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Juno’s Make-Or-Break Moment
published during a waning crescent moon.
07/01/2016

Juno

Visualization of the Juno spacecraft in orbit above Jupiter. Credit: NASA/JPL/Caltech

At 3:15 PM EDT this past Thursday, NASA’s mission controllers sent a message to their Juno spacecraft. The message, which took 48 minutes to travel to the Jupiter-bound probe, was simple: ‘You’ve made it, now it’s time to get to work.’

More specifically, the signal told the craft that it was time to begin the “orbital insertion phase” of its mission. A four day long period of pre-programed autoflight that will change Juno’s orientation and prepare it for what is arguably the most important moment in Juno’s flight plan: the orbit insertion burn. This crucial event will begin on July 4th at 11:18 pm EDT, last for 35 minutes, burn 447 kilograms of fuel, and slow Juno down just enough so that Jupiter’s gravity will trap it in an odd-looking elliptical orbit.

Visualization of Juno’s approach toward Jupiter as it delicately maneuvers through Jupiter’s radiation belts. Credit: NASA

 

Obviously, all spacecraft maneuvers, which are thought out years–to decades–in advance, are extremely precise and intended to hit a well-defined orbital target. The difference with Juno is that if it misses the target, it will not only ruin its orbital path, it could also destroy the scientific instruments on board as well.

The peril here springs from Jupiter’s powerful radiation belt—a lopsided donut-shaped region caused by a powerful magnetic field that surrounds the gas giant with rapidly moving electrons, protons, and ions. This magnetic field, scientists believe, is caused by two complementary phenomena. The first is the result of Jupiter’s composition—likely an extremely high-pressure/high-temperature form of hydrogen that is both metallic and liquid. The second is Jupiter’s fast rotation, which makes for rapid convection of that metallic material, not unlike the convection of metallic liquid iron of Earth’s core.

This magnetosphere prevents the stream of particles emitted from the sun—solar wind—from interacting directly with Jupiter’s surface. Solar wind pushes the magnetic field into a weird shape, with a smaller region on the sun-facing side and a longer tale on the far side. Within this distorted shape, there are essentially two places where the magnetosphere is much weaker—the north and south poles, where the magnetic fields emanate from. These “holes” of radiation are the target Juno is preparing to hit.

Visualization of the Juno’s path through its orbital insertion to the beginning of its science orbits. Credit: NASA

The goal for the Juno mission is to get as close to the planet’s cloudy surface as possible, in order to study the composition and makeup of Jupiter’s colorful clouds, while also passing over every part of the planet, over the course of 37 science orbits. The solution allows Juno to pass through the “hole” of lower radiation over the north pole, pass below the radiation belt on the far side of the planet to avoid the long tail, and then back out the south pole, where it passes over the shorter side of the belt on the near side. Keeping most of its orbit on the near side has the added benefit of providing constant sunlight to Juno’s solar panels.

In a JPL press release, Scott Bolton, principal investigator of the mission, said “We are not looking for trouble, we are looking for data. Problem is, at Jupiter, looking for the kind of data Juno is looking for, you have to go in the kind of neighborhoods where you could find trouble pretty quick.”

Right now, Juno’s autopilot is taking it straight toward the north pole. When it reaches the closest point of its approach, it will begin that crucial 35-minute orbital insertion burn, allowing Jupiter’s pull to sling it around back to the other side. After that, a series of smaller burns will occur over a period of 101 days to get the craft into a series of slightly shorter orbits—these are the “science orbits,” where the bulk of the data will be collected.

Juno

Artists’ concept of Jupiter’s magnetic field. Credit: NASA/JPL/Caltech

Even with Juno’s innovative orbit, though, it will not be immune to the effects of radiation. According to Rick Nybakken, Juno’s project manager from NASA’s Jet Propulsion Laboratory, the space probe “will be exposed to the equivalent of over 100 million dental X-rays” over the course of its mission. That’s why there will be a limited number of orbits. After that much radiation, the risk that the science instruments won’t be reliable is too high. At that point Juno will burn its engine one final time before falling into Jupiter’s surface, incinerating itself in the process.

Before that happens, and assuming the orbital insertion is a success, we will witness some pretty incredible science.