Correction: The distance from the sun Voyager 2 is estimated to reach in 2020 is about 11 billion miles.
SAN FRANCISCO--Thirty years after launch but earlier than expected, Voyager 2 has left the cozy realm of our solar system, where the stream of particles from the sun dominates space.
You might think that space billions of miles from the sun is a placid, empty domain. In fact, Voyager 2 has been heading outward in the same direction as the solar wind, charged particles streaming from the sun, but things started to get a lot more complicated on August 30, when the spacecraft was 7.8 billion miles from the sun.
There, the spacecraft passed into a new region, where the solar wind suddenly slams into the prevailing breeze and magnetic field left from a series of massive supernovas from 20 million to 30 million years ago, said Voyager project scientist Edward Stone of the California Institute of Technology during a news conference at the American Geophysical Union conference here Monday.
In this area, called the termination shock, the speed of the solar wind drops abruptly from about 250 miles per second to about 60, said John Richardson, a Massachusetts Institute of Technology researcher who's the principal investigator for the Voyager's plasma science work.
Voyager 1, which is traveling faster than Voyager 2 and in a different direction, already crossed the termination shock boundary in December 2004, though some of its elderly instruments are defunct and it crossed during a gap when data wasn't recorded. But Voyager 2 crossed the boundary while significantly closer to the sun, indicating that this region where the solar wind dominates, called the heliosphere, is not in fact a sphere but rather is squashed.
"The termination shock is 1 billion miles closer to the sun in the southern hemisphere than in the northern hemisphere," Stone said, referring to regions of space on either side of plane in which the planets orbit the sun. "There's something outside pushing in on the field of the heliosphere. We believe it's a magnetic field distorting an otherwise spherical surface."
It'll be a while--probably 7 to 10 years--before the Voyager spacecraft leave the solar system altogether and cross into interstellar space itself, Stone said. The researchers hope that will be before the Voyagers' radioactively powered batteries are estimated to run out of juice--sometime between 2020 and 2025, he added.
If you're disgruntled that your own batteries seem to expire much sooner, bear in mind that NASA shuts down most of the spacecrafts' instruments and that they transmit data back to Earth with a 20-watt transmitter. That's much less than most conventional light bulbs.
Their distance from the sun in 2020 will be about 14 billion miles for Voyager 1 and 11 billion miles for Voyager 2. For perspective, that's 148 and 122 times as far away from the sun as the Earth is, respectively. Voyager 1 is traveling about 10.6 miles per second and Voyager 2 at about 9.3 miles per second.
That's remarkable longevity. NASA bet on an initially modest mission to Jupiter and Saturn but planned for the spacecraft to lead much longer lives. With a gradually lengthening series of three-year budget extensions, the spacecraft have made it to Uranus and Neptune, and Stone and his colleagues are now writing the next three-year funding proposal.
The Voyager spacecraft cost $865 million to build and launch and $120 million so far to operate.
Others likely will follow in the Voyagers' footsteps. First will be the New Horizons mission, now launched and scheduled to visit Pluto in 2015. The next planned is Ibex, short for Interstellar Boundary Explorer, a spacecraft dedicated to investigate these outer reaches of the solar system that's scheduled for a June 2008 launch.
Ibex in particular is geared to investigate one mystery that Voyager 2 uncovered. The researchers expected the solar wind particles to be a toasty 1 million degrees in temperature, but in fact they are a relatively cool 200,000 or so, Stone said. "The thermal energy that was missing most likely went into the acceleration of ionic particles," he said.
Also to be studied are the particulars of the termination shock, a fast-changing region. Voyager 2 actually crossed the boundary at least 5 times on its way out from the sun, Stone said, as it traversed the turbulent region.