The Voyager 1 spacecraft's 35th anniversary is proving to be unexpectedly exciting, as scientists gathered this week to examine new hints that the spacecraft is on the verge of leaving our solar system.
Voyager 1 is now more than 11 billion miles away from Earth. It blasted off in September 1977, on a mission to Jupiter and Saturn. But it also carried a Golden Record filled with music and the sounds of our planet, in case it encountered intelligent life as it moved out toward the stars.
Scientists have been eagerly waiting for Voyager 1 to become the first human-made object to leave the solar system. And in recent weeks, the spacecraft has sent back intriguing signs that it might be getting close, to the delight of researchers who have been working on it for decades.
One of those scientists is Norman Ness, a professor at the University of Delaware who sent NASA a proposal for a Voyager instrument back in 1969. That December, he recalls, he received a telegram from NASA saying it had been selected.
He was ecstatic, because these probes were going to visit far-off giant planets. The prospect of leaving the solar system, and exploring interstellar space, was not really on his mind. After all, the spacecraft might not live long enough. And, Ness notes, "we didn't know if NASA was going to continue to support the mission after the primary mission had been accomplished."
But NASA kept funding it. And even though Voyager's technology is primitive by today's standards — for example, each spacecraft has an on-board eight-track tape recorder — the probes just keep working.
"So we're getting anywhere from five to eight hours of data every day from each of these spacecraft, and it's quite remarkable," says Ness.
Of the two, Voyager 1 is the farthest away from Earth. Don Gurnett of the University of Iowa, who has worked on the mission since the mid-1970s, says Voyager 1's radio signals now take more than 16 hours to reach us.
"Astronomers like to think of distances in terms of 'light-years.' Well, we're not anything like a 'light-year,' but we're now a substantial fraction of a 'light-day' from the Earth," says Gurnett. "And I just find that extremely impressive."
Out Beyond The Planets
When will Voyager 1 finally leave our solar system? It's already way beyond the planets — it even looked back and snapped photos of them, including a famous one of Earth, looking like a pale blue dot. For years, scientists have been waiting for Voyager 1 to exit the bubble of charged particles that stream out from our sun, because then it would truly be in the space between stars.
Just in the past couple of months, researchers have been getting some tantalizing hints that Voyager 1 might be almost there.
Ed Stone, the Voyager project scientist at Caltech, showed off some brand new data this week during a public lecture at NASA's Jet Propulsion Laboratory. Voyager 1 has been seeing decreases in the number of lower-energy particles that come from inside our solar system, he said. Meanwhile, it's been seeing increases in the high-energy particles that come from outside our solar system.
"So there is some kind of connection between where Voyager is and the outside, which lets the particles that are inside out, and lets particles outside in," explained Stone.
This is uncharted territory, and no one really knows just what the boundary with interstellar space is like. So Stone can't say how long it will be before Voyager passes through it.
"I can't tell you whether it's days, months or years. I really can't tell you," he said. "That's the nice thing. From a science point of view, there is so much that we're learning that we had no way of really understanding before Voyager."
Still, scientists are extremely excited that they could be getting there. Robert Decker of The Johns Hopkins University Applied Physics Laboratory, who has worked on Voyager since 1980, says the rapid changes being seen now are extremely unusual, and they come after a number of years when not much seemed to be happening.
"And the way that the changes are taking place — that is, the low energy is going down and the high energy is going up — certainly is evidence to me that we're very close to the interstellar medium," says Decker, "because that is what you would expect to happen as you get into the interstellar medium."
In the weeks ahead, scientists will be looking for more indicators that Voyager 1 is finally moving out. For example, they'll analyze the direction of the magnetic field, to see if it has changed.
Voyager 1 can keep talking to Earth for about another decade. That's how long the plutonium that powers it should last. After it falls silent, it will still keep going. But it will be about 40,000 years before it wanders close to another star.