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A New Probe of Dark Energy

by Paul Gilster | Apr 29, 2006 | Deep Sky Astronomy & Telescopes | 2 Comments

What we know about dark energy can be pretty much summed up in these words: "We know that it dominates the universe. In fact, it comprises an estimated 73 percent of the universe, while so-called dark matter accounts for 23 percent, and matter of the familiar kind — the stars, galaxies, all known life — comprises only four percent." The speaker is David Lambert, a University of Texas at Austin astronomer and the director of UT's McDonald Observatory. And it has always seemed to Centauri Dreams that these numbers -- what we know and see of the universe is no more than four percent of the total -- should inspire a certain humility. Yes, we know more than those before us, but just how far we are from comprehending the nature of 'reality' seems obvious. There are real reasons to wonder whether the human mind is capable of ever understanding ultimate reality. Perhaps a quote from Martin Rees about the beginning of the universe is appropriate: "There are lots of ideas of what...

Cometary Breakup Continues

by Paul Gilster | Apr 28, 2006 | Outer Solar System | 1 Comment

73P/Schwassmann-Wachmann is proving to be a far more interesting object than first anticipated. The comet is closing toward the Sun and will swing around it on June 7, passing the Earth along the way at a distance of 11.7 million kilometers. The fascination comes from watching its ongoing disintegration, which has broken the comet into more than 30 separate fragments. Nor is the show over. The larger fragments appear to be continuing their breakup. In the image below, taken by the Hubble Space Telescope, you can see one of the major fragments breaking into smaller chunks, dozens of which trail behind the main piece. The chunks are evidently pushed back along the tail by outgassing from their Sun-facing surfaces, and the smallest of them seem to be dissipating completely over a multi-day period. Image: The second image from a three-day observation with Hubble showing the breakup of Comet 73P/Schwassmann-Wachmann 3's Fragment B. Credit: NASA, ESA, H. Weaver (APL/JHU), M. Mutchler and...

Via Solar Array to the Outer Planets

by Paul Gilster | Apr 27, 2006 | Advanced Rocketry | 6 Comments

New Scientist is covering the work of Rudolph Meyer (UCLA), who envisions a vehicle that sounds for all the world like a cross between a solar sail and an ion engine. And in a way, it is: Imagine a flexible solar panel a solid 3125 square meters in size, and imagine this 'solar-electric membrane' weighing no more than 16 grams per square meter, far lighter than today's technology allows. I'll be anxious to see the paper when it's published in Acta Astronautica, but the gist of the design seems to be this: the solar membrane would power an ion engine array which, conventionally enough, draws xenon ions through a powerful electric grid to create thrust. The membrane, stabilized by additional ion engines at the corners, could reach remarkable speeds. Meyer talks about 666,000 kilometers per hour -- that's one year to Pluto, and an obvious invitation out into the Kuiper Belt. No show stoppers here, but clearly a design heavily dependent on advances in thin film arrays. I always listen to...

A Powerful Resource on ‘Hot Jupiters’

by Paul Gilster | Apr 27, 2006 | Exoplanetary Science | 0 Comments

That oh so interesting planetary system around 51 Peg continues to fascinate exoplanet hunters. After all, this was the first planetary discovery around a main sequence star, and the formidable dimensions of the radial velocity dataset accrued before and since the discovery may lead to other planets in the same system. For the latest on 51 Peg, check the online proceedings of last August's colloquium, in which eighty astronomers provide their thoughts on 'hot Jupiters' and discuss recent observational facts about this intriguing system. Centauri Dreams is always delighted to see conference proceedings made available online. The only catch here is that the format is PDF -- yes, it's a standard of sorts, but there has to be a better way...

Ultraviolet Limits on Habitable Worlds

by Paul Gilster | Apr 26, 2006 | Exoplanetary Science | 2 Comments

Habitable zones, and our idea of what constitutes them, change over time. We know, for example, that the habitable zone around a given star should migrate outward as main sequence stars become brighter with age. Thus the notion of the 'continuously habitable zone' (CHZ) has emerged, the region where a planet remains in habitable conditions for a specified period of time. If you want to look for technological civilizations, that time frame might be 4 billion years, paralleling the experience of life on our own planet. If you're content to look for microbes, as little as a billion years might suffice, perhaps less. Among the numerous factors involved in creating the CHZ, ultraviolet radiation is significant. A new paper points out the need to assess UV and the limits it places upon emerging biospheres. Get too much of it and you inhibit photosynthesis, as well as damaging DNA and various proteins. Get too little and you dampen a primary energy source for the synthesis of biochemical...

Modeling Black Hole Mergers

by Paul Gilster | Apr 25, 2006 | Deep Sky Astronomy & Telescopes | 3 Comments

Any guesses as to what the most powerful event in the universe is? According to a team of NASA scientists working with breakthrough computer modeling techniques, it's the merger of two massive black holes. When the event occurs, gravitational waves muscle out from the collision site at the speed of light. Each such merger generates more energy than all the stars in the universe combined. John Baker at Goddard Space Flight Center is lead author of the paper, which appears in the March 26 issue of Physical Review Letters. Of course, although the modeling works (via the largest astrophysics calculations ever performed on a NASA supercomputer), one problem is that gravitational waves have yet to be detected directly. But there is reasonable expectation that that will change through observations from the Laser Interferometer Gravitational-Wave Observatory and the joint NASA/ESA Laser Interferometer Space Antenna, a proposed mission that should be able to do the job. Einstein's theory of...

Eberhardt Rechtin: Death of a Pioneer

by Paul Gilster | Apr 24, 2006 | Communications and Navigation | 2 Comments

Centauri Dreams is saddened to learn of the death of Eberhardt Rechtin, a pioneer of deep space exploration. The list of his accomplishments is long: Rechtin served as CEO of Aerospace Corporation (El Segundo, CA), as chief engineer of Hewlett-Packard, and as director of the Defense Advanced Research Projects Agency. But his career is best remembered for his work on the Deep Space Network that today allows us to track probes at the edge of the heliosphere. Building that network was a towering achievement, and one that seemed unlikely in the 1950s, when Rechtin developed Microlock, a system of receivers that, when deployed at different ground stations, could compensate for the shifting frequencies produced by rockets in flight. Those of us who remember the days right after Sputnik may recall that there was an intense effort to expand America's ground tracking capabilities thereafter. Microlock was at the heart of it, expanding to the system that tracked the first US satellites. The...

A Boost for Innovative Interstellar Explorer

by Paul Gilster | Apr 22, 2006 | Advanced Rocketry, Missions | 9 Comments

The Innovative Interstellar Explorer mission discussed recently in these pages has received new support in a study of alternative propulsion concepts. IIE, you may remember, would use radioisotope electric propulsion (REP), tapping xenon as propellant. The mission's goal is to deliver a scientific payload to 150-200 AU within a 15 to 20 year time frame; the concept thus tracks earlier mission concepts built around solar sails and allows useful comparisons beween the various propulsion methods that have been proposed for such deep space work. In a paper to be published as a chapter in a book on NASA 'Vision' missions this summer, Thomas Zurbuchen (University of Michigan) and a team of researchers discuss the specifics of powering such a probe by nuclear methods and find them wanting. The paper is so rich that I want to discuss several issues from it in coming weeks. For now, though, let's consider the propulsion dilemma as seen by scientists running the numbers using existing...

Three Planets, and a New Category

by Paul Gilster | Apr 21, 2006 | Exoplanetary Science | 3 Comments

Three new planets have just been announced, but contrary to expectations, the number of planetary detections has not been picking up in recent times. The peak rate was 34 planetary discoveries in 2002, with the years since showing about 25 planets per year. There are a number of reasons for the slowdown, among them the fact that readily detectible short-period planets for the most obvious candidate stars have already been detected, and it will take years for enough data to accumulate to snare the presumably numerous outer planets in these systems. Greg Laughlin's Systemic site provides the details. But we do have a nice set of three new worlds delivered to us courtesy of the radial velocity method, which remains the primary detection scheme as we tune up transit and microlensing searches. HD 224693 offers a 0.7 Jupiter mass world in a 27-day orbit and HD 33283 shows a Saturn-class world in an 18-day orbit. HD 86081 is the most intriguing; it seems to have a 1.5 Jupiter mass planet in...

Antimatter’s Advantages (and the Catch)

by Paul Gilster | Apr 20, 2006 | Antimatter | 6 Comments

One of the beauties of antimatter is its efficiency. A fission reaction uses up about 1 percent of the available energy inside matter, whereas the annihilation of antimatter and matter converts 100 percent of the mass into energy. No wonder tiny amounts of antimatter can have such powerful effects. Put a gram of matter together with a gram of antimatter and you release the equivalent of a 20 kiloton bomb, about the size of the one that destroyed Hiroshima. And if you really want to see antimatter's potential, consider what it does to mass ratios, which compare the weight of a fully fueled spacecraft with that of an empty one. In his book Mirror Matter: Pioneering Antimatter Physics (New York: John Wiley & Sons, 1988), Robert Forward spoke of antimatter-driven spacecraft with mass ratios of 5 to 1 (by contrast, the Apollo missions operated with a ratio of 600 to 1). Indeed, Forward believed that a 1-ton probe to Alpha Centauri would require roughly four tons of liquid hydrogen and...

A Practical Positron Rocket?

by Paul Gilster | Apr 19, 2006 | Antimatter | 222 Comments

Antimatter seems the boldest -- and newest -- of propulsion concepts, but in fact Eugen Sänger's work on the uses of antimatter in rocketry goes back to the 1930s. The German scientist thought it would be possible to reflect gamma rays produced by the annihilation of electrons and positrons to produce thrust. His work wowed the Fourth International Astronautical Congress in 1952, but there was a catch: the gamma rays created by this reaction seemed too energetic to use the way Sänger hoped -- they penetrated all known materials and could not be channeled effectively into a rocket exhaust. Which is why most antimatter designs since have focused on antiprotons. When antiprotons and protons annihilate each other, they produce not only gamma rays but pi-mesons, short-lived particles also known as pions. Many of these are charged as they emerge from the proton/antiproton annihilation, and can therefore be controlled by sending them through a strong magnetic field. Early designs by Robert...

A Targeting Strategy for Optical SETI

by Paul Gilster | Apr 18, 2006 | Astrobiology and SETI | 1 Comment

Optical SETI has generally adopted the conventions of conventional SETI by targeting nearby, Sun-like stars. It's a strategy that makes sense, but given the number of potential transmitting stars and the need for broader surveys, what we'd ultimately like to find is a strategy for optimizing our chances, a way of looking for optical signals from other civilizations that both we and the transmitting civilization could deduce. That's the challenge Seth Shostak (SETI Institute) and Ray Villard (Space Telescope Science Institute) take on in a paper called "A Scheme for Targeting SETI Observations." So what makes immediate sense as a method of star targeting? Something that is sufficiently repeititive to be used as a kind of pointer. Shostak and Villard argue for planetary transits as a way of providing temporal synchronization between distant civilizations. A transmitting society could time its signal to be sent during the transit as observed from the transmitter, or timed to arrive when...

The Riches of an ‘Empty’ Field

by Paul Gilster | Apr 17, 2006 | Deep Sky Astronomy & Telescopes | 0 Comments

The image below is merely a marker -- it leads to something far grander. For what you're looking at is a small part of a vast image of 'empty' space, made with over 64 hours of observations using the Wide-Field Camera on the 2.2-meter La Silla telescope in Chile. Rather than linking to a simple enlargement of this fragment, I've linked instead to a zoomable imaging tool set up by the European Southern Observatory, where you can roam the galaxies in any direction you please in what is called the Deep 3 Field. Image (click to use the zoom tool): Part of the Deep 3 Deep Public Survey field, showing the brightest galaxy in the field ESO 570-19 (upper left) and the brightest star UW Crateris. This red giant (upper right) is a variable star that is about 8 times fainter than what the unaided eye can see. An 'S'-shaped ensemble of galaxies is also visible in the lower part of the picture. Credit: European Southern Observatory. The Deep 3 field is located in what appears to the naked eye as...

A Key Paper from an Astounding Source

by Paul Gilster | Apr 15, 2006 | Sail Concepts | 3 Comments

Most papers about interstellar flight appear in serious venues like Acta Astronautica or the Journal of the British Interplanetary Society. The latter, in fact, has emerged as the leading arena for such discussions, and the growth of the arXiv site has brought many new ideas to light in the digital realm. It may be surprising, then, to find that the popular Astounding Science Fiction was once a key player in interstellar theory with the publication of an article that brought solar sails to the attention of the public -- and to many scientists -- for the first time. But the magazine, in the hands of the capable John Campbell, was often home to science essays, and none more prescient than this one. "Clipper Ships of Space" appeared under the byline Russell Saunders in Astounding's issue of May, 1951. 'Saunders' was in reality an engineer named Carl Wiley, who we may speculate wrote under a pseudonym to avoid any damage to his reputation -- many scientists and engineers read science...

Microlensing and Its Limits

by Paul Gilster | Apr 14, 2006 | Exoplanetary Science | 2 Comments

Recent exoplanet detections like the 'super Earth' found orbiting a red dwarf 9000 light years away have put the spotlight on gravitational microlensing. The phenomenon occurs when light from a background star is deflected by the gravity of an intervening object; in other words, one star passing quite near or in front of a far more distant one (as seen from Earth) will cause a lensing effect that can be studied. We've seen that a distant quasar can be lensed by a foreground galaxy, producing eerie, multiple images of the same quasar. But things get trickier when it comes to microlensing within our own galaxy using individual stars. We can't resolve the images created by these events with current telescopes, but the lensing does produce a measurable amplification of the distant star's light. And any planets in orbit around the intervening star can perturb that lensing effect enough to signal their presence. The beauty of this is that microlensing is sensitive to planets down to...

Updating the SETI Hunt

by Paul Gilster | Apr 13, 2006 | Astrobiology and SETI | 2 Comments

I see that SETI@home is concerned about being able to continue its matching funds program from the University of California and is actively soliciting donations. It's a terrific project, of course, and the numbers are staggering: with early expectations of raising 100,000 participants, SETI@home wound up with 5.4 million volunteers who donated 2.4 million years of processing time. A new data recorder at Arecibo and juiced up operating software make the program more potent than ever, and certainly worthy of support. Also on the SETI front is the Planetary Society's dedication of the first telescope exclusively devoted to optical SETI (OSETI). The Harvard-based observatory includes a 72-inch primary mirror that is larger than any U.S. optical telescope east of the Mississippi. Performing one trillion measurements per second and expanding existing optical searches by 100,000-fold, the new installation will search for laser signals that can far outshine the light of a nearby star even...

A Warm ‘Saturn’ Around 51 Peg?

by Paul Gilster | Apr 12, 2006 | Exoplanetary Science | 0 Comments

A brief heads-up on the ongoing work on 51 Peg, which Greg Laughlin and team are studying to see if additional planets can be found in the voluminous data. 51 Peg, remember, was the first example of an extrasolar planet being found around a main-sequence star. The dataset goes back ten years and is far more extensive than those used in most other planet discoveries via radial velocity measurements. Using the Systemic Console, Laughlin sees evidence of a possible second planetary companion to 51 Peg, and it's a beauty: a Saturn-class world in an Earth-like orbit. "Does it really exist, this room-temperature Saturn?," Laughlin writes. "Is it really out there? Do furious anticyclonic storms spin through its cloud bands? Does it have rings? Does it loom as enormous white crescent in the deep blue twilight sky of a habitable moon?" 51 Peg c is a breaktaking, beautiful thought, but Laughlin is quick to caution that this cannot yet be described as an exoplanet discovery. Plenty of work...

10th Planet a Highly Reflective Puzzle

by Paul Gilster | Apr 12, 2006 | Outer Solar System | 2 Comments

We know little enough about our system's tenth planet (once called 2003 UB313 and now unofficially called Xena), but as new observations come in, we're learning that it is a mysterious place indeed. The latest news comes from the Space Telescope Science Institute, which pegs the object's size at five percent larger than Pluto. That would make Xena roughly 2400 kilometers in diameter, a smaller world than originally thought based on how much light it reflects. Not exactly startling news, given Xena's distance from the Sun and the difficulty in observing it, but the newly confirmed size means that the planet is so reflective that it bounces back fully 86 percent of the light that reaches it. No other planet can match this, and in the entire Solar System, no object other than Enceladus is so reflective. The latter is apparently constantly re-coated with ice by its active geysers. Image: An artist's concept of Kuiper Belt object 2003 UB313 (nicknamed "Xena") and its satellite...

Astronomical Breakups in the News

by Paul Gilster | Apr 11, 2006 | Outer Solar System | 4 Comments

The sky seems to be full of interesting objects that are breaking apart. They're always worth studying, as we learned through the impacts of the famous 'string of pearls' comet -- Shoemaker-Levy 9 -- on Jupiter in 1994. For one thing, the celestial display they afford is uncommonly interesting; for another, they are a reminder of the kinds of debris that we need to track just in case any of it turns out to be on an Earth-crossing trajectory. No apparent chance of that with 60558 Echeclus, a 50-kilometer 'centaur' out beyond the orbit of Saturn. Centaurs are icy planetoids that, in the case of both 60558 Echeclus and Chiron (the first centaur to be discovered), seem to display cometary properties. 60558 Echeclus is now even more interesting with the news that a large chunk of the object seems to have broken away. The resultant dust cloud is 100,000 kilometers across as both centaur and breakaway fragment blow off dust and gas, with more coming from the fragment than the main body (and...

On Interstellar Dust and Fast Probes

by Paul Gilster | Apr 10, 2006 | Interstellar Medium, Missions | 1 Comment

Here's why we'll have to know a great deal about the interstellar medium -- the stuff between the stars -- before we ever send out probes at a substantial fraction of lightspeed. The gas and dust forming into dark, concentrated knots in the image below creates so-called 'Bok globules,' named after astronomer Bart Bok, who hypothesized their existence back in the 1940s. They're hundreds of light years in size and, when perturbed, can form concentrated pockets of gas and dust that have the potential of turning into stars. But not all of them do become stars. In the image, we're looking at NGC 281, a nearby nebula and star-forming region some 9,500 light years away from Earth in the direction of Cassiopeia. In this region, many of the dust knots seem to be dissipating before stellar formation actually occurs. You can also see the bright blue stars of a cluster called IC 1590, whose young and massive stars put out enough solar wind to energize the surrounding hydrogen gas of the nebula...

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In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).

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