The Voyager 1 space craft, which was launched in 1977 to explore outer planets, has entered a new region on its way out of our solar system. It's now more than 11 billion miles (18 billion km) away from Earth and it detected "two distinct and related changes in its environment on August 25, 2012," according to a paper published in Geophysical Research Letters today and reported by Reuters earlier this week. "The probe detected dramatic changes in the levels of two types of radiation, one that stays inside the solar system, the other which comes from interstellar space."
From the paper abstract:
At the Voyager 1 spacecraft in the outer heliosphere the intensities of both anomalous cosmic rays (ACR) and galactic cosmic rays (GCR) changed suddenly and decisively on August 25th (121.7 AU from the Sun). Within a matter of a few days, the intensity of 1.9-2.7 MeV protons and helium nuclei had decreased to less than 0.1 of their previous value and eventually the intensities decreased by factors of at least 300-500. Also on August 25th the GCR protons, helium and electrons increased suddenly in just 2 or 3 days by factors of up to two. The intensities of the GCR nuclei of all energies from 2 to 400 MeV then remained essentially constant with intensity levels and spectra that may represent the local GCR. The suddenness of these intensity changes indicate that V1 has crossed a well-defined boundary for energetic particles at this time possibly related to the heliopause.
A statement from Edward Stone, Voyager project scientist at Caltech, responding to reports that Voyager 1 has left the solar system:
It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space. In December 2012, the Voyager science team reported that Voyager 1 is within a new region called 'the magnetic highway' where energetic particles changed dramatically. A change in the direction of the magnetic field is the last critical indicator of reaching interstellar space and that change of direction has not yet been observed.
And from NASA:
The Voyager spacecraft were built and continue to be operated by NASA's Jet Propulsion Laboratory, in Pasadena, Calif. Caltech manages JPL for NASA. The Voyager missions are a part of NASA's Heliophysics System Observatory, sponsored by the Heliophysics Division of the Science Mission Directorate at NASA Headquarters in Washington.More on Voyager at the NASA mission home page.