Looking back at 2013. Part II – Voyager 1 Leaves the Solar System

The American news channel CNN have selected what they consider to be the ten most important news about natural science and space in 2013. Gladly, planetary systems research top the list, with four different news. Other news concern fundamental physics (two), paleoanthropology and biology (two), cosmology (one) and climate (one). The four planetary news are about a fresh water lake on Mars, the spacecraft Voyager 1 that is leaving the Solar System, the space telescope Kepler looking for exoplanets, and the spectacular meteorite impact in Chelyabinsk, Russia. In four posts I will comment on each of these four top news of 2013.

Of all man-made objects, the U.S. spacecraft Voyager 1 is the one that is farthest away from Earth. It has left the planets behind long ago and has now even crossed the cocoon of plasma that the Sun surrounds itself with, and has gone out into the interstellar medium. The spacecraft is still completely in the gravitational grip of the Sun, and there are many bodies in our Solar System that are much further away than Voyager 1, such as comets in the Hills and Oort clouds. So in that sense the spacecraft is still deep within the Solar System and will remain here for tens of thousands of years. Not until the gravitational pull of our closest neighboring stars start to become comparable to the gravitational force of the Sun, is it possible to say that Voyager 1 truly has left the Solar System. What has happened now is that the spacecraft, for the first time ever, has entered the gas and dust that fills the space between the stars in our galaxy, the Milky Way, and also penetrates deep into the outer regions of the Solar System. I will therefore take this opportunity to describe the interstellar medium, with a focus on how it looks like in our immediate surroundings. First, however, a few words about the spacecraft itself.

Voyager 1

Voyager 1 was built by the Jet Propulsion Laboratory (JPL), which is still responsible for communications with the spacecraft. It was launched from Kennedy Space Center in Florida in September 1977. Only a year and a half later the spacecraft flew by Jupiter, the largest planet of the Solar System, located 5.2 AU from the sun. One astronomical unit (AU) is the average distance between the Sun and the Earth, and is the unit of length used to measure distances in the Solar System – it is equivalent to roughly 150 million kilometers. Jupiter had previously been visited by Pioneer 10 and 11. In November 1980 the spacecraft reached Saturn at 9.5 AU from the Sun, which previously only had been visited by Pioneer 11. After almost 37 years in space, Voyager 1 has reached a distance of just over 126 AU from the Sun. It is therefore far beyond the outermost planet Neptune, which is located at 30 AU, and also outside the Edgeworth-Kuiper belt that extends from 30 to 48 AU. Space is not empty at these great distances – this region is home to a little-known population called the Detached Scattered Disk. This population includes the transneptunian (90377) Sedna, which never comes closer to the Sun than 76 AU and whose orbit is so elliptical that its aphelion, the maximum distance to the Sun, is as large as 936 AU.

After the exploration of Jupiter and Saturn, Voyager 1 has primarily been studying the solar wind, which is an outflow of material from the Sun that fills our planetary system. The material mainly consists of ionized hydrogen and helium, which means that the negatively charged electrons, which normally revolve around the positively charged nuclei, have been detached. Such a mixture of free electrons and atomic nuclei is called a plasma. The solar wind rushes outward with an average speed of 400 kilometers per second and is extremely rarefied – at the current distance of Voyager 1 there are only 2000 particles per cubic meter, which is equivalent to 0.002 particles per cubic centimeter. This should be compared to the vacuum reached in the best of our laboratories – it contains about ten million molecules per cubic centimeter, which is still vanishingly small compared to the density of ordinary air. The temperature in the solar wind reaches about one million degrees. The plasma from the Sun fills a large region called the heliosphere. This bubble cannot become arbitrarily large – its current size is the result of a balancing act between the gas pressure within the bubble, and an equally large pressure from the outside. That external pressure is provided by the interstellar medium, which is much denser and cooler than the plasma in the heliosphere.

Voyager 1. Image credit: NASA/JPL/KSC

Voyager 1. Image credit: NASA/JPL/KSC

The interstellar medium

The space between the stars in our galaxy, the Milky Way, is not completely empty, but filled with the interstellar medium. It consists of both gas and solid dust particles. About 90% of the galactic mass is tied up in stars, while the interstellar medium constitutes about 10% of the mass. The interstellar medium is partly a leftover from the infancy of the Universe that has not been used to form stars, and partly material that has left the stars more recently in the form of solar wind or has been thrown out into space when the stars have exploded as supernovae. The interstellar medium is extremely important, because this is the material that forms new stars and planets. It is therefore useful to know its composition, since it will determine how both stars and planets are composed.

Composition

The interstellar medium is dominated entirely by the chemical elements hydrogen (H) and helium (He). It is common to measure the concentrations of elements relative to that of silicon (Si). Counted accordingly there are 24,000 hydrogen atoms and 2300 helium atoms for each silicon atom. After hydrogen and helium comes oxygen (O) and carbon (C), with roughly 14 and 7 atoms per silicon atom, respectively. Neon (Ne) and nitrogen (N) are both about twice as common as silicon. Magnesium (Mg) and iron (Fe) are about as common as silicon, while the level of sulfur (S) is almost half as large. A handful of elements have concentrations in the range 5-10% relative to silicon, namely, argon (Ar), aluminum (Al), calcium (Ca), sodium (Na) and nickel (Ni). This is 15 chemical elements in total, which completely dominate in terms of number. All other elements are also present, but they are very few. It is these few building blocks that make up all the stars and planets we see around us.

Hydrogen, helium, neon, nitrogen and argon will mainly be in gaseous form in the interstellar medium, along with parts of the oxygen and carbon. This represents about 99% of the interstellar medium mass. The remaining percent is made up of dust grains, which typically are a few tenths of a micrometer in size or less (a micrometer is one thousandth of a millimeter). These dust grains consist of various minerals made up of the common elements that are not in gaseous form. A distinction is made between silicate grains, graphite grains and PAHs. Silicate grains are dominated by oxygen, silicon, magnesium, iron and sulfur. Ultimately, these are the grains that will form planets like Earth – about 95% of Earth’s mass is made up of these five elements. The silicate grains also bind the bulk of the interstellar medium content of aluminum, calcium, sodium and nickel. Graphite grains are mainly composed of carbon and binds about 60% of the species. The rest of the carbon is evenly split between the gas and a type of organic molecules called Polycyclic Aromatic Hydrocarbons, or PAHs. The simplest PAH is called benzene and consists of six carbon atoms that forms a ring, and where each carbon atom is attached to a hydrogen atom. More complex PAHs are formed by joining such rings into large chunks of carbon and hydrogen.

Different phases of the interstellar medium

The interstellar medium density and temperature vary greatly between different parts of the Milky Way. The most common form, in terms of volume, is called the warm neutral medium, which fills about two-thirds of the space between the stars. A characteristic feature is that hydrogen atoms have not teamed up to form molecular hydrogen (H2), but move about freely. These hydrogen atoms are not ionized, so that each hydrogen nucleus (which usually consists of a single proton with no neutrons attached) has one electron in orbit around it. The warm neutral medium has a density of 0.2-0.5 atoms per cubic centimeter, and has a temperature of between 6000°C and 10000°C.

This warm neutral medium can be converted into other types of interstellar medium, either by cooling and compaction, or by being heated it up and thinned out. There are two different types of denser interstellar medium. The cold neutral medium (also referred to as diffuse clouds) generally has 10 to 100 atoms per cubic centimeter and a temperature of between -220°C and -170°C. The molecular cold neutral medium (also called molecular clouds) are dense enough for hydrogen atoms to merge, thereby forming molecular hydrogen (H2). The density is typically 100-1000 molecules per cubic centimeter and the temperature is about -250°C. About half of the interstellar medium mass is in the form of diffuse clouds and molecular clouds – but since they are so compact, they only make up few percent of the interstellar medium volume.

In the vicinity of very hot stars, the ultraviolet radiation so intense that the interstellar medium becomes ionized. Such bubbles of ionized gas are called Strömgren spheres after the Swedish astronomer Bengt Strömgren (1908-1987), or HII regions. However, the destructive effects of the radiation also reach far beyond the immediate stellar vicinity, thereby giving rise to the so-called warm ionized medium. Here, the density is around 0.2-0.5 ions per cubic centimeter and the temperature is about 8000°C.

The most extreme type of interstellar medium is called hot ionized medium, and is formed when one or more supernovas explode. In the hot ionized medium there are less than 0.01 ions per cubic centimeter and the temperature can reach a million degrees.

The interstellar medium in our neighborhood

The Solar System and our nearest neighbors among the stars are situated within a large region of hot ionized medium, known by the rather unpoetic name the Local Bubble. The bubble has a size of about 400-600 light years in the Milky Way plane, but extends twice as far in the directions perpendicular to this plane. In comparison, our nearest neighbor among the stars is just over four light years away from the Sun, and the Milky Way disk has a thickness of about 3000 light years. The density and the temperature of the Local Bubble are rather uncertain. The reason is that these parameters are measured by observing X-rays emitted by the plasma emits, but this radiation cannot easily be distinguished from the X-rays emitted by the solar wind in our own Solar System. Depending on what fraction that actually coming from the Local Bubble, the density may be as low as 0.003 ions per cubic centimeter, or as high as 0.04 ions per cubic centimeter. Similarly, the temperature may be as high as a million degrees, or as low as 20,000°C.

The supernova explosions that created the Local Bubble, have probably been located in the so-called Scorpi-Centaurus Association, a group of very massive stars that includes the constellation Scorpio’s brightest star, Antares.

The Local Bubble is not homogeneous, but also contains a number of wisps of hot ionized medium, that constitute 5-20% of the volume. The Solar System happens to be located between two such more or less attached clouds. In the direction of the Milky Way center, we find the Galactic Cloud (often referred to as G cloud), an elongated formation with a length of about 20 light years whose temperature is between 4800°C and 5600°C. In the opposite direction we have the Local Interstellar Cloud, measuring about five light years, and having a temperature of between 5900°C and 8500°C. The warm ionized medium within the Local Bubble typically has a density of about 0.3 hydrogen atoms per cubic centimeter. About a third of these are ionized, so there are about 0.1 electrons per cubic centimeter.

The measurements of Voyager 1

There are three instruments on Voyager 1 that made it possible to determine that the heliopause – the boundary between the heliosphere and the interstellar medium – has been crossed. The first instrument, the Low-Energy Charged Particles (LECP), detects solar wind particles. Before July 28, 2012, LECP had steadily registered about 30 particles per second, but on that day the number suddenly dropped to one third of that. After a short time, the count rose to the usual level again, but after a series of such fluctuations, it dropped to only a few particles per second on August 25, 2012, and has remained there ever since. This is considered to be the date when Voyager 1 finally took the plunge out of the heliosphere and entered interstellar territory.

The second instrument, Cosmic Ray System (CRS) measures the presence of cosmic rays. The cosmic rays primarily consist of hydrogen and helium nuclei traveling nearly at the speed of light. They have accelerated to these high speeds during supernova explosions within the Milky Way. The magnetic field in the heliosphere protects the Solar System from these cosmic rays to some extent, why it was expected that the number of detections would increase at the passage of the heliopause. Indeed, about 1.7 detections per second were made prior to May 2012, but then the amount of cosmic rays stated to increase, and by the end of August 2012 the count stabilized at around 2.3 detections per second.

The third instrument is called Plasma Wave System (PWS), which has been able to measure the density of the plasma in the interstellar medium. It utilizes the fact that Langmuir waves sometimes form in plasma. This happens, e.g., when the number of electrons per unit volume, for some reason, becomes larger than the ion density. This produces electrical forces that try to reduce the electron density, thereby making the region electrically neutral. The problem is that the reduction often becomes excessive, why the electrical forces then strive to increase the electron density again. The result is very rapid oscillations in the electron density – this is the Langmuir wave. PWS can measure the number of oscillations that the plasma performs every second and one can show theoretically that this frequency depends on the plasma density. PWS recorded an outbreak of plasma oscillations during October and November 2012, which showed that the density was 0.06 electrons per cubic centimeter. During April and May 2013 another outbreak was recorded, showing that the density had increased further, to 0.08 electrons per cubic centimeter. This density is typical for the warm ionized medium (observations from ground indicates about 0.1 electrons per cubic centimeter), and this density is about 40 times higher than that just inside the heliopause.

For the first time in human history, we have thus been able to measure the properties of the interstellar medium in situ. This is an enormous technological achievement, and it is surprising that the spacecraft still works after nearly 40 years in space, given the extreme conditions that prevail there. It is also very important from a scientific point of view – the measurements made in situ show that observations from Earth, and our interpretations of these observations based on physical theories, actually are correct. It gives us confidence and shows that previous research findings are reliable. Continuing observations will increase our understanding of this interstellar environment further. But there is also another value, in addition to technology and science. Voyager 1 says something important about us humans – the spacecraft is out there because we are curious, rational, capable, tenacious and hard-working creatures. It gives us the feeling that everything is possible and that we do not have to passively accept a life of ignorance and uncertainty about the world around us. It is a source of inspiration, and something every person can be proud of – because if you are curious, you are also a part of this adventure.

Literature

Ferrière, K. M. (2001). The interstellar environment of our galaxy. Reviews of Modern Physics 73 (4), 1031-1066.

Gurnett, D. A., Kurth, W. S., Burlaga, L. F., Ness, N. F. (2013). In Situ Observations of Interstellar Plasma with Voyager 1. Science 341, 1489-1492.

Redfield, S. (2009). Physical properties of the local interstellar medium. Space Science Reviews 143, 323-331.

Welsh, B. Y., Shelton, R. L. (2009). The trouble with the Local Bubble. Astrophysics and Space Science 323 (1), 1-16.

JPL’s website about the Voyager spacecraft: http://voyager.jpl.nasa.gov/index.html