Jupiter-bound Juno spacecraft breaks solar power distance record

Technicians work on one of the Juno spacecraft's three solar panels before launch. Credit: NASA/JPL-Caltech/KSC
Technicians work on one of the Juno spacecraft’s three solar panels before launch. Credit: NASA/JPL-Caltech/KSC

NASA’s Juno spacecraft bound for a rendezvous with Jupiter in July has set a record as the most distant solar-powered space probe ever flown.

Juno broke the record previously held by the European Space Agency’s Rosetta spacecraft Jan. 13 as it flew about 493 million miles (793 million kilometers) from the sun, NASA said in a press release announcing the milestone.

When it enters orbit around Jupiter on July 4, the Juno spacecraft will become the first solar-powered mission to reach the gas giant, beginning 20 months of commissioning and science observations aimed at studying the structure of the planet’s interior and atmosphere.

Juno carries three large solar panels, each one 9 feet wide and 29 feet long, to generate electricity.

Shortly after the mission’s launch in 2011, the solar power system could convert sunlight into 14 kilowatts of energy. When Juno reaches Jupiter, which orbits five times farther from the sun than Earth, the spacecraft must operate on 500 watts.

Half of Juno’s electricity budget goes to its thermal system to keep the spacecraft at a comfortable temperature. The balance goes toward communications, computers, propulsion and operating the probe’s seven scientific instruments and color camera.

All eight probes that previously explored the realm of the solar system now occupied by Juno relied on nuclear power sources, but the U.S. government’s stockpile of space-grade plutonium was diminishing when engineers designed Juno, prompting a decision to build three huge solar panels for the spacecraft.

Juno’s solar panels are arranged in a triangular shape in space. All together, the panels have an area of about 635 square feet, larger than most studio apartments.

Juno’s solar panels consist of 18,698 individual cells, each measuring approximately 3.7 inches by 2.25 inches. The rectangular cells are made of silicon and gallium arsenide.

Artist's concept of the Juno spacecraft at Jupiter. Credit: NASA/JPL-Caltech
Artist’s concept of the Juno spacecraft at Jupiter. Credit: NASA/JPL-Caltech

A rocket firing will steer the Juno spacecraft into a polar orbit around Jupiter, kicking off about 20 months of science observations comprising more than 30 laps around the huge gas giant.

Juno will initially be captured in a long, egg-shaped orbit that takes 53.5 days to complete one trip around Jupiter. A follow-up maneuver in October will reduce the high point of Juno’s orbit, moving it closer to the planet to allow the craft to complete one lap every 14 days.

That is a change from Juno’s original flight plan, which called for the probe to first go into a 107-day orbit, then switch to an 11-day orbit.

Scientists say the new plan allows Juno to complete a first look at meeting the mission’s science goals, which are centered on Jupiter’s interior, atmosphere and magnetic field, in shorter time than previously anticipated. But it means Juno will need 20 months, five months longer than originally planned, to collect the mission’s full data set.

Besides the challenges imposed by the distance from the sun to Jupiter, engineers had to account for the harsh radiation environment around the giant planet.

Engineers conducted extra testing before Juno’s launch to ensure the solar cells, computers and wiring in the spacecraft’s power and control system can endure Jupiter’s doughnut-shaped radiation belts as it passes as close as 3,100 miles over the planet’s cloud tops, closer than any mission before.

Juno will encounter radiation equivalent to receiving 100 million dental X-rays during its mission at Jupiter.

Juno’s flight computers, avionics and commanding system are wrapped inside a solid titanium box on top of the spacecraft. Known as the vault, the radiation-shielding container is about the size of a microwave oven and weighs nearly 500 pounds fully loaded.

But the solar panels could not be put in the vault, so engineers doubled the thickness of the cover glass over each cell to protect them from radiation. Otherwise, Juno’s solar cells are off the shelf, meeting the same qualification levels required for other space missions.

Engineers expect the solar cells to degrade over time as they are zapped by radiation, but not within Juno’s 20-month primary mission.

Future robotic missions to Jupiter will also employ solar panels, building on technologies demonstrated by Juno.

ESA’s first spacecraft to visit the outer solar system, the Jupiter Icy Moons Explorer, set for launch in 2022 will rely on solar power for its mission to study three of the planet’s largest moons. JUICE will eventually swing into orbit around Ganymede, becoming the first mission to enter orbit around one of Jupiter’s moons.

NASA’s Europa mission, an uncrewed probe in development to repeatedly fly past Jupiter’s most famous moon, will also use solar power generators. Managers chose solar power over nuclear power for the mission in 2014.

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