The US space agency has denied a claim made in a scientific study that its Voyager 1 spacecraft had left the solar system, describing the report as "premature."
Scientists are eagerly awaiting signs that the craft, which was launched in 1977 on a mission to study planets, has become the first man-made object to leave the boundaries of our solar system.
A scientific paper that purported to describe this departure appeared on the American Geophysical Union's website.
It said Voyager 1 "appears to have traveled beyond the influence of the Sun and exited the heliosphere," or the magnetic bubble of charged particles that surround the solar system.
Researcher Bill Webber, one of the article's authors, acknowledged that the actual location of the spacecraft - whether in interstellar space or just an unknown region beyond the solar system - remained a matter of debate.
"It's outside the normal heliosphere, I would say that," said Webber, professor emeritus of astronomy at New Mexico State University in Las Cruces, according to the AGU's website.
"We're in a new region. And everything we're measuring is different and exciting."
Shortly after the study appeared, Nasa spokesman Dwayne Brown told AFP the report was "premature and incorrect."
The Voyager science team reported in December 2012 the craft was in a new region called the "magnetic highway," but changes in the magnetic field to show a departure from the solar system have not yet been observed, Nasa said.
"The Voyager team is aware of reports today that Nasa's Voyager 1 has left the solar system," said Edward Stone, Voyager project scientist based at the California Institute of Technology, Pasadena, California.
"It is the consensus of the Voyager science team that Voyager 1 has not yet left the solar system or reached interstellar space," he said.
"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."
Voyager 1 and its companion Voyager 2 set off in 1977 on a mission to study planets. They have both kept going, and both are on track to leave the solar system, Nasa has said.
For months, experts have been closely watching for hints that Voyager 1 has left the solar system and most have estimated that this will happen in the next year or two.
Nasa has described Voyager 1 - now 11 billion miles (18 billion kilometers) away from the Sun - and its companion Voyager 2 as "the two most distant active representatives of humanity and its desire to explore."
The Voyager craft are both carrying gold-plated phonograph records and cartridges on which to play them.
They contain 115 images of Earth life, sounds made by whales, thunder and surf, spoken greetings in various languages and printed messages from former US president Jimmy Carter and former UN chief Kurt Waldheim.