by Paul Gilster | Nov 17, 2006 | Asteroid and Comet Deflection, Missions, Research Tools |
The recent National Science Foundation report recommending scaling back support for the Arecibo radio telescope raises eyebrows here. Arecibo has just been instrumental in identifying the near-Earth asteroid 1999 KW4 as a binary, one that provides useful information about the mass, shape and density of its components and hence about near-Earth asteroids in general. That’s the kind of knowledge we need as we ponder how to analyze Earth-crossing objects to prevent future planetary disasters.
But while focusing on ongoing radio astronomy work, the report gives short shrift to Arecibo’s radar capabilities, which make this kind of investigation possible. In a letter to the NSF’s Division of Astronomical Sciences, Guy Consolmagno SJ, who is head of the Department for Planetary Sciences of the American Astronomical Society, had this to say:
There is in fact only one reference to radar in the entire 78 page document, and no mention at all of asteroids. But the Arecibo radar results are key to understanding near earth object sizes, shapes, and dynamics. Besides having a central scientific importance, both of themselves and as samples derived from the main asteroid belt, near earth asteroids may represent a significant hazard to Earth and also a potential source of future resources. To decommission one of our primary tools for studying them would deal a serious blow to both our science and our safety.
That’s troublesome news, though an article by Larry Klaes in the Ithaca Times provides a needed perspective. Cornell University has managed Arecibo since 1963, so it’s useful to know that Cornell astronomers remain optimistic about its survival. Klaes quotes astronomy professor Jim Cordes on the issue:
“Cornell has no plans to close Arecibo,” said Cordes. “In fact, the NSF has provided funds to maintain the facility in the form of $5 million to conduct a high-tech paint job on the telescope. This should go far in keeping the observatory operating for another 20 years.”
Let’s hope that optimism is well founded, as it seems to be. Arecibo’s powerful radar is vital for continuing studies of near-Earth asteroids, a point driven home by NASA’s interest in a possible manned mission to such an object. An asteroid mission would obviously provide a valuable read on these survivors from the early Solar System, while also offering a useful shakedown in a near-Earth environment for new space technologies. It’s a win/win proposition, and one that builds our database as we ponder possible strategies for avoiding catastrophic impacts.
by Paul Gilster | Nov 17, 2006 | Deep Sky Astronomy & Telescopes, Exotic Physics |
Webster Cash’s New Worlds concept, a starshade and telescope mission to directly image exoplanets, may not have received NASA Discovery funding this time around, but its creator isn’t daunted. In a recent e-mail, Cash called the concept “…so robust that we aren’t even viewing this as a setback. It’s more of a lost opportunity.”
But Cash also provided an interesting speculation — how about merging the starshade with the Joint Dark Energy Mission (JDEM)? Aimed at teasing out details about the mysterious repulsive force responsible for the universe’s continuing acceleration, JDEM is in its research and development phase, with three mission concepts currently under scrutiny. All involve close study of Type 1a supernovae, objects whose known luminosity makes them ideal for measuring the universe’s expansion.
While we wait to see whether synergy develops between exoplanet imaging and JDEM, the dark energy news continues to come in. We learned a bit more yesterday, when NASA presented Hubble results showing that whatever the force is, it has been present in the universe since the earliest days. In fact, evidence from these supernovae studies shows that dark energy was a factor nine billion years ago, and that its strength has not changed with time.
The battle between gravity and dark energy was initially won by gravity as the universe’s expansion rate slowed, but five to six billion years ago the repulsive force of dark energy began to overtake it. Crucial to the work — and the evidence for this seems strong — is the assumption that the ‘standard candles’ being used, the Type 1a supernovae JDEM will also study, have not changed in the past ten billion years. That being the case, we can draw conclusions that provide us with the deepest understanding yet of the strength of dark energy and its constancy.
Credit: NASA, ESA, and A. Feild (STScI)
While researching the dark energy announcement, I was pleased to see that Clifford Johnson (USC) is now hosting a weblog of his own called Asymptotia (you’ve doubtless read him on Cosmic Variance). Johnson notes that for all the usefulness of the Hubble study, we still don’t know what dark energy is. From his post:
It could well be the repulsive force given by a simple cosmological constant (energy density of the vacuum), or it could still be due to a dynamical field, as in a quintessence model. I suppose that this new data set serves the purpose of ruling out some dynamical models, but I’d imagine that it leaves a wide class of dynamical models as still candidates.
The Hubble data are useful, in other words, and indicative of the progress we’re making in understanding the effects of dark energy, but it is remarkable that so dominant a force on the cosmic scale should remain such a mystery. Johnson’s whole post, in which he casts a somewhat skeptical eye on NASA’s announcement strategy for the dark energy story, is well worth your time.
by Paul Gilster | Nov 16, 2006 | Deep Sky Astronomy & Telescopes |
Following up on this morning’s post re cosmic rays and the early Earth comes news that the Chandra X-ray Observatory has mapped cosmic ray acceleration in Cassiopeia A, a 325-year-old supernova remnant. The map, showing that electrons are being accelerated close to a theoretically maximum rate, provides evidence that supernova remnants are major contributors of energetic charged particles like cosmic rays.
“Scientists have theorized since the 1960s that cosmic rays must be created in the tangle of magnetic fields at the shock, but here we can see this happening directly,” said Michael Stage of the University of Massachusetts, Amherst. “Explaining where cosmic rays come from helps us to understand other mysterious phenomena in the high-energy universe.”
Image: This extraordinarily deep Chandra image shows Cassiopeia A (Cas A, for short), the youngest supernova remnant in the Milky Way. New analysis shows that this supernova remnant acts like a relativistic pinball machine by accelerating electrons to enormous energies. The blue, wispy arcs in the image show where the acceleration is taking place in an expanding shock wave generated by the explosion. The red and green regions show material from the destroyed star that has been heated to millions of degrees by the explosion. Credit: NASA/CXC/MIT/UMass Amherst/M.D.Stage et al.
The remarkable thing about the image above is that the glow generated by cosmic rays is brighter than the superheated gas caught in the supernova shock waves. The data demonstrate the effects of cosmic ray acceleration and have much to teach us about how supernova remnants change over time. And I like the simile used by team member Glenn Allen (MIT), who likened the motion of charged particles as they are accelerated to the action of a pinball machine (see the caption). Are we looking at a primal force in spurring life’s evolution on nearby worlds?
by Paul Gilster | Nov 16, 2006 | Astrobiology and SETI |
New work out of the Danish National Space Center (DNSC) suggests a startling connection between star-making in the Milky Way and the evolution of life on Earth. During a period of intense star-creation that began some 2.4 billion years ago, ocean-borne bacteria went through cycles of growth and decline of an intensity never since equalled. The Danish study links this variability with incoming cosmic rays that reach Earth from exploded stars. The star-making period in question was a time of numerous supernova explosions.
To reach these conclusions, the Space Center’s Henrik Svensmark studied the record of heavy carbon in sedimentary rocks. Growing bacteria and algae in ocean waters absorb carbon-12, leaving carbon-13 to enrich the sea; the latter begins to appear in the carbonate shells of sea creatures. By studying variations in carbon-13, Dr. Svensmark can see how much photosynthesis was going on when the shell-making species were alive.
And it turns out that the biggest fluctuations in productivity coincided with high star formation periods as well as cool periods in Earth’s climate. During a billion year period when star formation was slow, the Earth’s climate was warmer, cosmic rays were less intense, and productivity in the biosphere was all but unchanged. Says Svensmark: “The odds are 10,000 to 1 against this unexpected link between cosmic rays and the variable state of the biosphere being just a coincidence, and it offers a new perspective on the connection between the evolution of the Milky Way and the entire history of life over the last 4 billion years.”
Image: Cosmic radiation penetrating the atmosphere promotes the formation of clouds which have a cooling effect on Earth’s climate. Credit: Danish National Space Center.
How would cosmic rays affect biology on Earth? One possibility is a link between cosmic rays and cloud formation, the subject of recent experiments reported by the DNSC. Stronger winds during the cold periods caused by increased cloud cover would stir ocean waters and improve the supply of nutrients in surface water, creating greater fluctuations in biological activity. All this as the result of supernovae activity in nearby space.
The paper is Svensmark, “Imprint of Galactic dynamics on Earth’s climate,” Astronomische Nachrichten Vol. 327, Issue 9 (October 2006), pp. 866-870), with abstract here.
by Paul Gilster | Nov 15, 2006 | Exoplanetary Science |
Watching the population of nearby stars grow is a chastening exercise. It reminds us that even in our own stellar neighborhood, there is much we have to learn. Consider that since the year 2000, the population of known stars within 10 parsecs (roughly 33 light years) of the Sun has grown by 16 percent. That includes 20 new stars identified recently by the Research Consortium on Nearby Stars (RECONS), whose list of the 100 nearest star systems can be found here.
As you might have guessed, all twenty of the new objects are red dwarfs, and if you look throughout that 10-parsec volume, 239 of the 348 stars within it (other than our own star) are red dwarfs. That tallies nicely with earlier estimates that red dwarfs make up about 70 percent of the stars in the Milky Way, and points to the obvious fact that when you look up into the night sky, you’re getting an unbalanced look at what’s around us. None of the new stars are remotely visible with the naked eye.
Image: The binary red dwarf represented in this artist’s concept is SCR 0630-7643 AB, a system discovered and measured by the RECONS survey. The measured separation of the two stars is 0.90 arcseconds; at a distance from Earth of 8.8 parsecs (28.7 light-years) as obtained by RECONS, this equates to 7.9 Astronomical Units between the two, a bit less than the distance between the Sun and planet Saturn. The orbital period of the red dwarfs is roughly 50 years. Credit: Zina Deretsky/National Science Foundation.
RECONS has been working with telescopes at Cerro Tololo Inter-American Observatory (CTIO) in Chile to make observations using parallax, measuring the change in apparent motion of a nearby star as the Earth revolves around the Sun. That’s a method useful only for relatively nearby objects, of course, but I was surprised to learn that if you extend parallax measurements over several years, you can reach an accuracy of better than 10 percent out to 300 light years.
But what I like best about this announcement is a quote from RECONS Project Director Todd Henry (Georgia State University): “We expect to announce more systems within 10 parsecs in the future. The pool of nearby stars without accurate parallaxes is nowhere near drained.” Thus the essential cataloguing continues. RECONS is all about finding not just red dwarfs but brown dwarfs as well in the vicinity of the Sun. In terms of stellar classifications, that means type M (red dwarfs) and types L and T, covering the known range of brown dwarfs.
It goes without saying that close-by stellar systems make interesting hunting grounds for exoplanets; fine-tuning the target list is crucial to the success of future space-based searches. The paper is Henry et al., “The Solar Neighborhood. XVII. Parallax Results from the CTIOPI 0.9 m Program: 20 New Members of the RECONS 10 Parsec Sample,” Astronomical Journal 132:2360-2371 (December, 2006), with abstract available online.
