by Paul Gilster | Dec 30, 2009 | Interstellar Medium |
We always cite the Mars rovers as examples of missions that perform far beyond their expected lifetimes, but the two Voyager spacecraft are reminding us once again that we have instrumentation at the edge of the Solar System that is still functioning after all these years. Both Voyagers are now in the heliosheath, the outermost layer of the magnetic bubble we call the heliosphere. With Voyager 1 crossing into the heliosheath in late 2004 and Voyager 2 in the summer of 2007, we get an estimate of the size of the heliosphere, a useful finding because it tells us something about what lies beyond.
What’s out there has been known for some time. Indeed, the interstellar medium (ISM) houses some ten percent of the visible matter in the Milky Way, mostly in the form of hydrogen gas. The ISM is patchy, enough so that astronomers have been able to isolate a Local Interstellar Cloud through which our Solar System is moving, a cloud flowing outward from the Scorpius-Centaurus Association, a region of star formation. About thirty light years wide, this cloud is colloquially called the ‘Local Fluff.’
Image: An artist’s concept of the Local Interstellar Cloud, also known as the “Local Fluff.” Credit: Linda Huff (American Scientist) and Priscilla Frisch (University of Chicago).
The Voyagers have yet to reach the cloud, but they’re closing in on it, and therein hangs a tale. For what determines the size of the heliosphere appears to be the balance between the inflation of the ‘bubble’ by the solar wind and the compressive forces of the Local Interstellar Cloud. In a new paper in Nature, Merav Opher (George Mason University) uses Voyager data to study this balance. Some of the pressure exerted by the cloud is magnetic, and Opher’s measurements of the magnetic field help us to understand how the cloud continues to exist despite forces that should tear it apart.
The problem is that the ‘Local Fluff’ should have been dissipated by the effects of nearby supernovae that exploded some ten million years ago. These hot gases would break up the cloud were it not for its strong magnetic field, argues Opher, who goes on to phrase the issue starkly:
“Using data from Voyager, we have discovered a strong magnetic field just outside the solar system. This magnetic field holds the interstellar cloud together and solves the long-standing puzzle of how it can exist at all.”
The Local Interstellar Cloud is thirty light years across and, given its temperature and density, should not be able to resist the effects of the supernova gases around it. Opher’s finding is that the cloud is much more strongly magnetized than had been previously thought, between 4 and 5 microgauss. This is roughly twice previous estimates. “This magnetic field,” adds Opher, “can provide the extra pressure required to resist destruction.”
Image: Voyager flies through the outer bounds of the heliosphere en route to interstellar space. A strong magnetic field reported by Opher et al in the Dec. 24, 2009, issue of Nature is delineated in yellow. Image copyright 2009, The American Museum of Natural History.
The field is found to be tilted between 20 and 30 degrees from the interstellar medium flow direction (determined by the Sun’s motion), and is at an angle of 30 degrees from the galactic plane. The conclusion: The interstellar medium is turbulent, at least in the vicinity of our Solar System. If other nearby clouds are similarly magnetized, the heliosphere should vary in size as the Sun moves through them (on a timescale of hundreds of thousands of years), varying the protection the heliosphere offers the inner system against galactic cosmic rays.
The paper is Opher et al., “A strong, highly-tilted interstellar magnetic field near the Solar System,” Nature 462 (24 December 2009), pp. 1036-1038 (abstract).
by Paul Gilster | Nov 23, 2009 | Interstellar Medium |
One problem with journeys that are beyond today’s technologies is that we forget, in our zeal to get a payload to the target, how little we know about the regions we’ll pass through along the way. It’s amazing how little we know, for example, about the heliosphere around the Solar System, yet any probe pushing into interstellar space will have to cross from the region of space under the Sun’s influence into a zone where the interstellar medium flows around this ‘bubble,’ disturbing the solar wind and creating a secondary bubble, the heliosheath.
We don’t yet have a global view of what spacecraft will encounter in the heliosheath as the solar wind is heated and slowed by these interactions. Only recently have we gotten the Voyagers into these regions, and in any case these doughty vehicles can only produce single-point measurements. But we’ve got the IBEX (Interstellar Boundary Explorer) spacecraft making observations from near Earth, and now we learn that Cassini, our intrepid Saturn orbiter, has also produced excellent data on what happens at system’s edge.
NASA is now offering an interesting animation of the heliosphere and heliosheath, showing the interstellar medium flowing past, with the interstellar magnetic field moving around the bubble of hot, high pressure particles. The findings that produced this animation are intriguing because it was previously thought that the Solar System moved through the interstellar medium in a shape resembling that of a comet. Instead, scientists using Cassini’s Ion and Neutral Camera sensor on its Magnetospheric Imaging Instrument (MIMI) find that the heliosphere is less like a comet and more like a ball moving through smoke, the ‘smoke’ being the interstellar magnetic field.
This JPL news release uses a more vivid turn of phrase: “…the new results suggest our heliosphere more closely resembles a bubble – or a rat – being eaten by a boa constrictor: as the solar system passes through the “belly” of the snake, the ribs, which mimic the local interstellar magnetic field, expand and contract as the rat passes.”
Image: In this illustration, the multicolored (blue and green) bubble represents the new measurements of the emission of particles known as energetic neutral atoms. The energetic neutral atoms were streaming in from the thick boundary known as the heliosheath. The heliosheath is the region between the heliosphere, the region of our sun’s influence, and the interstellar medium, the matter between stars in our galaxy. Areas in red indicate the hottest, most high-pressure regions and purple the coolest, lowest-pressure regions. Credit: NASA/JPL/JHUAPL.
As to the heliosheath itself, the Cassini data indicate it’s somewhere between 40 and 50 AU thick. That’s a remarkable result from a spacecraft that has as its primary mission the investigation of Saturn and its moons. But Cassini’s studies of the energetic electrons and ions trapped in Saturn’s magnetic field, and of the energetic neutral atoms also produced in this environment, have produced a treasure trove of data that includes particles arriving from the outer Solar System.
Usefully, the picture that began to emerge from Cassini squared with what IBEX had already produced, thus deepening our understanding of this outer region. Says MIMI scientist Don Mitchell (JHUAPL):
“I was initially skeptical because the instrument was designed for Saturn’s magnetosphere. But our camera had long exposures of months to years, so we could accumulate and map each particle that streamed through the tiny aperture from the far reaches of the heliosphere. It was luck, but also a lot of hard work.”
The paper is Krimigis et al., “Imaging the Interaction of the Heliosphere with the Interstellar Medium from Saturn with Cassini,” Science, Vol. 326, No. 5955 (13 November 2009), pp. 971-973 (abstract).
by Paul Gilster | Oct 16, 2009 | Interstellar Medium |
Yesterday’s story on IBEX is now complemented by images from the Ion and Neutral Camera, part of the Magnetospheric Imaging Instrument on the Cassini orbiter. The Cassini data confirm the fact that the heliosphere isn’t shaped the way we’ve always thought. The assumption up to now has been that the collision of the solar wind with the interstellar medium would create a foreshortened nose in the direction of the Solar System’s motion, and an elongated tail in the opposite direction.
Both IBEX and Cassini argue otherwise. Stamatios Krimigis (Applied Physics Lab, Laurel, MD) notes the import of these findings:
“These images have revolutionized what we thought we knew for the past 50 years; the sun travels through the galaxy not like a comet but more like a big, round bubble. It’s amazing how a single new observation can change an entire concept that most scientists had taken as true for nearly fifty years.”
Amazing and invigorating, for we’re opening up serious new ground here. Put together, what Cassini and IBEX are telling us is that particle pressure and magnetic field density are what dominate the interaction between the interstellar medium and the heliosphere. We’re getting a look at how a solar system moves through the space around it through twin imaging programs that map an otherwise invisible boundary.
Image: The shape of our solar system moving through the interstellar medium was previously thought to be comet-shaped, with a head pointed into the stream, and a tail flowing downstream. New observations show the shape actually resembles something more like a slippery ball (the hot particles that exert pressure) moving through smoke (the interstellar magnetic field). As the “ball” moves through the “smoke,” the smoke bends and parts to let the ball through, then resumes its previous shape after the ball has passed on. At present, this is only hypothetical: New models will be motivated by these measurements, and will provide a more physically accurate basis for the interaction of the heliosphere with the interstellar medium. Credit: NASA/JPL-Caltech/JHUAPL.
Cassini has been mapping energetic neutral atoms not only near Saturn but across the entire sky. These ENAs, discussed yesterday, are produced by energetic protons that interact with the magnetic field of the interstellar medium. For more on ENAs, I turn to Mike Gruntman’s AstronauticsNow site (thanks to Centauri Dreams reader Carl for the tip). Gruntman is a professor of astronautics at USC, an author, and a mission co-investigator on IBEX. Of ENAs he writes:
The interaction between charged and neutral particles is a common phenomenon in space plasmas. Whenever an energetic ion undergoes a charge exchange process in a collision with a neutral background atom, an energetic neutral atom – ENA – is born. Ion-electron recombination and neutral atom acceleration by the solar gravitation may also contribute to an ENA population under certain conditions. ENAs are ubiquitous in space environment and their study opens a new window on various phenomena in space plasmas with a promise (already partially realized) to qualitatively improve our understanding of global magnetospheric and heliospheric processes.
Gruntman goes on to note that recording ENA fluxes as a function of observational direction allows us to create a global image, which is what we are doing with the interactions at our system’s edge. It’s a method that is paying off handsomely, as Edmond Roelof, a co-investigator on the Magnetosopheric Imaging Instrument, points out:
“Energetic neutral atom imaging has demonstrated its power to reveal the distribution of energetic ions, first in Earth’s own magnetosphere, next in the giant magnetosphere of Saturn and now throughout vast structures in space-out to the very edge of our sun’s interaction with the interstellar medium.”
You can see an animation showing the interstellar medium flowing past the heliosheath here (the heliosheath is a secondary bubble around the heliosphere formed by the interstellar medium’s interactions with the latter). Also note five papers published in Science Express this week on IBEX and Cassini, including Krimigis et al., “Imaging the Interaction of the Heliosphere with the Interstellar Medium from Saturn with Cassini,” (October 15, 2009). Abstract available.
by Paul Gilster | Oct 15, 2009 | Interstellar Medium |
Findings that are outside our expectations seem par for the course as we explore the Solar System. From the volcanoes of Io to the geysers of Enceladus, unusual things show up with each new mission. Why should IBEX be any different? The Interstellar Boundary Explorer is the first spacecraft expressly designed to study what happens at the edge of the Solar System, where nearby space meets the interstellar medium.
The ‘bubble’ around the Sun called the heliosphere comes about as charged particles in the solar wind move continuously away from the Sun. Although IBEX is far from the heliopause (it’s orbiting the Earth with an apogee of 322,000 kilometers and a perigee of 16,000 kilometers), its instruments are tuned to study energetic neutral particles (ENAs) swept up by the solar wind in the boundary between the edge of the heliosphere and interstellar space beyond. IBEX has been mapping this area since last October.
And here comes the surprise, as explained by IBEX principal investigator David J. McComas (SwRI):
“The IBEX results are truly remarkable, with emissions not resembling any of the current theories or models of this never-before-seen region. We expected to see small, gradual spatial variations at the interstellar boundary, some ten billion miles away. However, IBEX is showing us a very narrow ribbon that is two to three times brighter than anything else in the sky.”
Image: Accurate timing of the incoming ENAs allows the IBEX team to obtain a higher resolution in the latitudinal direction. The inset at right shows some of the fine detail of the ribbon. Credit: SwRI.
Yes, we’ve got two Voyager spacecraft out there, but IBEX is giving us the big picture, imaging the region not with photons but ENAs from its distant vantage in a highly elliptical Earth orbit. It’s focusing on the so-called ‘termination shock,’ where the solar wind collides with interstellar gas. As they pass through the region, neutral hydrogen and oxygen atoms are dragged by the plasma at the interstellar boundary, while neutral helium passes straight through. Tracing their different arrival directions tells the tale.
McComas adds:
“The most astounding feature in the IBEX sky maps — the bright narrow ribbon — snakes through the sky between the Voyager spacecraft, where it remained completely undetected until now.”
Image: This image illustrates one possible explanation for the bright ribbon of emission seen in the IBEX map. The galactic magnetic field shapes the heliosphere as it drapes over it. The ribbon appears to trace the area where the magnetic field is most parallel to the surface of the heliosphere (the heliopause). Credit: SwRI.
We’re learning that our Solar System’s interactions with the interstellar medium are more intense than previously believed. As helpful as Voyager’s point measurements are, we now see the region far more clearly. And because the ribbon these data reveal seems to be governed by the direction of the local interstellar magnetic field, it appears that the interstellar medium has a much greater influence on the heliosphere than we originally thought.
by Paul Gilster | Sep 30, 2009 | Interstellar Medium |
Below you’ll see that I’m running Mike Brown’s sketch of the ‘new’ Solar System, one I originally ran with our discussion of Joel Poncy’s Haumea orbiter paper, which was presented at Aosta in July. The sketch is germane on a slightly different level today because as we look at how our views of the Solar System have changed over the years, we’ve learned how many factors come into play, including one Brown’s sketch doesn’t show. For surrounding the planets and nearer regions of the Kuiper Belt is the heliosphere, that bubble of solar wind materials whose magnetic effects help protect the inner system.
Image: Our view of the Solar System has gone from relatively straightforward to one of exceeding complexity. Credit: Mike Brown/Caltech.
Look at the heliosphere diagram below and you’ll see that while the eight planets are comfortably within it, our Pioneers and Voyagers are pushing toward or through the termination shock on their way to the heliopause. Galactic cosmic rays are shown pushing from deep space in toward the bow shock. Our new view of the Solar System must include the fact that the heliosphere does not extend to all of it. The Oort Cloud, that vast sphere of comets, is well outside it, and so would be those Kuiper Belt objects that wander too far from the Sun.
Image: Components of the heliosphere. Credit: NASA Ames.
How about a future mission to Sedna? Better be careful, because this odd object moves out to about 990 AU at aphelion. Our intrepid astronauts, having solved the propulsion problem, would now face galactic cosmic rays without the helpful shielding effects of the heliosphere. Galactic cosmic rays are subatomic particles — protons and some heavy nuclei — that have been accelerated to high velocity by supernova explosions. They’re enough of a problem on the interplanetary level, but become even more of one beyond the system.
All this comes to mind because we’re seeing an increase in cosmic ray intensities, some 19 percent higher than what we’ve seen in the last fifty years, according to Caltech’s Richard Mewaldt, who adds “The increase is significant, and it could mean we need to re-think how much radiation shielding astronauts take with them on deep-space missions.” This NASA news release points to three culprits: a flagging solar wind, a decline in the Sun’s interplanetary magnetic field, and a flattening of the heliospheric current sheet where the polarity of the Sun’s magnetic field changes from a plus to a minus.
We’re safe enough where we are, of course, because the Earth’s atmosphere has allowed life to weather far worse cosmic ray fluxes. But if Mewaldt is right, we may have experienced a low level of cosmic ray activity for most of the space era. “We may now be returning to levels typical of past centuries,” says the scientist, reminding us how much we have to learn about the factors that make space flight within and without the system a hazardous enterprise.
by Paul Gilster | Apr 3, 2009 | Interstellar Medium |
“Interstellar travel may still be in its infancy,” write Gregory Matloff and Eugene Mallove in The Starflight Handbook (Wiley, 1989), “but adulthood is fast approaching, and our descendants will someday see childhood’s end.” The echo of Arthur C. Clarke is surely deliberate, a sign that one or both authors are familiar with Clarke’s 1953 novel about the end of human ‘childhood’ as we learn about the true destiny of our species in the universe. But becoming a mature species isn’t easy, nor is figuring out interstellar flight.
Awash in Hard Radiation
Consider just one layer of complexity. Suppose we somehow discover a propulsion system that gets us to relativistic speeds in the range of 0.3 c. That seems a minimum for regular manned starflight given the times and distances involved, but suddenly attaining it doesn’t end our problems. Interstellar space isn’t empty, and when we accelerate to cruising speed at a substantial percentage of the speed of light, our encounter with interstellar gas becomes a nightmare. Indeed, this haze of gas between the stars acts as a flow of nucleonic radiation bombarding the starship as we push ever higher into relativistic realms.
Image: As a playful example of science fiction mixing with science, this photo shows the luminosity from hot gas used in a hypersonic “super-orbital expansion tube X2” test rig at the University of Queensland, Australia. A toy model of the fictional starship Enterprise is subjected to a Mach 5 flows. (Credit: Tim McIntry, Queensland Physics Department/Marc Millis).
And let’s not forget high-energy cosmic rays and dust, all of which demand protection. Because sensitive electronics are susceptible to damage as well as humans, we have to work out the hazards whether our mission is manned or not. A non-relativistic capsule moving in interstellar space would, according to Oleg Semyonov (State University of New York at Stony Brook), experience a radiation dose of 70 rems per year, while the safety level for people is considered to be between 5 and 10 rems in the same period. Going relativistic drives the dosage level to far greater extremes.
Did I say ‘greater’? Try this much greater: Thousands or hundreds of thousands of rems per second, comparable to conditions in the core of a nuclear reactor. All this from a ship moving at high relativistic speeds through interstellar gas. But even slower velocities are a problem as we move through this medium.
Puzzling Out Shielding Options
Semyonov plots the radiation involved from encountering interstellar gas versus velocity and finds that at speeds much above a comparatively sedate 0.1 c, an astronaut could not be outside the hull without layers of shielding. Shielding the entire ship is problematic. A radiation-absorbing windscreen installed in front of the vehicle is possible, a titanium shield of 1-2 cm workable up to 0.3 c but becoming ‘dramatically thicker with acceleration.’
Water? It’s not a bad idea because the crew needs water anyway:
Placing a water tank (or an ice bulge) in front of a ship is advantageous in comparison with a shield made of metal or another solid material because it eliminates the damaging embrittlement of solids under intense nucleonic radiation; for a given cruising speed, the penetration depth of monoenergetic nucleons will be the same and a layer located near the penetration depth inside a solid shield will be largely damaged because all the nucleons deposit the bulk of their kinetic energy at the end of their penetration depth dislocating atoms from the lattice, weakening the material, and causing peeling or flaking.
On the other hand, our water shield adds significant mass to the vessel, and at speeds close to that of light, it would need to be tens of meters thick to form an effective barrier. So we can contemplate titanium or aluminum hull shielding up to about 0.3 c, but 0.8 c demands several meters of titanium or the water barrier.
The Cosmic Ray Hazard
Cosmic rays are a hazard to any interstellar mission, relativistic or not. Water is again an option, but Semyonov notes that a ship would require a ’round shell of water of 5 m in thickness,’ a huge increase in mass, and still insufficient for absorbing the highly penetrating secondary gamma and muonic radiation that will bathe the ship, demanding an additional shield of its own. Usefully, cosmic rays become increasingly beamed as we increase velocity, so that a frontal shield for interstellar gas can also absorb them.
More on this:
Isotropic cosmic rays are subjected to relativistic beaming when a starship is moving with a relativistic speed. For the ship’s velocities closer to the speed of light, most of cosmic rays form into a beam directed toward the front of the spaceship. While they do present a hazard, they can be easily absorbed or deflected by a frontal shielding system that is required anyway protecting the crew and electronics against the hard radiation of the oncoming flow of interstellar gas. Cosmic dust will also contribute to the radiation hazard, because the dust particles are actually lumps of high-energy nucleons at relativistic velocities. A serious problem will be the sputtering of a ship’s bow or a radiation shield by the relativistic dust particles. Nevertheless, the shielding of relativistic starships from hard ionizing radiation produced by interstellar gas and cosmic rays does not seem to be far beyond existing technology.
In other words, if we can figure out the key question of propulsion, we should be able to overcome the shielding issue. Semyonov considers a range of options including combinations of material and magnetic shielding in arriving at this conclusion. His discussion is a wide-ranging and sobering reminder of how many barriers interstellar flight presents beyond tuning up the right kind of engine. Childhood may end, but we biological life-forms remain fragile creatures indeed when flung into the interstellar deep.
The paper is Semyonov, “Radiation hazard of relativistic interstellar flight,” Acta Astronautica 64 (2009), pp. 644-653.
