Astrophysicists confirm that the Voyager 2 probe has entered interstellar space

Begun in 1972, the Voyager program aims to study planets outside the Solar System. In 1977, the Voyager 1 and Voyager 2 space probes were launched and, for several years, will fly over the giants and their satellites, collecting valuable data on these planets. In September 2013, NASA confirmed that Voyager 1 left the heliosphere and entered interstellar space. And, according to new studies jointly published on the data collected by Voyager 2, the latter also officially entered the interstellar space in November 2018.

After a careful analysis of the data, the astrophysicists confirmed it: like Voyager 1, the space probe is now out of the heliopause and sinks into the interstellar space. Through five articles in the journal Nature Astronomy , astrophysicists confirm that Voyager 2 penetrated interstellar space on November 5, 2018, at a distance of 119 astronomical units (17.8 billion kilometers) from the Sun.

And, since Voyager 1's plasma study instrument was broken during the heliopause crossing six years ago, this is the first time researchers have been able to study a complete set of in situ profile data. plasma of this important boundary.

The two Voyager probes were launched in 1977 to study the External Solar System. Voyager 2 was sent first, with a two-week lead, but Voyager 1 had a shorter trajectory in the Solar System.

Mission Voyager Interstellar : en route to the interstellar medium

In addition, Voyager 2 was slowed down by its flight over Neptune in 1989, Voyager 1 was ahead of schedule. After this 1989 survey, the two probes had achieved their main objective, but they were far from having finished their work. " At that time, the mission became the Voyager Interstellar mission, " said astronomer Ed Stone of Caltech.

Nobody knew how long it would take probes to reach interstellar space. Via a supersonic wind of ionized plasma, the Sun creates a bubble around the Solar System. This bubble is called the heliosphere and its limit - where the external pressure exerted by the solar wind is no longer strong enough to oppose the wind of interstellar space - is called the heliopause.

At present, both Voyager 1 and 2 probes have entered interstellar space. This phase is the last leg of the Voyager Interstellar mission. Credits: NASA / JPL

" This contact surface is the limit, and we try to both understand the nature of the latter, where these two cosmic winds meet and mix, and how that happens,  " says Stone. Voyager 1 officially passed the heliopause on August 25, 2012, at a distance of 121.6 astronomical units (18.1 billion kilometers).

Valuable data on heliosphere dynamics and heliopause

When the probe made its historic crossing, the researchers could confirm this fact only eight months later, through oscillations of electronic plasma to infer an interstellar plasma density. Astronomers did not really know when Voyager 2 would do the same - the heliosphere is a little flickering and changes shape slightly frequently - but in October of last year, it began recording an increase in cosmic radiation similar to that of Travel 1 in 2012.

This time, the detection of plasma density was done directly. And, interestingly, what the five Voyager 2 instruments captured shows a smoother, finer heliopause, with a stronger magnetic field. According to plasma observations, the probe passed through the heliopause in less than a day. The Voyager 2 cosmic ray instrument also detected something that had not been detected with Voyager 1: the existence of a layer between heliopause and interstellar space, where both winds interact.

The valuable data collected by the two probes allows astrophysicists to better understand the complex dynamics of the heliosphere and its interaction with the interstellar medium. Credits: NASA

Voyager 1 detected galactic cosmic rays and the interstellar magnetic field invading the heliogaine. Voyager 2 discovered that the interstellar magnetic field wrapped around the heliopause and that the cosmic rays inside the Solar System were moving along it. This indicates that the heliopause is not a simple smooth contact boundary, but is much more complex and dynamic.

Different travel conditions for Voyager 1 and Voyager 2

The reasons for the differences between the results of the two probes are not entirely clear, but there are a number of possible explanations. Time was running out - Voyager 1 crossed this boundary as the Sun entered its maximum of solar activity, the active peak of its 11-year cycle, when the solar wind is much stronger. Voyager 2 made this crossing while the Sun is just coming out of its minimum activity.

The heliopause is only a limit of the influence of the Sun. The gravitational influence of our star is much, much larger, extending through the cloud of Oort up to 100'000 astronomical units (15 trillion kilometers). Unfortunately, it is extremely unlikely that Voyager probes will remain operational at this distance. Nevertheless, the data collected by the two probes are extremely valuable to better understand the dynamics of the Sun and the astrophère of other stars.


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