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Voyager Probes Aim For Interstellar Space, Four Decades Of Travel

NASA is on the brink of putting a man-made craft into interstellar space for the first time, as Voyager 1 speeds toward the outer edge of our solar system. The Voyager program's chief scientist, Dr. Ed Stone, spoke with NPR's Steve Inskeep about that feat, and what it means for NASA.

But they also talked about how the two Voyager spacecraft are still running, 34 years after their launch. And as you might expect, our two ambassadors to the galaxy are sporting the finest technology of 1977, the year they were launched.

"The computers onboard this spacecraft — it's a totally automated spacecraft — the computers have 8,000 words of memory," Stone tells Steve.

Steve reacts by saying, "That's, well, nothing" — to which Stone replies, "Yes. It's nothing."

Today's smartphones are exponentially more powerful, a fact that has been remarked upon in recent years, as processing power reached new heights in size and efficiency.

The Voyager probes send their data back to Earth via a large dish antenna, which stays pointed at their home planet.

Voyager 1 and Voyager 2 are now about 11 and 9 billion miles from the sun, respectively. In total, here's how far the the two spacecraft have traveled since their launch, as of Dec. 9:

  • Voyager 1: 14,714,066,931 miles
  • Voyager 2: 14,061,424,348 miles

"We have a 23-watt transmitter, transmitting from 11 billion miles out," Stone says.

It's worth noting that that means the Voyagers' transmitters are about 8 times stronger than, once again, a cellphone.

But despite their now-humble technology, the probes require energy to operate — and they're running out of juice. They're powered by heat from the natural decay of plutonium-238, which is translated into electricity by thermocouples.

"Very simple, robust power supply," Stone says. "The radioactive decay half-life is 88 years. And that's one reason that the spacecraft are still working. Because we have a very long-life battery, if you like.

But that battery is running down, and the Voyager craft are losing power. NASA scientists believe they'll be able to get data from the two probes until some time after 2020.

"We do know that our power level drops by 4 watts every year," Stone says. "And so, we have to systematically turn things off, one at a time. By about the year 2020, we'll have to turn off one of the science instruments. And by 2025, we'll have to have all the science instruments off. And that will be the end of the mission."

If they hang on that long, the two probes will have been in service for 48 years.

For anyone wondering about where Voyager stands in comparison to the Pioneer program, Voyager 1 passed Pioneer 10 to become the most distant man-made object in space on Feb. 17, 1998.

Copyright 2011 National Public Radio. To see more, visit

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