Random Walk
Voyager Nears Local Fluff
Our solar system is plunging through a vast cloud of wispy gas called the local interstellar cloud, also known as the “Local Fluff.” About 30 light-years wide, the Fluff is made of 6,000°C hydrogen and helium. The Fluff is about twice as dense as the interstellar meduim surrounding it, and what holds it together has been a mystery—until now, thanks to a discovery by JPL’s twin Voyager spacecraft.
By rights, the Fluff shouldn’t exist.
A group of nearby stars exploded about 10 million years ago, and the resulting blast of million-degree gas is now blowing past us. The Fluff is neither hot enough nor dense enough to withstand the onslaught, says Merav Opher, a former JPL postdoc now on the faculty at George Mason University. But in the December 24, 2009, issue of Nature, Opher and her colleagues reported that the latest data from Voyager 2 reveal a magnetic field strong enough to enable the Fluff to push back. “Voyager data show that the Fluff is much more strongly magnetized than anyone had previously suspected—between four and five microgauss,” Opher told Science@NASA. “This magnetic field can provide the extra pressure required to resist destruction.”
Previous estimates of the Fluff’s field had been in the 1.8 to 2.5 microgauss range. By comparison, Earth’s magnetic field is about half a gauss, or roughly a million times stronger.
Inside the Fluff—and encompassing us—is a 10-billion-kilometer-wide bubble called the heliosphere, which helps shield us from constant bombardment by high-energy cosmic rays from the depths of space. The heliosphere is kept inflated by the solar wind, a stream of charged particles emitted by the sun, so its size is determined by the balance of forces between the solar wind pushing out and the local interstellar cloud pressing back. In 2004 and 2007, respectively, Voyagers 1 and 2 crossed into the heliosphere’s outer layers, a region called the heliosheath. (See E&S 2008, No. 3.) Once there, they could measure the size of the heliosphere, allowing scientists to calculate how much pressure the Fluff is exerting on it. This pressure, in turn, partly depends on the strength of the Fluff’s magnetic field.
This discovery raises the possibility that other clouds in our galactic neighborhood are also strongly magnetized, and when the solar system collides with them, they will push back even harder. If the heliosphere is further compressed, more cosmic rays might reach Earth. “There could be interesting times ahead,” Opher says. But there’s no need to get out the tinfoil hats quite yet—we won’t run into the next cloud for hundreds of thousands of years. —MW
