Artist concept of the two Voyager spacecraft as they
approach interstellar space (Image credit: NASA/JPL)
The Voyager 1 spacecraft launched by NASA on September 5,
1977 continues to add to its impressive list of accomplishments.
Its “Grand Tour" through the Solar System has seen it become the
first probe to provide detailed images of the moons of Jupiter
and Saturn and earn the title of most distant human-made object
in the cosmos. After a 33-year journey, Voyager 1 has now
crossed into an area at the edge of our Solar System where there
is no outward motion of solar wind.
Now traveling at a speed of around 38,000 mph (61,155 km/h)
and some 10.8 billion miles (17.38 billion km) from the Sun,
Voyager 1 has reached a point where the velocity of the hot
ionized gas, or plasma, emanating from the sun has slowed to
zero. Instead of moving outward from the Sun, the solar wind has
been turned sideways with scientists suspecting the cause to be
the pressure from the interstellar wind in the region between
stars, indicating the spacecraft is drawing closer to leaving
the Solar System.
The sun gives off a stream of charged particles that form a
bubble around the solar system known as the heliosphere. For the
first 6.2 billion miles (10 billion km), the solar wind travels
at over 0.62 million mph (over a million km/h), but slows down
dramatically once it crosses a shockwave called the termination
shock. This marks the start of the heliosheath, a region where
the solar wind is slowed, compressed and made turbulent by its
interaction with the interstellar medium.
Voyager 1 crossed the termination shock into the heliosheath
in December 2004 and it took another three and a half years for
the solar wind to slow to zero. This occurred in June this year,
when Voyager 1 was about 10.6 billion miles (17 billion km) from
the Sun.
Scientists used data from the spacecraft’s Low-Energy Charged
Particle Instrument to deduce the solar wind’s velocity. When
the speed of the charged particles hitting the outward face of
Voyager 1 matched the spacecraft’s speed, they knew that the net
outward speed of the solar wind was zero.
Because the velocities can fluctuate, the scientists watched
four more months of reading to be convinced the solar wind’s
outward speed had actually slowed to zero. Data also showed that
the velocity of the solar wind slowed at a rate of about 45,000
mph (72,000 km/h) each year since August 2007, when the solar
wind was traveling outward at a speed of about 130,000 mph
(209,000 km/h).
The scientists believe that Voyager 1 has not yet crossed the
heliosheath into interstellar space because that would result in
a sudden drop in the density of hot particles. They currently
estimate the spacecraft will reach that milestone in about four
years but are putting data into their models of the
heliosphere’s structure to determine a better estimate.
As with reaching the point where the velocity of the solar
wind has slowed to zero, crossing into interstellar space will
add yet another amazing feat to Voyager 1’s long list of
achievements. It is estimated that Voyager 1 has enough
electrical power to operate its transmitters until at least
2025, so the list is sure to keep growing for at least a little
while longer.
Via
NASA