Sunday, June 30, 2013

Sientific models and Voyager 1

"Voyager 1's journey to solar system's edge upends theories"

The mysterious region 11 billion miles away proves to be even stranger than previously thought, according to Voyager's latest readings.


Monte Morin

June 27th, 2013

Los Angeles Times

As the Voyager 1 spacecraft speeds toward interstellar space at a rate of about a million miles a day, the NASA probe is causing scientists to jettison some long-standing theories on the nature of our solar system and life along its cold, dark edge.

In three studies published Thursday in the journal Science, Voyager researchers provided the most detailed view yet of a mysterious region more than 11 billion miles from Earth, where the sun's ferocious solar winds slow to a whisper and pieces of atoms blasted across the galaxy by ancient supernovae drift into the solar system.

The area, which has been dubbed the "magnetic highway," is a newly discovered area of the heliosphere, the vast bubble of magnetism that surrounds the planets and is inflated by gusting solar winds. Like Earth's magnetosphere, which shields us from radioactive solar winds, the heliosphere shields the solar system from many of the cosmic rays that fill interstellar space.

Scientists had long envisioned its outermost layer, the heliosheath, to be a curved, distinct boundary separating the solar system from the rest of the Milky Way. They theorized that once Voyager 1 crossed that threshold, three things would happen: The sun's solar winds would become still; galactic cosmic rays would bombard Voyager from every angle; and the direction of the dominant magnetic field would change significantly because it would be coming from interstellar space, not the sun.

All of those predictions have been turned on their head by Voyager's latest instrument readings.

Although Voyager 1 is equipped with video cameras, they were shut off more than 20 years ago to save power and memory. Instead the craft observes its environment with a fragile, lattice-work antenna that measures magnetic fields as well as a cosmic ray detector and a plasma detector. (Befitting a space probe launched in 1977, the data are stored on an eight-track tape recorder.)

Toward the end of July 2012, Voyager 1's instruments reported that solar winds had suddenly dropped by half, while the strength of the magnetic field almost doubled, according to the studies. Those values then switched back and forth five times before they became fixed on Aug. 25. Since then, solar winds have all but disappeared, but the direction of the magnetic field has barely budged.

"The jumps indicate multiple crossings of a boundary unlike anything observed previously," a team of Voyager scientists wrote in one of the studies. They labeled the new area the heliosheath depletion region.

Stranger yet, Voyager 1 detected an increase in galactic cosmic rays — but found that at times they were moving in parallel instead of traveling randomly.

"This was conceptually unthinkable for cosmic rays," said Stamatios Krimigis, a solar physicist at the Johns Hopkins University Applied Physics Laboratory in Laurel, Md., and leader of another one of the studies. "There is no cosmic ray physicist I know who ever expected that they would not all be coming equally from all directions."

The confusion hasn't ended there.

One Voyager project scientist reported in March that the spacecraft had entered interstellar space after more than 35 years of travel. The paper by Bill Webber, a professor emeritus of astronomy at New Mexico State University, triggered a media furor in the process.

Scientists including Krimigis and Edward Stone, a Voyager project scientist at Caltech, contended that the probe had not left the solar system. Voyager 1 remained within the sun's zone of magnetic influence, and therefore within the heliosphere, they said.

"We're not free yet," Krimigis said. "This is a new region that we didn't know existed. We have no road map, and we're waiting to see what's going to happen next."

Theorists are struggling to explain the data. Some say the unexpected increase in magnetic strength is the result of spiraling magnetic fields being compressed against the interstellar medium. Others say this is impossible since there is no solar wind to push them against that boundary, and that there must be another explanation.

Len Fisk, a professor of space science at the University of Michigan, described the studies' findings as "a complete surprise." He said Voyager 1's travels were proving to be both puzzling and exciting.

"It's causing a fundamental reconsideration of how the heliosheath interacts with the local interstellar medium," said Fisk, who was not involved in the new analysis.

One of the possible explanations for Voyager's peculiar magnetic readings is that the sun's magnetic fields have combined with the interstellar magnetic field in places — a process called magnetic reconnection.

Such reconnection has been observed between the magnetic fields of the sun and Earth, said Stone, a former director of the Jet Propulsion Laboratory in La CaƱada Flintridge. "Maybe that's what's happening here, but we really don't know," he said.

Adding still more mystery is the fact that Voyager 2 has yet to experience anything like its twin. Both spacecraft are headed toward the forward edge of the heliosphere, but are more than 9 billion miles apart.

Although Voyager 1 was launched 16 days after Voyager 2, it followed a more direct route toward the edge of the solar system. Since 1998, when it overtook Pioneer 10, it has been the farthest man-made object from Earth.

Voyager scientists say they're in no position to predict when the probe may finally exit the solar system. It could be months, or it could be years.

"I wouldn't dare to make an estimate," Krimigis said. "Voyager will probably prove us wrong, again."

"Voyager 1 finds that edge of solar system isn’t as defined as scientists once thought"


 Joel Achenbach

June 27th, 2013

The Washington Post

The edge of the solar system has no edge, it turns out. It has a fuzzy transitional area, not quite “solar system” and not quite “interstellar space.”

This basic fact of our star’s environment has been discovered by Voyager 1, one of the most remarkable spaceships ever built. Our premier scout of deep space, Voyager 1 is currently 11 billion miles from the sun, beaming data to Earth as it scoots at 38,000 mph toward the constellation Ophiuchus.

Scientists had assumed that Voyager 1, launched in 1977, would have exited the solar system by now. That would mean crossing the heliopause and leaving behind the vast bubble known as the heliosphere, which is characterized by particles flung by the sun and by a powerful magnetic field.

The scientists’ assumption turned out to be half-right. On Aug. 25, Voyager 1 saw a sharp drop-off in the solar particles, also known as the solar wind. At the same time, there was a spike in galactic particles coming from all points of the compass. But the sun’s magnetic field still registers, somewhat diminished, on the spacecraft’s magnetometer. So it’s still in the sun’s magnetic embrace, in a sense.

This unexpected transitional zone, dubbed the “heliosheath depletion region,” is described in three new papers about Voyager 1 published online Thursday by the journal Science.

“There were some surprises,” said Ed Stone, who has been the lead scientist of NASA’s Voyager program since 1972. “We expected that we would cross a boundary and leave all the solar stuff behind and be in all the interstellar stuff. It turned out, that’s not what happened.”
How big is this transitional zone at the edge of the solar system?

“No one knows,” said Stone, 77, a professor of physics at the California Institute of Technology and the former head of NASA’s Jet Propulsion Laboratory, Voyager’s home base. “It’s not in any of the models. We don’t know. It could take us a few more months, it could take us several more years to get through it.”

The dimensions and nature of the heliosphere are not a wholly esoteric matter. The sun’s magnetic field deflects much of the radiation coming from other parts of the galaxy that was created in supernova explosions. Interstellar space is not a benign environment. The heliosphere’s features make life easier for blue planets such as Earth.

Voyager 1 can be counted as one of the great exploratory craft in history, and none has gone farther, nor cruised steadily at such astonishing speed (a few have briefly gone faster while falling into the sun). Two Voyager probes were launched in 1977. Both spaceships carried a gold-plated record crammed with digital information about human civilization, including mathematical formulas, an image of a naked man and woman, whale vocalizations, and clips of classical and rock-and-roll music. (The famous joke was that the aliens listened to the record and replied, “Send more Chuck Berry.”)

The two Voyagers embarked on what was called the Grand Tour, taking advantage of an orbital positioning of the four outer planets that happens less than once a century. Voyager 1 flew by Jupiter and Saturn before angling “north,” as astronomers would describe it. (There’s no up or down in space, but there is a north or south relative to the orbital plane of the planets.) Voyager 2 went past Jupiter and Saturn and flew by Uranus and Neptune before heading “south.”

The images of those planets and their moons, now taken for granted, were stunning triumphs of the Voyager mission. And in 1990, Voyager 1 , nearly 4 billion miles from the sun, turned its camera toward Earth and took an image of what Carl Sagan called the “pale blue dot” of our home planet.

Now Voyager 1 is 124 astronomical units from the sun — one AU being roughly the mean distance from Earth to the sun, or about 93 million miles. Voyager 2 is at 102 AU.

These spacecraft are not immortal, even if sometimes they act like it. They have a power supply from the radioactive decay of plutonium-238, which generates heat. The half-life of that system is 88 years. Small thrusters occasionally are fired to keep Voyager 1’s 23-watt radio antenna pointed toward Earth, where the faint signals are picked up on huge arrays of radio telescopes in the United States, Spain and Australia. But Stone anticipates that weakening power will force scientists to start shutting down scientific instruments on Voyager 1 in 2020 and that by 2025, the last instrument will be turned off.

“It changed the way we view our place in the cosmos,”
said Bill Nye, the “Science Guy” who is chief executive of the Planetary Society in Pasadena, Calif. He said the new discovery by Voyager 1 is a classic example of why we explore: “What are you going to find over the unknown horizon? We don’t know. That why we explore out there.”

NASA’s associate administrator for space technology, Michael Gazarik, said of Voyager 1’s durability: “It is amazing, especially in the harsh environment of space. This piece of hardware has a life of its own.”

In 40,000 years, Voyager 1 will be closer to another star (with the romantic name AC+79 3888) than to the sun. And then what? It will just keep going — a silent, dark craft on a seemingly eternal journey.

“It will be orbiting the center of the Milky Way galaxy essentially for billions of years, like all the stars,”
said Stone of what has been, for him, the spacecraft of a lifetime.

"Voyager 1 Discovers Bizarre and Baffling Region at Edge of Solar System"


Adam Mann

June 27th, 2013


Not content with simply being the man-made object to travel farthest from Earth, NASA’s Voyager 1 spacecraft recently entered a bizarre new region at the solar system’s edge that has physicists baffled. Their theories don’t predict anything like it.

Launched 36 years ago, Voyager 1 and its twin Voyager 2 made an unprecedented tour of the outer planets, returning spectacular data from their journey. The first Voyager sped out of the solar system in 1980 and it has since been edging closer and closer to interstellar space. The probe is currently out more than 120 times the distance between the Earth and the sun.

Scientists initially thought that Voyager’s transition into this new realm, where effects from the rest of the galaxy become more pronounced, would be gradual and unexciting. But it’s proven to be far more complicated than anything researchers had imagined, with the spacecraft now encountering a strange region that scientists are struggling to make sense of.

“The models that have been thought to predict what should happen are all incorrect,” said physicist Stamatios Krimigis of the Johns Hopkins University Applied Physics Laboratory, who is lead author of one of three new papers on Voyager appearing in Science on June 27. “We essentially have absolutely no reliable roadmap of what to expect at this point.”

The sun produces a plasma of charged particles called the solar wind, which get blown supersonically from its atmosphere at more than 1 million km/h. Some of these ions are thrown outward by as much as 10 percent the speed of light. These particles also carry the solar magnetic field.

Eventually, this wind is thought to hit the interstellar medium – a completely different flow of particles expelled from the deadly explosions of massive stars. The extremely energetic ions created in these bursts are known as galactic cosmic rays and they are mostly blocked from coming into the solar system by the solar wind. The galaxy also has its own magnetic field, which is thought to be at a significant angle to the sun’s field.

Researchers know that Voyager 1 entered the edge of the solar wind in 2003, when the spacecraft’s instruments indicated that particles around it were moving subsonically, having slowed down after traveling far from the sun. Then, about a year ago, everything got really quiet around the probe. Voyager 1’s instruments indicated at the solar wind suddenly dropped by a factor of 1,000, to the point where it was virtually undetectable. This transition happened extremely fast, taking roughly a few days.

At the same time, the measurements of galactic cosmic rays increased significantly, which would be “just as we expected if we were outside the solar wind,” said physicist Ed Stone of Caltech, Voyager’s project scientist and lead author of one of the Science papers. It looked almost as if Voyager 1 had left the sun’s influence.

So what’s the problem? Well, if the solar wind was completely gone, galactic cosmic rays should be streaming in from all directions. Instead, Voyager found them coming preferentially from one direction. Furthermore, even though the solar particles had dropped off, the probe hasn’t measured any real change in the magnetic fields around it. That’s hard to explain because the galaxy’s magnetic field is thought to be inclined 60 degrees from the sun’s field.

No one is entirely sure what’s going on.

“It’s a huge surprise,” said astronomer Merav Opher of Boston University, who was not involved in the work. While the new observations are fascinating, they are likely something that theorists will debate about for some time, she added.

“In some sense we have touched the intergalactic medium,” Opher said, “but we’re still inside the sun’s house.”

Extending this analogy, it’s almost as if Voyager thought it was going outside but instead found itself standing in the foyer of the sun’s home with an open door that allows wind to blow in from the galaxy. Not only were scientists not expecting this foyer to exist, they have no idea how long the probe will stay inside of it. Stone speculated that the probe could travel some months or years before it reaches interstellar space.

“But it could happen any day,” he added. “We don’t have a model to tell us that.” Even then, Stone said, Voyager would not have really left the solar system but merely the region where the solar wind dominates.

For his part, Krimigis didn’t even want to speculate on what Voyager might encounter next because theorists’ models have so far not worked extremely well.

“I’m convinced that nature is far more imaginative than we are,” he said.

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