by Paul Gilster | Mar 7, 2007 | Deep Sky Astronomy & Telescopes |
By analyzing a carefully selected set of 544 distant galaxies, researchers are beginning to learn how galaxies take their mature forms, becoming the glorious objects we see today. Sandra Faber (University of California at Santa Cruz), puts it this way: “We are now well on our way to seeing how galaxies evolved over the last half of the age of the universe. This work is not over, but the outlines of a theory are emerging.”
The galaxies in question weigh in with redshifts in the range of 0.1 to 1.2, which translates to ‘look-back’ times of between 2 and almost eight billion years. These adolescent galaxies are far more disordered than nearby ones, but it turns out that the relationship between a galaxy’s mass and the orbital speed of its stars and gas is consistent over different types of galaxy and over billions of years of galactic evolution.
In other words, the more massive a galaxy is, the faster the stars and gas inside it move. That analysis includes a ‘dispersion component’ that adds to the disk’s rotational motion a way to analyze its disordered internal movements. The fit between stellar mass and internal velocities turns out to be tight, and it even works in the complex environment of galactic collisions and their subsequent mergers.
This new study extends and defines the so-called Tully-Fisher relation that had been used to correlate the luminosity of a spiral galaxy with its rotational speed. Now we’re correlating mass and rotation. But spirals and the remnants of collisions are only two types of galaxy. The third is the elliptical galaxy, where a different law had been observed: the more massive the galaxy, the faster the random motion of its stars. The new work pulls both regimes together and incorporates both rotation and random velocity.
Image: An irregular galaxy, the result of a galactic merger. This is one of many such galaxies studied by AEGIS (see below), whose data now reveal common ground in the principles governing the formation of spiral, elliptical and irregular galaxies. Credit: Hubble Space Telescope images taken by the DEEP2 Team, UC Santa Cruz and UC Berkeley.
Bringing ellipticals into the mix is significant. They’re largely free of star formation and made up of much older populations of stars than spiral galaxies, which show the blue light of hot, young stars. Now we learn that galaxies as diverse as these are more regular than we thought, emerging through the same set of generalized principles. What’s involved here may well date back to the Big Bang itself. Faber again:
“Galaxies began life as quantum fluctuations–tiny density fluctuations that created the seeds for the later coagulation of structure in the universe. When gravity took over, those seeds made galaxies, and we think that process is reflected in the Tully-Fisher relation.”
For more on all this, and in particular the remarkable galaxy study known as AEGIS (All-wavelength Extended Groth Strip International Survey), see the AEGIS site. A special issue of Astrophysical Journal Letters will be appearing to cover initial AEGIS results, including a paper describing the above work. It’s Faber et al., “Star formation in AEGIS field galaxies since z=1.1 . Staged galaxy formation, and a model of mass-dependent gas exhaustion,” with preprint available here.
by Paul Gilster | Mar 6, 2007 | Asteroid and Comet Deflection |
NASA is putting the number of potentially hazardous asteroids and comets at 20,000 in a report that will be released later this week, according to an AP story now circulating. And the report, reviewed at a Planetary Defense Conference in Washington yesterday, pegs the cost of finding 90 percent of these objects at $1 billion. That’s bad news for those worried about Earth-crossers. For AP quotes NASA Ames director Simon Worden: “We know what to do; we just don’t have the money.”
And as Larry Klaes wrote me this morning, “But just imagine the bill after a big space rock hits Earth.” NASA is already tracking some 769 objects in a search now described as behind schedule. From the story:
One solution would be to build a new ground telescope solely for the asteroid hunt, and piggyback that use with other agencies’ telescopes for a total of $800 million. Another would be to launch a space infrared telescope that could do the job faster for $1.1 billion. But NASA program scientist Lindley Johnson said NASA and the White House called both those choices too costly.
A cheaper option would be to simply piggyback on other agencies’ telescopes, a cost of about $300 million, also rejected, Johnson said.
“The decision of the agency is we just can’t do anything about it right now,” he added.
Deflection, which we often discuss in these pages, isn’t an option for objects we never see in the first place. Shouldn’t asteroid tracking receive higher budgetary priority? The public, perhaps jaded by alarmist rhetoric on too many fronts, seems to perceive this as only a science fiction scenario. Yet the real dimension of the threat won’t be known until we map the dangerous Earth-crossing materials and know where we stand.
by Paul Gilster | Mar 6, 2007 | Culture and Society
The seventh iteration of Philosophia Naturalis is now online at geek counterpoint. These ‘blog carnivals’ are increasingly helpful because they cluster articles of interest, and I always wind up learning about new things to read. This carnival’s find is Rob Knop (Vanderbilt), whose Galactic Interactions blog offers an intriguing entry on what he calls ‘The Greatest Mystery in All of Physics,” which turns out to be the link between gravitational and inertial mass. Another find: Cosmic Variance‘s take on relativity and why E=mc2. Which gets us into a thought experiment:
Think of a physicist, standing at one side of a large box, which itself is sitting on a perfectly frictionless surface (think of ice if you like). The physicist possesses a large cannon, which she is using to hurl heavy cannonballs across the box. What happens to the whole system?
The answer is informative and entertaining, particularly when you replace the cannon with a powerful laser. Read the rest at the site, and have fun exploring the rest of this month’s Philosophia Naturalis.
by Paul Gilster | Mar 5, 2007 | Sail Concepts
If you’re looking to shake out a solar sail design, a near-Earth asteroid (NEA) makes a tempting target. It’s relatively close and offers the opportunity of a landing and sample return. That helps us work out the age, evolution and other characteristics of a class of objects that are potentially dangerous to our planet. It’s no surprise, then, that when DLR, the German Aerospace Center, went into serious solar sail studies, it began to develop a dedicated mission via sail to one or more NEAs.
That was in August of 2000, and it built on DLR’s successful ground deployment of a square solar sail 20 meters to the side the previous December, conducted in a simulated weightless environment (see below). The DLR design is a square sail with four triangular sail segments, a valuable proof of concept in a time when little budgetary emphasis is being placed on sail designs by any of the major space agencies.
Image: DLR’s deployed solar sail, seen at the Center’s facility in Cologne. Credit: DLR.
For the asteroid mission, DLR now ponders a larger sail. Bernd Dachwald and colleagues, writing from DLR’s Institute of Space Simulation, provide an overview of such a sail in a recent paper, saying “…we consider a (70 m)² solar sail with a specific weight of about 20 g/m² to be a realistic, however still ambitious, near-term development goal.” Using such a craft, the authors believe it possible to return a sample from the NEA 1996FG3 within ten years of launch. The spacecraft would hold a payload of 300 kg including lander and return capsule (but excluding the sail assembly).
This mission, tagged ENEAS by its designers, focuses on 1996FG3 because of its scientific interest as well as its relatively accessible orbit. This NEA is a binary, with the primary body 1.4 kilometers in diameter and the secondary about a third of that. Getting to it involves, for the sake of time, a direct insertion into an interplanetary trajectory, after which the sail is deployed and oriented to follow the mission profile. At the NEA, the craft will hover in the hemisphere opposite the Sun, studying its gravitational field and deploying the lander and integrated return capsule.
A number of mission profiles are possible with this technology depending on the size of the sail and payload, with multiple NEA rendezvous and sample return missions analyzed in the paper. The authors also weigh against the sail design the possibility of using NASA’s NSTAR ion thrusters, noting that the ion option is faster but leads to larger launch costs. If the cost of longer ground operations is lower than the savings in launch cost, and if “…the mission duration plays a subordinate role with respect to cost,” the solar sail might prove to be the better option.
But here’s a key point, noted by the authors in their conclusion. No matter what the cost relationship turns out to be, “…on the way to more advanced solar sailcraft, as they are required for high-ΔV missions, the development of solar sails with moderate performance is an indispensable first stepping stone.” Because we need to gain experience with this technology just as we’re building the needed data on ion thrusters through missions like DAWN and SMART-1. So the solar sail asteroid rendezvous comes with powerful incentives.
The paper is Dachwald et al., “Multiple rendezvous and sample return missions to near-Earth objects using solar sailcraft,” Acta Astronautica 59 (2006), pp. 768-776.
by Paul Gilster | Mar 3, 2007 | Autonomy and Robotics |
If humans ever do establish a presence on Europa, it will surely be somewhere under the ice. Assuming, that is, that the ice isn’t too thick, and to learn about that we have to await further study, and probably a Galilean moon orbiter of some kind that can observe Europa up close and for lengthy periods. But assuming the ice is more than a few meters thick, it should provide radiation screening, and getting down into that presumed Europan ocean is where we want to be in the search for life.
Of course, the first undersea explorations on the Jovian moon will have to be robotic, and here we can talk about technologies under development today. NASA has funded a self-contained robot submarine called the Deep Phreatic Thermal Explorer (DEPTHX) that operates with an unusual degree of autonomy, navigating with an array of 56 sonar sensors and an inertial guidance system. Now a series of tests in Mexico at a geothermal sinkhole, or cenote, called La Pilita have tested out key components, proving DEPTHX can manage unexplored three-dimensional spaces.
Image: DEPTHX in the water at Cenote la Pilita. Credit: David Wettergreen/CMU.
What’s ahead for the technology is a much more challenging task: to explore the Zacatón sinkhole in the Mexican state of Tamaulipas in May. La Pilita seems easy by comparison. It’s about 100 meters deep, filled with overhanging rock and interesting biology. The depth of the Zacatón site is unknown. But Bill Stone (Stone Aerospace), leader of (DEPTHX) mission, sees La Pilita as a powerful proof of concept:
“The fact that it ran untethered in a complicated, unexplored three-dimensional space is very impressive. That’s a fundamentally new capability never before demonstrated in autonomous underwater vehicles (AUVs).”
Even so, don’t underestimate the challenge at Zacatón. From a Pittsburgh Tribune-Review story on the technology:
Divers have explored Zacaton for decades, but a turning point came on April 6, 1994, when cave-diving pioneers Jim Bowden and Sheck Exley strapped on scuba tanks and tried to reach Zacaton’s elusive bottom. Bowden made it to a depth of 925 feet — a world record for deep-water diving since broken — but tragedy overshadowed his feat. Exley did not return to the surface.
Using software called SLAM (Simultaneous Localization and Mapping) developed at CMU, DEPTHX maneuvered close to rocky walls at La Pilita and was able to take core samples. With Zacatón on the horizon, the robot’s ability to determine its position within 15 centimeters using sonar seems reassuring. Will technologies like this one day explore a Europan sea? Perhaps, but they’ll be just one part of a much larger challenge depending on how deeply we need to drill to reach liquid water.
And if any of this work on autonomous exploration technologies sounds familiar, it may be because DEPTHX’s software is being developed by CMU’s David Wettergreen, who was project leader for the four-wheeled Zoë robot recently tested in Chile’s Atacama desert. Think of the Atacama as a Mars analogue, while Zacatón reflects — at least in some respects — our exploratory needs on Europa. Autonomy is the key, operating unassisted in the remotest environments, and this work may one day ensure that when we do get to Europa, we’re up to the challenge.
