When Bill McAlpine set out to test how well the electronics on NASA’s Juno spacecraft could withstand Jupiter’s intense radiation, he had to be a bit resourceful.
It turns out that particle accelerators with the properties needed to simulate Jupiter’s intense radiation belts aren’t too common. “Getting the right facility for the right job turned out to be more difficult than you would think,” says McAlpine, who is Juno's radiation control manager, based at NASA's Jet Propulsion Laboratory in Pasadena, Calif.
In the end, McAlpine's team had to scatter its tests among a few facilities around the world, including a linear accelerator at the Curie Institute in Paris, where team members pulled all-nighters on a machine that’s used to treat cancer patients during the day.
It was one of a few challenges the engineers faced as they worked to design tough spacecraft radiation protection on a tight budget. Now the team can sit back and see how it all worked out. Juno launched today from Cape Canaveral in Florida atop an Atlas V rocket, beginning a five-year journey to the gas giant.
Once Juno reaches Jupiter in 2016, the spacecraft will spend a year in orbit, collecting data on the planet's magnetic fields, gravity, and atmosphere before ending its life with a plunge into the Jovian clouds. Planetary scientists hope the data collected by the $1.1 billion mission will yield new insights into how the solar system formed 4.5 billion years ago.
But that won't happen unless the spacecraft can survive long enough to collect its measurements. Jupiter’s magnetic field, which is 20,000 times the strength of Earth’s, creates conditions that are especially hazardous to spacecraft, because it can whip large numbers of charged particles to speeds that can easily penetrate a spacecraft’s body and damage electronics.
To help minimize this damage, Juno will take a highly elliptical path around the planet, roaming far out into space then zooming in low over the planet’s poles (you can see a dizzying view of the orbit from Juno's perspective here).
Over the course of its mission, Juno will perform 33 of these close passes. Each time, every square centimeter of the spacecraft will be bombarded by as many as 100 million electrons each second. Many will have energies that far exceed those of electrons in Earth's orbit.
To protect against this high “instantaneous flux” over the course of the mission, McAlpine and his colleagues needed to ensure that Juno’s electronics could survive an exposure of up to 50,000 rads, about 50 times the radiation hardness of commercial electronics.
It's a level of radiation hardness that also exceeds the requirements for other recently launched spacecraft. The electronics aboard NASA’s Mars Reconnaissance Orbiter (MRO), for example, are rated to about 20,000 rads, McAlpine says.
Juno's budget wasn't big enough for the spacecraft electronics to be designed from scratch to be as radiation-hard as possible. So the team borrowed many of their electronics designs from MRO and other recent spacecraft. Over the course of about five years, the team analyzed each and every circuit, swapping in radiation-hard components where they could.
To enhance the radiation protection, the team also packaged many of Juno’s critical components in a 160-kg titanium “vault” (pictured above), a box with centimeter-thick walls. Those devices that could not fit in the vault, such as electronics for science instruments, star tracking, and propulsion, got custom shielding. Because there was a strict limit on the total amount of the spacecraft's mass that could be devoted to shielding, the engineers had to horse trade shield mass among the science instruments to make sure each one was protected enough to get an acceptable level of signal to noise.
Juno isn’t the first spacecraft to have to withstand Jupiter’s intense radiation environment. From 1995 to 2003, the Galileo spacecraft swept though the Jovian system on a tour of the planet and its moons.
Galileo was exposed to a bigger total dose of radiation than Juno will receive. But Juno’s radiation exposure will come in more intense bursts: the flux of electrons close to the planet is about 10 times as high as it is around Jupiter’s moon Europa, where Galileo conducted many flybys. “We penetrate all the regions of the Jovian magnetosphere, so basically we’re exposed to the highest flux that’s present in the Jupiter system,” McAlpine says.
Galileo’s electronics were more radiation-hard than Juno’s. In part that’s because it was easier to buy radiation hard components when Galileo was being designed and built, McAlpine says. He adds that Juno has made up some of the difference with shielding that's about three times as thick as the Galileo spacecraft body's.
With only a year in Jupiter orbit, it might just be enough.