A spacecraft that took off from Cape Canaveral 35 years ago is continuing its journey out of the solar system, Nasa said.
The Voyager 1 probe was fired into space to observe the outer planets and the mysterious interstellar medium that lies beyond the solar system on September 5, 1977, as Elvis was topping the charts with Way Down.
Sensors on the probe recorded a dramatic fall in radiation more than 18 billion kilometres from the sun, while the intensity of galactic cosmic rays soared.
The spacecraft passed what researcher Bill Webber called the "heliocliff" on August 25 last year, said a report in the journal, Geophysical Research Letters.
"Within just a few days, the heliospheric intensity of trapped radiation decreased, and the cosmic ray intensity went up as you would expect if it exited the heliosphere," said Webber, professor of astronomy at New Mexico State University. The heliosphere is the vast region of space that is dominated by the sun and the solar wind it produces. Interstellar gas and dust that spreads throughout the Milky Way surrounds the heliosphere.
Edward Stone, a Voyager project scientist based at the California Institute of Technology, said: "It is the consensus of the … 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."
The Voyager probes - there is a twin that trails far behind Voyager 1 - have survived their journey despite relying on aged technology. Each has only 68kB of computer memory; the smallest iPod nano, at 16GB, has more than 16,384,000kB of capacity.