by Paul Gilster | Feb 5, 2009 | Outer Solar System |
Asteroid (234) Barbara is an unusual object, a denizen of the main belt that may be a binary. The European Southern Observatory’s Very Large Telescope Interferometer is able to piece together two bodies, with diameters of 37 and 21 kilometers respectively, separated by a bit over 20 kilometers. But as seen from Earth, the objects seem to overlap, so we don’t know whether this is a true binary or an asteroid in the shape of a giant peanut. The former would be more interesting, for if we can calculate the orbits of these objects and combine them with diameter measurements, we’ll learn about their density.
This is why Sebastiano Ligori (INAF-Torino, Italy) calls Barbara “…a high priority target for further observations.” Ligori is one of the researchers who used the combined light from two of the Very Large Telescope’s 8.2-meter instruments to make these interferometric studies, creating a view as sharp as a single telescope whose diameter is as large as the separation between the two. The image of asteroid (951) Gaspra just below gives an idea of the method’s powers — compare it to the subsequent photo of Gaspra taken by the Galileo spacecraft.
Image (left): Shape model of (951) Gaspra projected on the plane of the sky at the time of the second VLTI visibility measurement. Credit: Marco Delbo/ESO.
From a broader perspective, these interferometric methods give us an unprecedentedly sharp view of distant asteroids, being able to resolve main belt objects down to about fifteen kilometers in diameter. Thus we increase the number of objects we can measure, giving us a better window into the early days of the Solar System, when debris was coalescing into larger bodies. According to this ESO news release, an observing campaign using interferometric methods will now begin in an attempt to characterize more of the smaller asteroids.
Asteroid specialists clearly have a long way to go. The European Space Agency’s Gaia mission should, within the next ten years or so, deliver mass information for 100 or so of the largest main belt asteroids, and according to the paper on the VLTI work, should be able to directly measure the size of all main belt objects larger than 30 kilometers (roughly a thousand asteroids). But the burgeoning study of binary asteroids suffers from these size limitations, and the new VLTI work offers one way to deliver accurate information about smaller objects. Neither adaptive optics nor radar is up to that challenge.
Image: A 1991 image of asteroid (951) Gaspra taken by the Galileo spacecraft. Credit: JPL.
The paper is Delbo et al., “First VLTI-MIDI direct determinations of asteroid sizes,” in press at the Astrophysical Journal and available here.
by Paul Gilster | Feb 4, 2009 | Exoplanetary Science |
The COROT mission’s 27-cm telescope has discovered the smallest exoplanet yet, with a diameter less than twice that of Earth. COROT-Exo-7b orbits a Sun-like star and highlights the ongoing space-based investigation into rocky worlds that is drawing ever closer to an Earth-mass object. This is the kind of work COROT was designed to do, flagging planetary transits across the face of a star from an orbital perch that allows long periods of uninterrupted observation and the chance to measure the size of the planets found. ESA’s Malcolm Fridlund discusses the significance of the find:
“This discovery is a very important step on the road to understanding the formation and evolution of our planet. For the first time, we have unambiguously detected a planet that is ‘rocky’ in the same sense as our own Earth. We now have to understand this object further to put it into context, and continue our search for smaller, more Earth-like objects with COROT.”
Finding a ‘super-Earth’ is one thing when you’re using radial velocity methods that peg its mass, but quite another when a transit allows a direct measurement of its size. COROT’s success reminds us that we’re at the dawn of the era of characterizing rocky worlds around other stars, an investigation that will tell us much about how common terrestrial planets are in an exoplanet population now dominated by gas giants.
What we know about COROT-Exo-7b is that it is a hot place indeed, orbiting its star in less than a day and close enough to it to boast temperatures between 1000 and 1500°C. Our online discussion began to develop overnight, after David Blank (James Cook University, AU) passed along the link to early data showing a world with 0.035 the mass of Jupiter and a radius 0.13 as large as that planet. That points to iron, and would make COROT-Exo-7b somewhat denser than Mercury.
This ESA news release speculates that the planet’s surface is covered in molten lava, or perhaps dominated by water vapor. In any case, its internal structure should provoke much further study. The paper on the new discovery is Léger et al., “Transiting exoplanets from the CoRoT space mission VII. COROT-Exo-7b: The first super-earth with radius characterized,” to be submitted to Astronomy & Astrophysics.
Addendum: Be sure to read systemic‘s take on COROT-Exo-7b, from which this:
Everything about CoRoT-7b reemphasizes the fact that planets are wont to turn up in every corner of parameter space to which observations are sensitive. In this case, a V=11 K0V star in the direction of the galactic anti-center displays 176 individual 1.5-hour 0.3 mmag photometric dips with a strict 0.854 day periodicity. These measurements suggest a 1.7 Earth-radii planet with a 20-hour year — a world that makes 51 Peg b look like Fargo North Dakota.
And this:
In any case, it’s a remarkable detection, and will be hugely influential as soon as the mass is confirmed. The planet is orbiting at only four stellar radii — with the star filling nearly a thousand square degrees of sky…
by Paul Gilster | Feb 3, 2009 | Astrobiology and SETI |
The Drake Equation in its various forms has been tormenting us for decades, raising the question of how to adjust variables that range from astronomical (the abundance of terrestrial planets) to biological (the probability of life’s emergence) and even sociological (the average lifetime of a technological civilization). Wildly optimistic estimates of the number of technological civilizations in our galaxy are now giving way to more sober reflection. Now Reginald Smith (Bouchet-Franklin Institute, Rochester NY) offers up a new analysis looking at how likely radio contact is given a civilization’s lifetime, and how widely that civilization’s signals can be clearly received. The key question: What if there is a reasonable horizon for the detection of a signal from an extraterrestrial sender?
Signals and Their Lifetime
This is useful stuff, because contact depends not just upon the density of communicating civilizations (CC) but their average lifetime and the maximum detectable distance for their signals. A CC with a short lifetime is unlikely to be heard unless its neighborhood is crowded with other civilizations. But a longer living CC increases the necessary density — a culture producing a continuous signal over a span of a million years is obviously much more likely to be detected than one producing that signal over a single millennium. And if Fermi’s paradox continues to plague us, Smith would argue that perhaps it shouldn’t. From the paper:
What is most interesting about this analysis is that it demonstrates it can be possible for many CCs in the same galaxy to never contact one another. For example, even assuming the average CC has a lifetime of 1,000 years, ten times longer than Earth has been broadcasting, and has a signal horizon of 1,000 light-years, you need a minimum of over 300 CCs in the galactic neighborhood to reach a minimum density. For example, if there were only 200 CCs in our galactic neighborhood roughly meeting these parameters, probabilistically they will never be aware of each other. This finding can give pause to both those who predict no other CCs or those who predict a high number of CCs in our galactic neighborhood.
A pause for reflection is not a bad idea, given the flexibility of the values in the Drake Equation. Smith continues:
Arguing that the lack of contact signifies the lack of CCs may be tempered with the fact that if there is a signal horizon, even a galaxy replete with life may have relatively isolated CCs in the absence of interstellar travel or extremely power[ful] signals. On the other hand, high estimates of CCs in our galactic neighborhood does not guarantee that there will ever be contact between them, especially reciprocal.
From Radio to Artifact
Hold the density of technological cultures down to a certain level and contact is unlikely. Or is it? Note what this presupposes about the cultures we might expect to exist out there. Smith himself says that the constraints on contact can be circumvented by interstellar travel or automated beacons. And if we are willing to move into the realm of von Neumann probes and other self-replicating technologies, we have to deal with the possibility that a single ETI could blanket the galaxy with its sensors given enough time. Frank Tipler famously estimated the needed span to be about a million years, and argued from that that ETI does not exist.
Let’s assume for the sake of argument that there are no von Neumann probes. If this were the case, given that we believe a sufficiently advanced civilization ought to be able to build them, it seems to suggest that ETI would have universally chosen not to build them because, as Carl Sagan once speculated, such probes would constitute so potent a viral-like menace that no sane species would introduce them. The other possibility, far starker, is that no civilization can survive long enough to reach the stage of technology needed to produce them. For in terms of physics, a self-reproducing von Neumann probe appears to be feasible, assuming we or our hypothetical ETI develop the tools to the needed point without turning them on ourselves.
Statistical Approaches to Drake
I’m wandering far from Smith’s original point, which is to show that variables like short lifetimes or small maximum distances for communications could mask the presence of other technological cultures. We’re left to wrestle with the implications that follow from this. The paper is Smith, “Broadcasting but not receiving: density dependence considerations for SETI signals,” submitted to the International Journal of Astrobiology and available online. Also significant here is Claudio Maccone’s “The Statistical Drake Equation,” which was presented at the 2008 International Astronautical Congress held last year in Glasgow.
Maccone has re-worked the Drake values by taking what had been single value estimates and converting them into statistical form, with intriguing results. And while I don’t want to turn this into a bibliography, I do need to mention Duncan Forgan’s “A Numerical Testbed for Hypotheses of Extraterrestrial Life and Intelligence,” accepted by the International Journal of Astrobiology and available here. Working with statistical treatments of SETI’s key parameters is opening up new insight into their uses. Both the Maccone and Forgan papers have been on the back burner here for longer than I intended, an issue I hope to remedy soon.
by Paul Gilster | Feb 2, 2009 | Culture and Society |
With the Kepler mission scheduled for launch this spring, we should see increasing attention in the media on the detection of terrestrial-class exoplanets and speculations on possible life upon them. But it’s easy to forget that Kepler has other important goals, taking estimates, for example, on the disposition of planets in multiple star systems, and studying the stars that have planets in orbit around them. Kepler will also be looking at planetary distribution, including ‘hot Jupiters,’ and examining their size, density and reflectivity.
A Deep Space Challenge for Bloggers
All of which is a tall order for a three and a half-year mission, but we can expect a successful run to result in an extended mission as Kepler keeps its gaze fixed on a region in space allowing it to monitor the brightness of more than 100,000 stars. Have a look at OrbitalHub‘s treatment of Kepler in the current Carnival of Space, where DJ runs through the mission parameters and examines the equipment. Looking for transits, Kepler will monitor more than 100,000 stars for the duration of the mission, with the capability of detecting terrestrial planets in Earth-like orbits. We’ll learn a great deal about the orbit, mass and temperature of many new exoplanets and may well find rocky worlds inside the habitable zone.
I’m glad to see bloggers taking on the challenge of longer posts to explain the background of developing missions like Kepler. And I notice that Ethan Siegel does much the same thing with the dark energy question at his Starts with a Bang! site. Siegel has the knack of explaining complicated things in everyday terms, as in his illustration of intrinsic brightness told in terms of 100 watt light bulbs, and the relation of that discussion to calculating cosmic distances. We use type Ia supernovae, in which a white dwarf draws mass from a companion star in sufficient amount to cause an internal collapse and subsequent explosion, as helpful distance markers and, as Siegel points out, they’ve given us evidence for dark energy.
Problems in Interstellar Propulsion Studies
What else can we do to promote education in deep space matters? I often think about this in terms of where we are in the discipline of propulsion studies. The second half of the 20th Century saw two distinct threads of interstellar propulsion concepts. The first centered on approaches workable through known physics — lightsails, magsails, beamed particle and pellet propulsion, fusion and, to the extent that we might one day learn how to create sufficient quantities of the stuff, antimatter. The second thread looks at more exotic concepts including the bending of spacetime through wormholes and the possibility of exploiting interesting and little understood quantum effects.
Both these approaches have their adherents, but interstellar propulsion research has taken place in a largely disjointed fashion, with researchers usually under-funded (if funded at all), doing this work in their spare time as they toiled at their day jobs. The recent publication of the AIAA volume Frontiers of Propulsion Science (edited by Marc Millis and Eric Davis) consolidates much current work on breakthrough concepts, and the appearance of the Tau Zero Foundation will, we hope, begin to draw the two strands of research into closer contact, seeking funding for practitioners whose peer-reviewed work includes near-term concepts where the physics is well understood and the chief problem is on the engineering (and economic!) side.
An Interstellar Agenda for Today
Uncertain economic times are doubtless going to put the brakes on many useful mission concepts, but we should continue to develop interstellar studies as a discipline by deepening our commitment to public education over the Internet. Interstellar studies also needs two things crucial to healthy debate. One is a regular journal, beginning on an annual basis, to offer a venue for current research, adding to what outlets like the Journal of the British Interplanetary Society and Acta Astronautica already do part-time. The second is the re-emergence of a Robert Forward/Eugene Mallove-style bibliography to consolidate useful papers and links in a readily available resource.
As we work on all this, public outreach will remain crucial for developing the support any robust space program needs. And so, more power to bloggers like DJ and Ethan Siegel, who along with their many colleagues around the globe continue to explain complex issues with both clarity and an insistence on scientific rigor. Getting to the stars will be a multi-generational challenge that will require long-term commitment and a willingness to look beyond the demands of the present to see the broader goal. And who knows, maybe the economic meltdown will shake us out of our complacency enough to remember that we’re building a future not just for ourselves but for our grandchildren’s children.
