WASHINGTON – The tireless Voyager 1 spacecraft, launched in the disco era and now about 19 billion km from Earth, has become the first man-made object to enter interstellar space, scientists said Thursday.
Interstellar space, scientists now know with certainty, is dense with particles, and the place is literally hissing — or maybe you could say it is whistling in the dark.
“It’s almost a pure tone — like middle C, but slightly varying, like your piano is not quite tuned right,” said Donald Gurnett, a University of Iowa physicist who has been working on the Voyager mission most of his adult life.
Gurnett is the lead author of a paper published Thursday in the journal Science that provides what seems to be the final, incontrovertible evidence that NASA’s Voyager 1 has crossed into a realm where no spacecraft has gone before.
Scientists have long thought that there would be a boundary out there, somewhere, where the 2.4-million-kph “solar wind” of particles would give way abruptly to cooler, denser interstellar space, permeated by charged particles from around the galaxy.
That boundary, called the heliopause, turns out to be 18.1 billion km from the sun, according to Voyager’s instruments and Gurnett’s calculations.
Beyond the boundary, space is — perhaps counterintuitively — much denser with particles. There are 80,000 particles per cubic meter in the region where Voyager 1 is now, Gurnett said.
The sun’s hot ejecta — a plasma of charged particles — forms a vast bubble, known as the heliosphere. In the outer regions of the heliosphere, particles are relatively few and far between, with just 1,000 per square meter in some regions, Gurnett said. But the heliosphere has an edge.
Voyager 1’s epochal crossing of the boundary into the cooler, denser plasma, took place on Aug. 25, 2012, according to the new report.
This confirms earlier findings, published in three papers in Science in June, that Voyager 1 on that date experienced a sudden drop in solar radiation and a spike in cosmic particles coming from all around the galaxy.
But the earlier data from the spacecraft had been somewhat ambiguous. The spacecraft continued to pick up magnetic signals that suggested that it was still within the sun’s magnetic field. Ed Stone, who has overseen the Voyager project since the early 1970s, suggested that Voyager 1 was flying through a transitional zone.
Now, however, scientists have a new set of measurements thanks in large part to a solar flare. On March 17 last year, the sun ejected a huge mass of particles, and when those solar particles arrived at Voyager more than a year later, on April 9, they triggered oscillations in the charged particles of matter — the plasma — surrounding the spacecraft.
From the frequency of those oscillations — essentially the sound of space itself — the scientists could interpret the density of the plasma. That density, much higher than anything registered before, offered compelling evidence that Voyager 1 had entered the interstellar zone.
“For the first time, we’ve actually measured the density of the plasma,” Stone said. He said he is convinced by the new data that his spacecraft has fully penetrated interstellar space.
The two Voyager spacecraft were launched in 1977. Voyager 1 flew by Jupiter and Saturn, the gravity of which helped slingshot the spacecraft toward the outer reaches of the solar system. Voyager 1 is now traveling at 61,000 kph relative to the sun.
Voyager 2 flew near Jupiter and Saturn and then went on to pass by Uranus and Neptune. It is not quite as far from the sun as its sister spacecraft.
Although Voyager 1 is now in interstellar space, it hasn’t technically left the solar system. That is because of the Oort cloud — a region of comets in orbit around the sun.
“We’ll get to the inner edge of the Oort cloud in about 300 years,” Stone said.
The spacecraft draws power from the radioactive decay of plutonium-238, and Stone thinks the dwindling power supply will force engineers to start turning off instruments in 2020. Voyager 1 probably will go dark by 2025.
Stone said the spacecraft will pass through the far side of the Oort cloud in about 30,000 years.
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