by Paul Gilster | Jun 6, 2010 | Asteroid and Comet Deflection |
These days we think of Giovanni Cassini in relation to Saturn, for obvious reasons, but the Italian astronomer (1625-1712) did a lot more than discovering the division in the rings of Saturn that would later bear his name. In addition to his studies of the Saturnian moons, Cassini shares credit for the discovery of Jupiter’s Great Red Spot, and in conjunction with Jean Richer, made parallax observations of Mars that allowed its distance to be determined in 1672.
But back to Jupiter, for in 1686 Cassini reported seeing a dark spot on the planet, one that from his description was roughly the size of the largest impact made by the Comet P/Shoemaker-Levy comet fragments in 1994. We’re dealing with crude telescopes and lack of corroborating information with Cassini’s observation, but Shoemaker-Levy left us with Hubble imagery when it struck the giant planet after breaking apart into more than twenty pieces enroute.
I mention Cassini’s early sighting because it’s possible he was also seeing the results of an impact, and that would be significant. Given what we know about asteroid and cometary impacts, the assumption has been that impacts with Jupiter should occur on a timescale of every few hundred to few thousand years. But they’re starting to mount up. Add to Cassini’s possible impact sighting an 1834 observation by the British astronomer George Airy, who reported a dark feature that was four times as large as the shadows cast by the Galilean moons.
And now, just sixteen years after Shoemaker-Levy, we have two more impacts, the first evidently the result of an asteroid rather than a comet. The differences between it and Shoemaker-Levy are instructive. The impact occurred on July 19, 2009, was spotted by Australian amateur Anthony Wesley and quickly followed up by observatories around the world and via the Hubble telescope. The impact site was elongated, indicating an object that descended at a shallower angle than the Shoemaker-Levy fragments, and a different direction of origin.
Image: Hubble image of Jupiter’s full disk taken July 23, 2009, revealing an elongated, dark spot at lower, right (inside the rectangular box). The photograph was taken four days after an amateur astronomer first spotted the scar. The unexpected blemish was created when an unknown object plunged into Jupiter and exploded, scattering debris into the giant planet’s cloud tops. The series of close-up images at right, taken between July 23, 2009 and Nov. 3, 2009, show the impact site rapidly disappearing. Jupiter’s winds also are spreading the debris into intricate swirls. Credit: NASA, ESA, M.H. Wong (University of California, Berkeley), H. B. Hammel (Space Science Institute, Boulder, Colo.), I. de Pater (University of California, Berkeley), and the Jupiter Impact Team.
Also telling is that while the 1994 impact showed a distinct halo around the site, evidently the result of fine dust rising from the cometary material, the 2009 impact showed no halo and left little contrast between the debris and Jupiter’s clouds. Scientists are interpreting this as evidence of a lack of lightweight particles, pointing to a solid impactor like an asteroid rather than a comet.
Wesley and fellow amateur Christopher Go, based in the Philippines, also lay claim to a June 3 impact this year, an event first noted by Wesley and confirmed by Go. Both caught the impact on video. This Ars Technica story has more, and a detailed breakdown of the events is offered in Emily Lakdawalla’s account for the Planetary Society. Follow-up observations may yield more information, but for now this video, with its one-second flash, makes it clear that Jupiter has pulled in another victim.
We can’t with certainty ascribe the 1686 and 1834 observations to impacts, but the fact that we now have three Jupiter strikes within fifteen years does raise the eyebrows. The 2009 impactor is thought to have been about 500 meters wide, probably having its origin in the Hilda family of asteroids that orbit near Jupiter. It’s chastening to hear that its strike created an event that was the equivalent of several thousand nuclear bombs exploding. Not as big as the largest Shoemaker-Levy impacts, but that fact in itself tells us that while Jupiter may do a good job cleaning out our system’s inner debris, one stray asteroid could do unthinkable damage on our planet’s surface.
The paper is Sánchez-Lavega et al., “The Impact of a Large Object on Jupiter in July 2009,” Astrophysical Journal Letters 715 (June, 2010), L155 (abstract). More on the 2010 impact as the investigation proceeds.
by Paul Gilster | Jun 4, 2010 | Outer Solar System |
Finding life on Mars would be a huge accomplishment, but finding life on Titan would be a fundamentally different kind of discovery. Martian life might well be related to us because of the exchange of materials between our two worlds, the inevitable result of planetary impacts and the scattering of debris. But Titan is a far more unearthly place than Mars, its chemistry exotic, its climate seemingly beyond the range of any life form we have ever discovered. Life on Titan should be evidence of that ‘second genesis’ planetary scientists dream of identifying.
Image: This artist concept shows a mirror-smooth lake on the surface of the smoggy moon Titan. Cassini scientists have concluded that at least one of the large lakes observed on Saturn’s moon Titan contains liquid hydrocarbons, and have positively identified ethane. This result makes Titan the only place in our solar system beyond Earth known to have liquid on its surface. Credit: NASA/JPL.
Now we have two papers based on Cassini data that show complex activity on the surface of Titan. Is there a precursor to life on the distant moon, or even an exotic life form based on methane? Darrell Strobel (Johns Hopkins) is the author of a paper now in press at Icarus that shows hydrogen molecules descending through the atmosphere and disappearing at the surface. Using data from Cassini’s composite infrared spectrometer and ion and neutral mass spectrometer, Strobel has studied the density of hydrogen in various parts of Titan.
Hydrogen molecules on Titan are thought to be a byproduct of ultraviolet sunlight breaking acetylene and methane molecules apart, a process that is understood and should theoretically distribute hydrogen evenly through the atmosphere. What Strobel has found is a flow to the surface at a rate of 10,000 trillion trillion hydrogen molecules per second. Says Strobel:
“It’s as if you have a hose and you’re squirting hydrogen onto the ground, but it’s disappearing. I didn’t expect this result, because molecular hydrogen is extremely chemically inert in the atmosphere, very light and buoyant. It should ‘float’ to the top of the atmosphere and escape.”
Is there an unknown mineral acting as a catalyst on Titan’s surface, converting hydrogen molecules and acetylene back into methane? The question lingers even as we look at the hydrocarbon mapping of Titan’s surface, which is being done by Roger Clark (U.S. Geological Survey), who notes that despite earlier predictions that acetylene would fall from the atmosphere to coat Titan’s surface, Cassini has detected no acetylene on the surface. Acetylene is important because it could be the best energy source for methane-based life forms on Titan. It is conceivable that surface acetylene is being consumed as food.
Moreover, the hydrogen flow to the surface is consistent with models showing life on Titan consuming hydrogen. Astrobiologist Chris McKay (NASA Ames), who has proposed a set of conditions for methane-based life on the moon, draws a parallel with our own planet:
“We suggested hydrogen consumption because it’s the obvious gas for life to consume on Titan, similar to the way we consume oxygen on Earth,” McKay said. “If these signs do turn out to be a sign of life, it would be doubly exciting because it would represent a second form of life independent from water-based life on Earth.”
Cassini’s spectrometer has detected an absence of water ice on the surface, but has found plentiful benzene and an apparently organic compound that is still unidentified. A film of hydrocarbons seems to be forming over surface water ice. All of this makes Titan an extremely lively place, as Clark notes:
“Titan’s atmospheric chemistry is cranking out organic compounds that rain down on the surface so fast that even as streams of liquid methane and ethane at the surface wash the organics off, the ice gets quickly covered again. All that implies Titan is a dynamic place where organic chemistry is happening now.”
On a surface where temperatures hover around 90 Kelvin (-183 degrees Celsius), liquid methane and ethane are the only substances available to serve as a liquid for life processes. It’s an exotic and fascinating place, this Titan, but we’re far too early in the game to rule out non-biological explanations. Chemical reactions involving mineral catalysts are very much in the running as we await further Cassini data from future flybys. On that score, note that Cassini will be making a close approach to Titan later today (early Saturday June 5 UTC), moving to within 2000 kilometers of the surface to view the north polar region and its lakes. Of special interest: Kraken Mare, Titan’s largest lake, covering an area larger than the Caspian Sea.
The hydrogen paper is Strobel, “Molecular Hydrogen in Titan’s Atmosphere: Implications of the Measured Tropospheric and Thermospheric Mole Fractions,” in press at Icarus. Clark’s hydrocarbon mapping paper will appear in the Journal of Geophysical Research.
by Paul Gilster | Jun 3, 2010 | Outer Solar System |
Ralph McNutt’s contributions to interstellar mission studies are long-term and ongoing. We’ve looked at the Innovative Interstellar Explorer concept he has been studying at the Applied Physics Laboratory (Johns Hopkins), but IIE itself rose out of earlier design studies for a spacecraft that would penetrate the heliopause to reach true interstellar space. One possibility for that earlier probe was a ‘Sun-diver’ maneuver, a close pass by the Sun to gain a gravitational slingshot effect, followed by an additional kick from an onboard booster.
The thinking a few years back was to reach 1000 AU in less than fifty years, but Innovative Interstellar Explorer has lost the Sun-diver maneuver and focuses on a more realistic 200 AU, as part of a NASA Vision Mission study that contemplates a gravitational assist at Jupiter and the use of radioisotope electric propulsion. IIE is subject to the same funding constraints as any other mission of this nature but it’s well worth perusing its specs on the site, for McNutt is both scientist and visionary, a man who looks beyond the ‘lifetime of a researcher’ limit for mission duration.
That has taken him into interesting intellectual terrain, writing in a study for NASA’s now defunct Institute for Advanced Concepts of a future technology that could reach speeds of 200 AU per year. That’s fast enough to get you to Epsilon Eridani in 3500 years, approximately the lifetime of the Egyptian empire. Writes McNutt:
“A more robust propulsion system that enabled a similar trajectory toward higher declination stars such as Alpha Centauri could make the corresponding shorter crossing in a correspondingly shorter time of ~1400 years, the time that some buildings have been maintained, e.g., Hagia Sophia in Constantinople and the Pantheon in Rome. Though far from ideal, the stars would be within our reach.”
Human Expeditions to the Gas Giants and Beyond
Given these musings, where does McNutt stand on human exploration of the Solar System itself? We learn the answer in an interesting piece that has just appeared in the Johns Hopkins APL Technical Digest where, writing with Jerry Horsewood and Douglas Fiehler, he notes the sharp constraint that radiation exposure places upon mission designers. We know we can reach the outer Solar System — our unmanned probes continue to demonstrate the capability — but humans in deep space have to cope with solar energetic particles from the Sun (SEPs) and galactic cosmic rays (GCRs). That means getting to the destination quickly.
The article looks at optimized trajectories to Callisto, Enceladus, Miranda, Triton and Pluto, five expeditions that each demand one-way flight times of no more than two years, with a total mission time of five years. Solar energetic particles can be shielded against, but running the numbers on galactic cosmic rays shows they would require a huge mass penalty for shielding. To approximate the shielding effect of the Earth’s atmosphere would involve a shield massing thousands of tons. Limiting flight times seems the only solution.
To make this happen, McNutt envisions a nuclear electric propulsion system with an overall power level of 100 MWe, with the electricity generated by the nuclear reactor being used to power up the plasma stream that propels the vehicle. The Neptune mission, targeted for a 2075 launch, would achieve 197.5 kilometers per second with a thrust time of 1.2 years — compare that to the 16.2 kilometers per second New Horizons is currently managing on its trajectory to Pluto/Charon. And the trajectories of these five fast missions are themselves interesting:
The striking point for all of these trajectories, and especially for the three outermost targets, is the lack of curvature. To date, planetary transfer trajectories make use of near-Hohmann-transfer orbits (minimum-energy solutions), albeit sometimes with intermediate planetary gravity assists. Propulsive maneuvers typically are used for gravitational capture at the target, rather than slowing down from faster-than-required transfer orbits. The “straight” trajectories are driven by the requirement of a fixed transit time; without the interplanetary deceleration period before reaching the target planet, the spacecraft in each case would escape from the solar system.
Demands of the Journey
It’s the radiation constraint that pushes our propulsion technologies well past current capabilities, shortening acceptable trip times and demanding speeds that in our current context are almost surreal. Back in 1968, Clarke and Kubrick’s 2001: A Space Odyssey sent the ‘Discovery 1’ mission to Jupiter without evident regard for radiation shielding, and young optimists like me in the audience assumed that the outer planets would be within reach some time in the early 21st Century. Now we’re talking about putting together a set of missions that vaguely resemble Clarke and Kubrick’s a century later than the film had supposed.
Interestingly, by McNutt’s calculations, these expeditions would be mounted in a vehicle offering a habitable volume about twice that of the spaceship in 2001 if we assume a crew of ten (a crew of six is also considered in the paper). And if 2001 didn’t concern itself with enroute radiation, another thing it didn’t dwell on was the method for constructing the interplanetary craft. To build such a vehicle, we’ll need something like the extremely heavy lift launch vehicles (EHLLVs), or ‘Supernovas,’ that were originally studied in the 1960s. McNutt discusses lifting a thousand tons to low-Earth orbit with each launch for assembly of the outer system spacecraft in space. The study envisions 30 Supernova launches for the five expeditions.
Costs of an International Venture
All of this adds up to huge costs, some $4 trillion, which compares to a US GDP of $13 trillion in 2006 and a world GDP in the same year of $48 trillion. The five expeditions to the outer planets would clearly demand an international initiative, one that would cost 1.5 times the U.S. cost of World War II in 2006 dollars. From the study’s summary:
A 5-year round-trip mission will require ~10 t per person of expendable supplies with a likely crew of at least six people and an extremely reliable vehicle with an extremely dedicated and stable crew. Infrastructure capable of putting tens of thousands of metric tons of materials into LEO will be required as well. Such a project is potentially achievable at the cost of at least 10% of the current world GDP. With current investment in human space activity in the United States, even with growth projected on the basis of the growth of the overall U.S. economy, a dedicated, international effort will likely be required if the entire solar system is to have an initial reconnaissance by human crews by the beginning of the 22nd century.
Getting a human presence to the outer planets by the end of the century is going to be tough even if we assume the propulsion advances that can achieve 200 kilometers per second — or in the case of Pluto/Charon, over 300 kilometers per second. But this is exactly the kind of study we need to place our current technology in context. We can’t assume anything about future breakthroughs. We can only define the problems we face so that in that context, future work may produce solutions that can lower travel times and costs to acceptable levels.
The report is McNutt, Horsewood and Fiehler, “Human Missions Throughout the Solar System: Requirements and Implementations,” available online. McNutt’s Phase I and II studies for the NASA Institute for Advanced Concepts are still available on the NIAC site.
by Paul Gilster | Jun 2, 2010 | Deep Sky Astronomy & Telescopes |
Students now getting their degrees in astronomy and even postdocs working in the field have come along at a time when datasets are widely shared. It was not always so, as Alexander Szalay can attest. A professor of physics and astronomy at Johns Hopkins, Szalay was an early player in the Sloan Digital Sky Survey, leading the design of the archive and becoming involved with the statistical tools needed to analyze its holdings.
Back in the 1990s, Szalay recalls in the Chronicle of Higher Education (thanks to Regina Oliver for the tip), the astronomy community had no tradition of making data from projects like the SDSS public. In fact, astronomy at the time was a more tightly controlled enterprise. Telescope time, as always, was difficult to get, and no scientist wants critical findings to be claimed by someone else. Szalay remembers that era and the changes that quickly followed:
One incident demonstrates the mood at the time. A young astronomer saw a dataset in a published journal and wanted to reanalyze it, so he asked his colleague for the numbers. The scholar who published the paper refused, so the junior scholar took the published scatterplot, guessed the numbers, and published his own analysis. The original scholar was so upset that he called for the second journal to retract the young scholar’s paper.
Mr. Szalay said that astronomers changed their minds once the first big datasets hit the Web, starting with some images from NASA, followed by the official release of the first Sloan survey results in 2000.
“Once they saw the first data release, and they also saw that it was easy to use, I think they started turning around,” he said.
The Science of Shared Datasets
Quick as it was, that change looks to be lasting, and the astronomy community was in the forefront of it. We are in the era of large, shared datasets, a fundamental change in the way science is done that maximizes computer resources on desktops around the world. We’ve all become familiar with projects like SETI@Home, but the Galaxy Zoo is broadening the notion of ‘crowd science,’ categorizing images from the same Sloan Digital Sky Survey that Szalay helped structure. The Galaxy Zoo is about active participation, asking volunteers to examine and categorize images. And now there’s a Moon Zoo that lets them devote computer time to the tiniest lunar features.
Image: The 60-inch reflector at Mount Wilson, built in an era when astronomy was a solitary profession. Credit and Copyright: Wally Pacholka (TWAN), courtesy Mike Simmons (AWB).
The word ‘crowdsourcing’ is one of those ungainly coinages so common in the computer era that come into existence because we have no other good way to describe a phenomenon. It’s spreading into biology (the Gene Map Annotator and Pathway Profiler) and, excitingly, into oceanography. There are interesting parallels between space exploration, biology and what goes on under the oceans, especially in the fact that huge amounts of data are becoming available for widespread use. From the article:
Another gush of data is happening deep in the Pacific Ocean, as a series of thousands of sensors strung along an underwater fiber-optic cable, along with new self-guided mobile sensors that can beam back data, promises to make oceanography the next field to embrace the data revolution and a crowd approach.
Mr. Lazowska, the computer scientist at the University of Washington who focuses on data-driven science, says that at the moment oceanography is “expeditional,” meaning that data are hard to come by because only a few organizations can afford the equipment to probe the depths. But new technologies, like those mobile sensors, promise to pipe in more data than scientists can manage without a shared database, like what the Sloan project did for astronomy.
Spreading Space Mission Science
We’ve seen these issues play out in the space community. Even when a mission is compromised, as Galileo was enroute to Jupiter when it was discovered that its high gain antenna could not be deployed, the data returned from its instruments take years to sort through, and will increasingly be handled by computers or armies of human volunteers. As for Szalay, he’s involved with an attempt to link large telescope datasets called the National Virtual Observatory. The emerging paradigm is all about putting eyes on data that used to be read by a single researcher. Papers emerge with more authors than ever listed and careers are shaped around building the tools to mine these data in powerful new ways.
Science archive centers for NASA mission datasets created the need for these developments and the digital sky surveys like Sloan and 2MASS demonstrated how online datasets could be tapped. The National Virtual Observatory concept emerged as early as 1999 and was fleshed out at a series of workshops and conferences leading to a 17-member organization that is building the needed infrastructure, along with a similar effort in Europe called the European Virtual Observatory. Here’s a description from the NVO materials available online:
The VO will enable a new way of doing astronomy, moving from an era of observations of small, carefully selected samples of objects in one or a few wavelength bands, to the use of multi-wavelength data for millions, if not billions of objects. Such datasets will allow researchers to discover subtle but significant patterns in statistically rich and unbiased databases, and to understand complex astrophysical systems through the comparison of data to numerical simulations. The VO will provide simultaneous access to multi-wavelength archives and advanced visualization and statistical analysis tools.
When the Galaxy Zoo came online in July of 2007, its server was swamped, and within 24 hours of launch, the site was receiving 70,000 galaxy classifications per hour, with more than 50 million received during the first year. Numerous projects are now in motion using these data (see the Zooniverse site for more). What’s exciting about the newly emerging initiatives is that, unlike SETI@Home, they demand active investigation by their audience, as opposed to simply letting a screensaver run when the PC is not otherwise in use.
The Quiet of the Mountaintop
I’ve written before in these pages about ‘Hanny’s Voorwerp,’ the unusual object now believed to be a gas cloud that was spotted by Dutch schoolteacher Hanny van Arkel. That was a Galaxy Zoo find, and van Arkel’s name is on papers about its significance in explaining the life cycle of quasars. These are exciting times for those with a good Net connection and a yen to participate in cutting-edge science. And as the Chronicle notes, Alexander Szalay, while a highly regarded astronomer, hasn’t looked through a telescope in almost ten years. Large-scale collaborations are moving to supplant the solitary scientist on a mountain top.
Image: Edwin Hubble at work in an era without computerized datasets. Credit: Mount Wilson Observatory.
Did I say ‘solitary’? Here’s a description of Edwin Hubble using the 100-inch instrument on Mt. Wilson, drawn from Elaine Bartusiak’s wonderful The Day We Found the Universe (Pantheon, 2009):
Observing with the 100-inch was a choreographed dance within the monumental dome a hundred feet high and nearly as wide. Sometimes Hubble could just lean back in a bentwood chair, his favorite, and serenely smoke his pipe in the darkness while taking a photograph. But other times he was perched high in the air on a platform that could adjust to any height via rails set on either side of the dome opening. With the telescope’s clock drive shifting the telescope as the nighttime sky slowly moved overhead, he and his assistant made sure the advance stayed in synchrony with Earth’s rotation… “This was the astronomical observing experience at its best,” noted Mount Wilson astronomer Allan Sandage, “a dark, quiet dome, a silently moving monster telescope, and mastery of the dangerous… platform, all in the interest of collecting data on a problem of transcendental significance.”
And if the night turned cloudy? Hubble said “You begin with the deskwork, later you turn to heavy reading, and later, to a detective story.” The solitary explorer on the mountaintop is a romantic vision that’s part of how we all conceive of astronomy, and it’s a glorious part of the science’s history, but in today’s world, the astronomer puts down the detective story and dips into the worldwide data flow.
by Paul Gilster | Jun 1, 2010 | Sail Concepts |
The crescent Earth is lovely in this ultraviolet photo taken by the Japanese Akatsuki probe, now enroute to Venus. The shot was made at a distance of about 250,000 kilometers and keeps me in mind of the IKAROS solar sail demonstrator, which was launched along with Akatsuki and several other payloads on May 20. It’s been tricky keeping up with IKAROS (let’s just say my Japanese is not up to speed, and neither is Google Translate), but a ‘tweet’ from JAXA yesterday said that four cameras aboard the spacecraft had captured images of deployed tip masses, a cause for applause in the IKAROS control room. Photos of that deployment (on May 28) are available on this JAXA site. The sail deployment procedure begins with release of the tip masses and proceeds through stages, as shown below.
Image: Deployment procedure for IKAROS.
We’ll follow IKAROS with great interest as we move toward full sail deployment. Meanwhile, word from Louis Friedman at the Planetary Society is that LightSail-1 is probably not going to be launched until the second quarter of 2011 at the earliest, the delay caused by the need to piggyback aboard an existing mission that can reach the higher orbit the sail requires. The Planetary Society is working with NASA Ames to find launch opportunities.
Suddenly we’re in very interesting times for solar sails, after a long period when the technology seemed ready for testing in space but the opportunity to do so stubbornly failed to emerge. LightSail-1 is the first of three sail missions, with a demonstration of sail technologies to be followed in the second mission by refinements with a larger payload and finally, with LightSail-3, movement into interplanetary space to take on early-warning duties for geomagnetic storms from the Sun. The latter will involve stationing the sail at the Sun-Earth libration point, L1.
LightSail-1 sports a sail area of 32 square meters in the form of four triangular blades. Built around three Cubesat spacecraft, the vehicle will mass less than five kilograms and should offer a lower mass-to-area ratio (and thus higher acceleration from sunlight) than the Society’s Cosmos-1 sail, which was lost in a launch accident in 2005. A recent review by the LightSail project team has resulted in simplication in some aspects of the design, such as using a single-channel radio rather than the planned two-radio system. A critical design review is upcoming, after which the design will be frozen and the team will begin to build the spacecraft.
Hopes are high for LightSail-1 and luck seems to follow the project, as Friedman reports:
…colleagues have a high-sensitivity, commercially developed accelerometer from Lumedyne Technologies they want to test. With LightSail-1, we can supply a flight test, they get the data they need, and we get an advanced set of microminiaturized, high-performance accelerometers for our spacecraft. We’ve just signed an agreement with Millennium Space Systems to that end; the company will provide the software and special processing algorithms to utilize these accelerometers for the LightSail mission.
Image: The Planetary Society’s LightSail-1 will test out solar sail technologies in Earth orbit as a prelude for later missions including solar storm monitoring at L1.
As Friedman says he is committed to the idea of onboard acceleration data, the Lumedyne technology should prove hugely useful in demonstrating the effect of momentum transfer from photons, the basic mechanism for solar sail operations. Friedman told Thomas Mallon in a 2009 Atlantic article that “We’re not trying to be another NASA…but we do show the value of being able to come up with clever things that interest the public, that are scientifically solid.” A successful LightSail-1 will demonstrate the premise, but as to not being another NASA, that may be to Friedman’s advantage. NASA has been working on solar sail technology for a long time now, but a privately funded and built mission will be the next solar sail to fly.
