Voyager 1—one of two sibling NASA spacecraft launched 44 years ago and now the most distant human-made object in space—still works and zooms toward infinity.
The craft has long since zipped past the edge of the solar
system through the heliopause—the solar system's border with interstellar
space—into the interstellar medium. Now, its instruments have detected the
constant drone of interstellar gas (plasma waves), according to Cornell
University-led research published in Nature Astronomy.
Examining data slowly sent back from more than 14 billion
miles away, Stella Koch Ocker, a Cornell doctoral student in astronomy, has
uncovered the emission. "It's very faint and monotone, because it is in a
narrow frequency bandwidth," Ocker said. "We're detecting the faint,
persistent hum of interstellar gas."
This work allows scientists to understand how the
interstellar medium interacts with the solar wind, Ocker said, and how the
protective bubble of the solar system's heliosphere is shaped and modified by
the interstellar environment.
Launched in September 1977, the Voyager 1 spacecraft flew by
Jupiter in 1979 and then Saturn in late 1980. Travelling at about 38,000 mph,
Voyager 1 crossed the heliopause in August 2012.
After entering interstellar space, the spacecraft's Plasma
Wave System detected perturbations in the gas. But, in between those
eruptions—caused by our own roiling sun—researchers have uncovered a steady,
persistent signature produced by the tenuous near-vacuum of space.
"The interstellar medium is like a quiet or gentle
rain," said senior author James Cordes, the George Feldstein Professor of
Astronomy. "In the case of a solar outburst, it's like detecting a
lightning burst in a thunderstorm and then it's back to a gentle rain."
Ocker believes there is more low-level activity in the
interstellar gas than scientists had previously thought, which allows
researchers to track the spatial distribution of plasma—that is, when it's not
being perturbed by solar flares.
Cornell research scientist Shami Chatterjee explained how
continuous tracking of the density of interstellar space is important.
"We've never had a chance to evaluate it. Now we know we don't need a
fortuitous event related to the sun to measure interstellar plasma,"
Chatterjee said. "Regardless of what the sun is doing, Voyager is sending
back detail. The craft is saying, 'Here's the density I'm swimming through
right now. And here it is now. And here it is now. And here it is now.' Voyager
is quite distant and will be doing this continuously."
Voyager 1 left Earth carrying a Golden Record created by a
committee chaired by the late Cornell professor Carl Sagan, as well as
mid-1970s technology. To send a signal to Earth, it took 22 watts, according to
NASA's Jet Propulsion Laboratory. The craft has almost 70 kilobytes of computer
memory and—at the beginning of the mission—a data rate of 21 kilobits per
second.
Due to the 14-billion-mile distance, the communication rate
has since slowed to 160-bits-per-second, or about half a 300-baud rate.