May 25, 2005 — Voyager 1, a spacecraft launched 28 years ago to study Jupiter and Saturn, is well on its way into a new frontier: interstellar space, scientists said Tuesday.

"We have a totally new region of space to explore," said Edward Stone, NASA's former director of the Jet Propulsion Laboratory in Pasadena, Calif., and the chief scientist of the Voyager missions.

Now moving at nearly one million miles a day, a speed equivalent to flying from New York to Los Angeles in four minutes, Voyager 1 has reached the edge of the solar system and begun crossing into the vastness of interstellar space — the first human-made object to do so.

Some researchers believed Voyager had reached this domain at the end of 2003, but subsequent data collected by the probe seemed to contradict those findings.

Now, the scientific community is in agreement that Voyager is beyond a point known as the termination shock region, where the sun's influence ends and the solar winds crash into the thin gas between the stars.

The boundary is marked by an abrupt slowdown of the solar wind, the 700,000- to 1.5-million mph flow of electrically charged particles that stream continuously from the sun. Identifying the boundary is difficult, however, because the region can expand, contract and ripple depending on changes in the speed and pressure of the solar wind and because the exact environment of interstellar space is unknown.

"Voyager's observations over the past few years show the termination shock is far more complicated than anyone thought," said NASA scientist Eric Christian.

Based on data from the probe's magnetometers, scientists now believe Voyager has passed into an area known as the heliosheath, which is beyond the termination shock zone. Voyager 1 is about 8.7 billion miles from the sun and Sistership Voyager 2 is about 6.5 billion miles away.