Voyager 2 probes interstellar space
November 7, 2019 - One year ago NASA’s Voyager 2 sent back its first measurements from interstellar space, collected after travelling some 18 billion kilometres. Now, Voyager 2’s data are giving scientists new insights into the boundary region between the Sun’s sphere of influence and interstellar space.
Voyager 2 escaped the outer reaches of the heliosphere -- a massive bubble of charged particles that shield the Solar System from the Milky Way galaxy’s high-energy cosmic rays -- on November 5, 2018, more than six years after the probe’s pioneering twin, Voyager 1.
The probe crossed the boundary between the heliosphere and interstellar space at 119 astronomical units (AU) from the Sun. One AU is the average Earth-Sun distance which is about 150 million kilometres, which puts this boundary -- known as the heliopause -- at some 17.85 billion km from the Sun.
Voyager 1 made the crossing at nearly the same distance, 121.6 AU. This distance is surprising said Voyager project scientist Ed Stone of the California Institute of Technology in Pasadena.
“The heliosphere itself is breathing in and out,” said Stone, referring to the 11-year solar cycle. Voyager 2’s crossing occurred at a solar minimum when the solar activity is the least, Voyager 1 crossed at a solar maximum, when coronal mass ejections blast vast amounts of plasma out into space.
Voyager 2 confirmed that solar particles extend more than 160 million km beyond the heliopause. The data also show interstellar particles exert up to 10 times as much pressure on the heliosphere than previously thought.
Voyager revealed that within 225 million km of the heliopause, the solar plasma slows, heats up, and doubles in density. This is where the solar wind meets “interstellar winds from supernovae that exploded millions of years ago,” said Stone. On the other side of the boundary, interstellar plasma is between 30,000 to 50,000 degrees Celsius -- much hotter than expected -- and 20 times denser than solar plasma.
The Voyagers are each powered by three radioisotope thermoelectric generators (RTGs), which convert heat generated by the radioactive decay of plutonium-238 into electricity.
The mission team has already turned off heaters and scientific instruments to lower the power needs. Voyager 2 retains five working devices out of its original 10. The twin probes can probably collect and return data for just five more years or so, said Stone. Voyager 1 and Voyager 2 are currently about 148 AU (22.2bn km) and 122.4 AU (18.4bn km) from Earth, respectively.