by Paul Gilster | Feb 9, 2005 | Outer Solar System
Only time and energy for an abbreviated post today — I’m down with the flu! But Cassini comes through in the pinch. Below is a spectacular image of Mimas seen against the blue northern latitudes of Saturn.
From the JPL description (more of which can be found here):
Mimas drifts along in its orbit against the azure backdrop of Saturn’s northern latitudes in this true color view. The long, dark lines on the atmosphere are shadows cast by the planet’s rings.
Saturn’s northern hemisphere is presently relatively cloud-free, and rays of sunlight take a long path through the atmosphere. This results in sunlight being scattered at shorter (bluer) wavelengths, thus giving the northernmost latitudes their bluish appearance at visible wavelengths.
At the bottom, craters on icy Mimas (398 kilometers, or 247 miles across) give the moon a dimpled appearance.
Image credit: NASA/JPL/Space Science Institute.
by Paul Gilster | Feb 8, 2005 | Breakthrough Propulsion, Deep Sky Astronomy & Telescopes |
Most readers of Centauri Dreams will be familiar with SETI@home, the huge distributed computing project that taps the power of millions of PCs to process data from the Arecibo radio telescope. Distributed computing offers vast amounts of processing power, and it’s the cornerstone of a new project called Einstein@home, which has been created to apply the same kind of computing muscle to the study of gravitational waves.
The Laser Interferometer Gravitational Wave Observatory (LIGO) is behind this project, which will launch in February. Part of Einstein’s general theory of relativity includes the prediction that gravity waves should permeate the universe. Researchers at LIGO are looking for hard data to prove the prediction, using sites in Louisiana and Hanford, WA. You an read more about the background of the project in this Nature.com article. A fine backgrounder on gravitational waves is available here.
What exactly is LIGO looking for? A cosmic source that creates regular waves of gravitational energy. From the Einstein@Home Web site:
Gravitational waves are ripples in the fabric of space and time produced by events in our galaxy and throughout universe, such as black hole collisions, shockwaves from the cores of exploding supernovas, and rotating pulsars, neutron stars, and quark stars. These ripples in the space-time fabric travel toward Earth, bringing with them information about their origins, as well as invaluable clues to the nature of gravity.
For the purposes of the study, the most likely sources would be dense, rapidly rotating stars such as neutron stars or the still more elusive quark stars. Some of these objects may not be precisely spherical, which could lead to their emitting significant gravitational waves.
Image: LIGO’s Hanford WA facility. Credit: Laser Interferometer Gravitational Wave Observatory.
And like the SETI search, the problem is that vast amounts of data need to be combed through to find the possibly significant vibration that really is a gravitational wave rather than interference. LIGO needs supercomputing-style power and simply doesn’t have it, which is where the resources of millions of networked machines come into play.
Interesting to see that Einstein@Home has enlisted the help of David Anderson, who developed the SETI@Home software — the latter project has provided computing power far in excess of any supercomputer ever built to the study of extraterrestrial radio sources.
by Paul Gilster | Feb 7, 2005 | Sail Concepts
If we’re looking for an operational solar sail mission that is within our current capabilities — as Colin McInnes discusses in the quote from his book in yesterday’s entry — GEOSTORM seems just the ticket, and indeed, McInnes has contributed significantly to its design and orbital dynamics. The mission was first conceived at Goddard Space Flight Center and proposed to the National Oceanic and Atmospheric Administration (NOAA) in the 1990s. NOAA requested a mission concept study from the Jet Propulsion Laboratory in 1996.
GEOSTORM is conceived as a warning system for geomagnetic storms, which are the result of violent events that release plasma from the solar corona. Predicting them is important because they can affect satellite communications and damage geostationary spacecraft, as well as wreaking havoc with power grids on Earth. But GEOSTORM is also a mission that could advance the state of the art in solar sails as we look toward future deep space missions, including probes to the near interstellar medium outside the heliosphere.
Already, non-sail satellites like the Advanced Composition Explorer (ACE), launched in 1997, have proven useful in providing solar wind data. ACE flies in a ‘halo’ orbit around the L1 Lagrange point some 1.5 million kilometers Sunward from the Earth. L1 is a natural point of equilibrium where the gravitational forces of the Sun and the Earth are in balance. This allows a stable position for ACE with a continuous view of the Sun.
GEOSTORM would move significantly closer to the Sun, thus providing much better monitoring of solar activity and longer warning times for geomagnetic storms. But stabilizing the payload well Sunward of the L1 point at suborbital speeds is something only a solar sail can do. That seems to fit McInnes’ notion of “…a small, low-cost and low-risk solar sail mission for which there is either no feasible alternative form of propulsion or no alternative option of comparable cost” quite nicely.
Significantly, GEOSTORM would require sail technologies that are now on the shelf. The sail would not be huge, on the order of 67 meters to the side. As for materials, McInnes says that the sail concept “…allows the use of commercially available 7.6 µm Kapton film for the sail substrate, and does not require the manufacture of specialized thin films.” (McInnes, Solar Sailing: Technology, Dynamics and Mission Applications (Chichester UK: Springer/Praxis, 1999), p. 233).
What’s ahead for GEOSTORM? Before any operational mission can fly, we need to evaluate thrust performance, attitude control, structural dynamics and a host of other issues in Earth orbit. You can view NASA’s 15-year solar sail roadmap here. Also be aware of Inflatable Structures Taking to Flight, an article in Aviation Week & Space Technology, available on the Web site of solar sail researcher L’Garde. A useful backgrounder on NASA solar sail work is An Overview of NASA’s Solar Sail Propulsion Project, by Gregory Garbe and Edward Montgomery (PDF warning).
by Paul Gilster | Feb 5, 2005 | Sail Concepts
“In order to advance solar sailing, proponents need to step back from their enthusiasm which can give the mistaken impression that it is an elegant idea which should be funded for the sake of aesthetics. A cold look at the strengths and weaknesses of the technology is required in order to build a convincing case for support. In particular, it is the weaknesses of solar sailing, either real of perceived, which need to be addressed. While the obvious advantage of potentially unlimited velocity change is perhaps the greatest benefit, it is useless if the first operational solar sails fail to deploy. Historical problems with the deployment of even modest space structures can unfortunately taint solar sailing by association. Similarly, competition from solar-electric propulsion is still a threat, although the new institutional approach to advanced technologies provides a welcome opportunity for exploitation. Given these factors, it seems that what is required is a small, low-cost and low-risk solar sail mission for which there is either no feasible alternative form of propulsion or no alternative option of comparable cost. It is also a key requirement that there is an absolutely compelling mission application which will demand the development of solar sail technology to flight status. If these criteria are met, then mission planners and their political masters will be cornered into developing solar sail technology and so bring to fruition the dreams of Tsander, Tsiolkovsky and many others.”
— Colin McInnes, Solar Sailing: Technology, Dynamics and Mission Applications (Chichester, UK: Springer/Praxis (1999), p. 11.
Centauri Dreams note: Robert Forward called Colin McInnes’ book “…the reference book on solar sailing.” And that it certainly is, with in-depth studies of various mission scenarios, orbital dynamics, sail configurations, performance metrics and an exhaustive background analysis of how sails work. The only down side is that few copies were printed, and those that are available are expensive (I can find only one copy on Amazon, and it runs over $70 — I paid almost $100 for mine two years ago). But if you don’t want to spend so much, find a good engineering library and check this one out. McInnes is a fine writer with a gift for clarity coupled with the insights of a seasoned aerospace engineer. He is now a professor in the Department of Mechanical Engineering at the University of Strathclyde.
by Paul Gilster | Feb 4, 2005 | Deep Sky Astronomy & Telescopes
When you hear the word ‘baryon,’ you can think of neutrons and protons, though the term really covers any subatomic particles that use the strong nuclear force for their interactions. We know a surprising amount about baryons in the early universe, including the fact that a large fraction of their number — almost half — cannot be accounted for by current theory. What happened to the missing baryons?
A paper in the February 2005 issue of Nature may shed some light on the matter. Using computer simulations of galaxy formation, Fabrizio Nicastro of the Harvard-Smithsonian Center for Astrophysics and colleagues write that the baryons could well be contained in ‘warm-hot intergalactic matter’ (WHIM), clouds of gas out of which galaxies and galactic clusters first formed.
This work was based on observations made by the Chandra X-ray satellite on the quasar Markarian 421 (located in Ursa Major, the Big Dipper). A key player in these investigations was Ohio State associate professor of astronomy Smita Mathur, who gathered the first evidence about the composition of the gas. Markarian 421 was the light source Mathur needed to analyze the clouds — its light shone directly through them. This allowed the team to take X-ray spectra of the gas that produced the new findings.
Have we really found the missing baryons? The spectrum says yes. “”This is such a wonderful spectrum that there is just no doubt about it,” Mathur said. Made of carbon, nitrogen, oxygen and neon, the clouds maintain a temperature near 1 million degrees Celsius in a band some 2 million light years thick.
Image: The active galaxy Markarian 421, one of the brightest quasars known. Credit: Aimo Sillanpaa (Nordic Optical Telescope).
Factoring such clouds into new estimates of the baryon count proves consistent with the missing baryon puzzle. So the baryons seem to be out there, only in a gas that is so hot that it shines in high-energy X-rays instead of visible light, making it impossible to see with optical telescopes. Left unanswered is the question of how the baryons got to be where they are. The current candidate: dark matter, which some believe provides a kind of gravitational structure into which normal matter flows on a galactic scale.
The Nature paper is Nicastro, Zezas, Mathur et al, “The far-ultraviolet signature of the ‘missing’ baryons in the Local Group of galaxies.” Nature 421 (13 Feb 2003) 719 – 721.
