The orbiter was traveling some 125,000 mph as it closed in on the king of the planets. It was aiming for an area of space just a few miles wide, trying to hit that target within the span of a few seconds. If it missed, the probe might have zipped right past Jupiter, burned up in the gas giant's atmosphere or set itself on an orbital trajectory where the planet's intense radiation would destroy the spacecraft's instruments.
Scientists at NASA's Jet Propulsion Laboratory in Pasadena, Calif., sat crowded around computer screens, anxiously waiting to hear a few musical tones. The spacecraft's instruments had mostly been shut off in preparation for the engine burn that would slow it down enough for a close encounter with Jupiter. Juno was programmed to send home a simple series of 10-second signals to let mission control know it had succeeded.
At 534 million miles away from Earth, Juno communicated with its creators with an agonizing 48-minute delay. Scientists didn't even know the 35-minute burn had started as planned until 13 minutes after it finished. Just before midnight, they heard the tones they were waiting for: The engine burn had succeeded. Juno entered its orbit within one centimeter of the target, just one second later than the moment NASA had aimed for.
“NASA did it again,” principal investigator Scott Bolton of the Southwest Research Institute said during a news briefing early Tuesday. “It’s almost like a dream come true. … and now the fun begins. The science.”
Jupiter is so massive that it's heftier than everything else in the solar system (except for the sun) put together. More than 1,000 Earths could fit inside it. It's a mystery wrapped in an enigma — both its raging surface storms, which include the Great Red Spot, and its hidden core remain largely unknown to science — and learning more about the strange planet could help us understand the building blocks of life on our own planet and beyond.
More than 4 billion years ago, our solar system was a cloud of dust and gas known as a solar nebula. It spun and collapsed under the force of gravity, eventually coalescing into our golden sun. The rest of the solar nebula built up our solar system, with Jupiter as the sun's first child.
Juno isn't the first spacecraft to visit Jupiter, but it's very different from the Galileo probe that went exploring 21 years ago. The robot's crucial computer components are encased in titanium armor, a 400-pound vault like nothing ever used on a space probe before. This protective gear will allow Juno to circle Jupiter for an entire year without being fried by the planet's radiation, some of the strongest in the solar system.
During its 32 planned orbits, the probe will come within 2,700 miles of Jupiter's cloud cover. The probe's orbit is designed to keep it in areas of relatively low radiation. At the most active parts of Jupiter's magnetic field, electrons rocket up and down the planet's poles at nearly the speed of light, tearing through any matter they encounter.
Juno will sneak close enough to Jupiter's cloud cover to probe under the gas giant's roiling surface. Scientists expect to answer countless questions about the massive world; questions no one has even thought to ask yet. The answers they seek could help us understand the origin of the entire solar system, and by extension the origin of our own planet.
Launched in 2011, the $1.1 billion Juno is designed to unravel Jupiter's greatest mysteries.We know very little about the gas giant. We don't know whether the planet, which is mostly made up of hydrogen and helium gas, has a rocky core at its center. We don't know how much water is in its atmosphere, which is a crucial clue in understanding its formation. We don't know the extent of its rocky rings, a fact that could have proved disastrous if Juno had slammed into unforeseen debris. We don't fully understand its magnetic field, which is 20,000 times more powerful than the one that protects Earth from the sun's rays. Most importantly, we don't know what changed between the formation of the sun and the formation of Jupiter from what was left over, and that has critical implications for the formation of our own planet.
"One of the primary goals of Juno is to learn the recipe of solar systems," Bolton said during a news conference in June. "Jupiter holds a very unique position in figuring out that recipe because it was the first to form."
Jupiter looks remarkably like the sun. It's gaseous and incredibly radioactive, producing much more heat for itself than it gets from the sun. Scientists often say that the Jovian system is like a miniature solar system of its own — its largest moons may even be habitable. In fact, planets like Jupiter are almost indistinguishable from brown dwarves, stars often called "failed" for being too small to produce energy by way of fusion. The only real difference is that Jupiter, like other huge gas giant planets, formed in the wake of a true star. But despite its similarities to its host star, it contains traces of "heavy elements" — the ones that are only formed as stars age. These heavy elements are the stuff of life as we know it, the backbones of basic life-forms and of the planets that can support them.
"Jupiter is enriched with these heavy elements compared with the sun," Bolton said. "We don't know exactly how that happened, but we know it's really important."
The birth of our sun used up most of the building blocks available in the gas and dust cloud that once filled our region of space. Jupiter took most of the leftovers, and Earth was formed from the leftovers of the leftovers. Something happened between the time when the sun formed and the time when Jupiter formed that allowed it to be enriched with these elements. Figuring out what exactly Jupiter is made out of and how it was built could help solve the mystery.
"It is becoming increasingly clear that the formation of Jupiter was the defining event of our solar system," Yale University astrophysicist Gregory Laughlin, who isn't part of the Juno mission team, told The Washington Post. "The discovery of thousands of alien solar systems over the past two decades has shown us that Jupiter, with its large mass and its relatively distant circular orbit, is somewhat unusual. We may, in fact, owe the existence of Earth’s habitability to Jupiter’s sculpting influence on the Earth’s formation, and it is imperative to peel some of the mystery from our mute, strange, and gargantuan planetary neighbor.”
Laughlin refers to the theory that Jupiter — in all its hulking glory — once bullied several planets out of the solar system. The popular hypothesis suggests that Jupiter's incredible gravitational clout pulled another gas giant out of its orbit, leading the unstable smaller planet to tear apart or hurtle out into interstellar space. That would have made room for the rocky worlds that sit close to the sun in modern times, including Earth, and would explain why the assortment of planets in our solar system doesn't quite fit models of solar system development.
Scientists can finally take a deep breath knowing that Juno is safely in orbit, but their work is far from done. NASA won't start receiving data in earnest from Juno until late August, the next time the spacecraft makes a close sweep of Jupiter. Until then, they'll be combing through the data they managed to collect during Juno's approach, before its systems were turned off for the engine burn. With so many open questions about this kingly planet, there's no knowing when the first big surprises will make their way home.