Magnetic anomaly that cast doubt on Voyager 1's entry of interstellar space explained

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Artist's concept of Voyager 1, which was launched 38 years ago(Credit: NASA)

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One question that has been vexing space scientists for the past three years is whether NASA's Voyager 1 spacecraft is or isn't in interstellar space. The unmanned explorer was supposed to have passed out of the sphere of the Sun's influence and into galactic space in August 2012, but a magnetic anomaly threw a question mark over the event. Using data from other space missions, a team led by the University of New Hampshire (UHN) has found a clue as to what may have caused the anomaly and produces new insights into the nature of the region where the Solar System and the outer Universe meet.

Voyager 1 is one of the two Voyager deep space probes sent by NASA to study the outer planets and the edge of the Solar System. The 722-kg (1,590-lb) spacecraft was launched on September 5, 1977 and is powered by three radiothermal generators. In 1979, it flew by Jupiter and in 1980 it visited Saturn. As it did so, the giant planets sent Voyager on a one-way slingshot trajectory toward interstellar space at a speed of 17.043 km/s (38,120 mph) relative to the Sun. In August 2012, it became the first manmade object to enter interstellar space, though not without raising contention as to whether this record was recognized prematurely.

For astronomers, the Solar System’s boundary with interstellar space is the point where the Sun's solar winds of charged particles are no longer able to push out the incredibly tenuous bubble of gas that surrounds it, called the heliosphere, and where the Sun's magnetic sphere no longer dominates. This boundary is called the heliopause and it was this that Voyager 1 was supposed to have crossed in 2012.

Upon exiting the heliopause, the local measurements of the magnetic field by Voyager 1 differed by 40 degrees from interstellar “true magnetic north”(Credit: UNH/EOS)

Evidence that it did so is based on two major indicators. The first is that the cosmic rays striking the spacecraft showed a dramatic shift from the Anomalous Cosmic Rays (ACR), which are cosmic ray particles that have become trapped by the Sun’s magnetic field to Galactic Cosmic Rays (GCR), which are cosmic rays from outside the Solar System.

The second indicator is where the problem lay. As Voyager 1 went through the heliopause, the magnet field should have shifted, which is did, but the shift was 40º off from what was expected. This led some critics to say that the probe is still inside the heliosphere

"There are still naysayers out there regarding Voyager 1 crossing through the heliopause — the edge of the heliosphere," says astrophysicist Nathan Schwadron of the UNH Institute for the Study of Earth, Oceans, and Space and department of physics. "And the reason for this doubt is that when the spacecraft supposedly broke through the heliopause we should have seen some sort of distinctive shift in the magnetic field from one medium to the other."

Artist's concept of the IBEX satellite(Credit: NASA)

The UHN team's approach to solving this problem was to study the observations taken by four other spacecraft, including the SOlar and Heliospheric Observatory (SOHO), Ulysses, and the Interstellar Boundary Explorer (IBEX). Launched into high-Earth orbit in 2008, IBEX was particularly important because its ability to map energetic neutral atom (ENA) emissions demonstrated a very bright "ribbon" of energy, the center of which corresponds to "true north" in the pristine interstellar magnetic field.

By triangulating the observations of IBEX and the other spacecraft, the team discovered that when this ribbon approaches the heliopause, it bends around it like an elastic band wrapped around a beach ball. This distortion of the magnetic field in the region where Voyager 1 is currently travelling throws off the direction of "north" and will continue to do so until the probe enters pristine interstellar space around 2025. This indicates that not only that Voyager 1 has left the heliosphere, but that the region is much more complex than previously thought.

"What’s the nature of the galactic environment in terms of cosmic rays and magnetic fields?" asks Schwadron. "We are beginning to paint a picture of what our local interstellar environment is really like and we can tie that to what’s happening in a much broader environment within the galaxy. When Voyager 1 crosses that next boundary we will be poised to probe many longstanding mysteries."

The team's results were published in Astrophysical Journal Letters.

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