After a year of conflicting assessments, scientists for NASA's Voyager 1 spacecraft believe the probe crossed into interstellar space in August 2012.
A key milestone, 36 years in the making, was announced yesterday. The Voyager 1 spacecraft, having traveled outward 12 billion miles from the Sun (more than three times Pluto's mean distance) since its launch in 1977, has signaled its arrival in interstellar space. NASA associate administrator John Grunsfeld called it "a first in the history of exploration."
It has long been speculated that Voyager 1 would eventually cross the heliopause, the outer limit of the Sun's solar-wind bubble, but predicting exactly when has not been easy. It started picking up radio static from the turbulent boundary with the interstellar medium decades ago, and in 2004 it noted that the solar wind had decelerated to subsonic speed.
Then, in August 2012, the craft relayed that cosmic-ray counts had jumped — exactly the effect that space physicists expected to find in interstellar space. But, curiously, there was no corresponding change in the direction and intensity of the magnetic field. So for months researchers convinced themselves that the spacecraft had merely reached a kind of on-ramp to the heliopause.
Although the Voyager team continued to insist that the craft was not yet in interstellar space, others were more convinced. And the expected change in the magnetic field proved to be "something of a red herring," says Marc Swisdak (University of Maryland). In the September 1, 2013, issue of Astrophysical Journal Letters, he and two coauthors offer detailed modeling of the magnetic interactions along the heliopause. They conclude that the magnetic fields at the edge of the heliosphere and local interstellar medium must intersect at a very small angle.
Interestingly, Swisdak's magnetic-field simulations suggest that Voyager 1 crossed the heliopause very close to July 25, 2012, when the last reversal of the field's direction occurred. That's a month earlier than the cosmic-ray flux spiked and the solar-wind counts plummeted. The later events, he says, result from a mixing of the solar-wind and interstellar plasmas in
Now the Voyager team agrees that the changes seen in August 2012 truly did herald the long-awaited crossing. The evidence is compelling, if circumstantial.
In yesterday's online edition of Science, a quartet of mission scientists led by Donald Gurnett (University of Iowa) report that the craft has recorded pronounced changes in the plasma waves in its vicinity. These waves are sensitive indicators of how much plasma is present. In fact, Gurnett notes, the electron density scales as the square of the frequency. So if the frequency goes up, then the charged-particle density must be increasing as well.
Fortunately, two strong solar-wind blasts from the Sun, which reached the spacecraft in October-November of last year and then again in April-May 2013, triggered a pulse of detectable plasma waves. (The spacecraft was also equipped with an instrument to measure the plasma concentration and composition directly, but it failed in 1980.)
Since the interstellar medium's plasma is denser that the tenuous solar-wind plasma at Voyager 1's distance, the increase recorded over the past year is a clear sign that the spacecraft has exited the heliosphere. "The plasma density turned out to be about 100,000 electrons per cubic meter — "almost exactly what we expected in the interstellar plasma," Gurnett notes.
"It's a very exciting time for me," admits Gurnett, who is one of several investigators who have tended to the spacecraft's scientific instruments from the outset.
"We made it — while we still had enough power," says a relieved Edward Stone, the Caltech physicist who has served as mission's project scientist from the outset. Engineers estimate that Voyager 1's plutonium-fueled power system can provide adequate electricity for current operations only until about 2020. After that they'll have to switch off its instruments one by one.
In deference to some who've posted comments about this article, Voyager 1 has not yet left the Sun's realm entirely. For example, it has yet to traverse the Oort Cloud, a reservoir of trillions of comets that extends outward to perhaps 50,000 a.u.
Nor will the Sun lose its gravitational grip until the spacecraft is at least a few light-years away. (It's headed in the general direction of Gliese 445, an M-type star in Camelopardalis that's 17.6 light-years from us.)
Nonetheless, Voyager 1 is finally living up to its billing as an interstellar mission. Voyager 2, also headed toward the stars, is currently 9.5 billion miles (102 a.u.) from the Sun.
Read more details in NASA's press release.