by Paul Gilster | Oct 13, 2008 | Exoplanetary Science |
Computer simulations are showing us how to detect the signature of Earth-like planets — indeed, planets nearly as small as Mars — around other stars. That interesting news comes out of NASA’s Goddard Space Flight Center, where a supercomputer named Thunderbird has been put to work studying dusty disks around stars similar to the Sun.
Varying the size of the dust particles along with the mass and orbital distance of the planet, the team led by Christopher Stark (University of Maryland) ran 120 different simulations.
“It isn’t widely appreciated that planetary systems — including our own — contain lots of dust,” Stark says. “We’re going to put that dust to work for us.”
Indeed. Useful and observable things happen as dust responds to the forces acting upon it. For one thing, starlight can exert a drag that causes dust particles to move closer to the parent star. More to the point, particles spiraling inward can become involved in orbital resonances with planets in the system. A resonance like this is what happens when periodic gravitational influences go to work on orbiting objects. Particles that make three orbits around their star as a planet makes one, for example, will over time settle into identifiable structures, a complex dance that is played out with vast numbers of such particles.
Image: A planet twice Earth’s mass forms a ringed dust structure in this simulation. Enhanced dust density leads and trails the planet and causes periodic brightenings. Credit: NASA/Christopher Stark, GSFC.
Stark believes that this work will be useful as we study dust structures around exoplanets, particularly via future space platforms like the James Webb Space Telescope. Certainly the idea appears promising, as it means using dust that might otherwise complicate direct imaging attempts as a solution rather than a problem, capable of detecting planets down to a few times the mass of Mars. The paper is Stark and Kuchner, “The Detectability of Exo-Earths and Super-Earths Via Resonant Signatures in Exozodiacal Clouds,” Astrophysical Journal 686 (October 10, 2008), pp. 637-648 (abstract). Stark’s Exozodi Simulation Catalog is available online.
by Paul Gilster | Oct 11, 2008 | Culture and Society |
If you can put together a consortium that takes in a variety of public and private organizations, then seed it with university expertise, you can start involving yourself in space research. Take a look at what Kentucky Space is all about. I’m reminded of its ongoing efforts by the fact that its blog is currently hosting the Carnival of Space, reporting in the introduction on its upcoming sub-orbital mission, scheduled for launch today from the Mojave desert. Kentucky Space’s projects have included KySat, a student-led initiative involving small satellites from design to launch and operation.
This is an active and interesting program well worth your attention, and its Web presence is ably enlivened by Wayne Hall, who presents the current Carnival materials. Of these, I point you to Colony Worlds and its enjoyable musings on dogs in space. Headed out for Mars for a couple of years, or perhaps planning on settling in a distant colony, maybe an O’Neill habitat somewhere out around L-5? If so, you’ll get a kick out of Darnell Clayton’s reasons why your dog may be your best traveling companion. All of which reminds me of one of the wilder dreams I’ve ever had, about one of my Border Collies being sent Laika-style aboard a spacecraft bound for Neptune…
Also intriguing from the mix is Ian O’Neill’s short piece on black holes, and the results of a computer simulation that rammed two black holes into each other at close to the speed of light. The question: What happens to the event horizon after so cataclysmic an event? Can a black hole exist without one? The results from this work by Emanuele Berti and team at the Jet Propulsion Laboratory were intriguing:
Unlike previous simulations examining lower-energy collisions, far more energetic gravitational waves were produced. So much so that 14% of the total masses of the colliding black holes were converted into gravitational wave energy. So far so good. If this extreme (and unlikely) scenario were to occur, perhaps we’d know what to look out for in the noisy LIGO data, and we might gain an estimate of how much mass black holes shed in these encounters. However, there’s another outcome to Berti’s research: black holes keep their event horizons no matter what is thrown at them.
An event as powerful as this is gravitationally interesting, but it’s also of note in relation to Roger Penrose’s musings on so-called ‘naked’ singularities, which suggest there is no way they can exist in nature. Exactly how such ‘cosmic censorship’ might work is an open question — Ian notes that you can work out the math to show that a singularity could exist without an event horizon — but in any case, we have no idea what a naked singularity would look like if it did exist, making this theorizing unlikely to move into the realm of observational data any time soon.
by Paul Gilster | Oct 10, 2008 | Astrobiology and SETI |
Why would you want to take pictures of Earth from a spacecraft in orbit around Venus? Aside from the wish to see a familiar place from a distant location, our planet can also become an interesting testbed for exoplanetary studies. We’ve run into this idea before in the EPOXI mission, which is the combined extended mission of the Deep Impact spacecraft. Here the cometary component of Deep Impact was recently augmented with observations of Earth that can suggest how to study the glint of light off distant oceans, or the signature of land masses.
The extrasolar component of EPOXI is called EPOCh, for Extrasolar Planet Observation and Characterization, and it primarily involves an examination of stars with known transiting planets, looking for other planets in the system (EPOXI can detect transits of objects down to about half the diameter of the Earth) or possibly moons around the known ones. Meanwhile, the spacecraft continues its journey to comet Hartley 2 for observations there, its extrasolar investigation limited by the exigencies of the cometary mission.
But Venus Express, now in orbit around the second planet, is likewise helping us learn about distant worlds by a closer examination of our own. As seen through the cameras aboard the spacecraft, the Earth spans less than a pixel, which is all we can expect to see when we get the technologies in place to isolate light from Earth-sized worlds elsewhere in the cosmos. We need to know how to interpret potential habitability, in other words, with precious little information, and Venus Express can help.
It will be useful to know, for example, whether green plants, bright in the near infrared, can be discerned. For that matter, spectral observations can reveal information about planetary weather systems and the location of glaciers and oceans. Thus far the team is finding the work a challenge. “We see water and molecular oxygen in Earth’s atmosphere, but Venus also shows these signatures. So looking at these molecules is not enough,” says Giuseppe Piccioni (IASF-INAF, Rome).
A necessary next step is to compare the spectra of Earth’s oceans with spectra taken when continents dominate the view. David Grinspoon (Denver Museum of Nature & Science), who suggested these observations, calls this “…the first sustained program of Earth observation from a distant platform,” unlike the fleeting EPOXI images.
Image: This image composite shows the signatures of methane (CH4), carbon dioxide (CO2), ozone (O3) and nitrous oxide (N2O), minor species of the Earth’s atmosphere but powerful greenhouse gases, detected by the Visual and Infrared Thermal Imaging Spectrometer (VIRTIS) on board ESA’s Venus Express at infrared wavelengths, while the spacecraft was pointing Earth along its orbit around Venus. Our planet was just a pixel in VIRTIS’s field of view. These observations are relevant as they proof that a distant planet such as an extra-solar planet can reveal to an instrument like VIRTIS the signatures of chemical compounds composing the atmosphere and surface. Credit: ESA/VIRTIS/INAF-IASF/Obs. de Paris-LESIA (Earth views: Solar System Simulator JPL-NASA).
Approximately forty images of our planet have been collected in both the visible and near-infrared regions of the spectrum. What can they tell us about Earth’s atmosphere and its life-bearing capacities? With Kepler on the horizon and COROT engaged in active work in space, we may one day soon find our attention riveted by a planet whose orbital characteristics point toward habitability. Planet hunter missions beyond these should, perhaps in fifteen years, be able to resolve exoplanets down to terrestrial size. Building the necessary tools will teach us how to proceed when that welcome day arrives.
by Paul Gilster | Oct 9, 2008 | Outer Solar System |
The last time Cassini flew past Saturn’s moon Enceladus (August 11), temperatures over one of the so-called ‘tiger stripe’ fractures at the south pole were lower than had been measured on an earlier flyby in March. Two October encounters, one of them scheduled for today, may provide enough additional data to help us understand what’s going on. The fracture in question is known as Damascus Sulcus, which showed temperatures between 160 and 167 Kelvin in August, but 180 degrees Kelvin during the March flyby.
Then again, nothing about Enceladus should surprise us any longer, including an apparent change in the intensity of the plume, within which trace amounts of organics have been detected. The October 9 approach takes us to a distance closer than any previous flyby of a Saturnian moon, a mere 25 kilometers from the surface, a key objective being to study the composition of the plume with the spacecraft’s field and particle instruments. Thus Tamas Gambosi (University of Michigan, Ann Arbor):
“We know that Enceladus produces a few hundred kilograms per second of gas and dust and that this material is mainly water vapor and water ice. The water vapor and the evaporation from the ice grains contribute most of the mass found in Saturn’s magnetosphere. One of the overarching scientific puzzles we are trying to understand is what happens to the gas and dust released from Enceladus, including how some of the gas is transformed to ionized plasma and is disseminated throughout the magnetosphere.”
Image: This graphic shows the trajectories for the Cassini spacecraft flybys planned for Oct. 9 (E5) and 31, 2008 (E6). During Cassini’s Oct. 9 flyby, the spacecraft’s fields and particles instruments will venture deeper into the plume than ever before, directly sampling the particles and gases. The emphasis here is on the composition of the plume rather than imaging the surface. Image credit: NASA/JPL.
The October 31 flyby, closing to 196 kilometers, will image the tiger stripe region again, with both encounters offering the chance to find still further changes around this deeply interesting object. With four more Enceladus flybys planned for the next two years of the Cassini Equinox Mission — the name for Cassini’s extended mission — we can hope to learn more about what powers its geysers. We may also find clues to the moon’s past, since the different isotopes found in that environment could help to identify the temperatures at work during the formation of Enceladus.
Note: NASA’s Cassini flyby blog is now active.
by Paul Gilster | Oct 8, 2008 | Asteroid and Comet Deflection |
Now here’s an interesting question. What would happen if a small asteroid like 2008 TC3, the three-meter object that exploded in the atmosphere late Monday, were headed for a large city? We were able to judge with a high degree of confidence that 2008 TC3 would pose no threat to the surface, and indeed, early reports suggest that its energies — 1.1 to 2.1 kilotons of TNT — were expended in the atmosphere. But even the most confident scientists might be hard put to sell the case for calm if the public started imagining worse case outcomes.
David Morrison (NASA Ames) has written about the public response to a small impact scenario, a fact I’m drawing from the recent update of NEO News sent to me by Larry Klaes. Also available is a report from spaceweather.com of a visual sighting of the event, sent along by Jacob Kuiper, general aviation meteorologist at the National Weather Service in the Netherlands::
“Half an hour before the predicted impact of asteroid 2008 TC3, I informed an official of Air-France-KLM at Amsterdam airport about the possibility that crews of their airliners in the vicinity of impact would have a chance to see a fireball. And it was a success! I have received confirmation that a KLM airliner, roughly 750 nautical miles southwest of the predicted atmospheric impact position, has observed a short flash just before the expected impact time 0246 UTC. Because of the distance it was not a very large phenomenon, but still a confirmation that some bright meteor has been seen in the predicted direction.”
The best case scenario I can imagine for getting us to develop the tools needed for asteroid deflection is having the occasional small event like this making news around the globe. And indeed, we should have no shortage of events to point to, according to Don Yeomans, who toils at the Near-Earth Object Office at the Jet Propulsion Laboratory:
“We estimate objects this size enter Earth’s atmosphere once every few months. The unique aspect of this event is that it is the first time we have observed an impacting object during its final approach.”
Even so, we had little lead time, with the object being discovered only a day before its encounter with Earth. And if we found a much larger object on an equivalent course? We know that Tunguska-class events may occur as frequently as every 100 years, a grim reminder that space debris is wildly variable in size and can create catastrophe where it falls. Let’s hope it doesn’t take another Tunguska to awaken the public to the need for robust space-faring technologies that can nudge incoming asteroids into safer trajectories.
Emily Lakdawalla did an outstanding job on this story for the Planetary Society weblog. Let me quote from her thoughts on public reaction to a larger object:
But of course we now have to ask ourselves: what would have happened if the object was much bigger than 2 meters in diameter? Reassuringly, the first thing that would have happened is that the detection most likely would have happened much earlier. The bigger and more hazardous an object is, the brighter it is, and the sooner we will detect it. We will likely have way more than 20 hours’ warning of an incoming dangerous object. Still, though, the warning time for a tens-of-meter-diameter object could only be measured in days. If we’d had three days’ warning of a dangerous impactor heading for Sudan, what could the world have done? The remote location of the impact would have been fortunate for humanity in general, but disastrous for the few people who lived out in that remoteness. Could the developed world have done anything to prevent yet another humanitarian disaster from befalling the Sudanese?
These are highly theoretical questions at the moment, but they could become far more pointed at any time. All the more reason to be thinking, as the Association of Space Explorers continues to do, about the possibilities of crafting an international response. That one is dependent upon politics more than technology, and an equally tough challenge. For more on the public response to small impacts, see Morrison, D. “The Impact Hazard: Advanced NEO Surveys and Societal responses,” In Comet/Asteroid Impacts and Human Society (P. Bobrowsky & H. Rickman, eds.) Springer, New York (2007).
