A Voyager goes the distance

Science News, March 7, 1998 by Ron Cowen

On Feb. 17, the venerable Voyager 1 spacecraft became the most distant explorer in the solar system. Breaking the record held by another old-timer, Pioneer 10, the 20-year-old Voyager 1 is now more than 10.4 billion kilometers from Earth. That's more than double Neptune's average distance from the sun and almost 70 times the distance between Earth and the sun.

Hurtling through the deep freeze of the outer solar system at 17 km per second, Voyager 1 is nearing the edge of the heliosphere, the vast bubble of gas in which the sun's magnetic field reigns supreme. No craft has ever reached that boundary, so both Voyager 1 and its sister craft, Voyager 2, which lags about 2.25 billion km behind, will gather the first evidence of its structure.

The sun maintains the heliosphere by expelling a steady stream of charged particles, known as the solar wind. This wind travels at supersonic speeds, but at the boundary of the heliosphere, where it encounters charged particles and magnetic fields from interstellar space, it slows abruptly. The two Voyager craft will cross this deceleration zone, called the termination shock, before they reach the heliosphere's edge.

Voyager 1 is likely to reach the termination shock, which lies 12 billion to 13.5 billion km from the sun, during the next 3 to 5 years, says Edward C. Stone, Voyager project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif. Voyager 2 is expected to follow suit about 5 years later.

Stone bases his estimates on the increasing intensity of cosmic rays detected by the craft. Cosmic rays arise from the interaction between charged particles and magnetic fields at the heliosphere's edge. An increase in this radiation indicates that the craft are approaching the boundary. In about a decade, adds Stone, Voyager 1 will leave the heliosphere and become the first known craft to enter interstellar space.

Both craft, which use 20-watt radio transmitters to relay data, have enough electric power and fuel to last until about 2020.