Nasa's Voyager 2 sends back its first message from interstellar space

Voyager 2

This illustration shows the position of NASA's Voyager 1 and Voyager 2 probes, outside of the heliosphere, a protective bubble created by the Sun that extends well past the orbit of Pluto. Here are the five biggest takeaways.

With Voyager 2, the plasma instrument was undamaged.

Nobody knew how long it would take for the probes to reach interstellar space.

As Voyager 1 crossed the heliosphere threshold, it detected particles from outer space - notably cosmic rays - racing the other way.

"But contrary to all expectations and predictions, the magnetic field direction did not change when Voyager 2 crossed the heliopause", Leonard Burlaga, a scientist at the NASA Goddard Space Flight Center and lead researcher for one of the studies, told AFP.

This showed that the spacecraft were passing from an environment with hot, lower density plasma characteristic of the solar wind and entering a region with the cool, higher density plasma thought to be found in interstellar space. The fact that both spacecraft left the solar system at pretty much the same distance, at two very different locations, is a source of confusion at the moment. Another surprising revelation was the magnetic field in the region just beyond the heliopause is parallel to the magnetic field inside the heliosphere.

"Think of a cold front that forms when a very cold air mass comes down to the USA from Canada", said Don Gurnett, professor of physics at the University of Iowa.

Voyager 1 and 2 have now both passed through the heliosphere.

Bill Kurth, a research scientist at the University of Iowa and one of the authors of the studies, said: "It implies that the heliosphere is symmetric, at least at the two points where the Voyager spacecraft crossed".

"We also have galactic cosmic rays, which are out in the interstellar space trying to flow in", Stone said, referring to the high-energy atomic particles whizzing around the universe. Eventually, measurements of local electrons and magnetic field shifts confirmed it was in interstellar space.

Astronomers expected there might be a difference between the magnetic field direction inside the heliosphere, where magnetic fields come from the sun, and outside of it, where magnetic fields come from exploding stars spewing their magnetic fields into surrounding material.

Comparing what Voyager 2 learned with the data available from the Voyager 1 crossing provides interesting similarities and contrasts, Stone said.

The boundary area between the heliosphere and the interstellar space is called the "heliopause", where solar wind meets interstellar wind. Turns out the sun's impact goes beyond its own borders.

In addition, Voyager 2 was slowed by its Neptune flyby in 1989, so Voyager 1 surged ahead as planned. "This is somewhat similar to what you might find out in the galaxy", says Gurnett.

A diagram describing the different parts of the heliosphere, the interstellar medium, and where the Voyager spacecrafts have travelled to. The plasma wave instrument is recording that the plasma density is rising, in data feeds from a spacecraft now so far away that it takes more than 19 hours for information to travel from the spacecraft to Earth.

Late previous year the probe punched through the protective bubble that surrounds the solar system.

The thrusters on both spacecraft are also degrading.

"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. Each probe is powered by radioisotopic thermoelectric generators heated by plutonium-238. The astronomer believes that the probes give important clues about the details and structure of the heliosphere. There are now no plans for a successor to the Voyager program (the only other spacecraft headed to such distances, New Horizons, will run out of power at 90 AU), but the success of the missions and the questions they raise will undoubtedly inspire these scientists and engineers to come up with new proposals to study the heliosphere and beyond. In as little as five years, the mission team suspect, they may no longer have enough power to call home, and the twins will go silent.

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