Voyager is now embedded in the gas, dust and magnetic fields from other stars. Voyager still feels a gravitational tug from the Sun, just as some comets do that lie even further out in space. But to all intents and purposes, it has left what most people would define as the Solar System.
The data that finally convinced the mission team to call the jump to interstellar space came from the probe's Plasma Wave Science (PWS) instrument. This can measure the density of charged particles in Voyager's vicinity.
Readings taken in April/May this year and October/November last year revealed a near-100-fold jump in the number of protons occupying every cubic meter of space.
Science - In Situ Observations of Interstellar Plasma With Voyager 1
Launched over 35 years ago, Voyagers 1 and 2 are on an epic journey outward from the Sun to reach the boundary between the solar plasma and the much cooler interstellar medium. The boundary, called the heliopause, is expected to be marked by a large increase in plasma density, from about 0.002 cm−3 in the outer heliosphere, to about 0.1 cm−3 in the interstellar medium. On 9 April 2013, the Voyager 1 plasma wave instrument began detecting locally generated electron plasma oscillations at a frequency of about 2.6 kHz. This oscillation frequency corresponds to an electron density of about 0.08 cm−3, very close to the value expected in the interstellar medium. These and other observations provide strong evidence that Voyager 1 has crossed the heliopause into the nearby interstellar plasma.
Voyagers Future Path
While Voyager 1 is commonly spoken of as having left the Solar System simultaneously with having crossed the heliopause, it remains well within the sphere of the Sun's gravitational dominion. It is presently less than one seventh the distance to the aphelion of Sedna, and it will take about 30,000 years to pass through the Oort cloud, the source region of long-period comets.
Voyager 1 is not heading towards any particular star, but in about 40,000 years it will pass within 1.6 light years of the star Gliese 445, which is at present in the constellation Camelopardalis. That star is generally moving towards the Solar System at about 119 km/s (430,000 km/h; 270,000 mph). Provided Voyager 1 does not collide with anything (and providing it is not retrieved by later generations of humans), the New Horizons space probe will never pass it, despite being launched from Earth at a faster speed than either Voyager spacecraft. New Horizons is traveling at about 15 km/s, 2 km/s slower than Voyager 1, and is still slowing down. When New Horizons reaches the same distance from the Sun as Voyager 1 is now, its speed will be about 13 km/s (8 mi/s). The close flyby of Saturn and Titan gave Voyager 1 an advantage with its extra gravity assist.
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