A Milestone in Space Exploration

NASA's Juno spacecraft, part of the New Frontiers Program of medium-class missions, has officially become the most distant solar-powered probe in the history of space exploration.  The new record was set last week at 2 p.m. EST, on Wednesday, January 13, when Juno reached a distance of 793 million kilometers (493 million miles) from the Sun—easily surpassing the 792-million-kilometer (492-million-mile) record set in October 2012 by the previous champ, the European Space Agency's Rosetta spacecraft.

The new record is an important one, but it will hardly be Juno's last.

During its 16-month science mission (which will begin on July 4 of this year), Juno will achieve a furthest operating distance of 832 million kilometers (517 million miles)—a remarkable achievement made possible by the spacecraft's energy efficiency and improved solar cells, which were designed to coax the utmost power out of the sun's rapidly diminishing output of photons.

"It just seems right that the sun is helping us learn about the origin of Jupiter and the other planets that orbit it," said Scott Bolton, the mission's principal investigator, in the NASA press release.

Juno's unique, tripartite design—consisting of three 9-meter-long (30-foot) solar panels—enables the spacecraft to unfold enough square footage of solar cells that it can cast a pretty wide net, even so far away, and soak up as much of the sun's energy as is needed to supply about 500 watts.

That may not seem like much, but for a solar-powered spacecraft at Jupiter's distance from the Sun, such a surfeit of power is truly a monumental achievement, and is more than enough to accomplish Juno's primary mission.

Artist's rendition of the Juno spacecraft in orbit about its destination. Credit: NASA
Future Possibilities

So far, all space exploration beyond the Asteroid Belt has been accomplished by means of nuclear-powered energy sources (RTGs, or radioisotope thermoelectric generators).  The benefits have been enormous, in terms of the sophisticated, power-hungry instrumentation and sensors that such engines make possible; but the costs, both financial and environmental, have been just as high.

Worse, NASA's reserves of Plutonium-238, which is used to power RTGs, have become critically low in recent years.

Which is why developing alternative forms of power for deep space probes is so important; the record-breaking voyage of Juno may demonstrate that one possible answer to NASA's energy woes is not so much in the development of new power sources, but rather in the more efficient use of pre-existing technologies.

Future emissaries to the outer planets may rely on energy-efficient electronics and novel solar cell designs, rather than expensive and exotic power sources, to squeeze every last erg out of the sun's vanishing light.

And, as Juno amply demonstrates, they may be considerably cheaper.

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