Jovian Electrodynamics

Jupiter is one of the most useful planets for planning spacecraft trajectories, and it’s home to some of the most interesting science targets in the Solar System. However, it also happens to be one of the most dangerous planets for spacecraft.

Jupiter is so dangerous because of two things: its magnetic field, and the moon Io. You see, Io is extraordinarily volcanically active – the only known extraterrestrial body with active volcanism, in fact – and constantly spurts all sorts of particles out from its interior. The energy of Io’s eruptions gives some of these particles escape velocity, and they end up orbiting Jupiter. Jupiter’s immense magnetic field or solar radiation can then strip electrons off these particles, creating a torus of raging ions around the planet. It’s the same phenomenon as Earth’s Van Allen radiation belts, but without the nuclear explosions and, well, Jovian in scale.

Jovian radiation belts. Sort of. My Jupiter model didn't work, so this picture looks terrible.

Jupiter’s radiation belt kills spacecraft. The Galileo probe, a Jupiter orbiter, eventually died when it crashed into the planet, a protective measure to prevent it from contaminating Europa with Earth life in the event its electronics became too fried to control the spacecraft. (Galileo contrasts quite a bit with the Cassini mission to Saturn, which has been considerably extended from its main science mission!) The next mission to orbit Jupiter, Juno, is encased in a titanium radiation shield. And you can pretty much forget human exploration of Europa.

But it’s not just tempting to send robotic probes to Jupiter – it’s practically a necessity! Any spacecraft headed to the outer Solar System, or trying to do wild orbit maneuvers, needs a lot of delta v (that is, the capability to change velocity – in magnitude, direction, or both). The Ulysses spacecraft needed a bunch of delta v to kick its orbit waaaaaaay up over the sun. And New Horizons needed a ton of delta v to get out to Pluto and beyond. One efficient way to get delta v is to perform a planetary swingby, or flyby, maneuver – and you can get more delta v from a bigger planet. Jupiter is king of the planets and gives space vehicles a huge boost. So, every spacecraft that has ever gone into the outer Solar System has visited Jupiter.

That magnetic field, though. Jupiter has a magnetic moment 18,000 times as large as the Earth’s. That sort of magnetic field is useful, if a spacecraft has the hardware to take advantage of it. For instance, an electrodynamic tether: a long conductive filament stretching from the spacecraft, along which the spacecraft runs a current. (Currents through the rarefied plasma filling the Jovian orbit environment complete the circuit.) The spacecraft will then experience a force proportional to the cross product of the current along the tether and the local magnetic field. If you don’t like vector math, don’t worry – just remember that the force is perpendicular to both the tether and the planet’s magnetic field near the spacecraft (which, near the planet’s equator, will run approximately parallel to the planet’s spin axis). In Earth orbit, there have been several missions testing tether physics, with applications including both electrodynamic propulsion and harvesting power from the Earth’s magnetic field. Around Jupiter, these methods will be even more powerful.

If a spacecraft is in circular orbit around Jupiter and orients a tether parallel to its direction of travel, then the force will be either directly toward or directly away from the planet’s center. This means that the spacecraft can, by running a current through a tether, generate a force that has the effect of either enhancing or weakening gravity. So a robotic probe performing a Jupiter flyby could get a much bigger gravity-assist boost with a little help from a current-carrying tether. Or, if the current is high enough, the net force could repel the spacecraft from Jupiter, changing the direction of the delta v it picks up in the flyby. The capability for spacecraft to perform that kind of maneuver could open up more launch windows for outer Solar System missions.

Another idea is to use electrodynamic tether propulsion to keep a Jupiter orbiter out of the worst parts of the radiation belt, so that it can get lots of data on the Galilean moons. If the probe has a slightly inclined orbit, then it could vary the current in its tether over the course of each orbit so that the spacecraft pushes its semimajor axis in and out each period. With the right parameters, this non-Keplerian spacecraft trajectory would skirt the ring of hard radiation around Io’s orbital radius.

A trajectory that skates around the radiation belt

Perhaps the spacecraft has some leeway into how far into the radiation belt it can venture – or its orbit is just bigger than the “danger zone.” Then, it could follow a Keplerian orbit (affected only by gravity) some of the time and use the passage of the tether through Jupiter’s immense magnetic field to generate electricity. If engineers can balance the numbers, then such an exotic orbit might come out power-neutral over the course of each orbital period, giving spacecraft a free, and safe, way to explore Jupiter’s Galilean moons for a very long time.

Speaking of power harvesting: Jupiter is far enough from the Sun that spacecraft around there can’t really get all the power they need from solar panels. Dragging a conductive tether around and letting the planet’s magnetosphere drive charges along the length of the filament would be one way to overcome that challenge.

These are kinds of technologies that we can develop in Earth orbit and deploy in the outer Solar System, to take advantages of the resources out there and allow us to learn more about the things in our backyard. After all, the more we understand about the different regimes of our own Solar System, the more we understand about our origins – and about the possibilities that exist in planetary systems around other stars.

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