems to correlate to the mass of the planetary core.
And Guillot, who has won the Harold C. Urey Prize from the Division for Planetary Sciences of the American Astronomical Society, points to future missions like Juno, a Jupiter-bound spacecraft that will measure the planet’s gravity with minute precision, not to mention the data expected from both COROT and Kepler, transit hunters that will soon take to the skies. COROT is scheduled for a December launch, with Kepler to follow in two years. A golden era of transit studies is upon us.
If you’re following gas giant research, Guillot’s Web site at the observatory offers papers, figures, images and other data. A useful starting point is his paper “The Interiors of Giant Planets: Models and Outstanding Questions,” in Annual Review of Earth and Planetary Sciences, Vol. 33, p.493-530, which is also available online.
by Paul Gilster | Oct 16, 2006 | Exoplanetary Science |
Watching the snowline descend to ever lower elevations as fall deepens into winter is one of the great pleasures of the Canadian Rockies, an area better suited to train travel than any on Earth. And an image of snow-topped mountains in Alberta came back to me as soon as I read about another kind of snowline, the boundary between the inner regions of a solar system, where rocky planets tend to form, and the outer depths, which become the domain of cold, gaseous worlds. The snowline holds clues to how ‘super-Earth’ planets form.
The paper, by Grant Kennedy (Mt. Stromlo Observatory, Australia) and colleagues, takes a hard look at M-class red dwarfs and contrasts them to solar-type stars. The latter show a relatively constant luminosity during planet formation, meaning conditions change little during this era. But red dwarfs fade dramatically as they evolve toward maturity, dimming to the point where what had been a warm inner disk begins to freeze. And that has implications for planetesimals forming in the disk, especially the so-called ‘super-Earths.’ Thus Kennedy:
“It’s like a massive cold front that sweeps inward toward the star. The ices add mass to a growing planet, and also make it easier for particles to stick together. The two effects combine to produce a planet several times the size of Earth.”
Microlensing observations suggest that planets in the range of five to fifteen Earth masses might be common around M-class dwarfs, normally occurring between 2.5 and 3 AU from their parent star. The assumption is that these are ice giants like Uranus and Neptune in our own Solar System. And what Kennedy and team are proposing is that they form out of the relatively meager disk material around red dwarfs, emerging from snowstorms lasting millions of years, storms that envelop and help to create the resulting planet.
But bear in mind the distances involved when you hear the term ‘super-Earth.’ These would be icy planets with no liquid water orbiting far out of a red dwarf’s habitable zone. In fact, little planet forming material seems to exist close in to an M-dwarf, leading to the supposition that planets larger than Mercury or Mars would be unlikely to emerge there. The good news is that a Mars-size planet is perfectly suitable for the development of life in such a close-in orbit.
The paper is Kennedy et al., “Planet formation around low mass stars: the moving snow line and super-Earths.” It’s slated for publication in the The Astrophysical Journal Letters, but the preprint is already available.
And here’s Centauri Dreams‘ take: Are we certain that the protoplanetary disks around red dwarfs contain proportionately less material than those around Solar-type stars? It seems a reasonable assumption, but any play in the numbers there could drastically effect what we’ll expect to find one day around stars like Proxima Centauri or Barnard’s Star. Nailing down those numbers will be a lengthy and perhaps controversial process, and the recent discovery around GJ 849 makes the outcome look more problematic than it did a few months ago.
