by Paul Gilster | Jan 26, 2006 | Exoplanetary Science
Some further thoughts on OGLE-2005-BLG-390Lb, the planet whose discovery portends numerous microlensing breakthroughs to come. Note the distance between the Sun and the red dwarf in question, which is variously reported as between 20,000 and 28,000 light years. The red dwarf is in the constellation Sagittarius and lined up between us and the center of the galaxy, a location that is a natural consequence of the microlensing method used to find the new world.
For microlensing relies upon a closer star passing in front of one farther away, thus causing the gravitational effects used to pinpoint OGLE-2005-BLG-390Lb. To maximize the chances of seeing such an event, you need to point toward the largest possible field of stars, which is exactly what the OGLE project does. OGLE stands for Optical Gravitational Lensing Experiment, a study whose telescopes scan the central Milky Way each night. The scans are fruitful, for more than 500 microlensing events are discovered every year.
Bear in mind that the original OGLE data had to be passed along to other projects, like PLANET (Probing Lensing Anomalies NETwork), a global collaboration that can take a single OGLE observation and study it in greater depth. Another key player was RoboNet, a UK operated network of robotic telescopes. And the good news from this coordinated effort, at least to this point, is that microlensing seems to imply the existence of a huge number of small, rocky planets.
“The new discovery provides a strong hint that low-mass planets may be much more common than Jupiters,” said co-author and PLANET researcher David Bennett of the University of Notre Dame. Until recently, most extrasolar planets researchers have found have been Jupiter-like gas giants. “Microlensing should have discovered dozens of Jupiters by now if they were as common as these five-Earth-mass planets. This illustrates the primary strength of the gravitational microlensing method: its ability to find planets of low-mass.”
Indeed, the possibility exists of finding planets even smaller than Earth using these methods. That makes microlensing the chief candidate for finding terrestrial worlds around other stars using today’s technology, although because of the nature of the work, they will be planets thousands of light years away. Our early imaging efforts will, obviously, be devoted to much closer targets, but in terms of establishing the frequency of rocky worlds, microlensing allows us to survey a vast population of stars. A new telescope used by the Japanese/New Zealand Microlensing Observations in Astrophysics project can, in fact, observe 100 million stars per night .
Another thought: 70 percent of the stars in the galaxy are red dwarfs, not unlike the parent star of OGLE-2005-BLG-390Lb. If we have already found a rocky world orbiting such a star, the chances of there being terrestrial-type worlds around other M-class stars seem magnified. We may indeed have to revise our thinking about red dwarfs, which could emerge as an even more likely place to look for life-bearing worlds than the far less numerous G and K-type stars.
by Paul Gilster | Jan 25, 2006 | Exoplanetary Science |
The most Earth-like planet yet found has been discovered by a team of astronomers in a collaborative effort that involved 73 scientists in 12 countries. When astronomers use the term ‘Earth-like’ in this context, they’re referring to planets whose mass is closer to Earth’s than previous discoveries. They’re also talking about surface conditions, for OGLE-2005-BLG-390Lb seems to have a rocky surface, and is only five times as massive as Earth.
And what a fascinating find from the standpoint of technique. This is no relatively nearby gas giant found through observing the radial velocity changes it caused in its star’s path through space. Instead, the new world, some 20,000 light years away, was found through microlensing, where the gravity of an intervening star magnifies the light of a more distant star. Close study of the brightening phenomenon can reveal defects that are the telltale sign of a companion object.
In this case, as the star and its planet passed in front of the background star, the light curve showed the clear signature of a planetary body, as seen in the image below. OGLE-2005-BLG-390Lb is not the first planet found through microlensing, but the previous two have been gas giants larger than Jupiter.
Image (click to enlarge): Data obtained by PLANET/RoboNet, OGLE, and MOA on the microlensing event OGLE-2005-BLG-390 together with a model light curve, showing the planetary deviation on its falling part, lasting about a day. Also shown are best-fitting models with a single lens and a binary source (long-dashed) and a single-source-single-lens light curve (short-dashed). Each point represents the brightness in a single image and the data are colour-coded in order to indicate the telescope. The regular cycle of colors shows how observing is taken over by the next telescope in turn as the night ends at each site. An enlargement of the planetary deviation is shown as an inset in the top right. Credit: European Southern Observatory.
But back to the new planet: it orbits a red dwarf star five times less massive than the Sun in roughly ten years. Astronomers working on the project calculate its surface temperatures at 220 degrees Centigrade below zero, and suspect that the world has a thin atmosphere and a rocky surface buried deeply beneath frozen oceans. On that score, the Solar System analogue to OGLE-2005-BLG-390Lb may not be Earth so much as Pluto.
A heartening conclusion now being drawn from this work is that smaller, frozen worlds like this are more common than Jupiter-class gas giants, which have tended to be, overwhelmingly, the kind of worlds found by radial velocity searches. “Indeed if Jupiter-like planets were as widespread, the microlensing method should have found dozens of them by now,” said David Bennett (University of Notre Dame, USA), a PLANET team member. PLANET, one of the groups behind the find, is a network of one-meter class telescopes stretching from Chile to Australia and South Africa.
The paper, soon to appear in the January 26 Nature, is J.-P. Beaulieu, D. P. Bennett, P. Fouqué et al., “Discovery of a cool planet of 5.5 Earth masses through gravitational microlensing” (abstract here). A Nature.com article is available, while audio and video podcasts on the discovery are online.
by Paul Gilster | Jan 24, 2006 | Culture and Society |
Science writer Larry Klaes wrote last November with a thought about New Horizons that I’ve been pondering ever since. Klaes was troubled to learn that the spacecraft — just the fifth mission ever launched that will eventually leave the Solar System — was carrying little that could communicate information about its makers. Its major relic was a CD disc with signatures collected from those who signed up at a Web site, along with an image of the mission team. Klaes added that the CD would itself would be rendered unreadable within decades or, at most, a few centuries.
Noting that the European Space Agency had included samples of 1000 human languages on its Rosetta comet probe, Klaes went on to say, “I think this is why an independent committee should be formed working with NASA and any other space agency that plans to launch probes into interstellar space to create messages/info carriers for those future vessels. This may help to avoid giving the mission teams any extra issues beyond the usual ones in making space probes, since they don’t seem all that able or interested in working on such projects.”
The Cassini probe was another case in point. It carried a DVD housing 615,000 signatures. As the launch of New Horizons approached, the lack of a genuine message for potential extraterrestrial discoverers of the spacecraft troubled other readers, among them space artist Jon Lomberg, who wrote not long after the spacecraft lifted off, “Putting unreadable CDs filled with equally unintelligible signatures on a spacecraft has never made any sense to me, apart from the ‘Kilroy was here’ motivation that makes people gouge their names into old redwoods.”
Now the Wall Street Journal has gotten into the mix in the form of a fine article by Jason Fry. The journalist takes note of precedent: both Pioneer 10 and 11 carried plaques that included diagrams of Earth’s position in the Solar System and Milky Way, along with drawings of male and female human figures. The Voyagers carried 12-inch, gold-plated copper phonograph records that held images and sounds from the natural world, human speech and other information, along with a cartridge, phonograph needle and visual instructions on how to make a player.
But as Fry notes, Jon Lomberg’s attempt to get a diamond wafer aboard the Huygens probe containing human and astronomical information never made it aboard the spacecraft. And Lomberg, who had been design director for the Voyager record, points out in the article that while a phonograph record or an ‘artificial fossil’ like the proposed Huygens wafer could be read by a species finding the spacecraft, a digital recording system like that used on the New Horizons CD is arbitrary and all but indecipherable. And even if an alien species could figure it out, the result would be 435,000 meaningless names.
Fry sums the case nicely, and with an appropriate sting in the tail:
In all likelihood, space probes will be the only things of ours that endure after our species is gone and our planet utterly changed — a few inert, pitted machines will be the sole clues that we ever existed, and the ancient messages they carry our only chance to explain who we were. It’s vanishingly unlikely that any being will ever find the Pioneers, Voyagers or the New Horizons probe in the billion-odd years during which their messages will remain readable. But though imagining such a discovery borders on an act of faith, it’s not impossible. And since it isn’t, shouldn’t the only trace of ourselves be something more substantive than an unbelievably ancient PR campaign? Don’t we owe ourselves a final testament that’s something more than space spam?
Long-term thinking, anyone? We are launching artifacts of our civilization that will last longer than any human creations ever made and we seem unable to summon the resolve to speak to the distant future they will one day reach. And if the answer is that no one is ever likely to find such spacecraft, Centauri Dreams‘ response is this: Our species does things both for tangible result and also for symbolic meaning, redefining its place in the universe by the nature of its acts. The monuments of Egypt spoke not just to later centuries but to their builders, who knowingly made a statement about life confronting a mysterious universe that resonates across the centuries. Our artifacts need to speak as well.
by Paul Gilster | Jan 23, 2006 | Sail Concepts |
If we chose to launch an interstellar probe using near-term technologies, it would almost surely be a solar sail, unfurled on a close approach to the Sun and flung outward at speeds that could get it to Alpha Centauri in perhaps 1000 years. That’s the thinking of Gregory Matloff, author of Deep Space Probes: To the Outer Solar System and Beyond (Springer, 2005) and a leading proponent of interstellar studies. Later technologies might also use sails, driven by laser or microwave or even particle beams. But as we take incremental steps toward an interstellar future, solar sails have to be developed first.
On that score, it’s interesting to note the contributions of the Deutschen Zentrum für Luft- und Raumfahrt (DLR), Germany’s aerospace research center and space agency. An early consolidation of sail studies, for example, came from a joint NASA/DLR feasibility study conducted in 1996-97. The working group recommended a demonstration mission that could help develop the basic principles of sail fabrication, packaging, storage, deployment, and control.
The DLR successfuly deployed a 20m² sail structure in December of 1999, a ground-based demonstrator that consisted of four booms made of carbon fiber reinforced plastic. In August of 2000, DLR proposed a dedicated mission to explore near-Earth asteroids (NEAs) using a solar sail-driven spacecraft called ENEAS. The mission concept was to rendezvous with an object called 1996FG3 with a payload (not counting the sail assembly) of 75kg.
Such proposals run into financial difficulties familiar to anyone who recalls NASA’s early solar sail work for a Halley’s Comet mission that never flew. Tight budgets have kept ambitious sail missions on the ground both within ESA and NASA, but concepts continue to multiply. A new paper by Bernd Dachwald and Wolfgang Seboldt at DLR proposes a mission called ENEAS-SR, a 70m² sail designed to return a sample from NEA 1996FG3 within ten years of launch, carrying a 300 kg payload including lander and sample return capsule.
Why, other than to shake out a new technology, should we go to a near-Earth asteroid? In addition to pure science, NEAs are significant because they are probably fragments of main belt asteroids, and therefore representative of them. The larger class that contains them is the group called NEOs, those asteroids and comets with orbits that intersect or pass near the Earth’s orbit. We need, say Dachwald and Seboldt, to know much more about them. From the paper:
It is today widely accepted that NEO impacts have caused at least one mass extinction (65 million years ago at the Cretaceous/Tertiary boundary), and they are suspected to have caused several global catastrophes before. Even NEOs that do not intersect the orbit of Earth may evolve into Earth-crossers, because their orbits are chaotic, having a relatively short dynamical lifetime of about 107 years. One day, it will become necessary to prevent a specific object from impacting the Earth by nudging it out of its orbit. To be able to do this, the bulk properties of NEOs (material strengths, composition, structure, moments of inertia, etc.) should be determined as soon as possible.
Solar sails seem ideal for this purpose. Once at NEA 1996FG3, the sail-driven craft can take advantage of its sail in novel ways. Even close in to such a small object, the solar radiation pressure on the sail would exceed the asteroid’s gravitational pull; the spacecraft would therefore be placed into a hovering position in the hemisphere opposite the Sun (the sail can be turned to vary the photon pressure upon it, making this possible). Although ultimately unstable, these hovering positions could be maintained for extended periods of time, long enough for the release of the lander and its Earth return capsule.
Dachwald and Seboldt go on to envision an even more sophisticated triple NEA rendezvous mission with a 75kg payload and speculate on the properties of sails up to 140m² in size. Missions like these make outstanding sense — they take us to objects like NEOs with highly inclined orbits, the kind of target difficult to reach with conventional spacecraft, and they allow us to develop space experience with lightweight structures for future propulsion systems. The paper is “Multiple near-Earth asteroid rendezvous and sample return using first generation solar sailcraft,” in Acta Astronautica 57 (2005), pp. 865-875.
by Paul Gilster | Jan 21, 2006 | Autonomy and Robotics
Self-repair in spacecraft has always been a fascinating subject, one that comes to the fore as we launch missions to the outer Solar System. It’s one thing to send commands to fix a stuck tape recorder, as was done with Galileo on the approach to Jupiter in 1995. In that situation, controllers worked with a 40-minute time lag — eighty minutes for each confirmation that a command had been executed. But what happens if something goes awry in the Kuiper Belt, or the Oort Cloud, or a third of the way to Alpha Centauri?
Clearly, probes beyond our Solar System will have to rely heavily on advances in robotics and autonomous computing that allow them to diagnose and fix their own problems. But they’ll also have to make use of self-healing materials that can repair damage caused by temperature variations and external collisions. Small cracks caused by micrometeorites, for example, could weaken the spacecraft as they accumulate over the course of a long mission.
But how about healing such problems the way nature does? The European Space Agency has funded a new study that suggests that possibility. “When we cut ourselves we don’t have to glue ourselves back together, instead we have a self-healing mechanism. Our blood hardens to form a protective seal for new skin to form underneath,” says Dr Christopher Semprimoschnig, a materials scientist at ESA’s European Space Technology Research Centre (ESTEC) in the Netherlands, who oversaw the study.
Image: Hollow fibers just 30 micrometres in diameter thread the new material. When damage occurs, the fibers break releasing liquids that seep into the cracks and harden, repairing the damage. Credit: ESA.
Semprimoschnig and company used a resinous composite material of the sort used in making spacecraft components. They replaced some of the fibers in the composite with new glass fibers containing adhesive materials. ESA uses the analogy of blood in the human body; when any of these glass fibers are broken, a liquid resin spills out and hardens, filling cracks and sealing the breach.
More information can be found at Self-Healing: Glass Fibre Reinforced Plastic, a page at the University of Bristol which also contains links to a PDF presentation on this work. ESA’s General Studies Programme is here.
