by Paul Gilster | Sep 25, 2006 | Deep Sky Astronomy & Telescopes
There are images that need to be appreciated slowly, the way you sip a rare wine. They should be carried with you a while, pondered, mulled over, and sometimes, as happened to me last night, they slip into your dreams. That dream was powerful enough to push the image below into today’s entry. It’s a snapshot of 28 galaxies, all of them close to 13 billion years old, factories of star formation whose intense blue light has been red-shifted to the ancient hue we see today. All told, the astronomers doing this work have found more than 500 galaxies that existed less than a billion years after the Big Bang.
This vista follows up an earlier story in these pages describing work done by Rychard Bouwens (UC-Santa Cruz) and colleagues, who worked with the Hubble Ultra Deep Field and the Great Observatories Origins Deep Survey fields in their analysis of early galaxy formation. They believe these galaxies were producing stars at a rate ten times faster than we observe in nearby galaxies. That’s a pace that should have sufficed to reheat the cold hydrogen gas that would have existed between galaxies in the cooling that followed the Big Bang.
Image: The galaxies seen here are a sample of the most comprehensive compilation of galaxies in the early Universe. Credit: NASA, ESA, R. Bouwens and G. Illingworth (University of California, Santa Cruz, USA).
“Seeing all of these starburst galaxies provides evidence that there were enough galaxies 1 billion years after the Big Bang to finish reheating the universe,” said team member Garth Illingworth of the University of California, Santa Cruz. “It highlights a period of fundamental change in the universe, and we are seeing the galaxy population that brought about that change.”
Reheating early interstellar hydrogen would have produced, after a few hundred million years, a hot transparent plasma, a ‘brief’ transitional event that changed the fundamental character of the cosmos. But there are earlier galaxies to find, and the next step in the exploration of this distant era will be the Hubble upgrade known as the Wide Field Planetary Camera 3, an instrument sensitive enough to detect starlight so remote that it has been stretched into the infrared as the universe expands. The team will present more on this work in the November 20 issue of The Astrophysical Journal.
by Paul Gilster | Sep 23, 2006 | Advanced Rocketry |
Whenever I think about Project Orion, I recall the ‘putt-putt’ experiments that tested the propulsion concept back in 1959. It was hardly an atomic spaceship, but the little putt-putt called ‘Hot Rod’ is as far as Orion ever got operationally. Using chemical explosives, Hot Rod rose 100 meters, a brief flight that nonetheless validated the idea that a spacecraft built around nuclear bombs, propellant and a pusher plate could be made to take stable flight.
An atomic spaceship. There was a time when the idea seemed to have interstellar possibilities. Freeman Dyson, a key figure in Orion, envisioned one version that used a copper pusher plate twenty kilometers in diameter. Driving the ship would be a nuclear arsenal of staggering proportions: 30 million nuclear bombs, each of which would explode 120 kilometers behind the vehicle at intervals of 1,000 seconds.
With a total acceleration time of five hundred years—and a comparable time for deceleration—this mammoth super-Orion would carry a colony of 20,000 Earth people to Alpha Centauri. Flight time: 1,800 years, making it a true multi-generation ship, where the distant descendants of the initial crew arrive at the target to make a new start for humanity. Later Dyson would ponder a pared down version that used 300,000 bombs to reach a final velocity of 10,000 kilometers per second, with arrival at Alpha Centauri in 130 years.
These days Project Orion’s interstellar capabilities seem vastly over-rated, even though its potential for travel to the outer Solar System was very real. Dyson himself now considers nuclear options unviable for missions to another star. When I was researching my book, I asked him his current views about Orion as a way to reach Alpha Centauri. Here’s a bit of what he said:
The Orion idea was exciting, but as far as interstellar trips are concerned, nuclear energy just doesn’t cut it… Youre using less than one percent of the mass with any kind of nuclear reaction whether it’s fission or fusion. The velocities you get are limited to much less than a tenth of lightspeed. Nuclear methods are great inside the solar system but not outside; in interstellar terms, they are not very interesting.
But what a story, and if you haven’t read George Dyson’s book about his father’s work, you’re missing out on a great experience. It’s Project Orion: The True Story of the Atomic Spaceship (New York: Henry Holt & Co., 2002). Online, the ever-reliable Anthony Kendall offers up a fine account of Orion. Here Kendall describes the vehicle, which would have dwarfed any rocket ever made:
A full-size Orion vehicle would have had a mass of 4,000 tons – about 40 times that of the Space Shuttle – and would include a “pusher plate” about 1-meter thick at the center. This solid mass of metal served to reflect the Orion craft away from the nuclear explosions, while at the same time protecting the passengers from the neutron radiation. The enormous shock absorbers between the pusher plate and the crew module would then distribute the 10,000 G’s of each nuclear blast to something much more comfortable for Orion’s passengers. In fact, an Orion launch would probably be much more comfortable than a conventional chemical rocket because of the sheer mass of the vehicle.
So vast were some Orion concepts that Ted Taylor, a weapons designer who became a guiding force behind the project, once considered installing a 4000-lb barber’s chair on the ship, thumbing his nose at the piddling chemical rocket designs that measured out payload in teaspoons. But of course, those chemical payloads got larger even as political currents made the nuclear option less realistic. The nuclear test ban treaty was but one of many blows that put an end to the program. Dyson talks about all this in Disturbing the Universe (New York: Harper and Row, 1979).
Be sure to read Kendall’s account for the overview, then George Dyson’s book, a volume I could hardly put down. And if you want to follow some of the interstellar references, start with Freeman Dyson’s paper “Interstellar Transport,” in Physics Today (October, 1968), pp. 41-45. The drama of Orion’s demise is told in Dyson’s “Death of a Project: Research Is Stopped on a System of Space Propulsion Which Broke All the Rules of the Political Game,” Science 149, No. 3680 (July 9, 1965), p. 141. And keep an eye on an Orion descendant called External Pulsed Plasma Propulsion, which may have much to teach us still.
by Paul Gilster | Sep 22, 2006 | Outer Solar System |
The imagery from Cassini’s twelve-hour pass behind Saturn turns out to have been productive indeed. We pause from things interstellar, then, to admire the beautiful photograph below. The occultation of the Sun put the spacecraft in position to see the rings with exquisite and detail-enhancing backlighting, providing striking visual evidence for their extensive interaction with some of Saturn’s smaller moons.
Image (click to enlarge): Wispy fingers of bright, icy particles reach several tens of thousands of kilometers outward from Enceladus into the E ring, while the moon’s active south polar jets continue to fire away. Credit: Jet Propulsion Laboratory/Space Science Institute.
This is Enceladus as we’ve never seen it before, moving through a highly visible E ring, to which it appears connected by feathery strands of ice crystals. These are surely coming from the moon’s south polar geysers, another of Cassini’s remarkable discoveries. No clearer evidence that moons like this one have a role in shaping the rings through which they move could possibly be offered.
It’s the backlighting, of course, that teases out the level of detail in this image, showing microscopic particles that would otherwise be all but invisible. The E ring itself, Saturn’s outermost ring, had only been imaged in sections until now, but the occultation allowed Cassini to map its structure in great detail. Another significant find: a new, diffuse ring found outside the brighter main rings (and inside the G and E rings) that coincides with the orbits of Janus and Epimetheus. Here the process at work seems to involve debris from impacts on the two moons, a stream of fine particles that continue to feed the ring.
by Paul Gilster | Sep 21, 2006 | Exoplanetary Science |
Brown dwarfs are clearly commonplace in the galaxy, but we know all too little about them. Thus the excitement about the recent imaging of a brown dwarf that orbits its star along with a planet. More information about that dwarf, HD 3651 B, has now surfaced thanks to a preprint passed along by Massimo Marengo (Harvard-Smithsonian Center for Astrophysics), a member of the team that discovered this interesting object. As we saw several days ago in these pages, an independent team led by Markus Mugrauer (University of Jena) has also submitted a later paper announcing the same object. Clearly, this faint brown dwarf has been the subject of much scrutiny, and deservedly so.
For the primary, HD 3651, has already been subjected to radial velocity analysis, turning up a planetary companion of somewhat less mass than Saturn. Located in the constellation Pisces, HD 3651 is a bit less massive than our Sun, and the behavior of its known planet is unusual — its orbit is highly elliptical, a tip-off that a distant, low-mass companion is affecting its orbit. The research team, led by Kevin Luhman (Penn State) used the Infrared Array Camera aboard the Spitzer Space Telescope to identify that companion as a T dwarf, a cool brown dwarf star about 50 times the mass of Jupiter.
Image: This is an artist’s concept of the star HD 3651 as it is orbited by a close-in Saturn-mass planetary companion and the distant brown dwarf companion discovered by Spitzer infrared photographs. The Saturn-mass planet was discovered through Doppler observations in 2003. Its orbit is very small, the size of Mercury’s, and is highly elliptical. The gravity of the distant brown dwarf companion may be reponsible for the distorted shape of the inner planet’s orbit. Credit: NASA/JPL-Caltech/T. Pyle (SSC).
“The orbit of the planet in this system is similar to Mercury’s, but the T dwarf has an orbit over ten times larger than Pluto’s,” said Brian Patten of the Harvard-Smithsonian Center for Astrophysics (CfA), a co-author. “Although HD 3651 B would be just beyond naked-eye visibility to an intrepid astronomer living on this system’s planet, the T dwarf makes its presence known through gravity.”
A second brown dwarf was found around HN Peg, a young star in the constellation Pegasus. This one is also intriguing because the youth of the star may provide clues to the formation of both objects. HN Peg shows a previously discovered debris disk; the interactions between it and the brown dwarf should provide unusually fruitful. The two dwarf companions thus become the sixth and seventh known T dwarf companions to stars, and the first T dwarfs discovered through Spitzer.
These brown dwarf imaging studies are valuable because with knowledge of the primary star, the age, distance and metallicity of the companion can be more readily measured. As the paper comments:
HD 3651 B is a virtual twin to another T dwarf companion, Gl 570 D, in terms of spectral type, luminosity, mass, temperature, and radius… As with Gl 570 D, these measurements have relatively high accuracy because of the companionship of HD 3651 B to a nearby, well-studied star, making it a valuable brown dwarf for calibrating methods of characterizing the physical properties of late T dwarfs…
Turning up brown dwarf companions thus helps us plug in values on the frequency of companions vs. properties like mass, separation and age, all useful in developing a sound picture of system formation. Moreover, the only previously known T dwarf companions circle stars that are older than one billion years and have no known planets. These helpful discoveries underscore the importance of direct imaging for the study not just of brown dwarfs but planets as well.
by Paul Gilster | Sep 20, 2006 | Astrobiology and SETI |
Galactic catastrophism — the idea that certain kinds of cosmic events can destroy life on a periodic basis and prevent the emergence of technological civilizations — comes in a number of variants. And some catastrophe theorists believe such events don’t necessarily rule out species survival because their effects change over time. As we saw yesterday, Israeli theorist Itzhak Shechtman believes super-civilizations do arise despite the hazards of periodic extinctions, and argues that we may well find traces of their activities.
I return to Shechtman today because his paper crystallizes this interesting debate, especially when we turn to gamma-ray bursts (GRBs) as the agent of catastrophe. Shechtman examines the work of James Annis, who speculated in 1999 that although gamma-ray bursts could be deadly, their rate of occurrence declines over time. If this is the case, the universe may move into a ‘phase transition’ when the time between GRBs comes to equal the time needed for the emergence of intelligence. Suddenly the cosmos allows intelligent life the time to develop — we may ourselves live in that epoch.
But Shechtman questions Annis on numerous grounds and plugs different values into the variables he considers, with the result that he arrives at an interval between bursts that is 30 times longer than the time needed for the emergence of intelligent life. In which case, the idea of GRBs as a limiting factor loses force, and the Fermi question arises again: Where are these surviving civilizations? Another issue is that even when frequent, gamma-ray bursts need not reset the history of complex life to zero. Here’s Shechtman on this point:
Life on Earth has endured 5 major mass extinctions, 23 less devastating extinctions and many other upheavals in the last 570 million years. The most severe, the End Permian extinction, even liquidated 96% of all marine species and more than 75% of all vertebrate families, but did not succeed in eradicating life completely from this planet. Life held out tenaciously, modified, persisted and developed into its present state when it enjoys the luxury of searching for life on other planets! Thus, though the history of life on Earth is a single-case statistics, it nevertheless shows that life cannot be so easily reset to zero by ‘run-of-the-mill’ catastrophes.
What would make for a true life extinguisher? A massive star exploding into a supernova could do the trick for nearby civilizations. Shechtman uses the example of Eta Carinae, 100-150 times more massive than Sol; its explosion would probably doom life on planets within several hundred light years of the star. But such events in nearby space are presumably not common, and Eta Carinae is a solid 7500+ light years away. All this is part of the argument that life will survive and must be widespread, and that intelligent life’s engineering may be observable.
Centauri Dreams‘ take: An exploding Eta Carinae recalls a similar scenario in Richard Cowper’s wonderful Twilight of Briareus (John Day, 1974), in which a supernova plays havoc with the world’s weather. Cowper was actually the son of writer John Middleton Murry; his deft and evocative prose made stellar catastrophism a wrenchingly real concept. Whether supernovae, GRBs or any variety of more local events can reset life to zero remains a matter of conjecture, but the debate is important. Either catastrophism gives us a solution to the Fermi Paradox or, if we must discount the effect of these periodic extinctions, then the Fermi question is more pointed than ever.
The Annis paper Shechtman discusses, now available here, is “An Astrophysical Explanation for the Great Silence,” JBIS 52 (1999), pp. 19-22. An abstract of the Shechtman paper is available online.
