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The majority of the interstellar gas and dust that we see was produced by star death.
Interstellar material also comes from stars which are still undergoing stable fusion reactions. Our own sun sends out streams of particles and radiation -- the solar wind -- that interact with particles flowing into our solar system. Given that all stable stars also produce a similar "wind", a substantial portion of the interstellar medium can be accounted for in this way. Clouds of interstellar dust have accreted (clumped together) over millions of years. This dust is a varied mix of compounds and elements; some interstellar clouds even contain organic molecules like acetylene and acetaldehyde, known precursors of amino acids. The atoms needed to create these clouds of molecules have generally come from past supernovae.
Many of the denser clouds become stellar nurseries
we are all made of stardust.
About 10 million years ago, a cluster of supernovas exploded nearby, creating a giant bubble of million-degree gas. The Fluff is completely surrounded by this high-pressure supernova exhaust and should be crushed or dispersed by it.
Changes in the sun's galactic environment, moderate or otherwise, must have taken place in the past. Indeed there is evidence on earth suggesting that the local galactic environment has not been stable. Ice-core samples from the Antarctic show spikes in the concentration of beryllium-10 (which has a half-life of 1.5 million years) during two events, one about 60,000 years ago and another about 33,000 years ago. What events could have caused these sudden increases in beryllium?
These isotopes are created when intense radiation hits the atoms in the upper atmosphere, suggesting that a blast of energy had once hit our planet from space.
Tazmanian devils, turtles, whales, sea lions, dolphins and fish are some of the species shown in the study to be suffering from elevated rates of cancer, in some cases threatening the very survival of the species.
In some cases, cancers could threaten populations by preventing reproduction in addition to killing individuals. Sea lions found off of California suffer widespread metastasizing genital carcinomas which prevent them from mating. Rarely seen before the 1980's, such cancers were found in over 18% of sea lions washed up on California beaches. In a separate study of sea lions killed during an unusual algal bloom, 6.3% were found to have cancer. Fortunately, sea lion populations have been growing in spite of the simultaneous increase in observed cancers. Ocean-going dolphins are also showing increased rates of genital cancers.
Cancer remains a leading killer of pets and people and it’s incidence has grown from a rate of approximately 1 out of every 500 individuals being affected in the year 1900 to a rate of 1 out of every 3 people and one out of every 5 pets (dogs and cats) being affected with cancer today.
Beryllium-7 is a naturally occurring gamma-emitting radionuclide which is produced during cosmic ray interactions with nitrogen and oxygen in the upper atmosphere.
Chilton Be-7 in 2009 rain ranges between 0.31 ± 0.17 and 2.41 ± 0.59 Bq l-1 for June, this elevated June result of 2.41 ± 0.59 Bq l-1 is more than double the 10-year mean and above the significance value. At present, there is no explanation for this anomalous result although analytical data have been verified.
These allow the air to flow around in a grand circle meaning birds can have fresh oxygen rich air in their lungs all the time
"For the last few decades, space scientists have generally accepted that the bubble of gas and magnetic fields generated by the sun – known as the heliosphere – moves through space, creating three distinct boundary layers that culminate in an outermost bow shock. This shock is similar to the sonic boom created ahead of a supersonic jet. Earth itself certainly has one of these bow shocks on the sunward side of its magnetic environment, as do most other planets and many stars. A collection of new data from NASA's Interstellar Boundary Explorer (IBEX), however, now indicate that the sun does not have a bow shock."
"We've seen one after another signature of a very strong magnetic field in the galactic environment," says Nathan Schwadron, a space scientist at the University of New Hampshire in Durham who is one of the authors on the paper. "That magnetic field influences the structure of the heliosphere and the boundaries themselves. That leads to a whole new paradigm."
," The heliosphere's boundaries lie roughly 10 billion miles away from Earth, but are nonetheless crucial for understanding our place in the universe. Indeed, the heliopause provides some protection for our solar system from the harsh, radiation environment surrounding it."
"Changes in the pressure or magnetic field of the local interstellar cloud could also cause the boundary locations to move. The scale length for changes in the local interstellar cloud is not known. The Sun is very near the local interstellar cloud boundary and will cross into another interstellar medium environment in a few thousand years, and scale lengths for variations may be smaller near this boundary"
"Now IBEX has surprised astronomers by showing that this force field-like structure, the heliosphere, is an unexpectedly dynamic, unpredictable boundary."If we've learned anything from IBEX so far, it is that the models that we're using for interaction of the solar wind with the galaxy were just dead wrong,"
"Sun's protective 'bubble' is shrinking.The protective bubble around the sun that helps to shield the Earth from harmful interstellar radiation is shrinking and getting weaker, Nasa scientists have warned."
In a briefing today at NASA headquarters, solar physicists announced that the solar wind is losing power."The average pressure of the solar wind has dropped more than 20% since the mid-1990s," says Dave McComas of the Southwest Research Institute in San Antonio, Texas. "This is the weakest it's been since we began monitoring solar wind almost 50 years ago."
"The solar wind is 13% cooler and 20% less dense." "What we're seeing is a long term trend, a steady decrease in pressure that began sometime in the mid-1990s," explains Arik Posner, NASA's Ulysses Program Scientist in Washington DC.
It's hard to say. We've only been monitoring solar wind since the early years of the Space Age—from the early 60s to the present," says Posner. "Over that period of time, it's unique. How the event stands out over centuries or millennia, however, is anybody's guess. We don't have data going back that far." Flagging solar wind has repercussions across the entire solar system—beginning with the heliosphere."
Every planet from Mercury to Pluto and beyond is inside it. The heliosphere is our solar system's first line of defense against galactic cosmic rays. High-energy particles from black holes and supernovas try to enter the solar system, but most are deflected by the heliosphere's magnetic fields."
The solar wind isn't inflating the heliosphere as much as it used to," says McComas. "That means less shielding against cosmic rays."In addition to weakened solar wind, "Ulysses also finds that the sun's underlying magnetic field has weakened by more than 30% since the mid-1990s," says Posner. "This reduces natural shielding even more."
there are controversial studies linking cosmic ray fluxes to cloudiness and climate change on Earth
The cause of the surge is solar minimum, a deep lull in solar activity that began around 2007 and continues today. Researchers have long known that cosmic rays go up when solar activity goes down. Right now solar activity is as weak as it has been in modern times, setting the stage for what Mewaldt calls "a perfect storm of cosmic rays." "We're experiencing the deepest solar minimum in nearly a century," says Dean Pesnell of the Goddard Space Flight Center, "so it is no surprise that cosmic rays are at record levels for the Space Age."
Mewaldt lists three aspects of the current solar minimum that are combining to create the perfect storm: 1. The sun's magnetic field is weak. "There has been a sharp decline in the sun's interplanetary magnetic field down to 4 nT (nanoTesla) from typical values of 6 to 8 nT," he says. "This record-low interplanetary magnetic field undoubtedly contributes to the record-high cosmic ray fluxes." [data] 2. The solar wind is flagging. "Measurements by the Ulysses spacecraft show that solar wind pressure is at a 50-year low," he continues, "so the magnetic bubble that protects the solar system is not being inflated as much as usual." A smaller bubble gives cosmic rays a shorter-shot into the solar system. Once a cosmic ray enters the solar system, it must "swim upstream" against the solar wind. Solar wind speeds have dropped to very low levels in 2008 and 2009, making it easier than usual for a cosmic ray to proceed. [data]
3. The current sheet is flattening. Imagine the sun wearing a ballerina's skirt as wide as the entire solar system with an electrical current flowing along its wavy folds. It's real, and it's called the "heliospheric current sheet," a vast transition zone where the polarity of the sun's magnetic field changes from plus to minus. The current sheet is important because cosmic rays are guided by its folds. Lately, the current sheet has been flattening itself out, allowing cosmic rays more direct access to the inner solar system."
Hundreds of years ago, cosmic ray fluxes were at least 200% to 300% higher than anything measured during the Space Age. Researchers know this because when cosmic rays hit the atmosphere, they produce an isotope of beryllium, 10Be, which is preserved in polar ice."
Now IBEX has surprised astronomers by showing that this force field-like structure, the heliosphere, is an unexpectedly dynamic, unpredictable boundary."If we've learned anything from IBEX so far, it is that the models that we're using for interaction of the solar wind with the galaxy were just dead wrong,"
According to Svensmark, cosmic rays seed low-lying clouds that reflect some of the Sun's radiation back into space, and the number of cosmic rays reaching the Earth is dependent on the strength of the solar magnetic field. When this magnetic field is stronger (as evidenced by larger numbers of sunspots), more of the rays are deflected, fewer clouds are formed and so the Earth heats up; whereas when the field is weaker, the Earth cools down."
Interestingly we were able to observe different kinds of new particle formation events. A few of the events appear to be related to ion-induced nucleation or ion-ion recombination to form stable neutral clusters. In these cases, a small but signiﬁcant fraction of new particle formation could be explained by ion processes"
"ESA’s quartet of satellites studying Earth’s magnetosphere, Cluster, has discovered that our protective magnetic bubble lets the solar wind in under a wider range of conditions than previously believed." www.esa.int...
This interstellar cloud has been given a variety of names by the scientific community such as Local Fluff and 'G Cloud' but the metaphysical community have referred to this as an 'Etheric' cloud and more recently it has been acknowledged as the arrival of primordial 'Adamantine' particles. It is widely accepted by those who understand the implications, that it is the arrival of this interstellar molecular cloud that has been effecting the sun in the last few decades as the sun has passed through into the outermost tenuous regions of this cloud, populated with more dense filaments of plasma. It is fair to say that astronomers are not able to judge accurately astronomical distances in space, or distinguish the arrival of a moderately dense, but still not inconsequential cloudlet, hence over the last 30 years or so, the estimated time of arrival of a dense interstellar cloud has been stated to be up to 50,000 years. Therefore, there has been confusion because it seems that a cloudlet has arrived and some astronomers believe we entered the tenuous outer edges of a moderately dense cloud in the early 1990s. The arrival of an interstellar cloud of dense dusty plasma or an 'etheric cloud' is understood to be the most significant driver for massive evolutionary change within our solar system. The following quote from a metaphysical source indicates that the astronomical community had been aware for many decades that our solar system was being approached by an interstellar molecular cloud but from a metaphysical viewpoint, humanity must expect there to be significant consequences for the spiritual evolution of mankind." www.susanrennison.com...
A recent research by the Radio Astronomy Centre, National Centre for Radio Astrophysics (NCRA) of the Tata Institute of Fundamental Research (TIFR) has found that there has been a steady weakening of the Sun's magnetic field and its associated solar wind in the interplanetary space. According to the study, these changes can have a greater impact on the earth's atmosphere than previously thought. If such a steady weakening of the Sun's magnetic energy continues for one or two solar cycles, it may lead to a 'mini'-ice age kind of situation, similar to that which occurred in the 17th Century, states the study
space.mit.edu... "A strong, highly-tilted interstellar magnetic field near the Solar System" M. Opher, F. Alouani Bibi, G. Toth, J. D. Richardson, V. V. Izmodenov, T. I. Gombosi "Magnetic fields play an important (sometimes dominant) role in the evolution of gas clouds in the Galaxy, but the strength and orientation of the field in the interstellar medium near the heliosphere has been poorly constrained." "We conclude that the interstellar medium field is turbulent or has a distortion in the solar vicinity."
Researcher and author Marshall Klarfeld discussed the ancient Researcher Robert Felix spoke about climate and the possibility we could be heading into an ice age. According to a study, the Himalayas have lost no ice in the last 10 years, and glaciers are growing in areas such as Mount Everest, he noted. Ice age cycles occur around every 11,500 years, and one sign that one is impending is increased volcanic activity, particularly underwater, he stated.
Two vulcanologists published a paper in 2008 suggesting that as climate change continues, the next decades could see more volcanic activity in regions such as Iceland that are now under ice. Read more at cleantechnica.com...
Based on readings from more than 30,000 measuring stations, the data was issued last week without fanfare by the Met Office and the University of East Anglia Climatic Research Unit. It confirms that the rising trend in world temperatures ended in 1997.
The evidence linking space weather and terrestrial weather is growing. The idea here is that cosmic rays can ionise dust particles, which then attract water vapor triggering the formation of clouds.
During the time it takes you to read this article, something will happen high overhead that until recently many scientists didn't believe in. A magnetic portal will open, linking Earth to the sun 93 million miles away. Tons of high-energy particles may flow through the opening before it closes again, around the time you reach the end of the page. "It's called a flux transfer event or 'FTE,'" says space physicist David Sibeck of the Goddard Space Flight Center. "Ten years ago I was pretty sure they didn't exist, but now the evidence is incontrovertible."
"We used to think the connection was permanent and that solar wind could trickle into the near-Earth environment anytime the wind was active," says Sibeck. "We were wrong. The connections are not steady at all. They are often brief, bursty and very dynamic."
There are many unanswered questions: Why do the portals form every 8 minutes? How do magnetic fields inside the cylinder twist and coil? "We're doing some heavy thinking about this at the Workshop," says Sibeck. Meanwhile, high above your head, a new portal is opening, connecting your planet to the sun.
sdo.gsfc.nasa.gov... six images from SDO, chosen to show a representative image about every six months, track the rising level of solar activity since the mission first began to produce consistent images in May, 2010. The period of solar maximum is expected in 2013. The images were taken in the 171 Angstrom wavelength of extreme ultraviolet light.
Raymond Bradley of UMass, who has studied historical records of solar activity imprinted by radioisotopes in tree rings and ice cores, says that regional rainfall seems to be more affected than temperature. "If there is indeed a solar effect on climate, it is manifested by changes in general circulation rather than in a direct temperature signal." This fits in with the conclusion of the IPCC and previous NRC reports that solar variability is NOT the cause of global warming over the last 50 years. Much has been made of the probable connection between the Maunder Minimum, a 70-year deficit of sunspots in the late 17th-early 18th century, and the coldest part of the Little Ice Age, during which Europe and North America were subjected to bitterly cold winters. The mechanism for that regional cooling could have been a drop in the sun’s EUV output; this is, however, speculative.
Indeed, the sun could be on the threshold of a mini-Maunder event right now. Ongoing Solar Cycle 24 is the weakest in more than 50 years. Moreover, there is (controversial) evidence of a long-term weakening trend in the magnetic field strength of sunspots. Matt Penn and William Livingston of the National Solar Observatory predict that by the time Solar Cycle 25 arrives, magnetic fields on the sun will be so weak that few if any sunspots will be formed. Independent lines of research involving helioseismology and surface polar fields tend to support their conclusion. (Note: Penn and Livingston were not participants at the NRC workshop.) “If the sun really is entering an unfamiliar phase of the solar cycle, then we must redouble our efforts to understand the sun-climate link,” notes Lika Guhathakurta of NASA’s Living with a Star Program, which helped fund the NRC study. “The report offers some good ideas for how to get started.”
A radiometric imager, deployed on some future space observatory, would allow researchers to develop the understanding they need to project the sun-climate link into a future of prolonged spotlessness. Some attendees stressed the need to put sun-climate data in standard formats and make them widely available for multidisciplinary study. Because the mechanisms for the sun’s influence on climate are complicated, researchers from many fields will have to work together to successfully model them and compare competing results. Continued and improved collaboration between NASA, NOAA and the NSF are keys to this process. Hal Maring, a climate scientist at NASA headquarters who has studied the report, notes that “lots of interesting possibilities were suggested by the panelists. However, few, if any, have been quantified to the point that we can definitively assess their impact on climate.” Hardening the possibilities into concrete, physically-complete models is a key challenge for the researchers.
Cloud edges are interesting properties of the ISM that are difﬁcult to study, and the Sun is near a cloud edge
The LIC belongs to an ISM ﬂow (the cluster of local interstellar clouds, CLIC) with an upwind direction in the local standard of rest (LSR) that is within 15 degrees of the center of the S1 shell, directed towards the center of the Loop I superbubble [5, 37]. The properties of the edge of the LIC depend on the adjacent ISM. The CLIC is embedded in the hot Local Bubble plasma. Since neutral gas ﬁlls 25%–40% of the sightlines towards nearby stars, or less if clouds consist of randomly distributed uniform spherical objects, the LIC may be bounded by million degree tenuous plasma. For this scenario, the LIC edge may consist either of a hot evaporative conductive interface, or of a turbulent mixing layer . Both phenomena disrupt the velocities disrupt cloud edges
The CLIC is decelerating. This property is shown by the velocity distribution of clouds in the ﬂow, some of which are accelerated towards the Sun, relative to the mean ﬂow velocity, in both the upwind and downwind directions [5, 37]. In a decelerating ﬂow, velocities in cloud boundary regions may be disrupted by collisions between clouds that create density enhancements similar to the Leo clouds discussed in Meyer et al. , or by other instabilities.Fluctuations in interstellar electron densities are identiﬁed over scales of ∼ 10 4km through 10 AU by their effect on radio wave propagation . Radio scintillation data showing that electron scattering screens are present within 10 pc has been interpreted to indicate the presence of local cloud collisions "
"Based on the trend for the ecliptic longitude of the LIC velocity vector to increase with time, it should be considered a real possibility that the Sun has sampled turbulence in the LIC velocity over the past several decades
There is no information about the scale size of the turbulence in the LIC, so according to these arguments, a plausible case can be made that this nominal 4.6 km s−1 turbulent component samples turbulence in the ISM between the Sun and the upwind edge of the LIC. In such a case, a systematic variation in the ﬂow direction is possible."
Three sets of data suggest that the Sun is in a fragment of the shell of the Loop I superbubble: (1) The Loop I model of Wolleben [55, 56] identiﬁes two radio continuum shells (called “S1” and “S2”) at different distances and locations. The Sun is located in the rim of the S1 shell according to his model, a result that agrees with prior estimates of the Loop I conﬁguration . (2) Interstellar clouds within about 20 pc of the Sun, e.g. the CLIC, have a bulk velocity in the LSR that is within 15◦ of the center of the S1 shell, suggesting a dynamical expanding shell conﬁguration [56, 5]"
In conclusion, there are many astronomical justiﬁcations for the hypothesis that the very local ISM varies over spatial scales comparable to the heliosphere dimensions. After all, the heliosphere, as well as other stars , disrupt the ISM and the ISMF over thousand-AU spatial scales"
If the Sun is in such a shell, it should be apparent in both the local interstellar magnetic ﬁeld and the distribution of nearby interstellar material. The properties of these subshells are compared to the interstellar magnetic ﬁeld (ISMF) and the distribution of interstellar Fe + and Ca + within ∼ 55 pc of the Sun. Although the results are not conclusive, the ISMF direction obtained from polarized stars within ∼ 30 pc is consistent with the ISMF direction of the S1 shell. The distribution of nearby interstellar Fe + with log N(Fe + )< 12.5 cm−2 is described equally well by a uniform distribution or an origin in spherical shell-like features. Higher column densities of Fe + (log N(Fe + )> 12.5 cm−2 ) tend to be better described by the pathlength of the sightline through the S1 and S2 subshells.
The location of the Sun in the rim of the Loop I superbubble has been inferred from radio continuum data, kinematical data on the ﬂow of local ISM away from the center of Loop I, data on gas-phase abundances in local ISM, and the coincidence of the velocity of ISM inside and outside of the heliosphere. Loop I is an evolved superbubble shell formed from stellar evolution in a subgroup of the Sco-Cen association, ∼ 4 − 5 Myrs ago (e.g. de Geus 1992; Frisch 1995, 1996; Ma´ız-Apell´aniz 2001). Both the original dimensions found for the Loop I bubble observed in 820 MHz (Berkhuijsen 1973), and more recent studies of Heiles (1998a,b, H98a,H98b) and Wolleben (2007), place the Sun in or adjacent to the rim of a magnetic superbubble shell for an assumed spherical geometry.
Both the kinematics and abundance pattern of local interstellar material (LISM) suggest that the Loop I remnant has expanded to the solar location (Frisch 1981). LISM abundances of the refractory elements Mg, Fe, and Ca, show the characteristic enhancement indicative of grain destruction in interstellar shocks (Frisch et al. 1999).
The evolved shell is thicker near the ISMF equatorial regions, where ﬁeld strengths are larger due to ﬂux freezing, than the polar regions of the shell where thermal pressure provides the main support for the shell. In media where magnetic pressure is weak, e.g. the ratio of thermal to magnetic pressure β > 10, the evolved bubble is more symmetric. Supernovae in Sco-Cen Association subgroups have contributed to the3 evolution of the Loop I superbubble during the past ≤ 14 Myrs. The Loop I superbubble (and S1, S2) expanded in a medium with a density gradient, because the initial supernova occurred in the molecular regions of the parent Scorpius-Centaurus Association subgroups, while the subsequent bubble expansion occurred in the low density interior of the Local Bubble cavity (Frisch 1981, 1995; Fuchs et al. 2006). In this case the external plasma β may have varied irregularly across the expanding shell, so that the topology of the present day S1 and S2 shells may deviate from axial symmetry as well as sphericity. The ISMF direction at the heliosphere provides the most direct measure of whether the Sun is embedded in the shell of the Loop I superbubble. Several phenomena trace the ﬁeld direction – the weak polarization of light from nearby stars (Tinbergen 1982; Frisch 2007a, hereafter F07), the ﬂield direction in the S1 subshell of Loop I (Wolleben 2007), the 3 kHz emissions from the outer heliosheath detected by the two Voyager satellites (Gurnett et al. 2006, F07), the observed angular oﬀset between interstellar H o and He o ﬂowing into the heliosphere (Lallement et al. 2005; Pogorelov & Zank 2006; Opher et al. 2007), and the 10 pc diﬀerence between the distances of the solar wind termination shock detected by the two Voyager satellites (e.g. Stone 2008). The orientation of the plane midway between the hot and cold dipole moments of the cosmic microwave background is also within ∼ 15 ◦ of the local ISMF direction (F07). 1
Cosmic rays pour down on Earth like a constant rain.
Cosmic rays are mostly high-energy protons, with some electrons, positrons and heavy nuclei mixed in. Their energies range over 14 orders of magnitude, with the most energetic cosmic rays flaunting a billion times more energy than is possible in man-made particle accelerators on Earth.
At present, the average human receives the equivalent of about 10 chest X-rays per year from cosmic rays.
"The increase in ultraviolet light is extremely damaging, and might be especially lethal to single-celled organisms," says Adrian Melott of the University of Kansas.
At sea level, the majority of cosmic ray secondaries are highly penetrating muons. About 10,000 muons pass through our bodies every minute. Some of these muons will ionize molecules as they go through our flesh, occasionally leading to genetic mutations that may be harmful.
In a similar vein, Melott and his colleagues found a possible link between the bobbing of our Sun up and down in the galactic plane and a 63-million-year cycle in fossil biodiversity. The hypothesis is that our solar system is exposed to more cosmic rays every time the solar system peaks out of one side of the galaxy.
"No one has calculated the full effects on the ground," he says.
In Greek mythology, Icarus (the Latin spelling, conventionally adopted in English; Ancient Greek: Ἴκαρος, Íkaros, Etruscan: Vikare) is the son of the master craftsman Daedalus. The main story told about Icarus is his attempt to escape from Crete by means of wings that his father constructed from feathers and wax. He ignored instructions not to fly too close to the sun, and the melting wax caused him to fall into the sea where he drowned. The myth shares thematic similarities with that of Phaëton—both are usually taken as tragic examples of hubris or failed ambition—and is often depicted in art. Today, the Hellenic Air Force Academy is named after Icarus, who is seen as the mythical pioneer in Greece's attempt to conquer the skies. The Lament for Icarus by H. J. Draper Icarus's father Daedalus, a talented and remarkable Athenian craftsman, built the Labyrinth for King Minos of Crete near his palace at Knossos to imprison the Minotaur, a half-man, half-bull monster born of his wife and the Cretan bull. Minos imprisoned Daedalus himself in the labyrinth because he gave Minos' daughter, Ariadne, a clew (or ball of string) in order to help Theseus, the enemy of Minos, to survive the Labyrinth and defeat the Minotaur.en.wikipedia.org...
Day 1 of the event began with astronaut Jan Davis discussing some of her experiences as an astronaut, and showing support for interstellar flight. Her last slide encouraged the community to continue research on breakthrough propulsion, which I found encouraging. We were treated to a number of excellent talks including a 15 year olds discovery of a Gamma Ray Burst through the use of his personal Geiger counter, Kelvin Long discussing starship designs, and the i4is, and a particularly interesting talk by Sam Lightfoot. Sam’s work examines how certain technologies, if ‘gifted’ to a culture from a more advanced culture, can have particularly negative ramifications on the less advanced culture. He gave the examples of spam, iron axes, snowmobiles, tobacco and chocolate, and several other seemingly innocuous technologies which had dramatically negative effects on the culture to which they were given.
The internal structure of these clouds is also of potential importance for interstellar mission design
. Information on spatial structure can be obtained by comparing the interstellar spectra of stars that are close together on the sky, or by observing temporal changes in the interstellar spectrum of a single star as the relative motions of the Sun and star cause the line-of-sight to probe different regions of the cloud. For denser, more distant, interstellar clouds there is now considerable evidence for signiﬁcant density and/or ionisation structure on scales of tens or hundreds of astronomical units , which could be a potential problem for interstellar space vehicles passing through them
I review those properties of the interstellar medium within 15 light-years of the Sun, which will be relevant for the planning of future rapid (vZ0.1c) interstellar space missions to the nearest stars. As the detailed properties of the local interstellar medium (LISM) may only become apparent after interstellar probes have been able to make in situ measurements, the ﬁrst such probes will have to be designed conservatively with respect to what can be learned about the LISM from the immediate environment of the Solar System
but the highest plausible densities when considering possible damage caused by the impact of the vehicle with interstellar material
Properties of the LISM which may impact on the design of an interstellar space vehicle such as Icarus include: The density of the interstellar gas along the proposed trajectory. The ionisation state of the gas. The density and size distribution of solid interstellar dust grains. The strength of the interstellar magnetic ﬁeld.
The LIC is only one of a several broadly similar interstellar clouds within a few light-years of the Sun: Redﬁeld and Linsky  identify six within 15 light-years (4.6 parsecs). These clouds are immersed in the empty (nHEne0.005 cm3 ), ionised, and probably hot (T10 6 K; but see  for an alternative view) Local Bubble (LB) in the interstellar medium [11,17]. The LB extends for about 60–100 parsecs from the Sun in the galactic plane before denser interstellar clouds are encountered, while at high galactic latitudes the LB appears to be open, forming a chimney-like structure in the interstellar medium, which extends into the galactic halo (e.g.  and references cited therein)
still supported by a number of observations and arguments , the presence of a 10 6 K plasma immediately surrounding the local clouds has recently been questioned. In response, Welsh and Shelton  have tentatively put forward an alternative model in which the LISM clouds are surrounded by a cooler (T20,000 K), denser (ne0.04 cm3 ), photo-ionised gas
The properties of the LIC, and other nearby clouds, have been determined by spectroscopic studies of interstellar absorption lines towards nearby stars, augmented in the case of the LIC by observations of interstellar matter entering the Solar System and interacting with the heliosphere (
This implies that stars are more likely to lie in low density inter-cloud (Local Bubble) material than within the clouds themselves, although there appears to be a greater concentration of clouds closer to the Sun (half of those identiﬁed lie within 5 parsecs).
Thus, although it is important to keep an open mind, current indications are that the physical properties of the LISM clouds are relatively homogeneous. Of course, ultimately, obtaining detailed knowledge of the internal structure of the local interstellar medium is one of the scientiﬁc issues that we would expect an interstellar space probe to address [
Voyager 2 measurements of the deﬂection of the solar wind in the heliosheath, Opher et al.  have recently argued that the local interstellar magnetic ﬁeld strength actually lies in the range 3.7–5.5 mG which, if conﬁrmed, would imply that the magnetic energy density dominates the overall LIC energy budget. Uncertainty remains as whether the CHISM properties represent the properties of the LIC or, as argued by Redﬁeld and Linsky , the properties of a narrow transition zone between the LIC and the G clouds, and therefore an average of the two
which, in the absence of any other information, should probably be taken as the ‘best guess’ value for the LIC
. As already noted, these are still uncertain: the long-standing model of a hot, empty LB (nH totEneE0.005 cm3 ; TE10 6 K ), while probably still viable (see Shelton ), has recently been questioned, and it is possible that the spaces between the LISM clouds may instead be ﬁlled with a cooler (T20,000 K), higher density (nH totEneE0.04 cm3 ) medium .
The issue of shielding an interstellar space probe from interstellar dust grains was considered in detail in the context of the Daedalus study by Martin . Martin adopted beryllium as a potential shielding material (owing to its low density and relatively high speciﬁc heat capacity) and found that several kg per square metre of this material would be eroded from exposed surfaces over the course of a six light-year ﬂight at a speed of 0.1c (with the exact amount depending on the density of interstellar material; see his Table 6). Longer durations, and/or higher speeds, would result in greater ablation of shield material.
However, from the point of view of interstellar spacecraft design there is a potentially worrying twist: the same spacecraft instruments that have identiﬁed the mean radii of interstellar grains entering the Solar System to be 0.3 mm [27,37] have also identiﬁed a high-mass tail to the grain population, extending to at least 10 13 kg (i.e. 2 mm radius) and perhaps as high as 10 12 kg (4.5 mm radius). Allowing for these larger grains, Landgraf et al.  inferred a total dust mass density in the LIC (strictly the CHISM) of 6.2 10 24 kg m3 (which is actually a lower limit, as the smallest population of interstellar grains, those with radii o0.1 mm, are deﬂected at the heliopause and never enter the Solar System). This estimate is currently being reassessed on the basis of spacecraft data collected since the original measurements were made , but as of mid-2010 this value is still the best available for the local interstellar dust density (Dr. H. Kruger, personal commu- ¨ nication, 2010). It is a factor of about two higher than might be expected based on the astronomically determined grain size distribution of Mathis et al.
Even more worrying, from the point of view of interstellar spacecraft design, is the fact that the upper-bound to the size distribution of interstellar dust particles in the solar neighbourhood is not currently well constrained. Radar observations of meteors entering the Earth’s atmosphere [27,38,39] appear to have identiﬁed an incoming population of very large interstellar grains, having masses 43 10 10 kg (corresponding to radii 430 mm for a silicate density).
Thus, over a six light-year (5.7 10 16 m) ﬂight we might expect between 2 and 200 impacts per square metre with such large particles
, perfect mixing between gas and dust in these clouds may not be expected even for the small grains.With regard to the largest grains discussed above, the distance scale for coupling to the gas-phase of the interstellar medium would be hundreds or thousands of light-years  and no such mixing can be expected in the LISM. Rather, the ﬂuxes of these large particles in the solar vicinity may stem directly from their source regions (whatever these may be), with negligible interaction with the low density clouds of the LISM. In the context of planning interstellar missions such as Icarus, it follows that one should not assume a signiﬁcant decrease in the dust impact rate as the vehicle passes from a relatively high density medium (e.g. the LIC) into the surrounding low-density LB material.
(1) Many important properties of the LISM are still poorly deﬁned, including the distribution of nearby clouds,their physical properties (especially density and ionisation state), the size distribution of interstellar dust
grains, and the nature of the ‘inter-cloud’ material, which will occupy a signiﬁcant fraction of the pathlength towards many possible target stars.
(2) Given these uncertainties, it is important that the ﬁrst generation of interstellar space vehicles be designed conservatively with respect to key interstellar medium properties. That is, such studies should assume the lowest likely density if considering braking devices which rely on transferring momentum from the vehicle to the surrounding medium (such as magsails ), but the highest plausible densities when considering possible damage caused by impact of the vehicle with interstellar material.
(3) In order to maintain ﬂexibility with regard to the choice of target star, it would be a mistake to assume that the target will necessarily lie within one of the relatively high density LISM clouds whose properties are summarised in Table 4. This is probably a safe assumption for aCen (which appears to lie well within the G Cloud), but the situation regarding other possible target stars is uncertain
(4) In situ spacecraft detections of interstellar dust entering the Solar System imply a local interstellar dust mass density of 6.2 10 24 kg m3 . This is somewhat higher than previous estimates and will have to be taken into account by the dust protection system of any quasi-relativistic (vZ0.1c) interstellar space vehicle
(5) Observations over the last decade have revealed an unexpected high-mass tail to the local interstellar grain size distribution. Individual particles with masses as high as 10 12 kg (4.5 mm radius) are almost certainly present. Moreover, radar detections of interstellar dust particles entering the Earth’s atmosphere ( and references therein) imply a population of interstellar grains with masses 43 10 10 kg (corresponding to radii 430 mm)......a conservative planning assumption would be that particles aslarge as 100 mm radius (10 8kg), and perhaps larger, might be encountered in the course of a several lightyear journey through the LISM. The kinetic energy of such large particles striking an interstellar space vehicle with a relative velocity of 0.1c are considerable
(4.5 10 6J), and some kind of active dust detection and mitigation system may need to be considered.
Please cite this article as: I.A. Crawford, Project Icarus: A review of local interstellar medium properties of relevance for space missions to the nearest stars, Acta Astronautica (2010), doi:10.1016/j.actaastro.2010.10.016
"Some of those cloudlets might be hundreds of times denser than the local fluff," says Priscilla Frisch, an astrophysicist at the University of Chicago who studies the local interstellar medium. "If we ran into one, it would compress the Sun's magnetic field and allow more cosmic rays to penetrate the inner solar system, with unknown effects on climate and life.
"This cloud, although low density on average, has a tremendous amount of structure to it,” Frisch said. “And it is not inconsistent with our data that the Sun may eventually encounter a portion of the cloud that is a million times denser than what we’re in now.”
"We think the heliosphere might have been much larger before we entered the interstellar cloud,” said Frisch, “but that’s something we can’t say for sure.” "But if the solar system encountered the much denser cloud, Frisch estimates that the heliosphere could be compressed to within one or two astronomical units of the Sun, not much greater than the Earth’s distance from the Sun. “There would be dramatic effects on the inner solar system,” said Frisch. “It would immediately change the whole interaction between the solar wind and the interstellar medium.” Researchers have predicted increases in the cosmic-ray flux, changes in the Earth’s magnetosphere, the chemistry of the atmosphere and perhaps even the terrestrial climate."
"They hope to use the Hubble Space Telescope to answer some remaining questions that will confirm whether a dense cloud fragment does indeed lie in the upwind direction."
The primary mission of the Ulysses spacecraft was to characterize the heliosphere as a function of solar latitude. The heliosphere is the vast region of interplanetary space occupied by the Sun's atmosphere and dominated by the outflow of the solar wind. The periods of primary scientific interest is when Ulysses was at or higher than 70 degrees latitude at both the Sun's south and north poles. Ulysses launched on October 6, 1990 and in June 1994 it began a four-month observation from high latitudes of the complex forces at work in the Sun's outer atmosphere-the corona.
The stardust is embedded in the local galactic cloud through which the Sun is moving at a speed of 26 kilometres every second. As a result of this relative motion, a single dust grain takes twenty years to traverse the Solar System. Observations by the DUST experiment on board Ulysses have shown that the stream of stardust is highly affected by the Sun's magnetic field.
The pictures above show cut-aways of where interstellar dust is concentrated in the Solar System - high concentration: red/yellow, low concentration: blue/green (the planets are not shown). During solar minimum (top picture) most interstellar dust can be found above or below the Sun, while at the solar maximum (bottom picture) the dust is concentrated close to the Sun in the plane of the planets' orbits.
It is possible that the increase of stardust in the Solar System will influence the amount of extraterrestrial material that rains down to Earth.
23 September 2008: Data from the joint ESA/NASA Ulysses mission show that the Sun has reduced its output of solar wind to the lowest levels since accurate readings have become available. This current state of the Sun could reduce the natural shielding that envelops our Solar System.