by Paul Gilster | Nov 15, 2010 | Missions |
After all this time, I’m still trying to wrap my head around the idea of massive objects in space as lenses, their distortion of spacetime offering the ability to see distant objects at huge magnification. On Friday we saw how the lensing effect caused by galactic clusters can be used to study dark energy. And consider the early results from the Herschel-ATLAS project, conducted by ESA’s Herschel Space Observatory. Herschel is scanning large areas of the sky in far-infrared and sub-millimeter light. Many of its brightest sources turn out to be magnified by gravitational lenses, where light from a very distant object passes a galaxy much closer to the Earth, bending that light so that the image of the more distant galaxy is magnified and distorted.
Because Herschel has only covered one-thirtieth of the entire Herschel-ATLAS survey area, it’s likely that the project will uncover hundreds of gravitational lenses, offering astronomers the chance to probe galaxies in the early universe that would otherwise be hidden. Thus we learn about the evolution of galaxies from a time when the universe was only a few billion years old, not to mention the possibilities of studying dark matter and its effect on galactic lensing.
Lensing on the Small Scale
Closer to home, we’ve talked a lot in these pages about using the Sun’s gravitational lens for studies of everything from the cosmic microwave background (CMB) to exoplanets around nearby stars. The Sun’s lens is at 550 AU, but the focal line extends to infinity, meaning the spacecraft keeps moving outward while continuing its observation program. In fact, getting beyond 550 AU is a good idea, because it progressively diminishes the problem of solar coronal distortions.
The Tau Zero Foundation is continuing to advocate Claudio Maccone’s FOCAL mission, which would be the first attempt to get a spacecraft to our own Sun’s gravitational lens. In a recent visit, Maccone and I discussed the paper on FOCAL he had delivered at the International Astronautical Congress in Prague. Just as EPOXI’s second cometary pass has shown us how much a spacecraft’s mission can be extended by intelligent marshaling of its resources, so a mission to 550 AU offers up an entirely new set of observations as the vehicle continues to move outward from the Sun. These observations would be progressively more difficult, but they are worth examining for a potential mission trajectory into interstellar space.
For Maccone realized that even as observations of the Sun’s gravitational focus proceeded, a successor to the FOCAL spacecraft could, as it pushed ever deeper into space, tap the lenses of individual planets. The question of planetary gravitational lenses has come up on Centauri Dreams before, and Maccone has now gone into the specifics. If we must reach a minimum of 550 AU to make use of the Sun’s lens, how far do we travel to tap the lenses of the planets?
A Widening Series of Focal Spheres
Jupiter is the most massive planet, and we find that its focal sphere is about 1.1 light months out, or 6100 AU. That’s a useful number to remember, because it’s always possible that the Sun’s coronal effects may distort what we’re trying to look at on the other side of the Sun. If that is the case, we still have a lens at 6100 AU, and that becomes an obvious next target. Beyond this, Neptune’s focal sphere appears at 13,525 AU (2.6 light months). The fact that Neptune is next in line is due to the surprisingly high ratio of the square of its radius to its mass — Maccone demonstrates that the ratio of radius squared to mass is the key factor in this analysis.
Thus Saturn’s lens effect actually comes into play beyond Neptune’s, at 14,425 AU despite the difference in planetary size. As you see, we are now deep into the Oort Cloud, at a distance from the Sun farther than that of Proxima Centauri’s distance from Centauri A and B. Remarkably, the focal sphere of the Earth is found at 15,375 AU, closer than the focal sphere of Uranus, the point being that Earth is the body with the highest density (ratio of mass to volume) in the entire Solar System. Getting to the Earth’s gravitational lens would be useful because we know the composition of our planet’s atmosphere and surface better than that of any other planet. We would thus have maximum data for using its lens for observations.
Image: The complete BELT of focal spheres between 550 and 17,000 AU from the Sun, as created by the gravitational lensing effect of the sun and all planets, here shown to scale. The discovery of this belt of focal spheres is the main result put forward in this paper, together with the computation of the relevant antenna gains. Credit: C. Maccone.
A good part of Maccone’s presentation on the matter goes into the question of effective ‘gain’ — Maccone calculates numerical values for the gains at five selected frequencies, from the hydrogen line (1.420 GHz) to the CMB peak at 160 GHz, and evaluates each for planetary gravitational lenses as well as the Sun’s. Clearly, the Sun emerges as our primary target given the poor gain afforded by planets like the Earth, but if future antenna technologies emerge that make it possible to study the weak signatures of the latter, a number of advantages emerge.
The Beauty of Movable Lenses
Obviously, a FOCAL mission that could reach these distances would also qualify as a cometary observer, a spacecraft that would cross the inner Oort Cloud, and that has advantages of its own. But if we can develop the technologies for such a mission, we’ll also have an interesting new take on lensing. For if we start thinking in terms not of a single gravitational lens (the Sun’s) but a series of focal spheres between 550 and 17,000 AU — a series that the spacecraft would cross as it departs our system — then we can take advantage of the fact that we now have a selection of moving targets that paint the background sky with a broader brush.
From the paper:
…while the Sun does not move in the Sun-centered reference frame of the Solar system, all the planets do move. This means that they actually sweep a certain area of the sky, as seen from the spacecraft, so that a spacecraft enjoys a sort of moving magnifying lens. How many extrasolar planets would fall inside this moving magnifying lens? Well, we don’t know nowadays, of course, but the over 400 exoplanets found to date are a neat promise that many more such exoplanets could be detected anew by a suitably equipped spacecraft crossing the distances between 550 and 17,000 AU from the Sun thanks to the gravitational lenses of the planets.
A moving, magnifying series of lenses that we study as the planets sweep out their orbits, on a mission that offers not only observations of distant astronomical phenomena but direct exploration of the Kuiper Belt and the Oort Cloud along the way. Maccone adds:
…looking back to the work done thus far about the possibilities of a truly interstellar flight, it seem fair to say that all planners of the Alpha Centauri missions, in their efforts to reach 277,000 AU, have missed what was at hand at just 17,000 AU. Or, in terms of light time, in order to get all the way to 4.37 light years in a single shot, they have missed what was just three light months away, like the Earth’s focal sphere.
Pushing FOCAL to Its Limits
This is not the FOCAL mission we have discussed in these pages before. That mission is designed to be our first exploration of the Sun’s gravitational lens, one that will demand new developments in propulsion to accomplish its task within a fifty-year timeframe, but one that in the broader scheme of things is reasonably near-term. Think of the ‘moving lens’ mission as a follow-on, a more futuristic concept, one we can consider as a motivator to develop still faster technologies and the hugely sensitive antennae needed to pull down the data, not to mention the sophisticated communications demanded to relay the information back to Earth.
These are ambitious mission ideas, but it’s by thinking about what the universe offers us by way of observation and analysis that we set our goals. From the discovery of new exoplanets to the close study of galactic and extragalactic objects, the crossing of the space between 550 AU to 17,000 AU would be profitable in ways we have not before considered. Evaluate nearby space in terms of lensing opportunities and you begin to see the Solar System and neighboring stars in a different light, one that may even have SETI implications, as we’ll see in an upcoming story.
The paper is Maccone, “A New Belt Beyond Kuiper’s: A Belt of Focal Spheres Between 550 and 17,000 AU for SETI and Science.” I’ll update this with the complete reference when the paper is published.
by Paul Gilster | Nov 12, 2010 | Exotic Physics |
Abell 1689 is one of the most massive clusters of galaxies known, making it a superb venue for the study of dark matter. That’s because the cluster, some 2.2 billion light years away, creates gravitational lensing that magnifies and distorts the light from galaxies far beyond it. Astronomers used Abell 1689 in 2008 to identify one of the youngest and brightest galaxies ever seen, a galaxy in existence a mere 700 million years after the beginning of the universe. That find, A1689-zD1, turned out to be ablaze with star formation in an era when stars were only beginning to emerge.
New Hubble studies have now used Abell 1689 yet again to make some of the most detailed maps yet of dark matter. The idea is this: The cluster’s gravitational lensing bends and amplifies the light of objects beyond it. The researchers, led by JPL’s Dan Coe, go to work on the distorted images that result, figuring out the mass it would take to produce them. If the galaxies we see in the cluster were the sole source of gravity, the distortions would be much weaker. To straighten out the images, then, requires a great deal of dark matter within the cluster.
Image: Compass and Scale Image for Abell 1689 Dark Matter Map. Credit: NASA, ESA, D. Coe (NASA, Jet Propulsion Laboratory/California Institute of Technology, and Space Telescope Science Institute), N. Benitez (Institute of Astrophysics of Andulusia, Spain), T. Broadhurst (University of the Basque Country, Spain), and H. Ford (Johns Hopkins University).
The lensing effect is powerful, with the Coe team finding 135 multiple images of 42 background galaxies at distances ranging from 7 to 12 billion light years. The map of dark matter distribution that results from this work, if verified, would represent the highest resolution depiction of a galactic cluster’s dark matter distribution yet produced. It’s a particularly interesting result because the effects of dark energy, pushing against the gravitational pull of dark matter, should have had a disruptive effect on the growth of the cluster. The results parallel studies of other galactic clusters with dense cores, leading Coe to this conclusion:
“Galaxy clusters, therefore, would had to have started forming billions of years earlier in order to build up to the numbers we see today. At earlier times, the universe was smaller and more densely packed with dark matter. Abell 1689 appears to have been well fed at birth by the dense matter surrounding it in the early universe. The cluster has carried this bulk with it through its adult life to appear as we observe it today.”
Galaxy clusters, in other words, probably formed earlier than previously thought, before dark energy could go to work to inhibit their growth. Coe’s work with mathematician Edward Fuselier has produced new techniques for calculating the dark energy map, a feat the scientist likens to ‘cracking the code’ of gravitational lensing. Adds Coe:
“Other methods are based on making a series of guesses as to what the mass map is, and then astronomers find the one that best fits the data. Using our method, we can obtain, directly from the data, a mass map that gives a perfect fit.”
The analysis method in play is called LensPerfect, described this way in the paper on this work:
LensPerfect is a novel approach to gravitational lens mass map reconstruction. The 100+ SL features produced by A1689 present us with a large puzzle. We must produce a mass model of A1689 with the correct amounts of mass in all the right places to deflect light from 30+ background galaxies into multiple paths such that they arrive at the 100+ positions observed.
Most SL [strong gravitational lensing] analysis methods construct many possible models and then iterate to find that which best matches the data. LensPerfect instead uses direct matrix inversion to find perfect solutions to the input data. Using LensPerfect, we may, for the first time, obtain a mass map solution which perfectly12 reproduces the input positions of all 100+ multiple images observed in A1689.
Gravitational lens work, then, involves reconstructing the actual mass distribution based on the highly magnified and distorted images produced by the lensing. It’s no small feat, but dark matter and dark energy are among the highest priority targets for modern science. Sharpening our tools for understanding what lensing is telling us is a step toward understanding both. This work studies dark energy through matter which, though dark, is increasingly within the grasp of study because of its profound effects on spacetime at the galactic cluster scale.
More clusters are to be studied in the same way through the Cluster Lensing and Supernova survey with Hubble (CLASH) program, which will examine 25 clusters over the course of the next three years. Conclusive evidence of early cluster formation may help us put some boundaries on dark energy in the early universe. Let’s hope so, for a universe of which we see and understand a mere four percent (the rest being dark matter and dark energy) is a challenge that energizes the very heart of physics.
The paper is Coe et al., “A High-Resolution Mass Map of Galaxy Cluster Substructure: LensPerfect Analysis of A1689,” The Astrophysical Journal 722 (2010), pp. 1-25 (abstract).
by Paul Gilster | Nov 11, 2010 | Outer Solar System |
Keep an eye on the EPOXI site at the University of Maryland. New images from the Hartley 2 comet encounter are coming in, some of them truly breathtaking, as is the one at left. The jets clearly visible in the image can be linked with distinct areas on the surface of the comet, the first time we’ve ever seen a comet with this degree of clarity. Image by image, the tiny comet is yielding its secrets. We now learn that spectral analysis of the material coming from the cometary jets shows it to be primarily carbon dioxide, along with dust and ice particles.
Image: This enhanced image, one of the closest taken of comet Hartley 2 by NASA’s EPOXI mission, shows jets and where they originate from the surface. There are jets outgassing from the sunward side, the night side, and along the terminator — the line between the two sides. The image was taken by EPOXI’s Medium-Resolution Instrument on Nov. 4, 2010. The sun is to the right. Credit: NASA/JPL-Caltech/UMD.
Jessica Sunshine (University of Maryland), the mission’s deputy principal investigator, puts the findings in context:
“Previously it was thought that water vapor from water ice was the propulsive force behind jets of material coming off of the body, or nucleus, of comet. We now have unambiguous evidence that solar heating of subsurface frozen carbon dioxide (dry ice), directly to a gas, a process known as sublimation, is powering the many jets of material coming from the comet. This is a finding that only could have been made by traveling to a comet, because ground based telescopes can’t detect CO2 and current space telescopes aren’t tuned to look for this gas.”
With new data arriving at the rate of 2000 images a day, we should have much more to look forward to from Hartley 2, but the amount of carbon dioxide escaping the comet has proven to be the biggest surprise so far. It’s fascinating to realize that the dry ice producing the jets we see in the images has most likely been inside the comet since the earliest days of the Solar System. The EPOXI findings are consistent with what Deep Impact found at Tempel 1 back in 2005, though mission scientist Tony Farnham (University of Maryland) explains why the Tempel 1 results were less conclusive:
“Tempel 1 was most active before perihelion when its southern hemisphere, the hemisphere that appeared to be enhanced in CO2, was exposed to sunlight. Unlike our Hartley encounter, during the flyby with Tempel 1, we were unable to directly trace the CO2 to the surface, because the pole was in darkness during encounter.”
Taking Deep Impact to a second comet, then, has paid big dividends in the Hartley 2 data, which now point to carbon dioxide rather than water as the driver for cometary surface activity here and likely on other comets. We’re learning about comets step by step — EPOXI’s mission marks only the fifth time we’ve had close-up imagery of one — but we’re finding things no ground-based telescope could show us. The montage below puts all five cometary investigations into perspective.
Image: This montage shows the only five comets imaged up close with spacecraft. The comets vary in shape and size. Comet Hartley 2 is by far the smallest and has the most activity in relation to its surface area. This jet activity can be seen extending from the comet’s surface and into its outer shell of gas and dust, or coma. This is the first time scientists have been able to link jets to the details of the surface. Credit: NASA/JPL-Caltech/UMD.
by Paul Gilster | Nov 10, 2010 | Deep Sky Astronomy & Telescopes |
“To a man with a hammer, everything looks like a nail,” said Mark Twain, one take on which is that the way we see problems shapes how we see solutions. That fact can be either confining or liberating depending on how open we are to examining our preconceptions, but in the case of Amy Mainzer (JPL), it leads to a natural way to describe a failed star. Mainzer, who is deputy project scientist on the Wide-field Infrared Survey Explorer mission (WISE), is an amateur jewelry-maker. For her, it’s easy to look at the image below and see gems. “The brown dwarfs,” says Mainzer, “jump out at you like big, fat, green emeralds.”
And that emerald below, dead center in the image, is hard to miss.
Image: The green dot in the middle of this image might look like an emerald amidst glittering diamonds, but it is actually a dim star belonging to a class called brown dwarfs. This particular object, named “WISEPC J045853.90+643451.9” after its location in the sky, is the first ultra-cool brown dwarf discovered by NASA’s Wide-field Infrared Survey Explorer, or WISE. WISE is scanning the skies in infrared light, picking up the signatures of all sort of cosmic gems, including brown dwarfs. Credit: NASA/JPL-Caltech/UCLA.
The brown dwarf in question is somewhere between 18 and 30 light years away in the constellation Camelopardalis (the giraffe), and is one of the coolest such objects known, with a temperature of roughly 600 Kelvin (326 degrees Celsius). This is one of those brown dwarfs that burn at temperatures close to a hot oven here on Earth, cool enough that it takes WISE’s infrared view from space to pick it up. In the image, we’re looking at three of the four WISE infrared channels, color-coded so that blue shows the shortest infrared wavelengths and red the longest. The methane in the brown dwarf atmosphere absorbs the blue-coded light and the faint object gives off little of the red, leaving green as the dominant color.
The best news about the new brown dwarf is that it turned up a mere 57 days into the survey mission, meaning that WISE is on track to find many more. Given how hard they are to spot, the possibility of an ultracool brown dwarf being in the Sun’s neighborhood, and perhaps closer than the Alpha Centauri stars, cannot be ruled out. In any case, mission planners think WISE will find hundreds of the objects within a few parsecs of the Sun. And who knows, we may yet find a perturbing body whose presence accounts for anomalous orbits like that of Sedna.
Remember this: Back in June, we learned that the Spitzer Space Telescope had found fourteen of the coldest brown dwarfs then known, so faint that visible light telescopes could not find them. The Spitzer study focused on a region in the constellation Boötes, whereas WISE will be looking at the entire sky. Peter Eisenhardt, a WISE project scientist at JPL, puts it this way:
“WISE is looking everywhere, so the coolest brown dwarfs are going to pop up all around us. We might even find a cool brown dwarf that is closer to us than Proxima Centauri, the closest known star. WISE is going to transform our view of the solar neighborhood. We’ll be studying these new neighbors in minute detail — they may contain the nearest planetary system to our own.”
We’ve only been able to confirm the existence of brown dwarfs for fifteen years, and we certainly aren’t through classifying them. T dwarfs are defined as being less than about 1500 Kelvin (1226 Celsius), but the colder class of Y dwarfs, found in stellar models but not yet confirmed, makes for prime hunting for WISE, which should be able to detect them. Are there more brown dwarfs within 25 light years of the Sun than normal stars? WISE should be able to tell us, and in doing so may tune up our target list for future deep space missions into the Oort Cloud and beyond.
by Paul Gilster | Nov 9, 2010 | Astrobiology and SETI |
Stephen Baxter’s “Turing’s Apples,” which originally ran in a collection called Eclipse Two (2008), is an intriguing take on SETI and the problem of extracting meaningful information from a signal. It’s a bit reminiscent of Fred Hoyle’s A for Andromeda (1962) in that the SETI signal received on Earth contains instructions for building something that may or may not pose a threat to our species. Sorting out the issue involves discussion of information theory and Shannon entropy analysis.
Say again? Best to handle this by quoting from the story. In this scene, the protagonist’s brother, who is obsessed with the signal his team has received from the direction of the Eagle Nebula and, ultimately, the galactic center, is explaining how information is being extracted from it. Shannon entropy analysis looks for relationships between signal elements. The brother goes on:
“You work out conditional probabilities: Given pairs of elements, how likely is it that you’ll see U following Q? Then you go on to higher-order ‘entropy levels,’ in the jargon, starting with triples: How likely is it to find G following I and N?
“As a comparison, dolphin languages get to third- or fourth-order entropy. We humans get to eighth or ninth.”
“And the Eaglets?”
“The entropy level breaks our assessment routines. We think it’s around thirty…It is information, but much more complex than any human language. It might be like English sentences with a fantastically convoluted structure – triple or quadruple negatives, overlapping clauses, tense changes.” He grinned. “Or triple entendres. Or quadruples.”
“They’re smarter than us.”
“Oh yes…”
And that reminds me of an old friend, an amateur linguist but an extraordinary one, who once at a memorable lunch pulled off a triple pun involving three different languages, one of which was proto-Hebraic! I had to be led through its complexities before I could begin to appreciate it (and that only after I thought about it for the rest of the day). Now that’s high-level entropy…
I had the pleasure of talking about the Baxter story with Claudio Maccone over dinner last weekend in Austin. Claudio was on his way to the SETI Institute to give a lecture on the latest work on his statistical approach to the Drake Equation. The potential encounter with intelligence far greater than our own is what accounts for the fascination of SETI. No one knows whether we are alone in the galaxy or simply outliers amidst a sea of extraterrestrial civilizations, but the engagement with the latter possibility energizes a search that is now fifty years old.
Last week saw global coordination among SETI researchers as a way of marking that 50th anniversary, with astronomers from thirteen countries on six continents observing several nearby stars including the two that started it all, Epsilon Eridani and Tau Ceti. It was in April of 1960 that Frank Drake used the Green Bank instrument in West Virginia to listen in on the latter two stars. Project Ozma drew its name from The Wonderful Wizard of Oz, so the ongoing Project Dorothy picks up the theme by associating itself with that story’s heroine. The Project Dorothy observations involve both radio and laser signals, unlike the radio-only Project Ozma.
But the project is something more than merely commemorative. If one day we do receive a signal that appears to be the real thing, we’ll need to coordinate observations on a global scale. The SETI Institute’s Douglas Vakoch makes the point:
“Astronomers can now do SETI research at observatories from South Africa to the Netherlands, from Argentina to India, from Japan to Italy, as well as from the longstanding American projects at the SETI Institute, the University of California at Berkeley, and Harvard University. The lessons learned through Project Dorothy provide critical preparation for the day we finally detect a signal from another civilization. By learning how to coordinate international SETI observations now, we’ll be better prepared to track a signal continuously, around the world, after first contact.”
This Washington Post story discusses SETI basics and the new international exercise, which includes observatories in Italy, India, Argentina, Australia, France, Germany, the United Kingdom, South Korea, Sweden, the Netherlands, and several in the United States and Japan. Behind the observations is Shin-ya Narusawa (Nishi-Harima Astronomical Observatory, Japan), who notes the value of studying Tau Ceti and Epsilon Eridani in the venture. “They remain the symbol of the project Ozma,” Narusawa said, “and so are two of the target stars for Project Dorothy.”
We have reasons for doubting either of these stars might support an extraterrestrial civilization, but for that matter we have no confirmed rocky planet in the habitable zone of any star (and yes, that includes the hypothetical Gliese 581g, whose existence seems more and more unlikely). If SETI ever does get its breakthrough, it will be because we didn’t narrow the search too drastically based on our own assumptions, but remained open to the possibility of surprise.
Read the Baxter story for one take on SETI and surprise (it’s in the 26th annual Year’s Best Science Fiction volume edited by Gardner Dozois). But if you read no other science fiction on the subject, read James Gunn’s superb The Listeners (1972), as fine a take on the wonder-injecting business of listening to deep space as has ever been penned. It was, among other things, an inspiration for Sagan’s Contact. Gunn on SETI and the human spirit:
“…perhaps it was not just the reality but the imagery, like poetry, that soothed their doubting souls, the bowl held up to catch Donne’s falling star, the ear cocked to hear the shout from the other side of the universe that faded to an indistinguishable murmur by the time it reached them. And one thousand miles above them was the giant, five-mile-in-diameter network, the largest radio telescope ever built, that men had cast into the heavens to catch the stars.”
