by Paul Gilster | Apr 7, 2005 | Deep Sky Astronomy & Telescopes |
Centauri Dreams continues to maintain that a major justification for interstellar research is the need of our species to protect itself. The record of life on earth is studded with extinction-level events evidently caused by asteroid or cometary impacts, and as technology matures, the danger of a man-made catastrophe cannot be ruled out. We know that life is fragile, as is underscored by the following story.
According to a new study from NASA and the University of Kansas, working with ‘what if’ scenarios and a finely-tuned model of Earth’s atmosphere, a gamma-ray burst from the explosion of a relatively nearby star could destroy up to half the atmosphere’s ozone layer. Remarkably, a burst that hit the Earth for only ten seconds could do the trick, damaging Earth’s only shield against powerful ultraviolet radiation from the Sun. With recovery time of no less than five years, that could have catastrophic effect on all surface species and destroy the food chain.
“A gamma-ray burst originating within 6,000 light years from Earth would have a devastating effect on life,” said Dr. Adrian Melott of the Department of Physics and Astronomy at the University of Kansas. “We don’t know exactly when one came, but we’re rather sure it did come — and left its mark. What’s most surprising is that just a 10-second burst can cause years of devastating ozone damage.”
Image: Gamma-ray bursts are the most powerful explosions known in the Universe, and most originate in distant galaxies. A large percentage of bursts likely arise from the explosion of stars over 15 times more massive than our Sun. Scientists say burst from a nearby star could cause severe damage to the Earth’s protective ozone layer. In this artists conception we see the gamma rays hitting the Earth’s atmosphere. (The expanding shell is pictured as blue, but gamma rays are actually invisible.) Credit: NASA.
Centauri Dreams‘ take: We don’t know enough about the frequency of gamma-ray bursts in the Milky Way, nor do we understand their sources well enough to assume that — in millennial terms — such a burst would not pose a future threat to life on Earth. That makes a responsible, balanced and continuing effort into interstellar research a basic insurance policy for the human future. It also underlines the need for continuing work on gamma ray-bursts and their sources.
What we know now: The bursts seem to arise from the explosions of stars at least 15 times more massive than the Sun, and may signal the birth of a black hole. They are hard to observe even though several occur each day as seen from Earth. The reason: they appear at random and last only a few milliseconds up to a minute.
The scientists estimate that it may have been such a burst that caused the Ordovician extinction 450 million years ago, which killed 60 percent of all marine invertebrates and left most life confined to the sea. That theory is the work of Dr. Bruce Lieberman, a paleontologist at the University of Kansas.
Meanwhile, the NASA-led Swift mission has measured the distance to two gamma-ray bursts coming from opposite parts of the sky, and has found that both were more than 9 billion light years away. These results are the first direct distance measurements of the Swift mission, obtained with the spacecraft’s Ultraviolet/OpticalTelescope (UVOT). Swift has only been operational since November, and it is hoped that such measurements will become routine. The mission has detected 24 bursts so far.
“Swift will detect more gamma-ray bursts than any satellite that has come before it, and now will be able to pin down distances to many of these bursts too,” said Peter Roming, UVOT lead scientist at Penn State. “These two aren’t distance record-breakers, but they’re certainly from far out there. The second of the two bursts was bright enough to be seen from Earth with a good backyard telescope.”
The good thing about Swift is that its three telescopes can study the burst afterglow, although it’s also true that not all bursts produce an afterglow, showing that our knowledge of burst sources is far from complete. As with cometary or asteroid impacts, the danger of an Earth-threatening event from a gamma-ray burst seems tiny, but that threat should be measured against what Tennyson called ‘the long result of time.’ Interstellar migration over the course of the next several thousand years could be the way mankind hedges its bets against the destruction of what is currently its only outpost.
Sources: The extinction story is discussed on this NASA page. You can read more about Swift’s recent detections in a Pennsylvania State University news release. NASA offers a backgrounder on gamma-ray bursts here. Also be aware of the Gamma-Ray Burst Real-Time Sky Map.
by Paul Gilster | Apr 6, 2005 | Outer Solar System
The mystery of Sedna’s spin seems to be solved. The enigmatic Kuiper Belt object whose orbit reaches as far as 500 AU from the Sun (and as close as 80) appeared to have an unusually slow rotation rate when first observed. Some astronomers speculated that an unseen moon could be the cause, even though the best Hubble images showed no such object.
It has taken a set of new measurements by Scott Gaudi, Krzysztof Stanek and colleagues at the Harvard-Smithsonian Center for Astrophysics (CfA) to solve the mystery. Sedna’s rotation period isn’t the previously thought 20 days, but ten hours, which is consistent with other planetoids in the Solar System. The need for the missing moon has vanished.
Not that Sedna doesn’t remain unusual. In addition to its highly elliptical orbit, the planetoid is one of the largest Kuiper Belt objects known, three-quarters the size of Pluto, or about 1,000 miles across. Another oddity: Sedna’s ruddy color, which remains unexplained.
Image: CfA astronomer Scott Gaudi and his colleagues have solved the case of Sedna’s missing moon. That distant solar system world (shown in this artist’s conception) spins more rapidly than originally thought, rotating once every 10 hours. Although Sedna is unusual in many other ways, its rotation period is normal, meaning that no moon is required to slow it down. Credit: David A. Aguilar (CfA)
From a CfA press release:
“Up until now, Sedna appeared strange in every way it had been studied. Every property of Sedna that we’d been able to measure was atypical,” said Gaudi. “We’ve shown that Sedna’s rotation period, at least, is entirely normal.”
The team used the 6.5-meter MMT telescope at Mt. Hopkins AZ to measure periodic brightening and dimming of the distant object, showing that earlier measurements had been in error. “The variation in Sedna’s brightness is quite small,” said CfA team member Matthew Holman, “and could have been easily overlooked.”
You can read a preprint of the team’s work, “On the Rotation Period of (90377) Sedna,” at the ArXiv site. The paper has been submitted to The Astrophysical Journal Letters for publication.
by Paul Gilster | Apr 5, 2005 | Exoplanetary Science
The planetary systems so far discovered around other stars have generally been dominated by huge, gas giant worlds, many with so-called ‘hot Jupiters’ that orbit extremely close to their parent star. And that makes sense, given that a major method used for detecting exoplanets relies upon the star’s ‘wobble’ as it is influenced by such high-mass objects. We’re not yet at the point where Earth-sized planets can be found, although we’ve reduced the detection size down to Neptune-class objects, with better things to come.
But don’t assume that even the systems discovered so far are without terrestrial planets. As many as half of them may harbor habitable worlds, according to work by Barrie Jones, Nick Sleep, and David Underwood at the Open University in Milton Keynes (UK), which was presented today at the Royal Astronomical Society National Astronomy Meeting in Birmingham. The team used computer models to analyze the gravitational effects of known exoplanets on other, undiscovered worlds in their solar systems. They then inserted an Earth-like planet into the mix to see if it survived in the star’s habitable zone, where water can exist at the surface.
The results showed the location of two ‘disaster zones’ where the gas giant planet would cause the terrestrial world either to collide with it or the parent star, or be ejected from the planetary system altogether. With these rules in hand, the model was then run against all known exoplanetary systems. The result:
They discovered that about half of the known exoplanetary systems offer a safe haven for a period extending from the present into the past that is at least long enough for life to have developed on any such planets. This assumes that “Earths” could have formed in the first place, which seems quite likely.
You can read more in this Royal Astronomical Society press release. Interestingly, these numbers go up if you take into advantage the changes to the habitable zone as the star ages, allowing outer planets a chance to develop life (as recently discussed in Centauri Dreams).
Again from the press release:
These scenarios of past extinction and future birth increase to about two-thirds the proportion of the known exoplanetary systems that are potentially habitable at some time during the main-sequence lifetime of their central star.
The paper in which this work will be presented is Barrie W. Jones, David R. Underwood, and P. Nick Sleep, “Prospects for habitable ‘Earths’,” which is scheduled to appear in the 1 April 2005 issue of The Astrophysical Journal. The RAS meeting Web site is here.
by Paul Gilster | Apr 4, 2005 | Breakthrough Propulsion
When Freeman Dyson recently addressed Flight School (a part of the PC Forum technology conference held in Scottsdale AZ in March), he cited one key driver for getting people into space in a big way: propulsion. “What you need,” Dyson said, “is a launch system that stays on the ground.”
A case in point that Dyson favors is laser propulsion, as exemplified in the ‘lightcraft’ concept of Rensselaer Polytechnic Institute’s Leik Myrabo. Back in October of 2000, a small test model of Myrabo’s design rose to a height of 233 feet, powered by a 10-KW pulsed carbon dioxide laser. Beamed energy means that future, full-scale versions of such technology will need only a small amount of on-board propellant, sharply reducing the mass of the vehicle.
The lightcraft models created so far reflect the laser beam from a parabolic mirror on the underside of the vehicle to superheat air to a temperature roughly ten times that of the surface of the Sun. The air explodes and propels the craft into motion, with each new laser pulse driving it further. Running out of usable air as it pushes toward the edge of space, a future full-sized vehicle would switch to an on-board supply of liquid hydrogen for propellant.
Dyson told the Arizona crowd that a full-scale lightcraft — one capable of launching people in a Volkswagen-sized vehicle — would need a 1,000 megawatt laser (the Airborne Laser system created by Boeing, Lockheed Martin, and Northrop Grumman for the U.S. Air Force, by comparison, involves a 5 to 10 megawatt-class chemical laser).
Image: A lightcraft under laser propulsion. Note the bright ring of superheated air under the ring surface. Credit: Rensselaer Polytechnic Institute.
But Myrabo knows bigger lasers are coming. His goal is a thousand-fold reduction in the cost of reaching orbit. When I spoke to him while researching Centauri Dreams, he was emphatic about not standing still while waiting for laser technology to catch up:
There is a way to make significant progress with flight platforms using alternative fuels, waiting for that big laser. So I’m exploring alternatives. One other problem is dynamic stability control issues, which we have to address, but I think the future is wide open. And I don’t see any other propulsion technology that’s really in the running for extremely reliable and cheap access to space. Single stage to orbit is very difficult to do with chemicals.
On beamed propulsion, see the classic paper by Arthur Kantrowitz, “Propulsion to Orbit with Ground-Based Lasers,” in Astronautics and Aeronautics 10, no. 5 ( May 1972), pp. 74-76. Also useful for background is Leik Myrabo and Dean Ing, The Future of Flight (New York: Baen Books, 1985). Myrabo’s Lightcraft Technologies Web page is rarely updated, but may be worth the occasional check. “I don’t talk about what I’m doing until I do it,” the scientist says. Finally, be aware of the article “Orbital Oddballs: Unusual Ideas for Future Space Travel,” which ran last November in Strange Horizons.
by Paul Gilster | Apr 2, 2005 | Exoplanetary Science
Sky & Telescope is reporting that the purported planet around GQ Lupi may not be a planet but a brown dwarf. The magazine evidently draws this conclusion from a study of the paper by Ralph Neuhaeuser and colleagues that is to run in an upcoming issue of Astronomy & Astrophysics. “The newly released paper by Neuhauser and his colleagues suggests that the the object in question could be as much as 42 Jupiter masses. Brown dwarfs are, by definition, between 13 and 74 Jupiter masses,” the magazine reports.
Meanwhile, data on the possible GQ Lupi planet can be found at the Extrasolar Planets Encyclopedia. The preprint of the Neuhaeuser paper, “Evidence for a co-moving sub-stellar companion of GQ Lup,” is available here. A key excerpt from the paper:
The most critical point in the mass determination of the companion (candidates) of GQ Lup and 2M1207 are the models, which may be off by an unknown factor for low ages (few Myrs); they need to be calibrated, before the mass of such companions can be determined confidently.
Image: VLT-NACO Ks-band image of GQ Lup and its 6 mag fainter companion candidate 0.7325 ± 0.0034?? west, as found in Neuhaeuser et al.
Sky & Telescope goes on to quote Ben Zuckerman (University of California, Los Angeles) on this point:
“The plus or minus 1 Jupiter mass seems like an incredibly tiny error bar to me,” he says. Noting that he has not yet seen Neuhauser’s paper, Zuckerman points out that the planet’s estimated mass must be based on theoretical models and the host star’s estimated age, both of which are fraught with uncertainty for such a young star.
Centauri Dreams‘ take: Zuckerman’s concerns seem valid, and he makes a further point that is telling: both star and ‘planet’ are young enough that they remain deeply enshrouded in a cloud of gas and dust, making calculations of the fainter object’s true luminosity difficult. Since such calculations affect the object’s mass estimate, it is too early to consider this a confirmed image of a planet.
