Our solar system is taking the road less travelled from the rest of the Milky Way galaxy, say scientists who used radio signals from two spacecraft nearing interstellar space to map the unexpected route. The researchers determined that the magnetic field in interstellar space is propelling our solar system along at a 60-degree to 90-degree angle from the rest of the galaxy. That's happening because the part of the interstellar magnetic field that comes closest to our system is not parallel to the spiralling arms of the galaxy, as it appears to be elsewhere. As a result, our solar system has taken on a bullet-shaped appearance as it soars through space, say Merav Opher, with George Mason University in Fairfax, Va., who published his research in this week's issue of the journal Science.
Our Solar System is unevenly squashed by distortions in the interstellar magnetic field, according to data from the distant Voyager space probes. Previously, the Solar System was thought to be spherical. But the Voyager II space probe has detected its boundary on one side closer in than Voyager I detected the boundary on the other side, suggesting it is asymmetrical.
Read more 3-D rendition of the Solar System as seen from the outside. It is distorted into bullet shape in the direction of the interstellar magnetic field. The field lines are shown wrapping around the Solar System. Image: Opher et al.
On 16 August, the Voyager 1 spacecraft, the most remote artificial object ever launched into space, will reach a distance of 100 astronomical units (AU) from the Sun. (One AU is 149,957,870 km or 92,955,730 miles - the average distance of the Earth from the Sun.) This means that on 16 August, Voyager 1 will be almost 15 billion km from the Sun.
Voyager 1, became the first spacecraft to cross the termination shock at the edge of the heliosphere - the bubble in space created by the solar wind - and entered the outermost layer of the heliosphere about 14.4 billion km from the Sun. Voyager 1 is currently heading away from the Sun at a speed of about 17 km/s. It is expected to pass beyond the heliopause into interstellar space within 10 years, with Voyager 2 expected to follow about five years later. Flight controllers believe both spacecraft will continue to operate and send back valuable data until at least the year 2020.
Voyager 2 could pass beyond the outermost layer of our solar system, called the "termination shock," sometime within the next year, NASA scientists announced at a media teleconference Tuesday.
The milestone, which comes about a year after Voyager 1's crossing, would be earlier than expected and suggests to scientists that the edge of the shock is about 1 billion miles closer to the sun in the southern region of the solar system than in the north. This implies that the heliosphere, a spherical bubble of charged low-energy particles created by our sun's solar wind, is irregularly shaped, bulging in the northern hemisphere and pressed inward in the south. Scientists determined that Voyager 1 was approaching the termination shock when it began detecting charged particles that were being pushed back toward the sun by charged particles coming from outside our solar system. This occurred when Voyager 1 was about 85 AU from the sun. One AU, or Astronomical Unit, is the distance between Earth and the sun, or 150 million kilometres. In contrast, Voyager 2 began detecting returning particles while only 76 AU from the sun.
"This tells us that the shock down where Voyager 2 is must be closer the sun than where Voyager 1 is. The (magnetic) field is only one-100,000 of the field on the earth's surface, but it's over such a large area and pushing on such a faint gas that it can actually push the shock about a billion miles in" - Ed Stone, Voyager project scientist at the California Institute of Technology in Pasadena.
The researchers think that the heliosphere's asymmetry might be due to a weak interstellar magnetic field pressing inward on the southern hemisphere.
Both of the Voyager spacecraft were launched from Cape Canaveral Air Force Station in Florida: Voyager 2 headed out on Aug. 20, 1977, Voyager 1 on Sept. 5, 1977. Currently, Voyager 1 is about 8.7 billion miles from the sun and travelling at a speed of 3.6 AU per year, while Voyager 2 is about 6.5 billion miles away and moving at about 3.3 AU per year.
When Voyager 1 finally crossed the “termination shock” at the edge of interstellar space in December 2004, space physicists anticipated the long-sought discovery of the source of anomalous cosmic rays. These cosmic rays, among the most energetic particle radiation in the solar system, are thought to be produced at the termination shock — the boundary at the edge of the solar system where the million-mile-per-hour solar wind abruptly slows. A mystery unfolded instead when Voyager data showed 20 years of predictions to be wrong.
A new theory published in the February 17 2006 issue of Geophysical Research Letters by Dr. David McComas of Southwest Research Institute and Dr. Nathan Schwadron of Boston University explains why the energisation of anomalous cosmic rays is almost entirely absent where Voyager passed through the blunt nose of the termination shock. While the shape of the shock was formerly thought to be unimportant, the new theory explains how this shape is the major factor in particle energisation. McComas and Schwadron say that understanding the role of the termination shock’s shape in the energisation of anomalous cosmic rays may be a stepping stone to understanding the influence of shock shapes for energszation of particle radiation throughout the cosmos. Shocks energize many forms of this dangerous particle radiation, which pose significant hazards to astronauts on space missions, such as future manned missions to the Moon and Mars.
"Models showed we should see the source energy spectrum of anomalous cosmic rays at the termination shock. We were pretty sure we knew what we’d see, but when we got there it wasn’t what we expected and it clearly was not the source of the anomalous cosmic rays" - Dr. David McComas, senior executive director of the SwRI Space Science and Engineering Division.
Researchers were uncertain where the termination shock would even be found, but they knew there would be a jump in magnetic fields, a deceleration of plasma and other signs.
"It’s like walking across a field when you don’t know where the edge of the property is. You know you’re at the boundary when you finally see the fence" - Dr. David McComas.
This schematic diagram cuts through the termination shock at the equator. Inside the termination shock, the magnetic field line spirals out and connects to the shock. Also shown are the approximate positions of Voyager 1 at the “nose” of the termination shock and Voyager 2 farther back. Image credit: Geophysical Research Letters
The shape of the termination shock wasn’t thought to be important, so most researchers treated it as being circular, with the magnetic field from the solar wind spiralling out and piercing through it at a single point. McComas and Schwadron showed that acceleration of anomalous cosmic rays could be easily explained by including a more realistic termination shock shape.
"In fact, the termination shock couldn’t be circular because the solar system is moving through the galaxy, which would create more of a flattened egg shape. A flattening of the nose of the termination shock leads to a time dependant acceleration process" - Dr. Nathan Schwadron, assistant professor of astronomy at BU.
The production of anomalous cosmic rays requires a connection to the termination shock (the point where it’s pierced by the magnetic field line) and the ability for energetic particles to reside near that connection for up to about a year. Using the new model, simple calculations showed particles could remain at a connection point for about 300 days, further evidence of a valid model. Voyager 1 didn’t see the energetic anomalous cosmic rays when it crossed the termination shock.
"The 20-million-electron-volts-per-particle helium that we saw was less than 10 percent of what was predicted. Similarly, we saw only 5 percent of what was predicted for 4-million-electron-volts-per-particle oxygen. We weren’t off by 5 or 10 percent, we were off by factors of 10 and 20" - Dr. David McComas.
The new model shows that particles can indeed be accelerated at the termination shock, but not at the nose where Voyager crossed it.
"The particles don’t get accelerated up to the highest energies until the field line has moved a long way out and its ‘feet’ have moved back along the sides of the termination shock. This means the source of the energetic anomalous cosmic rays must be on the flanks" - Dr. David McComas.
The Voyager 2 spacecraft is also moving out of the solar system, making single-point measurements as it travels. It is expected to pass the termination shock, farther back from the nose, within the next 2–3 years.
"The explanation given here provides predictions that Voyager 2 should observe a larger jump in energetic particle fluxes and a more unfolded anomalous cosmic ray spectrum as it crosses the termination shock" - Dr. Nathan Schwadron.
The Interstellar Boundary Explorer (IBEX) spacecraft, scheduled to launch in the summer of 2008, will be the first to make global images of the interactions around the termination shock. At that time, researchers will be able to view global interactions at the termination shock’s nose, flanks and tail. Combined with data from Voyagers 1 and 2, these observations will enable researchers to understand the global interaction of the solar system with the galaxy for the first time.
"Even without IBEX, this is a big step in understanding what’s going on at the termination shock. We really feel that our answer to this mystery is just too simple to be wrong" - Dr. David McComas.
SwRI leads the IBEX science mission for NASA. The Goddard Space Flight Centre manages the Explorer Program for the Science Mission Directorate. The paper, “An Explanation of the Voyager Paradox: Particle Acceleration at a Blunt Termination Shock,” is available in the February 17 issue of Geophysical Research Letters.
A trio of surprise discoveries from the Voyager 1 spacecraft reveals intriguing new information about our solar system's final frontier. The findings appear in the Sept. 23, 2005 issue of Science.
The surprises come as the hardy, long-lived spacecraft approaches the edge of our solar system, called the heliopause, where the sun's influence ends and the solar wind smashes into the thin gas between the stars.
"These are just the most recent of many surprises Voyager has revealed in its 28-year journey of discovery. They tell us that the interaction of our sun with the surrounding interstellar matter from other stars is more dynamic and complex than we had imagined, and that there is more yet to be learned as Voyager begins the final leg of its race to the edge of interstellar space" - Dr. Edward Stone, Voyager project scientist at the California Institute of Technology in Pasadena.
Voyager 1 is expected to pass beyond the heliopause into interstellar space in eight to 10 years, with Voyager 2 expected to follow about five years later.
Voyager 1 has already passed the termination shock, where the million-mile-per-hour solar wind abruptly slows and becomes denser and hotter as it presses against interstellar gas. It was expected the wind beyond the shock would slow to a few hundred thousand miles per hour. But the Voyager scientists were surprised to find that the speed was much less, and at times the wind appeared to be flowing back inward toward the sun.
"This could mean that the outward pressure of wind was decreasing as the sun entered the less active phase of its 11-year cycle of sunspot activity" - Dr. Edward Stone.
Another surprise: the direction of the interplanetary magnetic field in the outer solar system varied more slowly beyond the termination shock. As the sun rotates every 26 days, the direction of the field alternates every 13 days. That field is carried out by the solar wind, with the alternating directions forming a pattern of zebra stripes moving outward past the spacecraft. One could imagine a zebra with giant "magnetic stripes" running past the spacecraft and Voyager 1 "observing" an alternating stripe every 13 days. After the shock, the "zebra" with its stripe pattern was moving at nearly the same speed as Voyager, so that it took more than 100 days for the stripe to pass the spacecraft and for the magnetic field to switch directions.
Perhaps the most puzzling surprise is what Voyager 1 did not find at the shock. It had been predicted that interstellar ions would bounce back and forth across the shock, slowly gaining energy with each bounce to become high speed cosmic rays. Because of this, scientists expected those cosmic ray ions would become most intense at the shock. However, the intensity did not reach a maximum at the shock, but has been steadily increasing as Voyager 1 has been moving farther beyond the shock. This means that the source of those cosmic rays is in a region of the outer solar system yet to be discovered.
NASA's Voyager 1 has passed into the border region at the edge of the solar system and now is sending back information about this never-before-explored area, say scientists at the University of Maryland.
"We have confirmed, for the first time, that Voyager 1 crossed the termination shock on Dec. 16, 2004. Until now there has been debate among scientists on whether Voyager 1 had crossed the termination shock as early as 2002 or not until December 16, 2004 " - Frank McDonald , a senior research scientist at the university's Institute for Physical Science and Technology, and a co-author on two of four Voyager 1 papers published in the Sept. 23 issue of Science.
The termination shock marks the beginning of a transition region at the edge of the solar system that is known as the heliosheath. McDonald, who co-authored "Crossing the Termination Shock into the Heliosheath: Magnetic Fields," and "Voyager 1 Explores the Termination Shock Region and the Heliosheath Beyond." Matthew Hill, George Gloeckler and Douglas C. Hamilton, scientists in the University of Maryland's Space Physics Group were among the co-authors of a third article, "Voyager 1 in the Foreshock, Termination Shock and Heliosheath," which presents other new observations on the spacecraft's entrance into the heliosheath. Gloeckler and his Space Physics group built the Low Energy Charged Particle (LCEP) instrument, one of the five main instruments on Voyager.
"The termination shock -- a shock wave in the solar wind, that marks the slowing of the supersonic solar wind to subsonic speed -- had been universally thought to be a prodigious accelerator of particles and our findings largely confirm that," said Hill, a research scientist in the department of physics.
"This paper describes a remarkable increase in particle intensity with energetic characteristics unlike anything we have seen before. In addition, the LECP instrument indirectly determines that the solar wind speed in the heliosheath is clearly sub-sonic."
However, Hill explained that one very surprising finding was that certain particles called "anomalous cosmic rays" do not appear to be accelerated at the termination shock, or at least not in the area where Voyager 1 crossed. This is despite the "near unanimity of opinion" among scientists that these rays would be accelerated.
"This finding has the potential to turn three decades of anomalous cosmic ray research on its head".
Our sun is surrounded by a bubble known as the heliosphere ("helio" means sun) that extends well beyond the solar system's outermost planets. This bubble is formed by the solar wind; electrically charged particles that blow out from the sun at a million miles per hour. As the sun races around the centre of our Milky Way galaxy at some 560,000 miles per hour, this bubble, or heliosphere, shoves through the clouds of dust, gas and charged particles that whirl between the stars.
At the outer edge of the heliosphere, is the heliosheath, a transition or border region where the solar wind is directly influenced by the pressure of the interstellar clouds through which our solar system travels. Uncertainly about when Voyager 1 entered the heliosheath, stems from the fact that the exact location and size of this transition region are not static, but change based on the relative pressures of the solar wind and the opposing interstellar clouds.
The beginning of the heliosheath region is marked by the termination shock, the point at which the solar wind abruptly slows. The termination shock gets its name from the shockwave produced by the slowing of the solar wind. This shockwave is similar to the sonic boom that occurs on Earth when an airplane crosses the subsonic-supersonic barrier. The outermost edge of the heliosheath is the heliopause, which marks the end of our solar system. At the heliopause, the force or pressure of solar wind is stopped, balanced by that of the pressure from the interstellar clouds.
Voyager 1 and its twin spacecraft Voyager 2 are now part of a NASA Interstellar Mission to explore the outermost edge of the sun's domain and beyond. Both Voyagers are capable of returning scientific data from a full range of instruments, with adequate electrical power and attitude control propellant to keep operating until 2020.
Maryland's McDonald, who has worked on the voyager missions from the beginning, said Voyager 1 should reach the heliopause, or end of the solar system in the next eight to ten years.
"When we started none of us thought of this mission lasting this long. Now it has gone 28 years, and there is no reason it can't go on for many more. When the Voyagers were launched in 1977, their mission was to explore the giant planets Jupiter and Saturn. However, after completing their initial assignments, the pair just kept going and going. Voyager 2 went on to explore Uranus and Neptune, the only spacecraft to have visited these outer planets. It now follows its twin and could cross the termination shock and enter the heliosheath by 2008".
The Voyagers each carry a message to any extraterrestrials they might encounter. Each message is carried by a phonograph record - a 12-inch gold-plated copper disk containing sounds and images selected to portray the diversity of life and culture on Earth. Experiments built by the University of Maryland space physics group are currently operating on 13 spacecraft, including the two Voyager spacecraft. Other missions carrying the group's sensors include Cassini, the Ulysses probe to the solar poles and near-Earth missions such as Geotail, the Solar Anomalous and Magnetospheric Particle Explorer (SAMPEX), WIND, the Solar and Heliospheric Observatory (SOHO), and the Advanced Composition Explorer (ACE).
Maryland's space physics group specializes in measurement of space plasmas and ions found in solar, planetary, and interplanetary environments. The work includes studies of the composition and ionization states of the solar wind, solar energetic particles, and interstellar neutral atoms, which have been "picked up" in the solar wind. This work, carried on at Maryland since the late 1960s, has given key insights into solar energetic particle acceleration and conditions in the solar atmosphere. Other work has provided fundamental information about the energizing of particles by travelling interplanetary shocks and such diverse topics as the origin of oxygen and sulphur ions in Jupiter's magnetosphere from the volcanoes on the moon Io and the composition and energy content of the Earth's radiation belts.
The plasma and energetic particle observations carried out by the Space Physics Group require novel instrumentation carried on Earth-orbiting satellites and deep-space probes. Instruments are designed and constructed on campus by the group's technical staff, with participation by graduate as well as undergraduate students.
Scientists have recorded the sounds of Voyager 1 as the probe crossed a turbulent boundary on the fringes of the Solar System. And here's your chance to hear it...
The encounter was recorded by a plasma-wave instrument aboard the ancient spacecraft, which faithfully relayed the data back to Earth, where it was picked up by the antennae of Nasa's Deep Space Network.
"Termination shock" is the field in deep space where interstellar atoms crash at high speed into the energy stream released from the Sun.
A snippet of the recording - a hiss and series of enigmatic clicks - can be heard HERE.
University of Iowa physicist Don Gurnett, who operates the plasma-wave instrument, said in a press release that Voyager 1 may still have another 10 years' travel before it is finally free of the Solar System. Exactly where this ultimate boundary, called the heliosphere, ends and yields to the relative serenity of interstellar space void has never been determined. It is impossible to accurately plot the boundary, known as the heliopause, from Earth and no man-made object, until now, has ever boldly gone that far.
Voyager Spacecraft Enters Solar System's Final Frontier:
NASA's Voyager 1 spacecraft has entered the solar system's final frontier. It is entering a vast, turbulent expanse, where the sun's influence ends and the solar wind crashes into the thin gas between stars.
"Voyager 1 has entered the final lap on its race to the edge of interstellar space." - Dr. Edward Stone, Voyager project scientist at the California Institute of Technology in Pasadena. Caltech manages NASA's Jet Propulsion Laboratory in Pasadena, which built and operates Voyager 1 and its twin, Voyager 2.
In November 2003, the Voyager team announced it was seeing events unlike any in the mission's then 26-year history. The team believed the unusual events indicated Voyager 1 was approaching a strange region of space, likely the beginning of this new frontier called the termination shock region. There was considerable controversy over whether Voyager 1 had indeed encountered the termination shock or was just getting close.
The termination shock is where the solar wind, a thin stream of electrically charged gas blowing continuously outward from the sun, is slowed by pressure from gas between the stars. At the termination shock, the solar wind slows abruptly from a speed that ranges from 700,000 to 1.5 million mph and becomes denser and hotter. The consensus of the team is Voyager 1, at approximately 8.7 billion miles from the sun, has at last entered the heliosheath, the region beyond the termination shock.
Predicting the location of the termination shock was hard, because the precise conditions in interstellar space are unknown. Also, changes in the speed and pressure of the solar wind cause the termination shock to expand, contract and ripple. The most persuasive evidence that Voyager 1 crossed the termination shock is its measurement of a sudden increase in the strength of the magnetic field carried by the solar wind, combined with an inferred decrease in its speed. This happens whenever the solar wind slows down.
In December 2004, the Voyager 1 dual magnetometers observed the magnetic field strength suddenly increasing by a factor of approximately 2 1/2, as expected when the solar wind slows down. The magnetic field has remained at these high levels since December. NASA's Goddard Space Flight Centre, Greenbelt, Md., built the magnetometers.
Voyager 1 also observed an increase in the number of high-speed electrically charged electrons and ions and a burst of plasma wave noise before the shock. This would be expected if Voyager 1 passed the termination shock. The shock naturally accelerates electrically charged particles that bounce back and forth between the fast and slow winds on opposite sides of the shock, and these particles can generate plasma waves.
"Voyager's observations over the past few years show the termination shock is far more complicated than anyone thought." - Dr. Eric Christian, Discipline Scientist for the Sun-Solar System Connection research program at NASA Headquarters, Washington.
The result is being presented today at a press conference in the Morial Convention Centre, New Orleans, during the 2005 Joint Assembly meeting of Earth and space science organizations.