If you’re trying to figure out how fast a gas giant rotates, you have your work cut out for you. Jupiter seems to present the easiest case because of the famed Red Spot, first observed by the Italian astronomer Giovanni Cassini. But gas giants are thought to have a relatively small solid core, one that is completely obscured by their atmospheres. Rotation involves atmospheric effects as the gases slosh and swirl. No wonder astronomers were glad to find Jupiter’s pulsating radio beams, discovered in the 1950s. Rotation of the planet’s inner core results in a magnetic field that produces these signals, offering our best estimate on the planet’s actual rate of rotation.
We now know that the largest of the planets is also the fastest rotating, completing one rotation every 9.9 hours. But even this turns out to be an average because the gaseous nature of the planet causes it to experience differential rotation. Head for the poles and you find a slightly slower rotation period than you do at the equator. The rotation speed of the Jovian magnetosphere is usually the rate cited, but Jupiter teaches how tricky gas giants can be. The equatorial bulge caused by its swift rotation even makes it tricky to measure the planet’s diameter, which will vary depending on whether you measure it from the equator or the poles.
The Problem with Neptune
University of Arizona planetary scientist Erich Karkoschka had to keep such issues in mind when he went to work on the rotation rate of Neptune, another planet whose surface is shrouded by a thick atmosphere. The scientist went on to analyze publicly available images of Neptune from the Hubble Space Telescope archive, studying 500 of them to record details of the atmosphere and track distinctive features over long periods of time. Two features in Neptune’s atmosphere drew his attention, rotating five times more steadily even than Saturn’s hexagon, an atmospheric feature previously thought to be the most regularly rotating feature of any of the gas giants.
The two Neptunian features are known as the South Polar Feature and the South Polar Wave, and the odds favor their being vortices in the atmosphere similar to Jupiter’s Red Spot. Using the Hubble imagery, Karkoschka was able to track them over the course of twenty years. He arrived at a rate for Neptune’s rotation that he believes to be 1,000 times better than earlier estimates. The regularity was remarkable: Both features appear exactly every 15.9663 hours, with less than a few seconds of variation. The regularity suggests these features have some kind of connection to Neptune’s interior, possibly the result of convection driven by warmer and cooler atmospheric areas, but at this point, the exact process remains to be determined.
Image: In this image, the colors and contrasts were modified to emphasize the planet’s atmospheric features. The winds in Neptune’s atmosphere can reach the speed of sound or more. Neptune’s Great Dark Spot stands out as the most prominent feature on the left. Several features, including the fainter Dark Spot 2 and the South Polar Feature, are locked to the planet’s rotation, which allowed Karkoschka to precisely determine how long a day lasts on Neptune. (Credit: Erich Karkoschka).
Karkoschka’s next move was to home in on Voyager imagery from 1989, which offers resolution higher than the Hubble instrument, searching the area around the two identified features:
I discovered six more features that rotate with the same speed, but they were too faint to be visible with the Hubble Space Telescope, and visible to Voyager only for a few months, so we wouldn’t know if the rotational period was accurate to the six digits,” the scientist added. “But they were really connected. So now we have eight features that are locked together on one planet, and that is really exciting.”
We have a great deal to learn about the interiors of gas giants, but this work is a step in the right direction. We have a measure of Neptune’s total mass. What we lack is information on how it is distributed. Karkoschka again:
“If the planet rotates faster than we thought, it means the mass has to be closer to the center than we thought. These results might change the models of the planets’ interior and could have many other implications.”
Saturn’s Rotational Challenges
A bit closer to home than Neptune, Saturn reminds us how quickly adding to our data can overturn established estimates. Voyager 1 and 2 found radio signals as they flew past Saturn that could be clocked at exactly 10.66 hours, seemingly iron-clad evidence of the planet’s rotation period. Then Cassini arrived. And while we were all caught up in the visual splendor of the scenes the spacecraft sent back, its sensors were detecting a change in the period of the radio signal of about one percent that could not have been the result of a change in rotation. Cassini went on to discover that Saturn’s northern and southern hemispheres seemed to be rotating at different speeds, another puzzle to add to the gas giant rotation problem.
We thus learn that the radio signals originating with Saturn’s magnetic field are not quite as reliable as we had thought, lagging behind the planet’s core as the interior rotates and drags the magnetic field with it. The sloshing gases of the outer planets continue to confound us, though the work of Karkoschka and others is helping us to pin down the problem areas. The new work is also a splendid example of how much science can be done with publicly accessible data, items long filed away that are ripe for further analysis and may contain clues to such mysteries.
The paper is Karkoschka et al., “Neptune’s Rotational Period Suggested by the Extraordinary Stability of Two Features,” in press at Icarus (abstract).
We’ve often speculated about the potential uses of the solar wind in pushing a ‘magsail’ to high velocities for missions beyond the Solar System. This isn’t solar sailing of the conventional type, in which the transfer of momentum from solar photons is the operating force. Instead of photons, a magsail would rely on the solar wind’s stream of charged particles, which can reach speeds of up to 800 kilometers per second. One problem, of course, is that the solar wind varies hugely, variations that might make managing a magsail a daunting task. In any case, before we can contemplate such missions, we have much to learn about how the solar wind operates.
Not all of that work is going to focus on our own Sun. We’re also learning how stellar winds operate in other star systems through careful observation, as new work from the European Space Agency’s XMM-Newton space observatory reminds us. The spacecraft recently observed a flare during a scheduled 12.5-hour observation of a system known as IGR J18410-0535, where a neutron star and a blue supergiant are in close proximity. The flare, which at X-ray wavelengths was almost 10,000 times the star’s normal brightness, resulted from the neutron star trying to absorb an enormous clump of matter flung from its companion.
Image: Artist’s impression of a neutron star partially devouring a massive clump of matter. Credit: ESA.
Enrico Bozzo (University of Geneva) says the flare was the result of ‘a huge bullet of gas that the star shot out,’ one that hit the neutron star, causing the gas in the clump to be heated to millions of degrees. Although expelling matter into space is a normal part of the stellar wind for all stars, the intensity of this particular X-ray flare shows that the blue supergiant star can release chunks of gaseous matter so large that most of it didn’t even hit the neutron star.
Without the nearby neutron star, such an event would not have been detectable from Earth, but with its help we can gain insight into the varied behavior of stellar wind patterns. The XMM-Newton spacecraft recorded the flare as lasting four hours, allowing astronomers to estimate the size of the clump of matter at some 16 million kilometers across, about 100 billion times the volume of the Moon, though containing only about 1/1000th of the Moon’s mass. The neutron star, thought to be about 10 kilometers in diameter, is the collapsed core of a once far larger star, now so dense that it generates a strong gravitational field. It now proves to be a useful detector for a stellar event of the kind we should be able to observe in other such systems.
We’ve taken useful measurements of our own star’s solar wind through the Ulysses and Advanced Composition Explorer spacecraft, not to mention the continuing work of our Voyagers as they study the solar wind’s behavior at the edge of the Solar System. Even with our own relatively stable star, we’ve learned that the solar wind is turbulent, with streams of material moving at different speeds and colliding to produce so-called Co-rotating Interactive Regions. Slower moving winds tend to come from regions overlying sunspots, while high-speed winds are associated with coronal holes, dark coronal regions often found at the Sun’s poles.
A slow-moving stream pushed by faster material behind it can produce shock waves that accelerate solar wind particles to high speeds, buffeting the Earth’s magnetic field and producing storms in our planet’s magnetosphere. We also find magnetic clouds produced when solar eruptions carry material off the Sun along with embedded magnetic fields (for more on all this, see this useful MSFC page on the solar wind). Now imagine trying to ride this wind using a magnetic bubble hundreds of kilometers in diameter — formed by injecting plasma into a magnetic field — as envisioned in Robert Winglee’s Mini-Magnetospheric Plasma Propulsion idea. Or take the idea a step further still, using a magsail to decelerate an interstellar probe by braking against the destination star’s stellar wind. The more we learn about how stars shed matter, the sooner we’ll discover how practical some of these concepts really are.
Image: Artist’s impression of a mini-magnetosphere deployed around a spacecraft. Plasma or ionized gas is trapped on the magnetic field lines generated onboard, and this plasma inflates the magnetic field much like hot air inflates a balloon. Credit: Robert Winglee.
Can magsails get their push from something other than the highly changeable solar wind? Dana Andrews (Andrews Space) has argued for some time that a magnetic sail could be pushed by a neutral plasma beam. That paper makes for fascinating reading. It’s “Interstellar Propulsion Opportunities Using Near-Term Technologies,” in Acta Astronautica Vol. 55 (2004), pp. 443-451. Robert Winglee (University of Washington) developed the Mini-Magnetospheric Plasma Propulsion concept in Phase I and II studies for NIAC. The Phase II study, “Mini-Magnetospheric Plasma Propulsion, M2P2” is particularly useful (full text). It is only one of a number of magsail concepts being investigated in the literature.
It’s always good to dream big, but sometimes dreams take you in unexpected directions. Growing up with science fiction, I reveled in tales of manned exploration of the Solar System and nearby stars, many of which I assumed would eventually become reality. But I never dreamed about personal computers. You can go through the corpus of science fiction in the first two-thirds of the 20th Century and find many a computer, but there are few tales involving personal computers on the desktop. An exception is Murray Leinster’s short story ‘A Logic Named Joe,’ which ran in the March 1946 issue of Astounding Science Fiction. Leinster invokes something like today’s massively networked computers in a story that anticipates the Internet.
How did science fiction fail to see something as huge as the PC revolution coming down the tracks? Maybe it’s because the future still surprises even those whose business it is to imagine it. I’m musing about all this because of my own desktop PC and the views it’s showing me, not to mention the continuous datastream that updates every mission I’m keeping an eye on. Surely this is a science fictional future as real as Leinster’s.
A widely distributed network lets us see things on demand, one of a kind things like an object that has never before been seen in detail as it slowly swims into focus in our cameras. We’ll have that experience in 2015 as New Horizons arrives at Pluto/Charon, but we’re also getting a taste of it right now as the Dawn spacecraft approaches Vesta. What we’re seeing in the early images has been taken for navigation purposes by Dawn’s framing camera, and as the video shows, we’re already looking at views that are twice as sharp as the best images previously available from the Hubble Space Telescope. Surface details are still a mystery, but Dawn will eventually swing as close as 200 kilometers around the asteroid.
With Vesta and then Ceres ahead of us, let’s also keep a close eye on Pluto, and we don’t have to wait until 2015. In late June, NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA) — a modified Boeing 747SP carrying a 2.5-meter telescope at high altitude — was able to observe an occultation as Pluto passed in front of a background star. This is a case where SOFIA’s airborne capabilities shone, for Pluto’s shadow fleetingly passed over a mostly empty stretch of the Pacific Ocean. SOFIA was able to position itself in the center of the shadow’s path to make the observations.
You wouldn’t think you could do much with Pluto from a mobile observatory here on Earth, but the science is actually quite rich, says Ted Dunham (Lowell Observatory), who led the team of scientists onboard SOFIA during the Pluto observations:
“Occultations give us the ability to measure pressure, density, and temperature profiles of Pluto’s atmosphere without leaving the Earth. Because we were able to maneuver SOFIA so close to the center of the occultation we observed an extended, small, but distinct brightening near the middle of the occultation. This change will allow us to probe Pluto’s atmosphere at lower altitudes than is usually possible with stellar occultations.”
I have no images of this one, but it’s easy to follow the exploits of SOFIA on the Net. It’s also worth noting that Dunham was a member of the team that originally discovered Pluto’s atmosphere by observing another stellar occultation using the Kuiper Airborne Observatory in 1988. SOFIA’s mobility allowed the scientists to quickly change position when it was learned that the center of the shadow would cross 200 kilometers north of the aircraft’s flight path. A revised flight plan and air traffic control clearance allowed the productive change of course.
Our space-based resources send us things we would never have imagined seeing, as witness ESA’s Mars Express, which was able to perform a special maneuver of its own to observe an unusual alignment of Jupiter and the Martian moon Phobos. The alignment occurred on June 1, when there was a distance of 11,389 kilometers between Mars Express and Phobos, and a further 529 million kilometers to Jupiter.
Image: Three frames from the series of 104 taken by Mars Express during the Phobos–Jupiter conjunction on 1 June 2011. Credits: ESA/DLR/FU Berlin (G. Neukum).
We often lament how the future that was imagined in the 1950s and 60s hasn’t materialized — where are the human missions to the outer planets we thought would be flying now? — but the big surprise that science fiction never showed us was what we could see by staying home. Now we wind up looking at imagery from robotic missions on demand, tapping cameras orbiting Mars and closing on major asteroids. Moreover, a high definition video stream studying Earth down to one-meter resolution from the International Space Station is scheduled to go online in 2012. No humans near Jupiter yet, but the view on our personal screens seems to be getting better all the time.
The return of the Genesis mission in 2004 was a spectacular event, its parachute failing to deploy upon re-entry, leading to a crash in the Utah desert that seemed to have destroyed the mission’s solar wind collectors. But Genesis was a tough bird and we’re getting good science from its remains. The latest news comes from study of an instrument designed to enhance the flow of solar wind onto a small target, with the aim of measuring oxygen and nitrogen. The Solar Wind Concentrator worked well and new papers out of Los Alamos National Laboratory have now appeared, with isotopic measurements of the Sun that illuminate our system’s formation.
It’s astonishing that we have these results — Genesis flight payload lead Roger Wiens (LANL) calls Genesis “…the biggest comeback mission since Apollo 13″ — but ponder what we’ve got here. The spacecraft spent two years at the Sun-Earth L1 Lagrange point, some 1.5 million kilometers from Earth, collecting atoms of the solar wind, the stream of charged particles ejected from the Sun that flows outward through the Solar System. The mission’s highest goal was to determine the abundances of the stable isotopes of oxygen and nitrogen in the Sun. Oxygen contains three major isotopes: 16O, 17O, and 18O, while nitrogen’s stable isotopes are 14N and 15N.
Image: The Solar Wind Concentrator is a special instrument built by a team at Los Alamos National Laboratory to enhance the flow of solar wind onto a small target to make possible oxygen and nitrogen measurements. Shown here, the target section of the concentrator, which produced essential samples of nitrogen and oxygen. Credit: Los Alamos National Laboratory.
After years of painstaking analysis, we now have striking results. Both oxygen and nitrogen from other parts of the Solar System differ from what we find on Earth. Oxygen and nitrogen samples from various meteorites, along with nitrogen sampled in Jupiter’s atmosphere by the Galileo probe as well as that collected from samples of lunar soil, vary 38 percent for nitrogen and up to 7 percent for oxygen compared to the Earth. We now turn to finding where Earth’s oxygen and nitrogen came from. Wiens comments on the significance of the differences:
“For nitrogen, Jupiter and the Sun look the same. It tells us that the original gaseous component of the inner and outer solar system was homogeneous for nitrogen, at least. So where did Earth gets its heavier nitrogen from? Maybe it came here in the material comets are made of. Perhaps it was bonded with organic materials.”
As to oxygen, the papers point to a mechanism called photochemical self-shielding, which may have modified the composition of space dust before it even began to form the planets. The Sun shows an enrichment of pure 160 relative to the Earth instead of differences in 160, 170 and 180 that are proportional to their atomic weight. That fact points to solar UV radiation uniformly enhancing the two rarer isotopes, 170 and 180, in the terrestrial planets. We have much to learn, but Genesis’ fine-grained isotope hunt is giving us clues to how the solar system formed. All this from a mission that crashed in the desert, and analytical techniques forged from necessity that drew results from apparent disaster.
The papers are McKeegan et al., “The Oxygen Isotopic Composition of the Sun Inferred from Captured Solar Wind,” Science Vol. 332 No. 6037 (24 June 2011), pp. 1528-1532 (abstract) and Marty et al., “A 15N-Poor Isotopic Composition for the Solar System As Shown by Genesis Solar Wind Samples,” Science Vol. 332 No. 6037 (24 June 2011), pp. 1533-1536 (abstract).
Marc Millis, Tau Zero’s founding architect, drawing on his experience with NASA’s Breakthrough Propulsion Physics project and the years of research since, offers us some ideas about impartiality and how scientists can hope to attain it. It’s human nature to want our particular theories to succeed, but when they collide with reality, the lessons learned can open up interesting alternatives, as Marc explains in relation to interstellar worldships and the possibilities of exotic propulsion.
by Marc G. Millis
The best researchers I know seem to be able to maintain their impartiality when reaching new conclusions. The more common behavior is that people get an idea stuck in their head and then try and prove themselves correct. I just learned that there is a term for this more common behavior: “Polemical.” Embedded in the word is the notion that controversial argument can turn aggressive, an inevitable result when people are defending what they consider their turf.
I mention this in the context of getting trustworthy results, and then acting on those reliable findings rather than just charging ahead based on unverified preconceived notions. If the overall intent is to make the best decisions for the future – then decisions rooted in reliable findings, rather than expectations, will be more in tune with reality. They will be better decisions.
The topic of interstellar flight affords opportunities for easier objectivity as well as the opposite – pitfalls where one can lose objectivity. Because interstellar flight is almost certainly farther in the future than the next Moon and Mars missions, it is easier to apply impartiality. The huge payoffs of interstellar flight (finding new human homesteads and new life) are far enough away that there is no need to sell a particular pet technology today or skew the results toward near-term promises.
That said, I surprised myself when my own assessments gave results different than my expectations. Case in point – estimating how far in the future the first interstellar missions are, based on energy (energy is the most fundamental currency of motion). Those findings and a refined sequel (to appear soon in the Journal of the British Interplanetary Society) indicate that the first interstellar missions might be 2-centuries away, albeit with huge uncertainty bands.[ref]Millis, Marc. (2010) First Interstellar Missions, Considering Energy and Incessant Obsolescence, JBIS Vol 63 (accepted, pending publication).[/ref] The first Centauri Dreamspost on those findings met with ‘energetic’ reaction, where many seemed disappointed that the prospects seemed so far in the future. Before I ran the numbers, I suspected that it would be much sooner too. The first calculations were done around 1996, and those results made me rethink what ‘next-steps’ were really required.
Rather than proceed with my prior notion, I had to stop and rethink things. The data said something unexpected. I knew I had conceived the methods to be impartial and fed the assessments with unbiased data, so the findings would be similarly unbiased. They were what they were. So, should I redo the analysis until I got the answer I wanted, or accept the results for what they were and then re-adjust my expectations? I decided to expose those results to other reviews, to check for errors and such, and then to accept the findings as they were.
Before that point, I thought the next step would be to use more detailed energy assessments to help pick the best interstellar propulsion options, but with two centuries of time to plan ahead, and many options whose numbers were still debatable, I realized that we need to abandon the idea of trying to pick the ONE best interstellar solution. Instead we need to focus on getting reliable data on the wide span of ideas (no salesmanship) – and to investigate the most critical ‘next-steps’ on as many of them as possible.
And this long lead time provides the topic of interstellar flight with the opportunity for more objectivity – the opportunity to take our time to reach sound decisions – to provide more trustworthy progress.
Colony Ships and Spaceship Earth
The other result that I was not expecting was that colony ships might be easier to launch than small, fast probes – at least in terms of energy. My prior expectations were that colony ships would need to be so immense and complicated that they would take longer to develop than a fast probe to Alpha Centauri. The energy study showed otherwise. Kinetic energy is linear in mass, and goes as the square of speed. That means if the ship is twice as massive, it requires twice as much energy, but if it goes twice as fast, it requires FOUR times the energy. Colony ships do not need to go fast. They only need to drift, carrying a segment of humanity. Up to that point, I thought colony ships would be a sequel, not a prequel to small, ultra-fast probes. Sometimes you just have to run the numbers.
Then it occurred to me, while I was drafting my first TEDx talk, that the notion of such slower interstellar world ships also provides a more impartial venue to discuss critical human survival questions. Colony ships allow us to consider these questions with NO dependence on conventions or biases. If designing a society from scratch, one is free to start anew to fit the facts as they are discovered. On Earth, however, when dealing with questions of population size, environmental stability, amount of territory per person, and governance model, the debates are typically won by cultural edict (e.g. no birth control) or warfare (quest for territory or power). So, after all that, I realized that colony ships merited far more attention than I originally gave them, and hence, we will need to track down suitable pioneers to cover those issues too as part of Tau Zero.
When it comes to one of my pet topics – propulsion physics and the quest for space drives – I ran into another facet of impartiality. I found that many physicists do not like to work on problems with potential applications since the application ‘taints the purity’ of the research. Instead they want to be driven by curiosity alone. In other words, they do not want to be biased. In the quest for propulsion physics, where I really hope a space drive method can be found, I have an ingoing bias. I want the results to turn out a certain way. This creates a conflict of interest in how I might view – or skew – the results. To make genuine discoveries, however, I must discipline myself to avoid imposing such biases. Although I can let my wishful musings help me pose the key questions, to get real progress I must also let the findings – unbiased findings – answer those questions. I must accept the results as they unfold.
Take the case of black holes in contrast to traversable wormholes or even warp drives. Studying black holes has revealed insights about spacetime warping, presumably without bias since no desired result is sought. But if one studies the very same physics in the context of faster-than-light wormholes or warp drives, one might get biased results because of wanting such devices to be feasible. Fortunately, much research published on these topics has maintained the rigor to avoid the taint of such biases. Insights into spacetime physics are also being learned by pondering warp drives and wormholes. These questions are even presented as homework problems in textbooks (e.g. Hartle (2003) Gravity: An Introduction to Einstein’s General Relativity).
The irony is that even the curiosity-driven research has implicit biases – that natural sense of ownership that a person has for their research ideas. There is an urge – even in this case – to have the findings prove the author’s pre-conceived point. This is just a human norm. High-quality physicists can discipline themselves to separate out this bias. In contrast, I’ve also seen physicists discuss their ideas with the same possessiveness as kids with toys on a playground. Regardless of our motivation in searching for new knowledge, we must maintain vigilance to avoid imposing our own biases on the findings, even the implicit bias of self.
In Centauri Dreams, Paul Gilster looks at peer-reviewed research on deep space exploration, with an eye toward interstellar possibilities. For many years this site coordinated its efforts with the Tau Zero Foundation. It now serves as an independent forum for deep space news and ideas. In the logo above, the leftmost star is Alpha Centauri, a triple system closer than any other star, and a primary target for early interstellar probes. To its right is Beta Centauri (not a part of the Alpha Centauri system), with Beta, Gamma, Delta and Epsilon Crucis, stars in the Southern Cross, visible at the far right (image courtesy of Marco Lorenzi).
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