"We're barreling down on Jupiter really quick," principal investigator Scott Bolton of the Southwest Research Institute said at a news briefing held at NASA's Jet Propulsion Lab in California on Monday. "It's been an amazing journey."
Around 1:30 p.m. Eastern, he said, Juno passed Europa – the Jovian moon that has subsurface oceans where future missions may look for signs of life. Around half an hour later, it passed Io, the innermost moon.
"In one Jupiter rotation, we'll be there," said Jim Green, director of planetary science for NASA. "What a wonderful day to celebrate. It's a milestone for our country, but also for planetary science."
Jupiter, a planet so huge that despite its gassy composition it has a mass greater than the rest of the objects in our solar system (save the sun) put together, is thought to hold some of the secrets of our own origins. Jupiter was formed from the sun's leftovers, creating a sun-like planet that almost has its own mini solar system surrounding it. The differences between Jupiter and the sun – and between Jupiter and the planets that followed it – could help scientists reverse-engineer the conditions of our early solar system.
"This is the beast we're after," Bolton said after putting a new image of Jupiter – one taken just before Juno turned its science instruments off several days ago – up on the screen. "And we're going to conquer it tonight."
Around midnight Eastern time, the spacecraft's engine will fire, slowing it down by over 1,700 feet per second. By slowing down at just the right spot — a target just a few miles wide — Juno will be set up to receive a gravitational hug that will tug it into the proper orbit.
The spacecraft's orbital path is designed to protect it from Jupiter's incredibly strong radiation. Jupiter has a magnetic field some 20,000 times as powerful as the one surrounding Earth, and at its strongest points it hosts electrons that bounce back and forth at nearly the speed of light.
"Juno is going into the scariest part of the scariest place," Heidi Becker, who leads Juno's radiation-monitoring investigation at the Jet Propulsion Laboratory, said during Monday's briefing. Its orbital path will keep it away from these danger zones most of the time, but it does have to pass through the most radioactive regions more than once during its insertion period.
When those electrons ricochet off Juno's instruments, Becker explained, they create a spray of proton "shrapnel" that could easily fry its computer systems. To protect the spacecraft during this period — and during its journey through the only slightly-less-terrifying quiet zones of Jupiter's magnetic field — its important instruments are encased in a 400-pound titanium vault.
But those radioactive particles aren't the only ones of concern. Much larger particles — pieces of dust and rock — could sabotage the mission as well.
Jupiter has a little-understood ring (like Saturn's), and scientists aren't sure how far it extends toward the planet's surface. Juno is plowing through this ring of debris with its engine nozzle cap off in preparation for Monday's burn. It would be unlikely for the spacecraft to hit debris, the mission team says, but because of how little we know about the ring's inner density, an unlucky encounter with a pesky piece of dust can't be ruled out. At a maximum speed of 165,000 miles per hour, Juno could theoretically be torn up by even a tiny speck of space dust if it were to hit the temporary breach in the spacecraft's defenses.
The engine burn will take about 35 minutes, and the sequence is fully automated — once it starts successfully, engineers on Earth can at least start to breathe a little more easily. But because it takes 48 minutes for signals from Juno to reach home via the Deep Space Network, there will be a 13-minute period between the end of the burn and our notification that it started as planned. Special tones will sound to alert the mission team of various milestones.
"I can tell you that when we receive that last tone that tells us the burn has been successful, it will be music to my ears," said Rick Nybakken, Juno's project manager. He's not too worried: The Juno computer is designed to automatically restart the orbital insertion burn if anything goes wrong.
"We need to get into orbit tonight, and I'm very confident that we will," he said. He just hopes it's the orbit they planned.
Bolton will hold his breath for a few minutes longer after that sweet tone comes.
"I won't inhale until we're sun-pointed again," he said. Juno is solar-powered – the farthest spacecraft from the sun ever to rely on its rays for energy. It receives just 1/25th of the sunlight we receive on Earth, and so its return to a sun-kissed position is crucial. The spacecraft is designed to cartwheel through orbit, and it should be facing the sun again around 12:30 a.m. on July 5.
Juno's most serious sleuthing won't start for a while. In late August, the orbiter will start to get close enough to Jupiter to "really delve in and have all its eyes and ears on," Bolton said. But, he added, measurements taken during the orbiter's approach are already being combed for useful data.