There are currently 24 active spacecraft exploring the solar system beyond low Earth orbit, ranging from the relatively nearby–Lunar Reconnaissance Orbiter is mapping our moon–to the far-flung. Voyager 1 has actually left the solar system and is currently 135 astronomical units from the Earth. (One astronomical unit is the average distance between the Earth and the sun; 150 million kilometers or 93 million miles.) But one is rapidly approaching a rendezvous with our largest planet, Jupiter. The Juno probe will fire its rocket engine on July 4th to place itself into orbit.
So what’s different about this mission? We’ve been to Jupiter before, more than 20 years ago, when the Galileo spacecraft explored not only the planet but its many moons as well. Remember this iconic picture of Io, the pepperoni pizza moon of Jupiter? This image is courtesy of the Galileo orbiter.
And this image of the planet itself, with Io at its side:
Somewhat less well remembered is a probe that the main spacecraft released into the Jovian atmosphere, floating down on a parachute until increasing heat and pressure caused instrument failure. Scientists had a pretty good idea of what they would find as the probe descended: successive cloud layers of ammonia, ammonium hydrosulfide, and water.
Surprise! The clouds just weren’t there—especially the water layer. What’s up? Are our models that far off? Or did the probe just happen to hit a dry and cloudless area? It’s as if we sent a probe to Earth expecting a water planet, and it landed in the Sahara Desert. How representative are our results?
This is the question that Juno is designed to answer. Jupiter emits microwave radiation from its hot interior. Water absorbs microwave energy (how your tea is heated in a microwave oven), so a microwave receiver on the spacecraft can map any water clouds below. The closer to the clouds we fly, the more clearly we can see. Doing so also allows us to peer deep inside Jupiter’s interior in ways that we can never see inside our own Earth.
But flying close to Jupiter definitely has its challenges. Jupiter’s magnetic field is by far the strongest of all the planets, and that results in intense radiation belts around its equator. These are super-sized versions of the Van Allen radiation belts around our own planet. (The International Space Station flies below these. Only the Apollo astronauts on their way to and from the moon passed through them, and then very quickly.) Juno’s orbit is polar, flying over the poles, dipping beneath the radiation belts, and moving most rapidly at its closest approach.
The orbital trajectory is shifted by the fact that Jupiter is gaseous and not perfectly round. It rotates so rapidly and is fluid enough so that it bulges at the equator.
This shifts the orbit over time so that Juno will eventually pass through the equatorial belt. Its scientific instruments are shielded inside a titanium box, but even so the intense radiation will likely kill them. It’s a tough environment in which to operate!
But while it does, Juno promises to give us new insight into the interior of Jupiter, the better to understand the origin and evolution of our home group of planets.