The Juno Spacecraft Reaches Jupiter

NASA has a habit of scheduling high-stakes maneuvers to coincide with patriotic holidays. On July 4, 1997, at lunchtime, the Mars Pathfinder mission successfully landed Sojourner, the first Martian rover. Eleven years ago, on the same date, the Deep Impact probe successfully collided with the comet Tempel 1, an achievement that the astronomer Iwan Williams described as being “like a mosquito hitting a 747.” And on Monday, at just before 9 P.M. Pacific Standard Time, after a thirty-five-minute engine burn, NASA’s Juno spacecraft successfully reached the end of its five-year journey and began circling Jupiter. For the engineers at the Jet Propulsion Laboratory, in Pasadena, California, it was not only Independence Day but also Orbital Insertion Day. As the operations team confirmed that the burn had gone to plan, all of mission control rose to its feet. To a standing ovation, a visibly relieved Scott Bolton, the mission’s principal investigator, said, “We just did the hardest thing NASA’s ever done.”

“This was a one-shot deal,” Bolton had said earlier. Not only was Juno moving faster than any human-made object ever has when its engine fired—more than a hundred and sixty-five thousand miles per hour—but its onboard electronics were operating in what J.P.L.’s Heidi Becker described as “a spray of radiation bullets”: charged electrons spiralling up and down the lines of Jupiter’s magnetic field at almost the speed of light. It was the mission’s riskiest moment since launch.

“Everything about Jupiter is the most extreme,” Bolton said. His colleague Rick Nybakker, the project manager for Juno, agreed. “It’s the most treacherous environment in the solar system after the sun,” he said. When NASA’s previous mission to Jupiter, Galileo, arrived at the planet, in 1995, its instruments were severely disrupted and, in some cases, irreparably damaged by radiation. This, combined with the partial failure of a key antenna, meant that Galileo was able to complete only seventy per cent of the scientific portion of its mission, leaving many questions about Jupiter unanswered.

NASA’s response to these challenges, Bolton said, was to “put together an armored tank.” Juno is a truly massive spacecraft, about the size of a professional basketball court and shaped something like the radiation-warning trefoil, with a hexagonal cog sitting at the junction of three arms covered in solar panels. (Compare that with NASA’s New Horizons spacecraft, which arrived at Pluto last summer and is only a little larger than a concert grand piano.) Juno’s command and data-handling electronics are housed in a central titanium vault, no bigger than the trunk of an S.U.V. Empty, the vault weighs five hundred pounds, and when full it is designed to reduce the radiation exposure of whatever’s inside by a factor of eight hundred. Loitering outside the vault for the duration of Juno’s mission, Becker said, would be like undergoing more than a hundred million dental X-rays, several hundred thousand times a human’s recommended lifetime radiation dose.

Despite Juno’s shielding, the team had anticipated that, as Monday’s insertion maneuver played out, radiation might cause the spacecraft’s guidance computer to hiccup and reset, stopping the engine and bungling the whole operation. Because radio signals take forty-eight minutes to travel the nearly five hundred and forty million miles between Earth and Juno, by the time engineers at J.P.L. found out about the shutdown, the window of opportunity for entering Jupiter’s orbit would already have closed. “We would not have ended up in a very exciting spot,” Nybakker said. As a result, they created a special override command, just for this portion of the mission, which would force the engine to re-start after only a few minutes. That safety net turned out not to be necessary, but in preparation for Juno’s closest approach to Jupiter, on August 27th, the team has programmed in another automatic re-start, this time of one of the spacecraft’s essential instruments: the magnetometer. Capturing a magnetic map of Jupiter is, according to Bolton, the mission’s most challenging science goal. The re-start plan insures that even if NASA temporarily loses data from all of Juno’s other instruments the magnetometer will recover in time to perform its function.

The result, Bolton and his team hope, will be the first accurate, detailed three-dimensional survey of Jupiter’s powerful magnetic field. The gas giant’s magnetosphere is nearly twenty thousand times more powerful than Earth’s, resulting in spectacular polar auroras larger than our entire planet. And its magnetic field extends so far into space that, if it glowed in visible light, Jupiter would appear to be twice the size of the full moon in our night sky. While Galileo captured stunning surface images of swirling ammoniac storms, Juno’s mission is to peek beneath the gaseous veil to see inside Jupiter. Scientists believe that the planet’s intense magnetism is generated by the churning of a deep interior ocean of hydrogen, under such intense pressure that the element becomes a shimmery liquid. (“Like a mirror, it reflects light, so if you were immersed in it, you wouldn’t be able to see anything,” Caltech’s David Stevenson explained in 2011, on the one-year anniversary of Juno’s launch.) A detailed map of the resulting field would offer researchers the clues they need to understand this hidden ocean’s depth and currents.

Right now, no one knows exactly what Jupiter’s internal structure looks like—not even whether it has a rocky core, like the rest of the planets in the solar system, or a gaseous one, like the sun. The scientists’ method for unravelling this question is particularly ingenious. It requires tracking minute variations in the radio transmissions between Juno and NASA’s system of antennas on Earth, the Deep Space Network. For the first time, the agency will point all nine antennas at its facilities in the Mojave Desert and Canberra, Australia, in the same direction: toward Juno. Tiny accelerations and decelerations in the spacecraft’s velocity as it is manhandled by Jupiter’s gravity will leave their mark on the frequency of its radio transmissions, in the same way that the Doppler-shifted honk of a car tells you whether it is moving closer or farther away from you. If Jupiter has a rocky core, Juno will speed up a little earlier than it otherwise would on its journey from the pole to the equator, and the resulting Doppler effect will make its radio signal higher pitched.

Meanwhile, by flying to within three thousand miles of Jupiter’s cloud tops, Juno will be able to measure the microwave radiation that escapes through the planet’s thick atmospheric haze, sniffing out things like how much water it contains and how deep the famous (and shrinking) Great Red Spot actually goes. The goal is to settle the question of how the planet formed, and, along with it, the question of how other worlds came to be. “What we really want is the recipe: how do you make the planets in the solar system?” Bolton said. Jupiter was the very first one to form. It consumed most of the helium and the hydrogen that was left over from the birth of the sun, but, based on earlier observations, we know that Jupiter also contains some elements, such as carbon and nitrogen, that are the basis for life on Earth. “Something happened between the time the sun formed and the time Jupiter formed that allowed it to be enriched in these heavy elements,” Bolton said. “But we don’t yet know what.” In the coming years, as scientists sift through the Juno data, they hope to resolve this mystery—and thus understand the early processes that led to the creation of the terrestrial planets, and, eventually, to us.

By this point, NASA is aware that a lofty goal, without visual imagery to accompany it, will not engage the public’s imagination. The agency’s solution is JunoCam, a device adapted from previous Mars missions. Freed from the burden of scientific responsibility, amateurs and enthusiasts will be able to vote online to determine where JunoCam points and which features it captures. “It will provide the very first views of Jupiter’s poles, and the most incredible closeup views of the planet ever seen,” Bolton said. “I expect we will probably discover some new moons, too. But it’s really a public camera.” Already, JunoCam’s discussion boards are alight: six-year-old Bee wants more photos of the Red Spot, while Hogarth-11 is voting for a closer look at greenish dots near Jupiter’s equator.

Juno will gather data for the next six months, although its first science orbit doesn’t begin until November 2nd, after another engine burn. As the spacecraft loops the planet every fourteen days, NASA expects radiation to start degrading its instruments: the mission is designed to end before the onboard computer becomes too unreliable. Some of Jupiter’s moons may be capable of harboring life, and NASA does not want to contaminate them with stowaways from Earth. So, after its thirty-seventh orbit, in February of 2018, Juno will fire its engines for the final time, taking aim straight at Jupiter and insuring its own fiery disintegration. It will continue capturing data even as it plunges to its certain end. “JunoCam could get a great image from under the clouds,” Bolton said. But, he added, as the spacecraft is hurled about in Jupiter’s turbulent atmosphere, its antenna will likely be wobbling too much to send that photo home. The mysterious giant of our solar system will retain some of its secrets yet.


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