For scientists and Jupiter groupies (like myself), the real Fourth of July finale took place a little after the official fireworks display. On Monday at 8:53 pm PST, a roomful of NASA scientists at the Jet Propulsion Lab in Pasadena, California exploded into cheers after NASA’s Juno spacecraft successfully entered orbit around Jupiter. The triumphant entry was a long time coming: we've been waiting nearly five years for the next chance to get up close and personal with the largest planet in our solar system.
Juno is the ninth spacecraft to see Jupiter up close, but only the second to ever go into orbit around it. The first was Galileo, which orbited Jupiter from 1995 to 2003. Since then we've made some great observations thanks to Cassini and New Horizons—both of which had Jupiter fly-bys—but Juno promises to provide the most intimate peek into the far-off Jovian system yet.
Juno’s primary science goals are to study Jupiter’s atmosphere and magnetosphere, and to probe its elusive interior to better understand how the gas giant originally formed. One of the biggest questions it hopes to answer: does Jupiter have a core, and if so, what is it made of? It's no coincidence that the probe is named after the wife of the Roman god Jupiter (known to the Greeks as Hera and Zeus, respectively). Juno the goddess could see through the clouds Jupiter draped around himself to keep her from discovering his mischievousness. NASA’s Juno, meanwhile, is equipped with instruments that are designed to penetrate Jupiter’s thick cloud layers and reveal the world underneath.
Launched in August of 2011, the Juno spacecraft travelled a total of 1,740 million miles from Earth to Jupiter, looping around the sun one and half times en route and getting a final gravitational assist from Earth in October 2013. Now, nearly five years later, it has oficially reached its final destination. At the time of its arrival, Juno was flying through the solar system at over 150,000 miles per hour—making it one of the fastest man-made objects ever.
Slowing down a spacecraft enough to drop a precise orbit around Jupiter is no small task. Jupiter orbital insertion (JOI) required Juno to execute a series of near-perfect autonomous maneuvers over a three-hour period. First the spacecraft rotated into position. Then it fired its main engine for 35 minutes, decreasing its speed by over 1,200 miles per hour and allowing it to be captured by Jupiter into a 53.5-day orbit.
Making things more complicated—and a lot more nerve-wracking, according to Principle Investigator Scott Bolton—was the fact that Juno had to turn away from the sun and the solar power it provides for the duration of JOI. Worse, turning away from the Sun also meant turning towards Jupiter, and more specifically, Jupiter’s ring—a hazardous source of dust particles that could have shut down Juno’s engine had it taken a direct hit.
On top of all that, Juno was operating on battery power for the majority of the process—well over an hour and a half—while everyone in mission control was holding their breath, awaiting each telltale beep from the spacecraft that meant all was well. From 6:13 pm PST to 9:16 pm PST, Juno switched all transmissions to from its high gain antenna to its medium and low gain antennas, meaning it stopped sending detailed data and instead communicated only in tones.
Some tones were at regular intervals to indicate “nominal status,” while others were at specific frequencies and durations to signal the beginning or end of programmed events. Each tone took approximately 48 minutes to travel the 540 million miles between Juno and Earth during this critical time. “When we get the tone (at the end of the 35 minute JOI burn) that will be music to my ears because it means we are exactly where we want to be,” said Rick Nybakken, project manager for Juno at JPL, at the press conference Monday morning.
In the press room, scientists and journalists alike kept watch over NASA’s Deep Space Network which visualized Juno’s transmissions to NASA’s Goldstone antenna located in the Mojave Desert, assuring us that things were going according to plan. At the post-orbital insertion press conference, Nybakken spoke again of those tones: “Tonight in tones, Juno sang to us and it was a song of perfection.”
Now that Juno has successfully performed its insertion maneuvers, it will complete two 53.5-day orbits and then transition into a 14-day orbit where it will remain until its mission ends in February 2018. During the two longer orbits, it will test out all the instruments aboard Juno before they go into official science mode for the remainder of the mission.
After zooming directly at the planetary giant, Juno has now swung around Jupiter into a polar orbit and is moving away from it. Around 50 days from now, it will begin another close approach, which is when the first detailed images should start rolling in. “Our official science collection phase begins in October, but we’ve figured out a way to collect data a lot earlier than that,” said Bolton. “Which when you’re talking about the single biggest planetary body in the solar system is a really good thing. There is a lot to see and do here.”
Juno is an exciting mission of firsts. It is the farthest solar-powered spacecraft sent from Earth, and the first to operate in the outer solar system (the others have all been nuclear-powered). At Jupiter’s distance from the sun, Juno’s solar arrays only get 1/25th the sunlight they would receive in Earth orbit. To compensate for this, each of the spacecraft’s three solar arrays is 24 square feet in area, giving Juno a “wingspan” of more than 65 feet and a footprint close to the size of a basketball court.
Juno is also the first mission designed to survive and operate in the heart of Jupiter’s radiation belts, which are best described as Earth’s Van Allen belts on steroids. During each orbit, Juno will pass through the strongest radiation zones not once but twice, crossing inside the magnetosphere to get the data it needs. To enable the spacecraft and its sensitive instruments to survive this harsh environment, Juno is the first mission to house its instruments in a titanium radiation vault. Without this essential shielding, Juno would receive “the radiation equivalent of 100 million dental x-rays each year,” in the words of Heidi Becker, Juno Radiation Monitoring Investigation Lead.
Even with the titanium vault “the highest energy electrons will penetrate the (barrier), creating a spray of secondary photons and particles,” Becker explained. “The constant bombardment will break the atomic bonds in Juno’s electronics”—hence Juno’s ultimately limited mission lifetime. But for now, scientists are enjoying the dawn of Juno’s residence around Jupiter, as we take another step down the path Galileo Galilei started us on more than 400 years ago.