Science

NASA’s Voyager Twin Probes Are Nearly 45 Years Old and Face Tough Decisions

On August 20, 1977, NASA’s Voyager 2 spacecraft was launched into space. Its twin Voyager 1 was launched 16 days later. Not only are they the most distant man-made objects today—at 12 billion and 14.5 billion miles (19.3 billion and 23.3 billion kilometers) from Earth, respectively—but also NASA’s longest-running mission, continuing to send data from its interstellar travels to edge of the solar system, approaching its 45th anniversary.

But each Voyager spacecraft is powered by a limited source of nuclear power, and both sources are depleted to dangerously low levels. Each spacecraft carries a stockpile of the radioactive isotope plutonium-238; when the isotope decays, it releases energy, which is converted into electricity by three radioisotope thermoelectric generators (RTGs). At launch, RTGs provided each spacecraft with 450 watts of power. Now they produce less than half that amount, and their electrical output is reduced by four watts every year.

“It takes about 200 watts to run a transmitter on a spacecraft to be able to send signals back to Earth, and we are currently at a power level where we only have about five to six watts of power headroom on each of them. spacecraft,” Suzanne Dodd, project manager for the Voyager interstellar mission, who is also director of the Interplanetary Network Office at NASA’s Jet Propulsion Laboratory (JPL) in California, told Space.com.

Related: 40 photos from NASA’s epic ‘Grand Tour’ mission

However, the show is not over yet. The Voyager crews strategically saved power by turning off certain subsystems on both spacecraft, such as certain heaters, while allowing others to run longer.

Miraculously, the remaining scientific instruments still work well even at low temperatures. “It’s great that we’re getting data far beyond what we thought we could handle the instruments cold,” Dodd said.

Between the launch anniversaries of each spacecraft this summer, the Voyager instrument teams will hold a meeting to discuss their recent findings. The combined data becomes the basis for a new model that will guide future plans for both spacecraft, including any instrument shutdowns. Dodd estimates that the spacecraft could operate with conservation methods for another five years if it were solely powered by energy. “And if we’re really lucky, maybe with some operations below some thresholds, we can get out into the 2030s,” she said.

But the advanced age of the spacecraft creates another problem: general hardware and software failure. For example, just a few months ago, Voyager 1’s Articulation and Attitude Control System (AACS), which orients the spacecraft and its communications antenna, began sending incorrect telemetry data back to Earth. However, mission personnel know that AACS is actually working fine because the signal from its antenna remains stable – it just seems to be confused about its location.

“We can control the spacecraft, and the spacecraft can send science data to us, so it really works as usual,” Dodd said. “It’s just that this one computer system can’t tell us that it’s running normally.”

Such anomalies are expected to appear as both Voyagers age, and a failure could eventually result in the loss of the spacecraft, even if it still has power. But no matter how long Voyager 1 and Voyager 2 have been working, the Voyager mission has already been a huge success. The original mission was to fly around the solar system’s gaseous planets and their moons and transmit data to Earth, tasks both spacecraft had accomplished by 1989.

Then the secondary interstellar mission Voyager was initiated, in which the Voyager spacecraft was tasked with obtaining information about the mysteries of space beyond the influence of the Sun. “How does everything change as you move further and further away from the sun?” Dodd said. “And how does the interaction of the magnetic field with interstellar space change as we move further and further away? How does the density of the plasma change as we get further and further away?

There is only one way to answer these questions, she noted. “The key here is to keep the spacecraft running and returning data for as long as possible.”

Follow Stefanie Waldek on Twitter @StefanieWaldek. Follow us on Twitter @Spacedotcom and on Facebook.

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