Date: March 27, 2012
Title: Astronomy Word of the Week : Heliopause
Podcasters: Dr. Christopher Crockett
Organization: United States Naval Observatory
Links: http://christophercrockett.com
http://astrowow.wordpress.com/
Description: We explore another word of the week as we travel along with the Voyager space probes to the very edge of the solar system and the boundary of interstellar space – the “heliopause”.
Bio: Dr. Christopher Crockett is an astronomer at the United States Naval Observatory in Flagstaff, Arizona. His research involves searching for planets around very young stars (“only” a few million years old). It is hoped that the results from this research will help constrain models of planet formation and lead to a better understanding of where, when, and how often planets form. Chris is also passionate about astronomy outreach and education and will talk for hours about the Universe if you let him.
Sponsor: “This episode of 365 days of Astronomy is sponsored in-kind by The Planetary Society.”
Transcript:
In late 1977, the twin Voyager probes – Voyager 1 and Voyager 2 – were launched from Cape Canaveral, Florida atop Titan IIIE rockets on a trajectory that would take them past the gas giant planets of our Solar System and then onward to interstellar space. Voyager 1 passed close to Jupiter and Saturn in 1979 and 1980, respectively. Voyager 2, after making similar flybys of the gas giants, continued on to a close encounter with Uranus in 1986 and then Neptune in 1989. And then….nothing. The Voyager probes sailed on into the dark void beyond the planets each heading out of the Solar System in a different direction. Though they continued to send back to Earth a trickle of data, little of consequnce occurred. In late 1998, Voyager became the most distant man-made object from Earth as it passed the Pioneer 10 probe roughly 7 billion miles from home. And then, in 2004, Voyager 1’s instruments detected a change in its environment. Something was happening! The Voyager probes were about to enter a place where no man-made object had ever tread. They were truly leaving home, leaving the Solar System. The Voyagers had begun the journey into interstellar space.
The Sun blows off of its surface a steady stream of charged particles – electrons and protons – which collectively are known as the solar wind. This wind blows past the Earth at roughly 900,000 mph. When it strikes our planet’s upper atmosphere, it creates the aurora – the northern and southern lights; an ethereal swirling pattern of color in the skies near the poles. The solar wind continues on past all the planets and leaves our Solar System ensconsed in a bubble within the interstellar medium – the tenuous clouds of hydrogen, helium, and dust grains that pervade our Galaxy. This bubble in space is called the heliosphere. Within it, the Sun rules. The particles and magnetic fields all emanate from the Sun.
But this can not continue forever. Eventually the solar wind must slow down and give way to the particles that flow through the Galaxy. It is this boundary – often considered to be the true edge of our Solar System – that has excited the Voyager spacecraft scientists and engineers. In 2004, Voyager 1 recorded that the solar wind around it was slowing down. Twenty seven years after its launch from Earth, Voyager 1 was approaching the boundary of the Solar System – a place called the heliopause.
The region around the heliopause is complex and poorly understood. Until Voyager 1, and then Voyager 2 three years later, made the first tentative steps into this unexplored domain, all our understanding of the Solar System’s outer edge was mostly theoretical. The Voyagers are currently in a zone called the termination shock. This is where the solar wind particles start slowing down, reacting to pressure from the interstellar winds. When Voyager 1 entered the termination shock, it was nearly 9 billion miles from Earth or about twice the distance that Neptune is from the Sun. Voyager 2 made its own passage into the termination shock in 2007 while it was much closer to Earth – about 2 billion miles closer! That small bit of information revealed something interesting: the shape of the heliosphere is not uniformly round, but rather it appears to be dented in on its south side. In truth, what the Voyagers were seeing was just a tiny sliver of a much larger picture. The boundary of our Solar System is probably not smooth at all, but actually quite lumpy, owing to local variations in the dynamics of the interstellar medium that envelops us.
In 2010, Voyager 1 recorded something truly monumental: the outward motion of the solar wind had stopped. 11 billion miles from home, the steady stream of particles from our Sun had hit a wall. It appears that the solar wind was no longer moving away from the Sun, but had now started flowing perpendicular to it. This is the heliosheath – the realm just inside the heliopause where the particle stream is compressed and becomes turbulent. Here lies a frothy mix of magnetic bubbles – each bubble being as large as the distance from the Earth to the Sun. Here, the magnetic field of the Sun has become detached from its parent. It has become apparent that Voyager 1 is rapidly approaching the heliopause and within the next year or so will leave the Sun behind for good.
This past December, Voyager 1 offered its first glimpse of what lies beyond the heliopause. Ultraviolet light generated from excited hydrogen atoms in star forming regions of our own Galaxy had begun to strike the veteran spacecraft’s detectors. This light is undetectable from Earth where it is overpowered by radiation from the Sun. But out here, beyond the Sun’s influence, the glow of young stars in the Milky Way could finally be seen.
What awaits the Voyager probes in the coming years? It is hard to say. Clearly the heliopause is fast approaching. Here the number of hot charged particles from our Sun will rapidly drop and Voyager will instead be bathed in a sea of much colder Galactic streams. And what then? It is thought that as the Solar System plows through the interstellar plasmas that it actually produces a wake not unlike that produced by a boat speeding through the water. This is called the bow shock – a region of compressed gas that trails out behind the Solar System. Bow shocks have actually been obeserved around other stars. The star LL Orionis in the Orion Nebula is home to what resembles a backwards letter ‘C’ in the nebular gasses that envelope it – the result of the star’s magnetic field ramming into gas flows within the nebula. Before reaching the bow shock, Voyager may pass through what astronomers call the “hydrogen wall” – a relatively narrow region of hot compressed hydrogen gas caused by the interstellar medium being stirred up by the heliopause. But this is still somewhat speculative. For now, we’ll just have to wait and see.
The Voyagers are intrepid little explorers. Over 34 years old, and more than 11 billion miles from home, they continue their journey into the unknown. Every minute takes them nearly 1500 miles further from Earth. It currently takes a radio signal over 16 hours to travel from Earth to the spacecraft (and another 16 hours for it to respond). Three decades after launch they continue to suprise us with discoveries at the very edge of our Solar System. Here at the heliopause, the familiar Sun is no longer king. The solar wind – still dazzling humans back home with brilliant auroral displays – is breaking down. The Sun’s magnetic field is fracturing. Humanity’s first emissaries into interstellar space are taking their first steps into the wilds of the Galaxy.
End of podcast:
365 Days of Astronomy
=====================
The 365 Days of Astronomy Podcast is produced by the Astrosphere New Media Association. Audio post-production by Preston Gibson. Bandwidth donated by libsyn.com and wizzard media. Web design by Clockwork Active Media Systems. You may reproduce and distribute this audio for non-commercial purposes. Please consider supporting the podcast with a few dollars (or Euros!). Visit us on the web at 365DaysOfAstronomy.org or email us at info@365DaysOfAstronomy.org. Until tomorrow…goodbye.