Scientists have confirmed that the 35-year old probe, launched to study the Jovian system, has crossed into a region of space that shows all the signs of it being the interstellar medium.

Around August 25, 2012, more than 19 billion kilometres from the Sun, the Voyager 1 space probe became the first human-made object to cross out of the Solar System and into interstellar space. The event was confirmed to have happened by a report published in the journal Science on September 12. It included an analysis, of the data beamed back by the probe, by scientists from the University of Iowa among others.

Voyager 1, a testament to durable engineering, was launched by NASA in 1977 alongside its identical sister probe Voyager 2 to study the outer Solar System and the interstellar medium – whatever occupies the gigantic chasms between stars in the universe.

Voyager 1 flew by Jupiter in 1979 and Saturn in 1980, concluding its primary mission. It was the first probe to provide detailed images of the two planets and their moons. In 1990, 9.6 billion km from Earth, it turned around and photographed the entire Solar System.

Ever since, it has been drifting toward its edge at some 17 km/s, surrounded by a hot ‘sea’ of charged particles called plasma. To detect the crossover, which isn’t at an exact boundary, scientists were looking for some telltale signs: the plasma’s density would increase, its temperature decrease, and the direction of the surrounding magnetic field would change.

A report published in Science on September 12 confirmed that these changes in Voyager 1’s environs had kicked in around August 2012, meaning the probe has been in the interstellar medium for the last year.

“It’s like the first time a satellite [Sputnik] went beyond the Earth’s atmosphere to an altitude of some 600 miles; Voyager is now leaving the solar bubble at an altitude of 11.3 billion miles. It’s another historic milestone,” announced Stamatios Krimigis, principal investigator for Voyager’s Low-Energy Charged Particle (LECP) experiment, in a NASA statement.

Since the probe doesn’t have a plasma sensor that can take readings continuously, scientists looked at the magnetic field direction. It hadn’t changed for a long time even though, by 2011, the intensity of particles originating from within the Solar System had begun to drop off.

Then, on March 2012, the Sun released a massive burst of charged particles into the Solar System, called a coronal mass ejection. When this wave of particles reached the plasma around Voyager in April this year, it set off disturbances in the medium that set off Voyager’s sensors.

“We literally jumped out of our seats when we saw these [disturbances],” said Don Gurnett, who led the analysis efforts from the University of Iowa, in the statement. This providential gift from the Sun exposed the plasma around Voyager to be much denser and cooler than what was found inside the Solar System. “Clearly we had passed through the heliopause, which is the long-hypothesized boundary between the solar plasma and the interstellar plasma,” Gurnett clarified.

A previous set of studies published in June this year had noticed some anomalous readings from Voyager 1 originating from the same date. The probe was concluded to have stumbled into a region of space previously unknown to astronomers, falling along the boundary between the Solar System and the region of space beyond.

Krimigis, who was part of those studies, had said in an interview to The Hindu then that he felt like “Columbus who thought he had gotten to West India when in fact he had discovered America!”

His comment reflects a knowledge inadequacy among the astrophysics community to address the finer details of what Voyager 1 is set to encounter. The probe is now just beyond the heliosphere, an imagined bubble of space beyond whose borders the Sun doesn’t have a dominating influence.

Scientists from NASA’s Jet Propulsion Lab, which built and now operates the probe, expect the particle science instruments on board Voyager 1 to continue to send radio signals home until 2020. These signals will be the first of their kind for the voyager has gone where no human-made machine has gone before.

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