by Paul Gilster | Mar 11, 2005 | Missions
Nature is reporting that the two Voyager missions — recently discussed here as our first active probes of the interstellar medium, if they live long enough to cross the heliospheric boundary — may be terminated in October. The decision is not yet final, and there is always the hope that it will spur enough reaction among space scientists and others to force a reprieve.
But if these missions end (along with six others, including Ulysses), the loss to science would be severe. Voyager 1 is the fastest man-made object, now leaving the Sun behind at over 17 kilometers per second, at a current distance of approximately 94 AU (14 billion kilometers from Earth). Voyager 2 is roughly 76 AU out. Both spacecraft should be able to continue transmitting until 2020 or later.
At $4.2 million per year, the Voyager program catches NASA’s eye as the agency ponders budgetary cutbacks. But to shut down two operational spacecraft as they approach the interstellar medium for the first time in history is unthinkable.
Robert Forward told a budgetary tale with some resemblance to this one in his 1990 novel Rocheworld. A human crew has been sent to Barnard’s Star, on a spacecraft powered by a laser sail. Forty years out, the question for Earth’s politicians becomes whether or not to finance the enormous laser installation that must be fired to allow the vehicle’s sail system to decelerate upon arrival (how Forward set up deceleration by laser beam from Earth is so ingenious that it deserves, and will get, a separate entry in the near future). Forward also solved the political problem, but then, he knew a thing or two about how committees work.
No one can discount the fiscal realities, but we have to hope that a formal scientific review of these proposed funding shutdowns will be initiated, as has been done in the past when issues of mission termination have arisen. That review should take into account not only the scientific value of the data both Voyagers can provide as they approach and cross the ‘termination shock’ and the heliopause, but also the less tangible philsophical return from the exploration of utterly uncharted territory. Our knowledge of history should remind us that pushing back boundaries is a human imperative.
Centauri Dreams has made the case that robotic exploration of the outer Solar System will lead to interstellar probes that will examine the Oort Cloud and, eventually, the nearest stars. The technologies for making such probes a reality — and the issues they raise — are on display in microcosm in the Voyager story. These two spacecraft have demonstrated long-life electronics and communication over vast distances. They have taught us much about how to improve in both areas, and how to create robotic systems that can assume greater spacecraft autonomy, as witness Cassini. On the propulsion front, we have learned to harness gravity ‘slingshot’ maneuvers around planets, and now we have flown ion engines in space. We are not far from flight testing a solar sail.
But even as we do these things, we have two functional probes pushing ever deeper into a region no instruments have ever seen. To abandon these forerunner craft, 10000 days out from Earth and still alive, would be to fall victim to the worst kind of short-term thinking. It would be to turn our backs on a gift to the human future.
The Nature article is “NASA Funding Shortfall Means Journey’s End for Voyager Probes,” in the issue of March 9. Also see “Space Science: NASA Plans to Turn Off Several Satellites” in Science 307, p. 1541.
by Paul Gilster | Mar 10, 2005 | Outer Solar System
The Cassini Imaging Team has published its first findings about Titan in the journal Nature. The complexity of Titan’s surface and the extent to which it is continually modified draws the most attention. Where are the craters that should have pocked its surface over the past billion years? Thanks to Cassini/Huygens, some answers are beginning to emerge. Working with the last eight months of imagery from the orbiter, the team reports that thirty percent of the satellite’s surface has now been mapped with resolutions high enough to pick out features as small as one to ten kilometers.
From a press release from the Cassini Imaging Central Laboratory for Operations (CICLOPS):
At this scale, what has been discovered are geologically young terrains with signs of tectonic resurfacing, erosion by liquid hydrocarbons, streaking of the surface materials by winds and only a few large circular features thought to be impact craters formed in the ice ‘bedrock’. (The largest of these – a 300-kilometer (190-mile) wide, double-annulus structure to the northeast of the large region called Xanadu – was recently imaged by Cassini’s Radar instrument, providing independent confirmation of an impact origin.)
That means large craters have been erased by tectonic activity, erosion and the flow of surrounding material. The tectonism shows up in the linear boundaries found between Titan’s light and dark areas as seen from Cassini’s close passes of the moon.
Dr. Alfred McEwen, imaging team member from the University of Arizona, said, “The only known planetary process that creates large-scale linear boundaries is tectonism, in which internal processes cause portions of the crust to fracture and sometimes move – either up, down, or sideways. Erosion by fluids may then serve to accentuate the tectonic fabric by depositing dark materials in low areas and enlarging fractures. This interplay between internal forces and fluid erosion is very Earth-like.”
Image: This map of Titan’s surface brightness was assembled from images taken by the Cassini spacecraft over the past year, both as it approached the Saturn system and during three closer flybys in July, October and December 2004. The map reveals complex patterns of bright and dark material on Titan’s surface. The large scale features, including Xanadu Regio — the large, bright feature that extends from approximately 80 degrees to 130 degrees west near the equator — have been observed from Earth over the past several years. Credit: NASA/JPL/Space Science Institute.
Intriguingly, cloud activity over Titan’s south pole indicates to some scientists that this is where a methane rain cycle, with accompanying channel carving, runoff and evaporation, may be most active. All of this means we may have the opportunity to observe surface changes on Titan within short time frames, something that would have seemed unlikely to many planetary scientists on any of the outer planets’ moons until 1979, when Voyager 1 revealed volcanic activity on Io, and ten years later, when Voyager 2 imaged the strange ‘ice volcanoes’ of Triton.
And another indication of change on Titan, also from CICLOPS:
In Titan’s hazy stratosphere, it looks as though modelers may have to go back to the drawing board. Voyager images of Titan detected a faint detached haze layer above Titan’s main stratospheric haze, at altitudes of 300-350 kilometers (190 to 220 miles). Cassini ultraviolet images, which are sensitive to scattering of sunlight by small particles, detect a similar detached haze layer, but at an altitude of 500 kilometers (310 miles) instead.
“The change we see in the detached haze over the 25 years since Voyager suggests that either the photochemical process that produce the hydrocarbon haze particles, or the atmospheric circulation that distributes them around the planet, may change with the seasons,” said imaging team member Dr. Bob West of the Jet Propulsion Laboratory, who designed all the Titan atmosphere imaging sequences for the Cassini mission. “It will be a challenge for models to be able to predict how and where these detached hazes occur,” he said.
The Nature article is Porco, Baker, Barbara et al., “Imaging of Titan from the Cassini spacecraft,” Nature 434, 159 – 168 (10 March 2005).
by Paul Gilster | Mar 9, 2005 | Culture and Society
The death of Hans Bethe has been covered by the media worldwide, and William J. Broad’s obituary in the New York Times seems among the most thorough and accurate of the accounts of his life. But to me, Bethe will always be seen through Richard Feynman’s eyes, and I think Broad misses the point of the one Feynman anecdote he tells.
Feynman first worked with Bethe at Los Alamos during the days of the Manhattan Project, and he recalls the physicist’s openness to debate, and his focus on the issue at hand rather than personality. Here, Feynman has just arrived in Los Alamos, and work had just begun, as told in the wonderful Surely You’re Joking, Mr. Feynman! (New York: W.W. Norton, 1984):
“Every day I would study and read, study and read. It was a very hectic time. But I had some luck. All the big shots except for Hans Bethe happened to be away at the time, and what Bethe needed was someone to talk to, to push his ideas against. Well, he comes in to this little squirt in an office and starts to argue, explaining his idea. I say, “No, no, you’re crazy. It’ll go like this.” And he says, “Just a moment,” and explains how he’s not crazy, I’m crazy. And we keep on going like this. You see, when I hear about physics, I just think about physics, and I don’t know who I’m talking to, so I say dopey things like “no, no, you’re wrong,” or “you’re crazy.” But it turned out that’s exactly what he needed. I got a notch up on account of that, and I ended up as a group leader under Bethe with four guys under me.” (p. 112).
So the point wasn’t that, as Broad says, “Colleagues often balked” at Bethe’s ideas. The point is that most colleagues wouldn’t speak up the way Feynman did. When I first read Feynman’s account, I recalled Norman Eliason, a world-class medievalist I had the pleasure of working with at Chapel Hill. He was pushing me to do my dissertation on Anglo-Saxon metrics, as found in such Old English poems as Beowulf, and the experience was much the same as what Feynman encountered with Bethe. Although widely feared by graduate students, Eliason wanted to be challenged; he would lift his eyes to the heavens when you did so, and try to take you down a notch, preferably in a classroom setting. It was all part of the game. What he didn’t want was for you to back down (all too many did).
Bethe and Feynman were key players in the development of quantum electrodynamics (Feynman won a Nobel Prize for this work in 1965). We could use more of their provocative, inquiring spirit, unhampered by ego and any motive other than pure love for knowledge — what Feynman called ‘the kick in the discovery’ — wherever research is conducted on any subject.
Bethe’s output was legendary: “If you know his work,” said John Bahcall of the Institute for Advanced Study, delivering his own appreciation, “you might be inclined to think he is really several people, all of whom are engaged in a conspiracy to sign their work with the same name.” And anyone who’s getting up in years will admire a man who, at the age of 83, set about probing the immensely difficult lattice gauge theory, which deals with the transformation of nuclear materials into plasma at high temperatures. Much of his retirement, in fact, was devoted to the study of astrophysics.
Finally, I note this wonderful description from Lee Edison’s 1968 profile of Bethe, as quoted in Broad’s obituary. Calling him “a tall, spare man with a deceptively distracted look,” Edison goes on to say:
“His graying hair seems permanently electrified; his shoes are scuffed, and his tie seems to have been studiously arranged to miss his collar button. He listens attentively, nodding his head as if in agreement, but – as devastated colleagues and adversaries have discovered – this habit is far from a sign of agreement. His ‘yes, yes, yes’ is rather a signal that his mental apparatus is receiving. What he does with the input is another matter.”
Cornell’s own press release on Bethe’s death is also worth reading. Bethe died in Ithaca at age 98. At his death he was emeritus professor of physics at Cornell, having joined the university in 1935 after fleeing Nazi Germany.
by Paul Gilster | Mar 9, 2005 | Missions, Outer Solar System
Has Voyager 1 left the heliosphere? The question is a reminder that the Voyagers are our first interstellar probes; they’ll still be returning data when they move into the interstellar medium. The heliosphere is a kind of bubble created by the solar wind from the Sun, that stream of high-speed charged particles constantly blowing into space at roughly 400 kilometers per second. Observing how Voyager 1 makes the transition across the boundary of the heliosphere will provide our first in situ study of interstellar space.
Some scientists believe that at roughly 90 AU from the Sun, Voyager 1 has already pushed up against the ‘termination shock,’ that region where the speed of the solar wind drops to subsonic levels. Now new data studied by French and Finnish researchers indicate that the shape of the heliosphere may be distorted, further complicating the question of just where the true interstellar medium begins.
Rosine Lallement and colleagues used data collected by the Solar and Heliospheric Observatory (SOHO), studying the direction of helium and hydrogen particles from the area where the heliosphere and interstellar space begin to interact. Their findings suggest that Voyager 1 remains within an elongated region well within the heliosphere. We’ll be learning much more about the heliosphere’s boundaries as Voyager continues its extended mission, returning data perhaps as late as 2020.
The paper is R. Lallement, E. Quemerais et al., “Deflection of the Interstellar Neutral Hydrogen Flow Across the Heliospheric Interface,” Science 307, pp. 1447-1449 (4 March 2005).
by Paul Gilster | Mar 8, 2005 | Advanced Rocketry
Sonoluminescence — the emission of light from bubbles in a liquid that has been excited by sound — is a mystery. How does a sound wave put enough energy into such a small volume as to cause light to be emitted? The concentration of energy needed is something like a factor of one trillion, according to this Los Alamos National Laboratory introduction to the phenomenon. And not only that; the spectrum of the emitted light implies extremely high temperatures. Fusion, anyone?
Well, not yet. But the slang term for sonoluminescence, ‘star in a jar,’ seems a little closer to reality now that the first direct measurements of the phenomenon are in. They show that the temperature inside a collapsing bubble can reach 20,000 degrees Kelvin, which is four times the temperature of the surface of the Sun. This work, by Ken Suslick and David Flannigan (University of Illinois at Urbana-Champaign), comes two years after controversial findings by an Oak Ridge National Laboratory team that found evidence of fusion in sonoluminescence experiments, but the Illinois researchers are more circumspect. They claim only that plasma can be formed in the process, and it is known that confined fusion reactions require a plasma.
Image: A cloud of gas bubbles in a liquid excited by ultrasound (generated by a titanium rod vibrating 20,000 times a second) can emit flashes of light (sonoluminescence) due to extreme temperatures inside the bubbles as they collapse. A single bubble can be trapped in an ultrasonic field and driven into oscillation will flash on every cycle when it reaches maximum compression. Photo by K.S. Suslick.
The explosive collapse of bubbles of gas in a liquid is known as cavitation. An article posted online at Nature.com explains sonoluminescence as performed by Suslick and Flannigan:
The sound waves (between 20 and 40 kilohertz) produce areas of high and low density within the liquid, making pressure at any one point oscillate between two extremes. Bubbles of gas in the liquid swell rapidly at lower pressures before being squeezed tight by the high pressure that follows.
The change in pressure is so fast that the bubble effectively implodes with enough force to generate tremendous heat, in a process called acoustic cavitation. “Compress a gas and you heat it, just like pumping up a bicycle tire,” explains Suslick. The heat separates electrons from their atoms, and as they snap back into position the energy they acquired is released as a burst of light.
The temperatures thus far recorded are remarkable, as noted in this UIUC press release:
“At 20,000 degrees Kelvin, this emission originates from the plasma formed by collisions of atoms and molecules with high-energy particles,” Suslick said. “And, just as you can’t see inside a star, we’re only seeing emission from the surface of the optically opaque plasma.” Plasmas are the ionized gases formed only at truly high energies.
Suslick and Flannigan worked with sulfuric acid containing traces of argon gas instead of the water used in previous experiments. The resultant release of light was 2700 times more powerful, and clearly points to continued investigation of different gas/liquid mixtures. Suslick offers a Web page on sonoluminescence at UIUC.
‘Bubble fusion’ has yet to be proven, but clearly sonoluminescence is becoming more and more promising in studying how highly energized plasma forms. Suslick and Flannigan’s paper “Plasma formation and temperature measurement during single-bubble cavitation” appears in the March 3 issue of Nature (Vol. 434, pp. 52 – 55); an abstract can be read here. The Oak Ridge team’s paper is Taleyarkhan et al., “Evidence for Nuclear Emissions During Acoustic Cavitation,” Science Vol. 295, pp. 1868-1873 (2002).
