"When we saw it drop, we said, 'Oh, oh, this it it.' It turns out it wasn't, but it was certainly the first time we've seen something that might have been it," said Edward Stone, a professor of physics at the California Institute of Technology in Pasadena and the chief scientist for the Voyager mission, run out of NASA's Jet Propulsion Laboratory at Caltech.
Instruments on the spacecraft measure the high-energy particles, accelerated to near-light speed by distant supernovas and black holes, that make up the cosmic rays seeping into the solar system from the interstellar region of the galaxy as well as the lower-energy particles within our solar system.
It is these measurements that help scientists determine how close to the edge of the solar system Voyager 1 is.
"The particles from inside [the solar system], they've been pretty steady for the last seven years, and then on July 28, in a matter of about 12 hours, their intensity dropped to half, and it remained at that lower level until Aug. 1," Stone said.
"That was the first time in seven years that we've seen anything like that. It was very dramatic."
Voyager 1 was launched from Earth on Sept. 5, 1977, and is now about 18 billion kilometres from Earth and 121 times as far from the sun as Earth is, the only human-made object to have travelled that far into space.
Its partner spacecraft, Voyager 2, launched Aug. 20, 1977, is about three billion kilometres behind Voyager 1.
"The latest data from Voyager 1 indicate that we are clearly in a new region where things are changing quickly," said Stone.
"This is very exciting. We are approaching the solar system's final frontier."
Jupiter was original target
Astronomers and physicists are excited to have Voyager 1 so close to the interstellar region of space because they want to learn about the cosmic rays that exist in that part of the galaxy, few of which make it into our solar system, and to understand the flow of the solar wind in that part of space, Stone said.
When the Voyagers launched, there was no way to accurately predict when they would exit the solar system, or if they would even continue to operate long enough to make the journey and send information about it back to Earth.
Voyager 1 was initially headed to Jupiter, and because of that planet's gravitational effect on its orbit, known as the sling shot effect, it picked up speed and swung over to Saturn, where it gathered even more speed.
Voyager 2 got a similar speed boost from Uranus and Neptune and is travelling about 30 degrees south of that plane.
The two spacecraft are currently in an area of the solar system referred to as the heliosheath, the outermost layer of the heliosphere.
The heliosphere is an area of charged particles originating at the sun. It can be thought of as a kind of bubble of charged particles surrounding the solar system.
The heliosheath is its outermost layer, where the solar wind is slowed by pressure created by interstellar gas, forming a kind of barrier at the edge of the solar system.
Voyager 1 and 2 have been in the heliosheath for almost eight years.
Once the two cross the edge of the heliosheath, known as the heliopause, they will be in the interstellar region of space, never to return to the solar system again.
"The two spacecraft will leave the sun behind and will orbit the centre of our galaxy basically for billions of years, along with all the stars," Stone said.
How will we know when it's gone?
Scientists have three signatures they look for when measuring whether Voyager has exited the solar system: they measure the type and amount of subatomic particles the spacecraft is encountering to determine whether they are coming from within or outside of the solar system, and they look at the nature of the magnetic field around the spacecraft.
Very few cosmic rays from the interstellar region make it into the heliosphere, so when their numbers increase dramatically and those of lower-energy solar particles drop close to zero, that will be a sign that Voyager has crossed into the interstellar space.
Scientists can identify which particles come from inside or outside the solar system in part because those from the interstellar region are composed of as much carbon as oxygen while those within the solar system are composed of only oxygen, Stone said.
They also look for a change in the magnetic field through which the spacecraft is travelling. The spiral magnetic field created by the solar wind from the sun is oriented in an east-west direction while outside the solar system, the field should have more of a north-south orientation.
"Once we actually leave the bubble, we should see a different orientation of the magnetic field than we've been seeing for the last 35 years," Stone said.
No way to predict when it will exit solar system
The Voyager mission scientists check the massive data sets sent back from the spacecraft every day and have been noticing a rise in the amount of cosmic particle activity and a drop in solar particles since about January 2009.
But the recent drop in July really got their attention, because it happened over such a short period and was so dramatic.
Nothing built on Earth has ever travelled as far as Voyager 1, and scientists say there's no way to predict when exactly the spacecraft will enter interstellar space.
"Voyager is now getting closer, but I can't tell you how close because none of our models can predict anything with this kind of scale — it's just too fine a scale to be seen in the model of the heliosphere," Stone said.
"So, we could literally cross the heliopause any day, or it could be up to a few more years, but I'll be surprised if it's much more than a few more years. It may well be a few days."
Voyager carries message from Earth
While the designers of the two Voyager spacecraft didn't know how long they would continue to relay information to Earth, they did build in a kind of time capsule intended to communicate information about our planet to any highly advanced interstellar travelers who might find them.
The Voyager message is carried on an analogue record — a 12-inch gold-plated copper disc containing sounds and music meant to give a sense of life on Earth.
"The launching of this 'bottle' into the cosmic 'ocean' says something very hopeful about life on this planet," astronomer Carl Sagan once said of the capsule.
Sagan headed the committee that chose the content for the "golden records," as they are affectionately known. They contain 115 images, as well as recordings of the sounds of surf, wind, thunder, whales and birds.
They also have music and greetings in 55 different ancient and contemporary languages.
Intended to be played at 16 and 2/3 revolutions per minute, the records come encased in a protective aluminum sleeve etched with instructions on how to play them. The package even contains a needle.
Sagan notes in Murmurs of Earth, a book about the creation of the golden records, that the Voyager probes and the messages they carry, "will be encountered and the record played only if there are advanced space-faring civilizations in interstellar space."