NASA confirmed Thursday that the Voyager 1 probe, launched 36 years ago, has achieved a milestone beyond any other man-made object. In August 2012, it broke free of the bonds of the sun and its dainty solar wind in a magnetic cage — its heliosphere — and now travels in the interstellar medium, where high-energy particles dart hither and yon. This is astonishing and marvelous.
But, no, it has not left the solar system — by at least one definition that's easy to support. Despite every headline you've read! Despite the fact that you may believe NASA has announced and retracted it leaving the solar system over and over again. (Which is hasn't, either. Thursday was the first time the NASA and JPL scientists concurred with outside researchers that the previously conceived-of model was incorrect, and it's gone beyond a critical point.)
This subtlety may have been lost after the cheesy Star Trek paraphrase and absurd music that led off the official NASA press conference on September 12th, timed to coincide with the release of a paper by University of Iowa scientists in the journal Science. The principal author is Don Gurnett, a designer of the Voyager instrument that took the critical measurements. Starting with measurements picked up in August 2012, there was increasing suspicion that Voyager 1 had marked a transition in the region of space in which it traverses. Today, it is 125 astronomical units (AU or an anthropocentric unit equal to the distance from the Earth to the sun) from the Earth.
As objects travel outward from the sun, they pass through several transition points, and Voyager 1 has allowed us to learn much more about those zones. The sun produces solar wind, or low-energy charged particles that travel rapidly outward from its surface along lines of magnetic force. The sun lines of force curve back onto themselves to form a massive magnetic bubble that lives an enormous distance beyond the planets and Kuiper Belt, which spans 30 to 50 AU, and is where Pluto and other dwarf planets are found.
It was long suspected that there was a termination shock beyond the Kuiper belt where the solar wind would abruptly slow and perhaps produce a physical shock wave much like breaking the speed barrier does on Earth. In fact, that point exists and was measured but Voyager 1 and 2 have, in their separate directions, passed it without a bump. (Voyager 2 extended its initial Jupiter and Saturn mission to swing by Uranus and Neptune, so is closer in, and has a different heading.)
Some time after Voyager 1 entered that region of slower solar wind, known as the heliosheath, it found a strange phenomenon that the JPL team dubbed the "magnetic highway." Lines of force connected from inside and outside the sun's heliosphere, its magnetic bubble, so that high-energy particles from the interstellar medium through which the solar system traverses away from other stars, would exchange with low-energy solar particles.
The Voyager mission's principal investigator, Edward Stone, in charge since its inception in the early 1970s, and the rest of the JPL team expected that the transition from the heliosheath to interstellar space, a termination point called the heliopause, would be marked by a change in the direction of magnetic force. In the plane of the elliptic, the sun's magnetic fields run east-west; scientists theorized based on measurement from NASA's Interstellar Boundary Explorer (IBEX), that the interstellar medium would be more towards the north-south orientation — a shift scientists described today as expected to be about 30° of change.
In February and July, scientists outside of NASA and JPL published papers relying on Voyager data, which is publicly released, that concluded the probe had left the magnetic bubble on August 25, 2012. However, Dr. Stone and his colleagues remained unconvinced as recently as June 2013, stating that one needed to see not just the exchange of charged particles, but the magnetic directional change, too. There it stood.
(Data is continuously recorded on an eight-track digital tape player that, nonetheless, can hold the equivalent of half a gigabyte. It is transmitted at 1400 bps every six months back to Earth. Amazing technology for the early 1970s; amazing that it continues to work.)
But then on April 2013, data were measured that tipped the team over. Voyager 1 has several remaining functioning instruments, and its plasma-wave detector allowed to take detailed measurements of the medium in which it traveled in April 2013 from a coronal mass ejection from March 2012. (Its plasma sensor had failed in 1980, which would have made this far easier to detect earlier! The plasma-wave detector was designed to pull data from Jupiter and Saturn about wave/particle interactions. The coronal ejection sent particles way out yonder a-thrummin’, making this instrument able to detect density directly.)
The density of plasma inside and outside the heliosphere has been determined from Earth-based instruments: the interstellar medium should have roughly 50 times the number of electrons per unit of volume (10000 per cubic meter versus 200 per cubic meter). The plasma-wave detector measured oscillations corresponding to about 8000 electrons per cubic meter, which was considered well within margins. Tracking back through the data and looking for consistent measures, the team pinpointed August 25, 2012 — the same as previous reports, but now with the added certainty of a measurement of density. Voyager 1 was then at 121 AU. The scientists are excited to figure out why the expected magnetic field rotation didn't occur — giddy, in the press conference about it.
So: hasn't it left the solar system? By the broadest definition, the solar system has various "edges," or points at which one can measure the end of things. This is true of magnetic influences, as with the termination shock and heliopause, and astronomical bodies, like the major planets or the Kuiper Belt.
But the true edge is often considered at the Oort Cloud, which primarily ends at 50,000 AU, or 400 times the current distance that Voyager 1 is from the sun, and which may extend out faintly to 100,000 or 200,000 AU. Beyond 50,000 AU, the sun's gravitational pull has declined to a point where it can no longer retain objects; within the Oort Cloud, or at least within 50,000 AU, the sun brings all its toys with it.
JPL mentions this in passing in its press release, which carefully refers to interstellar space. The sun's heliosphere is an egg, which keeps the majority of high-energy particles mostly out, but the space between the heliopause and the Oort Cloud is where the interstellar medium freely passes.
My friend David Blatner wrote poetically about how far away the Oort Cloud lies in his book, Spectrums: Our Mind-Boggling Universe From Infinitesimal to Infinity:
If Earth were the size of a grain of salt, our solar system (only out to Neptune!) would be 352 meters wide — that’s a grain of salt sitting inside about three and a half football fields of space. If you include the whole solar system (out to the Oort cloud), it’s more than 2,000 times more space: a grain of salt in a region about 450 miles wide. (That’s like flying from San Francisco to Seattle — a two-hour flight—and encountering virtually nothing but a few specks along the way.)
It's a major achievement for humanity (and the JPL mission scientists and related academics who planned it so long ago) to have a functioning probe reach this far. Ed Stone said today, "It is sailing the uncharted waters of a new cosmic sea, and it has brought us along for the journey."
The Voyager probes' power sources, radioisotope thermoelectric generators (RPGs), allow still-active instruments to function until about 2021, at which point they must be turned off one by one until all the components are off in 2025. Then they proceed into the inky depths alone and forever, our voyagers.
