By Sid Perkins
From Washington, D.C., at the American Geophysical Union meeting
Even though the two Voyager probes launched in 1977 passed the outermost planets in our solar system more than a decade ago, their sensors show that they haven’t yet outrun the influence of the sun.
Instruments on each spacecraft have detected several sudden surges in the solar wind since January 2001, says Robert B. Decker of the Johns Hopkins University Applied Physics Laboratory in Laurel, Md. These shock waves probably resulted from solar flares produced during the increased sunspot activity associated with the latest periodic surge of solar activity known as the solar maximum (SN: 1/13/01, p. 26: Stormy Weather).
In early January 2001, Voyager 2 detected a sudden jump in solar wind speed from 375 kilometers per second to 450 km/s. That pulse probably was associated with the Bastille Day solar burst, a torrent of charged particles that began to spill off the surface of the sun on July 14, 2000. A stronger shock wave, marked by a larger boost in solar wind speed, raced past the probe in mid-October 2001. That surge probably stemmed from the consolidation of several smaller shock waves that had been generated during a period of increased solar activity the preceding March and April.
Voyager 1 spotted a parallel boost in solar wind in early March 2002. However, the probe apparently didn’t detect a similar pulse from the Bastille Day activity. Decker says that’s probably because the shock from that single, yet massive, flare didn’t spread wide enough to strike Voyager 1, which is heading away from the sun in a slightly different direction from that of Voyager 2.
The Voyager probes are now the most far-flung spacecraft launched from Earth. Voyager 1, which visited Saturn in November 1980, is now over 12.5 billion km from the sun.
Voyager 2, which swooped past Neptune in August 1989, is just over 10 billion km away. Both have enough speed to escape into interstellar space, where the solar wind doesn’t reach.
