In most cases we call the corona the sun’s upper atmosphere, because that’s the part we see as the faint, outermost halo stretching from the bright orb, usually visible from Earth only during a solar eclipse.
But the sun’s influence stretches farther than that. The star also produces what’s known as the solar wind, a flurry of charged particles constantly streaming from the sun in all directions at a whopping 1 million miles (1.6 million kilometers) an hour.
This supersonic wind forms a bubble around our entire solar system called the heliosphere.
But at about 8.7 billion miles (14 billion kilometers) out, the wind speed suddenly drops, creating a warmer, turbulent layer of solar particles around the solar system called the heliosheath.
—Image courtesy NASA/JPL
NASA’s Voyager 1 probe, launched in 1977, crossed into the heliosheath in 2004 and began feeding scientists some of the first data on this far-flung region where the solar system meets interstellar space.
Now, scientists announced today, Voyager 1 has reached the part of the heliosheath where the outward speed of the solar wind has dropped—to zero.
“The solar wind has turned the corner,” Ed Stone, a Voyager project scientist based at the California Institute of Technology in Pasadena, California, said in a NASA release.
“Voyager 1 is getting close to interstellar space.”
In June Voyager was about 10.6 billion miles (17 billion kilometers) from the sun. At that point, scientists noticed that the speed of charged particles hitting the craft’s outward face matched the spacecraft’s speed.
In other words, the net outward speed of the solar wind had reached zero.
Later analysis showed that the solar wind had steadily slowed at a rate of about
45,000 miles (72,000 kilometers) an hour each year since August 2007, when the solar wind was clocked at about 130,000 miles (209,000 kilometers) an hour.
Since June, the outward speed has stayed at zero, the scientists announced during a meeting of the American Geophysical Union, currently being held in San Francisco.
“When I realized that we were getting solid zeroes, I was amazed,” said Voyager co-investigator Rob Decker.
“Here was Voyager, a spacecraft that has been a workhorse for 33 years, showing
us something completely new again.”
—Image courtesy NASA/JPL
The sudden slowdown, however, does not mean that Voyager has totally left home, so to speak.
If Voyager had reached the much chillier vastness of interstellar space, scientists would expect to see a sudden drop in the density of hot particles and an increase in cold particles.
Instead, the new data on wind speed can help scientists recalculate when Voyager will finally cross into that great unknown, currently estimated at four years from now.
“In science, there is nothing like a reality check to shake things up, and Voyager 1 provided that with hard facts,” said Voyager scientist Tom Krimigis. “Once again, we face the predicament of redoing our models.”