by Paul Gilster | Sep 23, 2010 | Outer Solar System |
It’s hard to believe it’s been over six years since the launch of the European Space Agency’s Rosetta mission, now well enroute to comet 67P/Churyumov-Gerasimenko, a ten year journey that will be completed in 2014. Upon reaching the comet, Rosetta will begin an extended encounter that includes an orbiter that will circle Churyumov-Gerasimenko for thirteen months as it moves toward the Sun, and a small lander that will investigate surface conditions. We learned more about the mission in today’s sessions of the European Planetary Science Congress, now being held in Rome.
Jeremie Lasue (Los Alamos National Laboratory) presented the latest Rosetta findings at the conference, drawing on computer models that predict the behavior of the comet’s nucleus over the course of the spacecraft’s operations. Landing on a comet is tricky business, because dust, ice and frozen carbon dioxide and carbon monoxide will be active as the comet’s tail begins to form. What Lasue and colleagues have done is to predict how heat is transferred through the nucleus and the rates at which the gases and dust in the cometary nuclei will be vaporized.
Image: the orbit of comet Churyumov-Gerasimenko and Rosetta during the encounter (not to scale). Credit: ESA/Jeremie Lasue/INAF.
According to Lasue, the studies make it clear that Rosetta’s lander, called Philae, will find the optimal landing site on the southern hemisphere of the comet:
“Churyumov-Gerasimenko is a time capsule holding material from the birth of the Solar System. The nucleus’s southern hemisphere has been heavily eroded, so Philae will not have to drill down far to find those pristine samples. At the time of Rosetta’s rendezvous, gas will be escaping mainly from the northern hemisphere, so it will be safer for Philae to touch down in the south. In addition due to the orientation of the comet, the southern hemisphere will be protected from extreme temperature variations at the time of delivery.”
Of course, nothing is static on such a changeable body. But at the time of landing, it’s the northern hemisphere that will be illuminated and emitting gas and dust, with the south relatively quiet. That southern hemisphere erosion is the result of several orbits in which the south pole has been most exposed to the Sun’s heat, all of which could make for a bonanza for Philae in terms of ancient cometary materials. The lander will be drilling down approximately thirty centimeters to collect the samples it will analyze with its onboard instrumentation. The scientists believe a dust mantle up to 20 centimeters deep will have formed in the landing region.
What a bizarre and fascinating place Rosetta is going to. Says Lasue’s co-author Maria Cristina De Sanctis (IASF-INAF, Rome):
“When Philae lands, temperatures at the equator may rise above freezing and could fluctuate by around 150 degrees Celsius. However, the regions close to the south pole will keep more stable temperatures. From our present results, we’ve concluded that the southern hemisphere promises the best landing sites. As more data on Churyumov-Gerasimenko becomes available to better quantify our results, we will be able to add to the picture and help prepare for a safe landing for Philae.”
Analyzing comets is an activity with a long pedigree. It was back in 1910 that spectroscopic studies of comet tails conducted by Sir William Huggins came into public discourse with the revelation that among the organic molecules found in comets was the gas cyanide. As the Earth was then expected to travel through the tail of Comet Halley, speculation ran rampant that people would be asphyxiated by the cometary materials. Gunter Faure and Teresa Mensing note what happened in their textbook Introduction to Planetary Science: The Geological Perspective (Springer, 2007):
During the night of May 18/19 of 1910, when the Earth passed through the tail of comet Halley, some people took precautions by sealing the chimneys, windows, and doors of their houses. Others confessed to crimes they had committed because they did not expect to survive the night, and a few panic-stricken people actually committed suicide. Enterprising merchants sold comet pills and oxygen bottles, church services were held for overflow crowds, and people in the countryside took to their storm shelters. A strangely frivolous mood caused thousands of people to gather in restaurants, coffee houses, parks, and on the rooftops of apartment buildings to await their doom in the company of fellow humans.
Image: This image of Comet Halley, taken in 1910, inspired dread in some of those who saw it. Credit: New York Times.
Needless to say, the speculation was groundless. The Earth passed through only a small part of the comet’s tail, but in any case, a more substantial passage through a comet’s tail had occurred in 1861 without incident. Cometary materials are far too tenuous to create deadly conditions on the surface of the Earth (a cometary impact is, of course, quite another thing). Looking further ahead, if astronauts ever get to the surface of a comet, all that hydrogen cyanide could be a problem, which is why David Brin and Gregory Benford invented ‘cyanutes’ — bioengineered microbes — to protect the crew in their novel Heart of the Comet (1986).
Today we’re more interested in the organic molecules that can be found in comets as having a role in the seeding of the early Earth with the chemicals needed for life to begin. Rosetta’s work around and on 67P/Churyumov-Gerasimenko should produce a full catalog of organic materials in the comet using mass spectrometry on both the orbiter and lander. Moreover, Rosetta’s studies of right- and left-handed amino acids in this primitive environment may help us learn whether the left-handed variety could have had a celestial origin.
by Paul Gilster | Sep 22, 2010 | Outer Solar System |
Imagine being Jean-Pierre Lebreton. The man behind the Huygens probe, Lebreton and the ESA team behind him hold the record for the most distant landfall in history, the 2005 descent onto the surface of Titan. I have no idea what dreams this man might have had in his childhood, but one of mine was descending through Titan’s thick atmosphere to see its never before glimpsed surface. Huygens pulled off the feat with astonishing ease, descending slowly and steadily while snapping panoramic views of the enigmatic moon. Doubtless Lebreton has much to share about all this at this year’s European Planetary Science Congress, now being held in Rome.
EPSC is the major meeting in Europe for planetary scientists, and it’s no surprise that Titan appears prominently in the news from the conference. Cassini’s Visual and Infrared Mapping Spectrometer instrument (VIMS) has been keeping an eye on Titan’s cloud cover since the orbiter first took up its vigil around Saturn, and 2000 VIMS images have now been used to construct a long-term study of Titan’s weather that was presented at the meeting today. VIMS has made over 20,000 Titan images in visible wavelengths and infrared, from which Sébastien Rodriguez (AIM laboratory – Université Paris Diderot) culled the 2000-image database for the study.
Have a look at seasonal change on Titan as shown in the image below. This is a world with seasons that last for seven terrestrial years, and the new work shows significant activity between July of 2004, a time of early summer in the southern hemisphere, and April of 2010, the beginning of northern spring. Cloud cover has decreased near both of Titan’s poles, which had been heavily overcast during the late southern summer until a few months before the equinox in 2008.
Image: Left: T43 flyby of Titan – 12 May 2008 – VIMS images a large cloud that caps the north pole of Titan (yellowish tones). Right: T63 flyby of Titan – 12 December 2009 – VIMS still observes a huge cloud system at 40°S (yellowish tones) and the north pole of Titan free of clouds, a few months after the equinox. Credit: NASA/JPL/University of Arizona/University of Nantes/ University of Paris Diderot.
Sébastien Rodriguez discusses the result:
“Over the past six years, we’ve found that clouds appear clustered in three distinct latitude regions of Titan: large clouds at the north pole, patchy cloud at the south pole and a narrow belt around 40 degrees south. However, we are now seeing evidence of a seasonal circulation turnover on Titan – the clouds at the south pole completely disappeared just before the equinox and the clouds in the north are thinning out. This agrees with predictions from models and we are expecting to see cloud activity reverse from one hemisphere to another in the coming decade as southern winter approaches.”
Rodriguez used a cloud model developed by Pascal Rannou (Institut Pierre Simon Laplace) to analyze cloud developments over time, all part of developing a global understanding of climate on the distant moon. The team believes that northern polar clouds of ethane form in Titan’s troposphere in winter at altitudes of 30 to 50 kilometers owing to ethane and aerosols being supplied from the stratosphere. In the southern hemisphere, mid- and high-latitude clouds are the result of upwelling surface air enriched with methane. The model thus far fits observation, but Rodriguez’ team will be able to use the Cassini extended mission (ending in 2017) to observe further seasonal changes from mid-winter to mid-summer in the northern hemisphere.
The VIMS instrument is a story in itself, a color camera that takes pictures in 352 wavelengths ranging between 300 and 5100 nm. By contrast, the visual spectrum is in the range between 400 and 700 nm, a small window in the larger region covered by VIMS. The instrument can measure the chemical composition of a surface or an atmosphere (or, for that matter, of objects in Saturn’s rings) by measuring this visible and infrared energy. Because clouds have different optical qualities at different wavelengths, we can use VIMS data to see how the atmosphere of Titan absorbs or reflects light to understand the composition of its changeable cloud cover.
Fine spring weather for the northern hemisphere on Titan, then, with Rannou’s global climate models seemingly on target in describing it. Right now Titan stands alone in offering an off-planet source for study of surface liquids and their contribution to weather patterns in a cycle somewhat reminiscent of Earth’s hydrological cycle, though based on methane. One day, given advances in our instruments, we’ll be studying similar cycles on planets around other stars, armed with the experience we’ve gained by mastering the frigid (94 K, or ?179 °C) seasons of Titan.
by Paul Gilster | Sep 21, 2010 | Outer Solar System |
I was fortunate enough to meet Joel Poncy (Thales Alenia Space, France) at last year’s deep space conference in Aosta, where he gave the audience the lowdown on an extraordinary mission concept, an orbiter of the Kuiper Belt object Haumea. Haumea is a tricky target, lacking an atmosphere that would allow aerobraking and pushing all our limits on propulsion and power generation. But Poncy’s team worked out a design using either electric or magneto-plasma technologies, assuming a gravity assist to shorten the journey for arrival around 2035.
In a later lunch conversation, Poncy talked to me about the benefit of probing the Kuiper Belt, and the collateral advances that such a mission would bring in terms of developing the orbiters, landers and deep-drilling capabilities we’ll need to explore planetary moons like Europa or Ganymede. Sometimes you choose targets, in other words, not only for their immediate payoff, but because they become part of the process of developing next generation tools. My two-part look at the Haumea mission, written last summer, begins here, with follow-up in this article the next day.
Solar Sails and Data Delivery
Now I see that Poncy is working on a unique solar sail concept he presented yesterday at the European Planetary Science Congress, which is taking place at the Pontifical University of Saint Thomas Aquinas in Rome. Poncy’s team has been studying ‘data clippers,’ spacecraft equipped with solar sails that are expressly designed to get large amounts of data back to Earth. And though we don’t often think of data return as a major problem, it becomes one as we push deeper into space and develop more sophisticated probes. Poncy puts it this way:
“Space-rated flash memories will soon be able to store the huge quantities of data needed for the global mapping of planetary bodies in high resolution. But a full high-res map of, say, Europa or Titan, would take several decades to download from a traditional orbiter, even using very large antennae. Downloading data is the major design driver for interplanetary missions. We think that data clippers would be a very efficient way of overcoming this bottleneck.”
The mention of Europa is particularly telling, given how much trouble we had with Galileo’s high-gain antenna, the lack of which caused the data flow from Europa and other Jovian targets to be much slower than anticipated. Moreover, a single orbiter with an equipment malfunction is, like Galileo, able to rely only on onboard backups and whatever software fixes can be sent.
So how do we proceed? The data-clipper would approach an orbiter, upload its stored data, and then make a flyby of the Earth, downloading the information to a station on the ground. Develop an entire of fleet of these vehicles and we could support planetary missions throughout the Solar System, including those interesting objects in the Kuiper Belt like Haumea. The work builds on recent solar sail successes like IKAROS but also looks ahead to magnetic sail possibilities. The latter would tap not solar photons but particles in the solar wind for their propulsive force. Needless to say, sail missions into the outer system would require hybrid propulsion, and it’s interesting to note that JAXA is already working on hybrid sail/ion propulsion concepts for a proposed Jupiter mission as a follow-on to IKAROS.
Image: Data clippers could move terabytes of information between planetary orbiters and ground stations on Earth. Credit: Thales Alenia Space.
Moreover, data-clippers take advantage of the huge advances in miniaturization we’ve seen in digital electronics, allowing us vast amounts of data storage and long mission times with a properly shielded craft. Let me quote Poncy on this again:
“Using the Sun as a propulsion source has the considerable advantage of requiring no propellant on board. As long as the hardware doesn’t age too much and the spacecraft is maneuverable, the duration of the mission can be very long. The use of data clippers could lead to a valuable downsizing of exploration missions and lower ground operation costs — combined with a huge science return. The orbiting spacecraft would still download some samples of their data directly to Earth to enable real-time discoveries and interactive mission operations. But the bulk of the data is less urgent and is often processed by scientists much later. Data clippers could provide an economy delivery service from the outer Solar System, over and over again.”
The first data clipper mission could launch in the late 2020s, meaning that mission planners will want to include the technology in any thinking about future missions, the roadmap for which must be generated many years in advance. Think of fleets of these data harvesters bringing back the imagery and scientific data on planets, moons and objects of interest throughout the Solar System, without having to work with the minuscule bandwidth offered by current methods.
Different Kinds of Bandwidth
Sometimes, it turns out, just picking up an object and taking it somewhere is the best way to proceed. I remember a discussion with Vinton Cerf’s team on the ‘interplanetary Internet’ project out at the Jet Propulsion Laboratory some years back. I was taking voluminous notes as the team discussed what is called ‘disruption-tolerant networking’ (DTN), a way of using not a continuous connection (think FTP) but store-and-forward methods to hold data for later transmission. All this can be automated to reduce the load on Earth-based receivers.
And somewhere in that meeting, Cerf noted “You can’t underestimate the bandwidth of a pickup truck full of CDs driving from one coast to the other.” That’s bandwidth, too, getting the data where they need to be in large quantity and with low overhead. Poncy’s data clippers make me think of that pickup truck, only in this case it’s loaded not with CDs but with exotic planetary data, delivered in such quantity that we could set up serious mapping of objects that today are no more than bright spots in our surveys. All this is decades away — we have to get orbiters to such places — but the mid-term future could hold a planetary data bonanza delivered to our door.
by Paul Gilster | Sep 20, 2010 | Astrobiology and SETI |
Is anyone out there in the galaxy aware of our presence? If so, it’s most likely through detection of our planetary radars, like those at Arecibo and Evpatoria that are used to detect and study nearby objects like asteroids, and provide a valuable part of our planetary defense. Sure, we’ve been pumping television and radio signals into the deep for a long time now, but Arecibo is the most powerful radar in the world, its 430 MHz transmitter offering a maximum total peak pulse output power of 2.5 MW. The planetary radars at Arecibo, Goldstone and Evpatoria are sending far more powerful signals than the faint traces of our early TV broadcasts.
It’s one of the hopes of SETI that we might detect a similar transmission from another civilization, but in saying that we run into all kinds of assumptions. How long a time-frame does a civilization have before it develops technologies far superior to planetary radars for studying nearby objects? For that matter, how long would any sort of electromagnetic leakage be detectable before the culture behind it moves on to the next stage of development? The SETI Institute’s Jill Tarter nails the problem in this recent article on Silicon.com:
“I can tell you how long it will take us to cover one million, ten million or 100 million stars,” Tarter said. “But I can’t guarantee that what we are looking for is the right thing.
“We may be doing a marvellous search for the wrong thing – we may not even have the technology today to look for the right thing because we haven’t invented it yet.”
SETI pushes on despite the questions, and despite Tarter’s analogy that all of the searching to date is no more than taking an eight-ounce glass of water out of all the Earth’s oceans. Our most intensive studies thus far have included about 2000 stars, and even though the Allen Telescope Array will help the SETI Institute extend the search to perhaps as many as ten million stars over the coming decade, we’re still talking about a fraction of the galaxy’s population, variously estimated at anywhere between 100 billion to 400 billion depending on where you look.
How to enhance our chances? The ATA, which will eventually expand to an array of 350 antennae from its current 42, will allow the SETI Institute’s search to proceed year-round, one wrinkle being that the search will now take in stars whose position means that any civilizations there would be able to spot a transit of Earth across our Sun with their telescopes. It’s good to see this idea get more public exposure. It was back in 2008 that Richard Conn Henry (Johns Hopkins) told an American Astronomical Society meeting that we’re more likely to receive a signal from a civilization that can spot us first, assuming that alien cultures share our interest in sending messages into space.
Henry is interested in the ecliptic, the plane of the Earth’s orbit around the Sun. The ecliptic is tilted about sixty degrees with relation to the galactic disk, and it takes up no more than three percent of the sky, offering a useful constraint on the area that a search using these methods would have to examine. A star system close to the ecliptic would be one that could have detected our transits across the Sun, and perhaps have learned that our planet is in the habitable zone. Spectroscopic analysis of our atmosphere would make it clear that life exists here.
Image: As the Earth orbits the Sun, it seems from the Earth that the Sun moves over the ecliptic (red) on the celestial dome. Astronomers on distant worlds that line up with the ecliptic would be able to see the Earth transit across the face of the Sun. Credit: Wikimedia Commons.
We fall back on Tarter’s comment that we don’t necessarily know what technologies an alien culture would use for such detections, but it’s certainly true that in our early efforts to find habitable planets around other stars, we’re using the transit method in missions like CoRoT and Kepler. Shmuel Nussinov (Tel Aviv University) has examined how we might use this approach to study where the ecliptic stands in relation to the galactic plane, the two areas of intersection being in Sagittarius and in Taurus. We’ll see how the idea plays out in the ATA investigations, which will target the entire ecliptic but probably pay special attention to Taurus and Sagittarius.
All of this reminds me that if you haven’t yet looked at the SETIQuest site, you should familiarize yourself with it. SETIQuest is an attempt to harness the power of the Net community to do solid science on behalf of the SETI effort, and that includes releasing not just data collected by the ATA but also the computer code that is used to analyze that data. You can look at an early release of SonATA (standing for ‘SETI on the ATA’) here, although I notice that this early version is only compatible with SuSE Linux (should run on other flavors, I suspect) or the Mac, with the powers that be looking for the necessary help to build a Windows version as well.
What can the open source community do with SETI code? The idea is that someone out there might have a great idea for a new approach, a set of algorithms that may make it possible to ferret a useful signal out of the noise, assuming one is actually found. We don’t have all the answers on digging signals out of interference and we can use work that questions older assumptions by getting down and dirty with the code to see what comes up. SETIQuest is going to be a fascinating project to watch as dish by dish, the ATA becomes fully operational.
A Johns Hopkins news release on Richard Conn Henry’s work is here. Shmuel Nussinov’s paper is “Some Comments on Possible Preferred Directions for the SETI Search” (preprint).
by Paul Gilster | Sep 17, 2010 | Exoplanetary Science |
While we’re this early in the game of detecting life signs from distant planets, it makes sense to focus on surface habitability, which is why oceans are so interesting. Sure, we can imagine potential biospheres under the ice of a Europa or even an Enceladus, but given the state of our instrumentation and the distance of our target, going after the most likely catch makes sense, and that means looking for oceans. Significant work from the EPOXI mission has given us some of the parameters for studying a planet like ours using multi-wavelength photometry.
EPOXI, you’ll recall, is the extended mission of the Deep Impact spacecraft that drove an impactor into Comet Tempel 1 in 2005 and is now enroute to Comet Hartley 2. Its views of Earth are being used to help scientists prepare for studies of terrestrial worlds around other stars. Planets with large bodies of water should reflect light from their star differently than dry planets, and as the observed planet goes through its phases as seen from Earth, the changes in that reflectivity can be measured. EPOXI showed us that we can make useful observations at different points in the Earth’s rotation. We’ve also seen specular glints on Titan, and now the focus is on what else we can learn to help us exploit this phenomenon.
Tyler Robinson (University of Washington) is involved in the study of such glints to help find Earth’s twin somewhere among nearby stars. Robinson’s team has been using the NASA Astrobiology Institute’s Virtual Planetary Laboratory, which allows them to model the Earth as it would appear to a distant observer tracking the planet’s progress through an entire orbit. It turns out that in a variety of simulations the ‘glinting Earth’ can be as much as 100 percent brighter at crescent phases than when modeled without the glint effect, a result that may be observable with the James Webb Space Telescope. Robinson describes the glint colorfully to BBC News:
“The glint I’m talking about is pretty much the exact same thing when you talk about gorgeous sunsets over the ocean. With the sun low on horizon, sun beams come in and glance off the ocean surface which is acting like a mirror and you get these beautiful red sunsets.”
Image: Glinting sunlight off Lake Erie (not an EPOXI image). Source: Image Science and Analysis Laboratory/NASA JSC.
And now we know that the glint effect (‘specular reflection,’ to be precise) produces major changes in brightness. For all its powers, though, the JWST wouldn’t be able to spot a glinting planet without the use of an external occulter, a shield that blocks starlight to reveal much fainter planets. And the new work tells us what wavelengths are the most likely to produce results. Here the authors discuss them in the context of Rayleigh scattering, the scattering of light by particles smaller than the wavelength of light, which must be incorporated in the analysis:
At crescent phases, pathlengths through the atmosphere are relatively large and optical depths to Rayleigh scattering can be larger than unity even at longer wavelengths. This indicates that observations which aim to detect the brightness excess due to glint should be made at wavelengths in the near-infrared range. Earth’s brightness drops by over an order of magnitude between 1-2 μm, arguing that searches for glint should occur below 2 μm for higher signal-to-noise ratio (SNR) detections. Since glint is a broad feature in wavelength space (it is the reflected solar spectrum, modulated by Rayleigh scattering, liquid water absorption at the surface, and atmospheric absorption), photometry can be used to detect glint provided that strong absorption features are avoided.
All this is helpful information as we add the items we need for detecting habitability to our tool chest. We can take into account the fact that the size of a ‘glint spot’ compared to the illuminated portion of a disk is highest at crescent phases and add in the fact that the reflectivity of water increases at glancing illumination angles, but as the authors do, we also have to factor in how the glint effect can be duplicated by liquid and ice crystals in high clouds. New work following up on high clouds and their uses in detection will be presented in October at the Division of Planetary Sciences meeting in Pasadena, and I’ll have more on it then.
The paper is Robinson et al., “Detecting Oceans on Extrasolar Planets Using the Glint Effect,” Astrophysical Journal Letters 721 (2010), L67 (preprint).
