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.
The mission's most challenging scientific objective is to map Jupiter's magnetic field:
Citizen science: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.
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.