The edge of the solar system has been sought out by humanity for hundreds of years. Saturn was thought to be where the solar system ended centuries ago by astronomers. Later discoveries showed that there are three planets beyond this "edge." Some mark the boundary there, while others mark it with "Planet X." However, recent theory suggests that there is a belt of comets four billion miles from the Sun. This is where Planet X is theorized to be. This strangest theory yet about the border of the solar system is the Oort Cloud. Jan Oort first proposed it in 1950 as a "land of comets," two light-years away. There, comet-forming nuclei are waiting for a little push from a passing star to begin their journey towards the sun.

Modern science glossaries define the heliopause as essentially constituting the edge of the solar system so plenty or research is being made into what causes this phenomena. The Ulysses spacecraft is conducting an extensive study of the sun, the solar wind, as well as investigating the Sun's polar regions. The Space Shuttle Discovery launched Ulysses in October 1990. In February 1992, it received a gravity boost from Jupiter to take it out of the plane of the ecliptic. It has now completed its main mission of surveying both of the sun's poles. Its objective has been extended for another orbit so that it can survey the sun's poles near the maximum point of the sunspot cycle, too. It’s a solar-studies spacecraft that's always further from the Sun than the Earth is!

The word heliopause is a noun derived from the Greek helios meaning sun and the English word pause for the boundary created between the solar wind and interstellar gas.

It’s really from an out of this world science! The sun is the heart of our solar system and the solar wind is a flow of electrically charged particles that stream progressively away from the sun. As this wind travels out into space, it generates a magnetized bubble of hot plasma around the sun, called the heliosphere. In time, the expanding solar wind encounters the charged particles and magnetic field in the interstellar gas. The periphery created between the solar wind and interstellar gas is the heliopause.

Voyager is without doubt the most fantastic probe ever to leave Earth, but what of them now? Ever since 1989 when Voyager 2 encountered Neptune, both spacecraft have been studying the surroundings of space in the outer solar system. The Voyager 1 and Voyager 2 spacecraft have passed the orbit of Pluto and begun to explore the environment of space. Nearly two decades after they left home, the spacecraft discovered the first direct evidence of the long-sought-after rim that separates Earth's solar system from interstellar space. Scientists have received signals from both spacecraft that indicates they are in the neighborhood of the heliopause. For the spacecraft to pass beyond the heliosphere, the Voyagers will initially pass through a termination shock, then enter interstellar space where no spacecraft has ever gone before.

Science instruments on both spacecraft are now sensing signals that scientists think are coming from the outermost margin of the sun's magnetic field that the spacecraft must pass through before they reach interstellar space.

    Both spacecraft are studying the vast bubble the Sun inflates around itself by outward pressure of the solar wind. The bubble has a boundary, called the heliopause, where this outward pressure is counterbalanced by inward pressure of the interstellar wind in our neck of the galaxy. The interstellar wind outside that boundary is a flow of atoms and other particles blasted from explosions of dying stars. The location of the heliopause varies with the level of solar activity during the Sun's 22-year sunspot cycle and with changes in the interstellar wind, Stone said. Some scientists suggest that, on a much longer time scale, the interstellar wind may occasionally press the boundary far enough inward to sway Earth's climate.
The solar wind blows a bubble in space inside the ambient interstellar medium, or partially ionized gas of the small local interstellar cloud that the sun travels through. This bubble, the heliosphere, is bordered by the heliopause and the surface is believed to divide solar and interstellar plasmas, which have not been detected yet. In a similar manner other stars also have winds and pass through interstellar clouds surrounded by asterospheres, and bounded by asteropauses.

The boundary marking the edge of the sun's influence where the solar wind and the wind from other stars meet is about 100 and 150 astronomical units from the sun. An astronomical unit (AU)is the distance between the Earth and Sun. In June 1993 radio signals were detected by the two Voyager spacecrafts. This passage through the heliopause is the genesis of interstellar exploration which is the ultimate goal of the Voyager Interstellar Mission.

    Both Voyagers are headed towards the outer boundary of the solar system in search of the heliopause, the region where the Sun's influence wanes and the beginning of interstellar space can be sensed. The heliopause has never been reached by any spacecraft; the Voyagers may be the first to pass through this region, which is thought to exist somewhere from 5 to 14 billion miles from the Sun. Sometime in the next 10 years, the two spacecraft should cross an area known as the termination shock. This is where the million-mile-per-hour solar winds slows to about 250,000 miles per hour—the first indication that the wind is nearing the heliopause. The Voyagers should cross the heliopause 10 to 20 years after reaching the termination shock. The Voyagers have enough electrical power and thruster fuel to operate at least until 2020. By that time, Voyager 1 will be 12.4 billion miles from the Sun and Voyager 2 will be 10.5 billion miles away. Eventually, the Voyagers will pass other stars. In about 40,000 years, Voyager 1 will drift within 1.6 light years (9.3 trillion miles) of AC+79 3888, a star in the constellation of Camelopardalis. In some 296,000 years, Voyager 2 will pass Sirius, the brightest star in our sky, at a distance of about 4.3 light years (25 trillion miles). The Voyagers are destined—perhaps eternally—to wander the Milky Way.
A scientist from the American Geophysical Union, Donald Gurnett of the University of Iowa offers this account: "a powerful solar flare event in May-June 1991 caused a surge of solar-wind particles which subsequently interacted with the heliopause, setting up huge radio bursts (at more than 10**13 Watts, the most powerful radio source in the solar system) detected by the Voyagers beginning in July 1992. High in power but low in frequency (2-3 kHz), the radio signals could not be detected in the inner solar system. However, Voyager 1, at a distance of 52 AU, and Voyager 2, at 40 AU, were well placed to made a measurement."

Dr.Gurnett and his research group have now designed and constructed instruments aboard Voyager I that have flown farther from the sun and instruments aboard the Helios 2 in 1976 that passed closer to the sun. No other space research group have achieved as much. Ralph McNutt a co-investigator on the Voyager plasma science experiment and a researcher at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. McNutt said that "from the timing of the signals the distance to the heliopause could be estimated to be between 80 and 130 AU." The heliopause distance is theorized as fluctuating in response to changes in interstellar matter and solar wind conditions. The breadth of the heliopause remains undefined but may be tens of astronomical units. Upstream of the heliopause, in the ISM suppositions put forward the idea of a bow shock where the incoming interstellar wind first encounters its impending collision with the sun's magnetosphere. Downstream from the sun is thought to be ‘a long, turbulent tail. The extent to which interstellar matter penetrates inside the heliopause is open to question but may be partially determined by the first interstellar probes.’

Researchers, Mike Gruntman of the University of Southern California in Los Angeles and Hans J. Fahr of the Institute for Astrophysics and Extraterrestrial Research in Bonn, Germany, predict that the heliopause will be found to be at least 125 times further from the sun than is the Earth:

    Mapping the heliopause from earth might be possible by deciphering reflected solar ultraviolet radiation. The heliopause is the boundary where the outgoing solar wind meets the incoming plasma of the local interstellar medium. The Voyager 1 spacecraft, now at a distance of 67 astronomical units, might actually sample the heliopause (at an expected distance of 125 AU or more) in the future, but scientists at USC and the University of Bonn contend that UV rays bouncing off the heliopause and returning toward the Earth might be brighter than the general UV background, and hence would offer a way of probing the heliopause shape, which is expected to be nonspherical, and the magnetic field of the interstellar medium. (Gruntman and Fahr, Geophysical Review Letters, 15 April, 1998.)
Beginning in August 1992, the radio antennas on the spacecraft, called the plasma wave subsystem, have been recording intense low-frequency radio emissions approaching from outside of the solar system. For quite a long time the cause of these radio emissions was a mystery. Our interpretation now is that these radio signals are created as a cloud of electrically charged gas, called a plasma, expands from the sun and interacts with the cold interstellar gas beyond the heliopause,” said Dr. Don Gurnett, chief researcher of the Voyager plasma wave subsystem and a professor at the University of Iowa.

"These radio emissions are probably the most powerful radio source in our solar system," continues Gurnett. "We've estimated the total power radiated by the signals to be more than 10 trillion watts. However, these radio signals are at such low frequencies, only 2 to 3 kilohertz, that they can't be detected from Earth."

The sun experienced a phase of intense solar activity in May and June 1992, which released a cloud of swiftly moving charged particles. The particles interacted violently with the interstellar plasma and produced the radio emissions, once the cloud of plasma arrived at the heliopause.

According to Gurnett, "We've seen the frequency of these radio emissions rise over time. Our assumption that this is the heliopause is based on the fact that there is no other known structure out there that could be causing these signals," Gurnett added that due to the Voyagers' unique positions in space, they serendipitously discovered and recorded the radio emissions. "Earth-bound scientists would not know this phenomenon was occurring if it weren't for the Voyager spacecraft."

Precisely where the heliopause is located is still one of the great-unanswered subjects in space physics. "It's this Voyager radio data combined with the plasma measurements taken at the spacecraft that give us a better guess about where the heliopause is. Based on the solar wind speed, the time that has elapsed since the mid-1992 solar event and the strength of the radio emissions, my best guess for the upper limit of the heliopause currently is about 90 to 120 astronomical units (AU) from the sun," said Dr. McNutt.

One can imagine the sun's heliopause as it moves through the local interstellar medium much like a boat moves on water, pushing a wake out in front. Where does the sun's influence end? Nobody is sure. Out past the orbits of Neptune and Pluto extends this region called the heliosphere where the sun's magnetic field and particles from the solar wind continue to govern. The surface where the solar wind falls below sound speed is called the termination shock and is so near that a Pioneer or Voyager spacecraft may soon glide through it as they exit the solar system.

For a nice picture that puts everything in perspective you can visit the following web site:
http://spaceprojects.arc.nasa.gov/Space_Projects/pioneer/f23.gif

Sources:

The American Institute of Physics Bulletin of Physics News:
www.aip.org/enews/physnews/1993/split/pnu131-1.htm

Astronomy Picture of the Day:
antwrp.gsfc.nasa.gov/apod/ap020624.html

Edge of the Solar System:
library.thinkquest.org/12713/textonly/endsolsy.html

Gruntman and Fahr, Geophysical Review Letters, 15 April. HELIOPAUSE AND ASTEROPAUSES:
despa.obspm.fr/OBS2000/absbook/node58.html

Press Release: Voyager-Heliopause, May 26, 1993:,BR> http://vraptor.jpl.nasa.gov/voyager/vgrhelio_pr.html

25 Years Later, Voyager Mission Keeps Pushing the Space Envelope:
awequest.com/astro-space/

Voyager:
voyager.jpl.nasa.gov/mission/interstellar.html

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