When NASA launched the Voyager spacecraft in 1977, the mission was to study the outer planets of the solar system. Now the probes are so far away, they are able to make measurements of our galaxy from an outsider's perspective.
For the first time, scientists have been able to measure a type of radiation streaming out from the Milky Way that in other galaxies has been linked to the birthplaces of young, hot stars.
There was no way to make our own galaxy's measurement of the radiation, known as Lyman-alpha, until the probes were about 40 times as far away from the sun as Earth -- any closer and the solar system's own emissions drowned out the fainter glow from the galaxy.
"The two spacecraft are moving out of the huge glow of Lyman-alpha radiation in which the solar system is bathed and, being less blinded by it, perceive the much fainter radiation that comes from the galaxy," astronomer and lead researcher Rosine Lallement, with the Paris Observatory in Meudon, France, wrote in an email to Discovery News.
"It is like beginning to see small candles within a brightly lit room," she said.
The Voyager probes are crossing boundary regions in space where the amount of gas from the sun is subsiding and the amount of gas from the galaxy is increasing.
The probes' power supplies are waning, which means many of the instruments, including a rotating platform for the spectrometer on Voyager 1, has had to be turned off, and which limits the amount of observing time, astronomer Jeffrey Linsky, with the University of Colorado in Boulder, told Discovery News.
The Voyager spacecraft generate electricity from heat emitted by the natural decay of radioactive plutonium, the same type of power system aboard Mars Science Laboratory, a NASA rover launched last week.
The Voyager probes should have enough power for communications with Earth until at least 2020.
The new data will allow scientists to test sophisticated computer models that have been developed to understand distant galaxies and perhaps aid the search for the universe's earliest stars
Lallement and colleagues' research appears in this week's Science.