A key goal of planetary science has been to understand the formation, evolution, and internal structure of the gas giants. Galileo and Cassini have studied the outer skin of Jupiter and Saturn, but the data returned has not provided answers to key questions on these issues. The Galileo atmospheric probe was intended to address many of the compositional questions (which relate to key questions of formation and evolution), but it was skunked by entering in a hot spot that both lacked clouds and water vapor.
In the last few years, plans have been made to address these questions with new and extended missions. The Juno orbiter to Jupiter will address all three questions by probing the composition and structure of the atmosphere and will study the deep interior through precise gravity and magnetic measurements. Its close orbit -- cloud skimming on the scale of the Jovian system -- enables these studies. The final stages of the extended Cassini mission will bring it equally close to Saturn for precise gravity and magnetic measurements. (Alas, Cassini lacks Juno's microwave radiometer for deep probing of the atmospheric structure and composition.)
Juno and Cassini will not be able to address all the key composition questions. Those lines of inquiry require the precise measurements that can only be made from within the atmosphere. This is considered so important that the original concept for what became Juno had both the orbiter and new atmospheric probes for Jupiter. Unfortunately, the technology to build and test heat shields for the extreme heating encountered in a Jupiter entry has been lost.
Fortunately, the challenges for entering Saturn's atmosphere are much less severe, and scientists are proposing an atmospheric entry mission for that planet. While Saturn is not identical to Jupiter, it is similar enough that measurements made there will answer questions relevant to both planets. (Even if new Jovian probes were programmatically possible, scientists would want probes for Saturn to compare the two worlds.)
Two presentations at a recent Outer Planets Panel for the Decadal Survey addressed the science rational and possible approaches for a Saturn probe mission. A fundamental challenge of the mission is that reaching the depths where water vapor will be present (a key measurement) requires operation to depths of 50 - 100 bars. (A bar is the pressure of Earth's atmosphere at sea level.) Not only does this require a sturdy pressure shell, it is difficult to maintain adequate communication rates with a relay spacecraft, and the battery would have to be large to provide power for an extended descent.
Several solutions to the problem of studying the deep atmosphere were proposed:
- Have the probe piggyback a microwave radiometer that would measure water abundances prior to entry from just above the atmosphere as Juno will do for Jupiter. The radiometer would be jettisoned just before entry. Alternatively, the carrier craft could carry the radiometer, although that would require a flyby or orbit that passes just above the atmosphere.
- Have a two stage probe where the larger, more instrument laden probe falls slowly on a parachute while a smaller probe with just an instrument or two falls quickly to the necessary depth
- Forgo the deep measurements and focus on multiple shallow probes to study several locations in Saturn's atmosphere
While no cost estimates were provided, one of the presentations states that a probe mission "may exceed" the cost of a New Frontiers (~$650M) mission and recommends a new class of $1.2-1.5B missions. (This sounds like a strong hint that a probe mission is likely to be closer to $1B than $650M).
Editorial Thoughts: In-depth (literally) exploration of the gas planets likely will be a continuing priority for decades. After Juno, the Cassini end of mission, and an eventual probe mission to Saturn, the science community is prioritizing missions in the same class to Uranus and Neptune.
In theory, a Saturn probe does not require a dedicated mission. Any spacecraft traveling to or passing by Saturn could drop off a probe. It's possible that Saturn may be a busy place in the coming decade with the proposed Argo mission passing through on its way to Neptune and the Kuiper belt, a possible Titan lake lander, and a small orbiter to continue the exploration of Enceladus and possibly Titan. Celestial mechanics may make piggy backing difficult in some of these cases. The Argo craft, for example, would need to thread a narrow path to get the gravity boost, and that path may not allow a probe delivery and relay. Even if celestial mechanics cooperate, carrying the extra weight of the probe and the communications relay equipment will add costs to a mission.
I personally would like to see ~$2B budget to continue the exploration of the Saturn system in the coming decade with a Titan lake lander, a small orbiter, and an atmospheric probe or two (in that order of priority). With approximately $7B of the coming decade's expected $12B budget for planetary missions, committing these funds would mean many other planetary targets would not be explored. (I'd also like to see $2B dedicated to Venus studies and $1B or so to small bodies studies...) It will be interesting to see where Saturn falls in the Decadal Survey's priorities.
Presentations to the Outer Planets panel (images in this post taken from the first presentation)