A postcard from Voyager 1, interstellar traveler

You can’t spot Voyager 1 as it leaves the solar system’s heliosphere, but the National Radio Astronomy Observatory can, and did it just for kicks.

Well maybe not for kicks, but scientists were interested in how well their array of 10 radio telescopes, stretching from Hawaii to St. Croix, Virgin Islands, could plot the spacecraft, which is more than 11 billion miles from Earth. Usually, those instruments home in on the faint radio signals from quasars, black holes and the like. They also routinely track NASA’s Cassini spacecraft, out around Saturn.

But in February and again in June, scientists trained the instruments on Voyager 1. At the time, scientists were hotly debating whether the craft, launched in 1977, had burst through the solar winds enveloping our planets and sun.

“It was mainly done to see what the instruments can do,” said Walter Brisken, a scientist at the radio observatory based in New Mexico.

At about 22 watts, Voyager 1’s main transmitter signal pulses like a police radio, or a refrigerator lightbulb if it were in the visible light spectrum. But that’s quite brilliant to the Very Long Baseline Array, and to the steerable, single-dish Green Bank Telescope in West Virginia.

“The signal is extremely strong,” Brisken said. “You can detect it in fractions of a second.”

Scientists used data from both instruments to plot Voyager on a celestial grid system that’s “almost like establishing mile markers on a highway,” Brisken explained. It’s not the kind of data that would resolve whether the craft left the heliosphere, but it did nail Voyager’s position to within fractions of an arc-second of where it had been predicted to be as it soared through space at some 38,000 miles per hour. (An arc-second is roughly the size of a penny viewed from about 2.5 miles away.)

Scientists analyzing a plasma signal from the craft announced Thursday that they believed Voyager 1 had entered interstellar space on Aug. 25, 2012. For the better part of this year, there had been substantial debate about whether the craft had passed that milestone. NASA now acknowledges that Voyager 1 exited the heliosphere.

But if you are picky about your definition of leaving the solar system -- and astronomers tend to be -- you could say Voyager 1 has as much as 30,000 years of flight ahead of it before it will pass the Oort Cloud, a distant shell that is believed to be the birthplace of many comets.

Either way, Voyager 1 has at least entered the record books, and sent back a belated postcard to boot.