Has Voyager 1 finally left the Solar System?

An answer to this question has been proclaimed so many times in the last few years that it has almost lost its effect. Part of the confusion lies in how we define “solar system.” Is it the edge of planetary orbits or the end of the Sun’s influence…or is there yet another definition?

Launched in 1977, the craft has been hurtling through space at an incredible 38,000 miles per hour, sprinting nearly 1,000,000 miles per day. It passed the orbit of the farthest planet Neptune on August 25, 1989 (at the time, due to its highly elliptical orbit, the then-planet Pluto was closer to the Sun than Neptune). Its twin spacecraft, Voyager 2, actually flew close to the planet itself. In 1990, with their planetary missions accomplished, both Voyager missions were renamed the Voyager Interstellar Mission. This consists of three phases: detection of the termination shock (the edge of the Sun’s magnetic influence, where the solar wind slows); exploration of the heliopause (the interface between the solar wind and the interstellar wind); and exploration of interstellar space (the region where the interstellar wind dominates). In December 2004, Voyager crossed the termination shock. Roughly ten years later, the craft was expected to transverse the heliopause, which many consider the edge of the Solar System.

And on August 25, 2012, 35 years after its launch and 12 billion miles (125.3 AU) from the Sun, Voyager 1 officially crossed into interstellar space.

The determination that the event actually occurred, however, did not come until last week. What took so long?

The Sun ejects plasma material (called the “solar wind”) out into a bubble called the heliosphere. The plasma outside that sphere comes from stellar explosions millions of years ago and has since been dispersed throughout the galaxy. The interaction between the heliosphere and plasma is the boundary between the two.

Voyager was looking to detect that boundary between plasmas; however, it could not do this directly because the plasma detector on Voyager 1 malfunctioned in 1980, just a few years after launch. Instead, scientists measured the magnetic field of the Sun and of the interstellar wind. The change did not manifest as expected, so scientists could not draw a definite conclusion. Another set of instruments on board, two antennae, are able to measure plasma—but only if it is moving in waves. A solar eruption in March 2012 sent a shock wave that took 400 days to reach Voyager, but caused the plasma to react in a way that Voyager could detect. This signal finally enabled the confirmation of the craft’s passage into interstellar space.

Sadly, our connection with Voyager will eventually end as its power runs out (its current power output is about that of a refrigerator lightbulb—try detecting that from 11 billion miles away!) The craft is expected to lose all power and cease its communications with Earth by 2025. With no friction to slow it down, however, Voyager will continue to drift on, indefinitely. It remains well within the sphere of the Sun’s gravitational dominion, but will take about 30,000 years to pass through the Oort cloud, the cometary halo extending about a light year or so from the Sun and the farthest-known objects orbiting the Sun. So although the plucky spacecraft has entered interstellar space and left the Sun’s magnetic influence, the Voyager team says it will not yet leave the Solar System until it passes through the Oort Cloud. Beyond that, it will take another 70,000 years to travel the 4.3 light year distance between us and the next closest star, Alpha Centari.

But let’s not underestimate the significance of this event. A man-made object has left the confines of the tiny speck of our galactic home for the very first time and entered the space between stars. We have physically entered a space greater than any explored before and taken the first step in ever visiting other star systems. True, it is a mere 16 light hours, but substantially farther than the 1.3 light seconds to the Moon, which is the farthest that humans have gone.

Voyager leads the way in a whole new frontier of exploration.

Elise Ricard is the Senior Presenter at the Morrison Planetarium and holds a master’s degree in museum education.

How large is the heliosphere? The region of interstellar space dominated by the Sun? The Voyager 1 spacecraft has a partial answer: much larger than expected!

The heliosphere, composed of the sun’s magnetic field and a high-velocity stream of charged particles called the solar wind, creates an enormous bubble around our solar system. The charged particles move at about a million miles per hour, only slowing down when they near the region where the pressure of interstellar gas dominates. We thought Voyager 1, our farthest spacecraft, had arrived at edge of the heliosphere, but there is something fishy about Voyager 1’s new data.

Launched in 1977, the twin spacecraft Voyager 1 and Voyager2 have both entered an area called the heliosheath, where the solar wind slows, even though they’re headed in different directions away from the Sun. Voyager 1 lies farthest away, 11 billion miles from Earth, and at this distance it encountered a “magnetic highway.” Here the Sun’s magnetic field connects with the interstellar magnetic field, allowing for an exchange of charged particles between inside and outside the heliosphere.

Voyager 1 measured the highest rate of change so far between incoming and outgoing particles. “We saw a dramatic and rapid disappearance of the solar-originating particles. They decreased in intensity by more than 1,000 times, as if there was a huge vacuum pump at the entrance ramp onto the magnetic highway,” said Stamatios Krimigis, the low-energy charged particle instrument’s principal investigator at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. In this same region, scientists first detected the low-energy cosmic rays that originate from dying stars.

This should indicate that the spacecraft has reached interstellar space, except scientists have not yet seen the final indicator: an abrupt change in the direction of the magnetic field.

“If you looked at the cosmic ray and energetic particle data in isolation, you might think Voyager had reached interstellar space, but the team feels Voyager 1 has not yet gotten there because we are still within the domain of the Sun’s magnetic field,” said Edward Stone, Voyager project scientist at the California Institute of Technology in Pasadena.

So how much farther does Voyager 1 need to travel until it reaches interstellar space? Scientists estimate several months or even years until Voyager 1 experiences a change in magnetic field direction. For now, they have named this strange zone the heliosheath depletion region. Catchy, eh?

Stay tuned for more Voyager discoveries from the edge of interstellar space!

Alyssa Keimach is an astronomy and astrophysics student at the University of Michigan and interns for the Morrison Planetarium.

Image: NASA/JPL-Caltech

Launched in 1977, Voyager 1 is the most distant human-made object in our galaxy, and is close to passing beyond the limits of our solar system.  This week, the team announced that Voyager 1 has entered the “magnetic highway,” a region between the heliosphere and interstellar space. Scientists coined the new term “magnetic highway” to describe the place where the Sun’s magnetic field lines connect with interstellar magnetic field lines. As Stone said at yesterday’s meeting, “The new region isn’t what we expected, but we’ve come to expect the unexpected from Voyager.”

Voyager has three instruments on board to measure changes in the magnetic environment—one that detects the low-energy particles that come from the solar wind within the heliosphere; one that detects the high-energy particles from interstellar space (remnants from supernovae explosions millions of years ago); and a magnetometer, which measures the strength and direction of magnetic fields.

How do the scientists know the magnetic highway isn’t just interstellar space? First, both low- and high-energy particles are detected.  Also, the magnetic field from the Sun runs east to west, and that should change dramatically once the spacecraft enters interstellar space.

Voyager 1 first merged onto this highway in late July, but then quickly exited. The same thing happened in early August, and finally Voyager entered for good in late August. Stone predicts that interstellar space can’t be too far for Voyager 1. “We believe this is the last leg of our journey to interstellar space,” Stone said. “Our best guess is it’s likely just a few months to a couple years away.” (Hopefully well before the spacecraft’s power is due to shut off in 2025.)

Because Voyager 1 is now located about 11 billion miles away from the Sun, the signal from the spacecraft takes approximately 17 hours to travel to Earth. Voyager 2, the longest continuously operated spacecraft, is about 9 billion miles away from our sun, headed in a completely different direction. While Voyager 2 has seen changes similar to those seen by Voyager 1, the changes are much more gradual. Scientists do not think Voyager 2 has yet reached the magnetic highway.

The two Voyagers are almost in interstellar space, at the edge of our Solar System. In a sense, they are the Star Trek Enterprise of satellites, “boldy going where no one has gone before.” Voyager 1 lies about 11 billion miles from the Sun; Voyager 2, a bit behind at 9 billion miles from the Sun. At these distances, on the boundary of the Sun’s influence, the two spacecraft provide amazing new discoveries.

The press conference at AGU on Monday described the kind of twilight zone Voyager 1 currently inhabits. The spacecraft has sped outside the heliosphere, a “bubble” of gas blow into space by our Sun. But Voyager 1 has not yet reached interstellar space. It’s detecting small amounts of both solar wind and interstellar wind and appears to be in a “stagnation region,” described in a press release as “a kind of cosmic purgatory.”

(More on the interstellar winds: supernovae explosions cause these winds and they tend to come from a direction near the center of our Milky Way galaxy.)

The two spacecraft travel one billion miles every three years, and researchers are unsure when they will reach interstellar space. Ed Stone, the Voyager Project Scientist from Caltech, estimates it could take anywhere from a few months to a few years. We can only hope that at least one of the spacecraft reaches interstellar space before they are both powered off. Power for all instruments exists through 2020, with both spacecrafts shut down by 2025.

The press conference participants (and many in the audience) gushed about the productivity of the two middle-aged spacecraft.

Last week’s Science publication reports on the spacecrafts’ detection of Lyman-alpha emissions coming from the Milky Way. Nature News describes:

Lyman-alpha emissions are produced when ultraviolet light hits neutral hydrogen atoms, splitting them into protons and electrons. When the two recombine, they can form an atom in an excited state that emits ultraviolet light at a characteristic wavelength, known as the Lyman-alpha band.

Because this high energy occurs in star-forming regions, scientists believe that these detections suggest the birth of stars in both our galaxy and beyond. The Lyman-alpha emissions from sources other than our Sun are virtually undetectable from within the solar system, so these detections come as exciting news for astronomers.