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November 6, 2019

Voyager 2 Illuminates Boundary of Interstellar Space

Schematic summary of the Voyager spacecraft’s findings in the region where the solar system meets the local interstellar medium

Schematic summary of the Voyager spacecraft’s findings in the region where the solar system meets the local interstellar medium. The current positions of NASA’s Voyager 1 (V1) and Voyager 2 (V2) spacecraft are shown in astronomical units, with 1 AU equal to 150 million kilometers or 93 million miles. At these distances, radio signals from Voyager 1 and Voyager 2, traveling at the speed of light, require 20 hours, 31 minutes and 16 hours, 58 minutes, respectively. Dimensions of various regions along the spacecraft trajectories are shown in the figure.

Credit: NASA/JPL-Caltech/Johns Hopkins APL

NASA’s Voyager spacecraft

This artist’s concept shows one of NASA’s Voyager spacecraft entering interstellar space, or the space between stars. This region is dominated by plasma ejected by the death of giant stars millions of years ago. Hotter, sparser plasma fills the environment inside our solar bubble.

Credit: NASA/JPL-Caltech

After more than four decades in space, the discoveries keep coming from NASA’s Voyager spacecraft and their sets of science instruments — including the durable charged-particle detectors developed at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland.

On Nov. 5, 2018, Voyager 2 became only the second spacecraft in history to leave the heliosphere — the protective bubble of particles and magnetic fields created by our Sun. At a distance of about 11 billion miles (18 billion kilometers) from Earth — well beyond the orbit of Pluto — Voyager 2 had entered interstellar space, or the region between stars. Five new research papers in the journal Nature Astronomy describe what scientists observed during and since Voyager 2’s historic crossing.

Each paper details the findings from one of Voyager 2’s five operating science instruments: a magnetic field sensor, two instruments to detect energetic particles in different energy ranges and two instruments for studying plasma (a gas composed of charged particles). Taken together, the findings help paint a picture of this cosmic shoreline, where the environment created by our Sun ends and the vast ocean of interstellar space begins.

The Sun’s heliosphere is like a ship sailing through interstellar space. Both the heliosphere and interstellar space are filled with plasma, a gas that has had its atoms stripped of their electrons. The plasma inside the heliosphere is hot and sparse, while the plasma in interstellar space is colder and denser. The space between stars also contains cosmic rays, or particles accelerated by exploding stars. Voyager 1 discovered that the heliosphere protects Earth and the other planets from more than 70% of that radiation.

When Voyager 2 exited the heliosphere last year, scientists announced that its two energetic particle detectors — including the APL-built Low-Energy Charged Particle (LECP) detector, an investigation led by Stamatios Krimigis of APL — noticed dramatic changes. The rate of heliospheric particles detected by the instruments plummeted, while the rate of cosmic rays (which typically have much higher energies than the heliospheric particles) increased dramatically and remained high. The changes confirmed that the probe had entered a new region of space.

Before Voyager 1 reached the edge of the heliosphere in 2012, scientists didn’t know exactly how far this boundary was from the Sun. The two probes exited the heliosphere at different locations and also at different times in the 11-year solar cycle, when the Sun goes through a period of high and low activity. Scientists expected that the edge of the heliosphere, called the heliopause, can move as the Sun’s activity changes, sort of like a lung expanding and contracting with breath. The big surprise was that the two probes encountered the heliopause at about the same distance from the Sun even though one occurred at high solar activity and the other one at low activity. “We don’t understand why that is,” Krimigis said.

The new papers now confirm that Voyager 2 is not yet in undisturbed interstellar space: Like its twin Voyager 1, Voyager 2 appears to be in a perturbed transitional region just beyond the heliosphere.

“The Voyager probes are showing us how our Sun interacts with the stuff that fills most of the space between stars in the Milky Way galaxy,” said Ed Stone, project scientist for Voyager and a professor of physics at Caltech. “Without this new data from Voyager 2, we wouldn’t know if what we were seeing with Voyager 1 was characteristic of the entire heliosphere or specific just to the location and time when it crossed.”

Leaking Particles

If the heliosphere is like a ship sailing through interstellar space, it appears the hull is somewhat leaky. The LECP detector showed that a trickle of particles from inside the heliosphere is slipping through the boundary and into interstellar space. Voyager 1 exited the heliosphere close to the very “front” of the heliosphere, relative to the bubble’s movement through space. Voyager 2, on the other hand, is located closer to the flank, and this region appears to be more porous than the region where Voyager 1 is located.

“Voyager 2 crossed in an area where material from inside the solar bubble was leaking upstream, into the galaxy,” Krimigis said. “That was very different than what happened to Voyager 1, which crossed at a different time and region where, and when, hardly any material was leaking out. We don’t yet know exactly why this occurred, and the models on this area of solar system haven’t been able to explain it.”

Plasma and Magnetic Measurements

In addition to the particle measurements, the Voyager spacecraft have now confirmed that the plasma in local interstellar space is significantly denser than the plasma inside the heliosphere, as scientists expected. Voyager 2 also measured the temperature of the plasma in nearby interstellar space and confirmed it is colder than the plasma inside the heliosphere.

An observation by Voyager 2’s magnetic field instrument also confirmed a surprising result from Voyager 1: The magnetic field in the region just beyond the heliopause is parallel to the magnetic field inside the heliosphere. With Voyager 1, scientists had only one sample of these magnetic fields and couldn’t say for sure whether the apparent alignment was characteristic of the entire exterior region or just a coincidence. Voyager 2’s magnetometer observations confirm the Voyager 1 finding and indicate that the two fields align.

The Voyager probes launched in 1977, and upon completing their Grand Tour of the planets, began their Interstellar Mission to reach the heliopause in 1990. Voyager 1 is currently more than 13.6 billion miles (22 billion kilometers) from the Sun, while Voyager 2 is 11.3 billion miles (18.2 billion kilometers) from the Sun.

Both spacecraft carry LECP instruments, which have far exceeded operational expectations. It includes a motor that rotates the instrument through 45-degree steps every 192 seconds, allowing it to gather data in all directions. The device, designed and tested to work for 500,000 steps and last four years, has been working for 42 years and close to 8 million steps, Krimigis said.

The findings and resulting, intriguing questions have scientists pondering other opportunities to explore this region beyond the work of the Voyagers. The APL-built and -operated New Horizons spacecraft, which flew by Pluto in 2015 and is currently surveying the Kuiper Belt some four billion miles from Earth, is on a path to eventually leave the solar system. And APL also leads a NASA-funded study for an Interstellar Probe, which would be the first mission dedicated to study the space between the stars.

“We do need another mission,” Krimigis said. ”We absolutely need more data. We have an entire solar bubble and only crossed in two points, very far apart. Two examples are not enough.”

Media contact: Michael Buckley, 240-228-7536, michael.buckley@jhuapl.edu

The Applied Physics Laboratory, a not-for-profit division of The Johns Hopkins University, meets critical national challenges through the innovative application of science and technology. For more information, visit www.jhuapl.edu.