At the outer limits of our solar system, a solar shock wave is about to
overtake NASA’s Voyager 1 spacecraft.

When Voyager 1 signals NASA, which it does almost every
day, there’s usually not much to report. The spacecraft is nearly 9
billion miles (14.5 billion km) from the sun, at the edge of our solar
system. It’s quiet out there, dark and uneventful.

A solar blast wave is heading for the spacecraft, and “it could arrive
at any moment,” says Ed Stone, project scientist for the Voyager mission
at the Jet Propulsion Laboratory.

Remember the solar storms of October and November 2003? Giant sunspots
unleashed some of the most powerful flares in recorded history; the
explosions hurled billion-ton clouds of gas, called coronal mass
ejections (CMEs), into the solar system. When the CMEs reached Earth,
auroras appeared as far south as Florida, and our planet gained a new
radiation belt that persisted for weeks.

Almost a year later, people are still talking about what happened. Well
– the storm isn’t over. Clouds spit out by the sun during those historic
weeks have been traveling through the solar system ever since, and
they’re about to overtake Voyager 1.

Other spacecraft have already been hit.

On Oct. 28, 2003, a CME swept past Mars Odyssey, in Mars orbit. Intense
radiation disabled one of the craft’s science instruments, the Martian
Radiation Environment Experiment (MARIE), designed, ironically, to study
solar storms and space radiation. In the weeks that followed, CMEs
buffeted Ulysses near Jupiter, and Cassini on its way to Saturn. Neither
spaceship was harmed.

When one of the CMEs reached Saturn, Cassini detected bursty radio
emissions signaling a magnetic storm around the ringed planet. Saturn
(like Earth and Jupiter) has a global magnetic field that wraps around
the planet, enveloping it in a protective bubble called the
magnetosphere. When a CME hits, the magnetosphere reverberates (“a
magnetic storm”); auroras appear; plasmas inside the magnetosphere begin
to emit radio waves – but the planet itself is safe.

“The blast wave was powerful enough to spark a magnetic storm all the
way out at Saturn, almost ten times farther from the sun than Earth.
That’s impressive,” marvels Stone.

October. November. December. “The CMEs kept traveling outward,” says
Stone. January. February. March. “Some of the CMEs merged, the faster
clouds having scooped up the slower ones.” April. “The shock wave hit
Voyager 2.”

Voyager 1 and 2 are the most distant spacecraft in the solar system.
They left Earth in the late 1970’s, visited Jupiter and Saturn (Voyager
2 also went to Uranus and Neptune), then headed for the stars. Voyager 2
is now 7 billion miles from Earth, and Voyager 1 is almost 9 billion
miles away.

Soon these spacecraft will reach the edge of the sun’s magnetosphere, or
“heliosphere,” a vast magnetic bubble containing all nine planets.
Outside the bubble lies interstellar space. Inside … the Voyagers are
still within range of solar storms.

The shock wave hit Voyager 2 traveling 600 km/s, or 1.3 million mph.
(For comparison, CMEs left the sun last October going 1500 to 2000 km/s,
“so there has been substantial deceleration,” notes Stone.) The physical
force was slight, less than the touch of a feather–the spacecraft
didn’t go tumbling. Neither did radiation cause problems. The storm had
diffused over such a great volume by the time it reached Voyager 2, that
“no damage was done,” says Stone.

In fact, the encounter was good. Voyager 2 measured (indirectly) the
speed of the shock, as well as its composition, temperature and
magnetism. These data are invaluable, says Stone. Combined with
measurements from Mars Odyssey, Ulysses, Cassini and other spacecraft,
they show how far-ranging CMEs evolve and dissipate. One day human
astronauts will be “out there,” and mission planners need to know what
to expect.

All that remains is Voyager 1.

Based on the velocity of the blast wave when it hit Voyager 2, “we
expected the shock to reach Voyager 1 on June 26th,” says Stone. ”
We’re still waiting.” It’s possible the wave, irregular in structure,
will simply miss Voyager 1.

But it won’t miss the edge of the heliosphere–it can’t. When the shock
wave gets there, Stone says, there might be a 2 to 3 kHz radio burst
signaling the impact, akin to the radio emissions Cassini detected when
the wave hit Saturn’s magnetic field, but at much lower frequencies.
Voyager 1 has a receiver on board that can record such bursts and report
them to Earth.

That’s not all: the blast wave will push the edge of the heliosphere
outward as much as 600 million km, Stone believes, and then there will
be a rebound. For months the outer layer of the sun’s magnetic bubble
might slowly sweep back and forth over Voyager 1.

For Stone and his colleagues, who’ve have been waiting decades for
Voyager 1 to reach the outer limits of the solar system, this is an
exciting time. Solar shock waves. Radio bursts. The heliosphere itself
bulging and rebounding.

It’s not so uneventful out there, after all.