Launched on August 20, 1977, NASA’s Voyager 2 spacecraft became the second human creation to break through the Sun’s heliopause and enter interstellar space, achieving the feat on Nov 5, 2018 – officially announced by the space agency on Monday (Dec 10).
While the heliopause is the boundary separating the Sun’s heliosphere from interstellar space, the heliosphere itself is a vast region surrounding the Sun that is dominated by its continuously expanding plasma known as the solar wind.
It is because of this solar wind that objects within this vast bubble of heliosphere, including Earth, are relatively better protected from the impact of galactic cosmic rays that are far more dominant beyond the heliopause – in interstellar space.
While most of the material inside the heliosphere originates from the Sun, a majority of those found outside the heliopause come from stars that exploded billions of years ago.
The sudden drop in Voyager 2’s plasma readings of the solar wind and the corresponding increase in hits from galactic cosmic ray particles were strong indicators of the Nov 5 crossover.
Voyager 1 experienced this transition on August 25, 2012, more than six years before Voyager 2, making it the first spacecraft to achieve the feat even though it was launched 16 days after (Sep 5, 1977) its twin – thanks to its superior speed.
Explaining the difference between the two heliopause crossings at a press conference, Voyager project scientist at Caltech and former JPL director, Ed Stone, said that the team would have been “ amazed” had both the crossings appeared the same.
“We’re in a different place — one is in the northern hemisphere and the other is in the southern hemisphere — and it’s a different time in the solar cycle,” he said.
“Comparing data from different instruments aboard the trailblazing spacecraft, mission scientists determined the probe crossed the outer edge of the heliosphere on Nov. 5,” said a NASA press release.
“This boundary, called the heliopause, is where the tenuous, hot solar wind meets the cold, dense interstellar medium,” said the release.
“Its twin, Voyager 1, crossed this boundary in 2012, but Voyager 2 carries a working instrument that will provide first-of-its-kind observations of the nature of this gateway into interstellar space,” it added.
Voyager 2 is now more than 11 billion miles from earth, getting farther and farther away as it hurtles through the interstellar void at 34,191 miles per hour (55,025 kph).
However, it is still 300 years short of entering the disc-shaped inner Oort cloud and another 30,000 years away from exiting the spherical outer Oort cloud, completely beyond the influence of the solar system.
“The boundary of the Solar System is considered to be beyond the outer edge of the Oort Cloud, a collection of small objects that are still under the influence of the Sun’s gravity,” NASA said.
“I think we’re all happy and relieved that the Voyager probes have both operated long enough to make it past this milestone,” Suzanne Dodd, Voyager project manager at NASA’s Jet Propulsion Laboratory (JPL), was quoted as saying in the Dec 10 release.
“This is what we’ve all been waiting for,” she said, adding that the team was “now looking forward to what we’ll be able to learn from having both probes outside the heliopause.”
Also, now that both the Voyagers are in interstellar space – beyond the range of the protective forces of the heliosphere – scientists are looking forward to doing a comparative analysis between readings from their onboard instruments and those from spacecraft and instruments that are currently within the heliosphere.
“I’ve been studying galactic cosmic rays for many years from within our heliosphere,” said Georgia Denolfo, a NASA space scientist at the Goddard Space Flight Center.
“So it’s especially exciting to be able to think that we will be having a mission in the very same space that I have been studying, and many others, from afar,” she said.
Both the Voyagers have been operational for more than 41 years now, with Voyager 1 successfully achieving its primary objective of flybys of Jupiter and Saturn and the latter’s largest moon Titan, studying the planets’ weather, magnetic fields, and rings.
In fact, Voyager 1 was the first spacecraft to capture detailed images of the planets’ natural satellites.
Having completed its mission in November 1980, Voyager 1 made the historic heliopause crossing in 2012, as mentioned earlier.
It will continue traveling through the vastness of interstellar space until its radioisotope thermoelectric generators are no longer capable of keeping the onboard instruments operational, which is expected to happen sometime in 2025.
Traveling 4,000 miles per hour slower than its twin, Voyager 2 did take longer to reach Jupiter and Saturn but it achieved what no other spacecraft had done before; that is, explore the Uranus and Neptune systems, achieving the historic feat in 1986 and 1989, respectively.
And now, as we know, it has successfully made the second heliopause crossing in history and, like its twin, is flying through the galactic cosmic ray particles of interstellar space.
It continues to maintain contact through the NASA Deep Space Network (DSN) and has already started beaming data on interstellar plasma temperature and density.
“Both spacecraft are very healthy, if you consider them senior citizens,” said Dodd, adding that “they are operating just fine.”
As the power-generating capabilities of the two spacecraft continue to decline, mission officials may have to sacrifice some of the relatively less important instruments on them.
“The difficult decisions are going to be made by Dr. Stone and the science team on which instruments to turn off first,” she said, adding that “those decisions will be made with getting the most science value back.”
Dodd is looking forward to squeezing out, at least, ten more years from the two probes.
“My own personal goal would be to get these spacecraft to last 50 years,” she said, adding: “If we get out to 2027, that will be a 50-year mission. I think that would be fantastic.”
“I often get asked, ‘Is this it for Voyager?’” said Nicky Fox, director at NASA’s heliophysics division.
“Absolutely not. This is really for me the beginning of a new era of heliophysics science,” he said.