LibriumArcana.Games

After 36 years of space travel and months of heated debate among scientists, NASA confirmed Thursday that Voyager 1 has indeed left our solar system and had entered interstellar space more than a year ago.

"Voyager has boldly gone where no probe has gone before, marking one of the most significant technological achievements in the annals of the history of science," said John Grunsfeld, NASA's associate administrator for the Science Mission Directorate.

At a Thursday news conference in Washington, D.C., officials said the belated confirmation was based on new "key" evidence involving space plasma density. The evidence was outlined in a paper published online Thursday in the journal Science.

Lead author Don Gurnett, an Iowa State plasma physicist and a Voyager project scientist, said the data showed conclusively that Voyager 1 had exited the heliopause — the bubble of hot, energetic particles that surrounds our sun and planets — and entered into a region of cold, dark space called the interstellar medium.

"When we got that data, I and my colleagues just looked at each other and said, 'We’re in the interstellar medium.' It was just that clear to us," Gurnett said.

Gurnett calculated that Voyager crossed the edge of the heliosphere, or heliopause, at or around Aug. 25, 2012.

"Even though it took 36 years, it's just an amazing thing to me," said study coauthor Bill Kurth, a radio and plasma researcher at the University of Iowa.

Scientists had begun to vigorously debate Voyager’s whereabouts earlier this year, when it was clear that the probe was being bombarded by an increasing number of galactic cosmic rays and that the number of high-energy particles from inside the heliosphere had plummeted.

However, NASA scientists said they could not be certain Voyager had left the solar system until surrounding magnetic fields changed direction. After waiting for that change for more than a year, however, officials conceded that the magnetic field change was not a necessary indicator.

“It’s a big surprise, and it's another mystery,” said Ed Stone, a Voyager project scientist at Caltech and former chief of the Jet Propulsion Laboratory in La Canada Flintridge. “This is not what our models were telling us. We have to address this issue, but right now ... we don’t understand.”

Confusion over Voyager’s whereabouts has a lot to do with the failure of one specific piece of equipment, the spacecraft's plasma science experiment, or PLS. The device, which was developed at MIT, measures the electron density of space plasma: ionized gas that is ejected from our sun as well as other stars.

Cool plasma, the product of stars that exploded millions of years ago, populates interstellar space and has a high density: about 100,000 electrons per cubic yard of space. Super-heated plasma, like the solar wind that flows from our sun, fills the heliosphere and is much less dense, only about 1,000 electrons per cubic meter, Gurnett said.

A functioning PLS would have been able to sense the rising density change as Voyager exited the heliosphere.

“The instrument failed in 1980, so the spacecraft is sort of instrument-challenged,” Gurnett said. “That’s really one of the major failures we’ve had. There really aren't that many.”

Voyager does, however, have two plasma wave antennas that stretch from its base in a wide V shape. The antennas, which are connected to a radio transmitter, detect the oscillation, or vibration, of excited plasma particles. The device will convert the oscillations into an audible noise that is recorded on Voyager's vintage eight-track tape recorder.

The frequency of the noise is associated with a specific density of plasma. The higher the frequency, the denser the plasma.

The only trouble is that something has to excite the plasma for it to “ring,” something like a large solar flare. Waiting for a solar flare can take years during a solar minimum (a period of low solar activity).

Also, when a flare does occur, it can take as long as a year for the shock wave to reach Voyager 11.6 billion miles away.

Fortunately for Voyager scientists, the antennas picked up two long-lasting oscillations. The first was in October and November of 2012 and the second was in April and May of 2013. In both cases, the frequency suggested that the plasma was cold and dense. Voyager was in interstellar space.

“It was key evidence,” Stone said. “We really needed to measure plasma to know if we were inside or outside the heliosphere. Everything else is more of a proxy.”

Gurnett and his colleagues arrived at the crossing date of Aug. 25 by extrapolation.

Plasma density was increasing in a linear fashion as Voyager moved further from the heliosphere and into the interstellar medium. The frequency measured in the fall of 2012 was 2.2 kilohertz, and by the spring of 2013 it had risen to 2.6 kilohertz. Previous research told Gurnett that the frequency of the radio signal at the crossing point should be 2 kilohertz, and so by plotting each point on a line, he was able to arrive at a date.

Scientists are hoping that many gaps in our understanding will be filled in by Voyager 2. The sister spacecraft, which was also launched in 1977, is nearing the edge of the heliosphere via a different path and is expected to encounter interstellar space sometime within the next several years.

Unlike Voyager 1, however, Voyager 2 has a fully functioning plasma science instrument and has been sending back density readings throughout its journey.

“I think it's going to teach us even more about this region,” Stone said.

(CNN) -- At the edge of the heliosphere, you wouldn't know by looking whether you left the cradle of humanity behind and floated out into interstellar space. You would just see unfathomably empty space, no matter which side of the invisible line you were on.

But scientists now have strong evidence that NASA's Voyager 1 probe has crossed this important border, making history as the first human-made object to leave the heliosphere, the magnetic boundary separating the solar system's sun, planets and solar wind from the rest of the galaxy.

"In leaving the heliosphere and setting sail on the cosmic seas between the stars, Voyager has joined other historic journeys of exploration: The first circumnavigation of the Earth, the first steps on the Moon," said Ed Stone, chief scientist on the Voyager mission. "That's the kind of event this is, as we leave behind our solar bubble."

A new study in the journal Science suggests that the probe entered the interstellar medium around August 25, 2012. You may have heard other reports that Voyager 1 has made the historic crossing before, but Thursday was the first time NASA announced it.

The twin spacecraft Voyager 1 and 2 were launched in 1977, 16 days apart. As of Thursday, according to NASA's real-time odometer, Voyager 1 is 18.8 billion kilometers (11.7 billion miles) from Earth. Its sibling, Voyager 2, is 15.3 billion (9.5 billion) kilometers from our planet.

Technically, if you include the distant comets that orbit the sun, then Voyager 1 has not left "the solar system," Stone said. For that, we'll have to wait another 30,000 years.

Another milestone for long after we're gone: The probe will fly near a star in about 40,000 years, Stone said.

How do we know?

Voyager, currently traveling at more than 38,000 miles per hour, never sent a postcard saying "Greetings from interstellar space!" So whether it has made the historic crossing or not is a matter of controversy.

"The spacecraft itself really doesn't know," Stone said. "It's only instruments that can tell us whether we're inside or outside."

Further complicating matters, the device aboard Voyager 1 that measures plasma -- a state of matter with charged particles -- broke in 1980.

To get around that, scientists detected waves in the plasma around the spacecraft and used that information to calculate density. Vibrations in the plasma came from a large coronal mass ejection from the sun in 2012, resulting in what Stone called a "solar wind tsunami." These vibrations reached the area around Voyager this spring.

Measurements taken between April 9 and May 22 of this year show that Voyager 1 was, at that time, located in an area with an electron density of about 0.08 per cubic centimeter.
This illustration shows NASA\'s Voyager 1 spacecraft entering the space between stars.
This illustration shows NASA's Voyager 1 spacecraft entering the space between stars.

In the interstellar medium, the density of electrons is thought to be between 0.05 and 0.22 per cubic centimeter. The particles of interstellar plasma were created by the explosions of giant stars, and carry the magnetic field of the galaxy, scientists said.

Last year, between October 23 and November 27, researchers calculate that Voyager 1 was in an area with an electron density of 0.06 per cubic centimeter. That's still within the interstellar space range, and it means that over time the spacecraft passed through plasma with increasing electron density.

The study suggests that the plasma density is about 30 times higher in the interstellar medium than in the heliosphere, which is close to what scientists thought based on other kinds of measurements. The boundary is called the heliopause.

When did it happen?

Scientists have been using several kinds of measurements to figure out if and when Voyager 1 had reached the interstellar medium.

Evidence from particle data had already pointed toward the conclusion that the probe succeeded. In late July and early August of 2012, scientists saw dips in the concentration of particles made in the solar system, and peaks in particles made outside.

"If you just looked at that data, you'd think it's pretty clear that we've actually crossed a boundary. We're no longer in the place where the solar system particles are being made, and we're actually out in the interstellar medium," said Marc Swisdak, associate research scientist in the Institute for Research in Electronics and Applied Physics at the University of Maryland. Swisdak was not involved in the new study, but has worked with Voyager data.

Magnetic field measurements suggested otherwise. Researchers had expected to see stark changes in magnetic field direction when the probe crossed out of the heliosphere, but that wasn't supported by measurements from the probe.

Swisdak and colleagues published a modeling study suggesting that the particle data is more relevant, and that the magnetic field might not change as much as people thought. They proposed a crossing-over date of July 27 -- about a month sooner than the new study.

The specific date will likely be debated for some time, Swisdak said. One possible explanation is that if the heliosphere is analogous to an air-conditioned room, Voyager stepped through the doorway into a hot room on July 27. For a month it was in a metaphorical room with a mixture of hot and cold air, and finally entered the truly hot part on August 25.

Puzzles still surround the magnetic field at the edge of the heliosphere, Stone said, and "We're going to be prepared to have more surprises."

What else is out there?

Voyager 1 has only 68 KB of memory on board -- far less than a smartphone, said Suzanne Dodd, Voyager project manager. Scientists communicate with the spacecraft every day.

"It's the little spacecraft that could," she said in a NASA press conference.

The probe now has a totally new mission, Stone said.

"We're now on the first mission to explore interstellar space," he said. "We will now look and learn in detail how the wind which is outside, that came from these other stars, is deflected around the heliosphere."

Wind -- made of particles -- from these other stars has to go around the heliosphere the way a water in a stream flows around a rock, Stone said. Scientists are interested in learning more about the interaction between our solar wind and wind from other stars.

Natural radioactive decay provides heat that generates enough electricity to help Voyager 1 communicate with Earth. The first science instrument will be turned off in 2020, and the last one will be shut down in 2025, Stone said.

Both Voyager probes carry time capsules known as "the golden record," a 12-inch, gold-plated copper disc with images and sounds so that extraterrestrials could learn about us. Let's hope they can build appropriate record players.

Voyager 2 will likely leave the heliosphere in about three to four years, Stone said.

Its plasma instrument is still working, Stone said, so scientists can directly measure the stellar wind's density, speed and temperature. That also means that when it crosses out of the heliosphere, Voyager 2 will send a clearer signal.

At that time, it will join its twin in the vast nothingness between stars that used to be beyond our reach.

General Havoc wrote:Didn't Pioneer 10 and 11 already leave the solar system?

Josh wrote:I think I'm going to start referring to it as 'V'ger' now, just so I'm ready.

General Havoc wrote:Didn't Pioneer 10 and 11 already leave the solar system?