According to data received via the antennae of NASA’s Deep Space Network, the exploratory spacecraft Voyager 1, nearly 35 years into an extended mission, appears to be crossing the border -- the seemingly amorphous line known as the heliopause between the ends of the Sun's influence at the Solar System's edge and the increased encroachment of the charged cosmic particles of interstellar space.
In a release from the Jet Propulsion Laboratory at the California Institute of Technology in Pasadena, where Voyager 1's progress is monitored, project scientist Ed Stone noted: “The laws of physics say that someday Voyager will become the first human-made object to enter interstellar space, but we still do not know exactly when that someday will be. The latest data indicate that we are clearly in a new region where things are changing more quickly. It is very exciting. We are approaching the solar system’s frontier.”
That Voyager 1 had entered a new region became clear in early May, when the spacecraft's two High Energy telescopes saw a spike in the charged particles coming from beyond the Solar System. Although galactic cosmic rays had increased gradually to about 25 percent since 2009, signaling that the craft was closing in on the interstellar boundary, there was a "very rapid escalation in that part of the energy spectrum," Stone explained. "Beginning on May 7, the cosmic ray hits have increased five percent in a week and nine percent in a month."
The increased particle activity is just one of three separate indicators that NASA is studying to see if Voyager 1 has indeed found the heliopause. The second measure, also taken from the High Energy telescopes, tracks the solar wind (its limits referred to as the heliosphere), which is expected to drop off dramatically once the interstellar boundary is met. The third data set that scientists believe will be an indicator of a true crossing will be a shift in the magnetic field lines surrounding the spacecraft itself. While inside the heliosphere, the magnetic lines have entertained an east-west orientation. Interstellar space is expected to redirect the field lines along a north-south orientation.
A year ago, Tom Krimigis of Johns Hopkins University Applied Physics Laboratory in Laurel, Md., prinicipal investigator for Voyager's low-energy charged particle instrument and Cassini's magnetospheric imaging instrument, calculated that Voyager 1 had entered a "transition zone" and could cross into interstellar space at any time. The boundary was expected to be anywhere from 11 billion miles to 14 billion miles out from the Sun.
In June 2011, when the recalculations were announced, Voyager 1 had reached the 11 billion-mile limit. It now has moved over 11.1 billion miles away from the Sun.
The spacecraft's original mission had been to explore the Solar System's largest planets, Jupiter and Saturn, but saw the mission altered in 1990 by NASA when it was decided to use the vessel to further explore the limits of the Solar System itself. Along with its sister exploratory spacecraft, Voyager 2, which went on to explore Uranus and Neptune, the two make up the Voyager Interstellar Mission.
At present, both Voyagers are in the Heliosheath, which is the outermost section of the Sun's reach. Voyager 2 is about 9 billions miles out from the Sun, moving at a slightly slower pace than Voyager 1, which is traveling at about 35,000 miles per hour.
Even in 1977, the Voyager mission was seen as an interstellar undertaking. Just in case either -- or both -- of the spacecraft encountered alien life, they were equipped with a 12-inch golden record that contained 115 images from Earth along with greetings spoken in 55 different languages.
(photo credit: NASA, Wikimedia Commons)