gravity lord, Jupiter.
There is much to learn about Jupiter's internal composition. For example, we don't actually have a very good idea of the water content of its atmosphere. When the Galileo mission dropped its probe into the abyssal gloom of the Jovian clouds for a 7 hour long plunge (for which only the first hour maintained communication, as expected) it found things much drier than anyone had suspected. Although this data barely penetrated 0.3% of the way into Jupiter, the absence of much water was and still is a bit of a puzzle. It is quite likely that the probe simply entered a region with few water vapor clouds, perhaps a product of severe downdrafts in the atmosphere, but it's critically important to understand on a global scale. In essence the water content of Jupiter offers a probe of the formation pathway for the planet. Jupiter should have formed with the same kind of oxygen to hydrogen (and hence water) ratio as was in the proto-planetary disk of material surrounding our young Sun 4.5 Gyr ago. A real deviation from this would indicate either some mechanism of sequestration or something funny about Jupiter's formation history.
Then there is Jupiter's powerful magnetic field. Some 20,000 time stronger at the poles than Earth's own field it profoundly effects the environment within the entire jovian system. Intense particle radiation rains down on moons like Europa, influencing surface chemistry and appearance. Within the polar regions of Jupiter itself then the great aurora make Jupiter glow in the ultraviolet and pump out radio emission. Understanding the geodynamo inside Jupiter as well as its external manifestations are high up on the list for exoplanetary science.
Another visit to Jupiter is long overdue. Quick flybys by missions like New Horizons, on its way to Pluto and the Kuiper Belt, offer tantalizing glimpses of the system but we want to get up close again.
Sitting in a clean room in Denver is humanity's next voyager to the gas giant. Juno - the Roman goddess, wife to Jupiter, among other dodgy attributes - is due to launch in August this year. It's a terrific example of a well focused mission with some very specific goals. After a 5 year journey it will enter into an elliptical polar orbit around Jupiter, complete about 32 of these orbits and then be sent to a gaseous doom within the atmosphere. While it flies from pole to pole it will probe Jupiter with a suite of instruments that include a microwave radiometer to peer deep into the upper atmosphere, optical and infrared imagers, an ultraviolet imager/spectrograph, particle detectors, and a magnetometer. By carefully observing Doppler effects the entire spacecraft will also serve as a gravitational plumb-bob - feeling out the internal mass distribution of the giant planet. To protect against the intense radiation at Jupiter then Juno also hides it's electronics away in a shielded vault - a first for this kind of mission and likely a useful pathfinder for later attempts to explore this environment.
Remarkably this is all being done using solar power. Juno is the first deep-space probe to manage this - sunlight is a good 30 times fainter out around Jupiter than it is at the Earth. Advances in solar cell technology aren't just for humans and Juno's three panels give the craft a great three-winged shape, and total span of over 60 feet.
There's a good chance that Juno will reveal much to us about Jupiter's deep interior structure and atmospheric dynamics. This will serve as an extremely important datum for our models of gas giant planets in general. While every new planet found around a distant star is something to celebrate, our own planetary system has a vast amount to still teach us, and Juno is likely to unearth some surprises.