|Artist's impression of the Galileo mission to Jupiter.|
In a previous post, I summarised some of the stories of Voyager's Grand Tour of the outer solar system, as discussed by John Casani at the Alpbach summer school in 2012. After the Pioneer missions to Jupiter in the 1970s, NASA began to think about its next voyage to the outer solar system, and conceived the Pioneer-class Jupiter Orbiter with Probe mission, a spin stabilised platform with a despun component to allow remote sensing. At an early stage, the Galileo project was forced to move away from a Titan launch vehicle like the Voyagers, and were instructed to use the Space Shuttle as the launch system, in a move away from the expendable launch systems of the past to reusable technologies. From the first planned launch in 1982 to the final launch in 1989, Galileo was continuously delayed by problems with the shuttles launch schedule and deteriorating performance. When it finally arrived in 1995 to drop its probe into the churning atmosphere of Jupiter, it was ten years later than planned, a lesson for all future planners of outer solar system missions! To accommodate the space shuttle launch, Galileo went through several costly redesigns, including the introduction of a Probe Carrier, a Mars Flyby Module and a Centaur upper stage which never saw the light of day. Ultimately, it may have been less costly to stick with the original expendable launch vehicle!
Galileo's main hurdle to overcome was a broken antenna, which failed to unfurl in the early days of the mission, considerably limiting the amount of data returned. For example, remote sensing coverage in the infrared was restricted to small postage stamps of discrete regions if the atmosphere, rather than the global coverage that is still desired by we Jupiter scientists. In the thermal infrared, the PPR instrument featured a rotating wheel used to change the filter wavelengths, but this got stuck and meant that most of our Jupiter observations are restricted to just two wavelengths. Indeed, with the exception of the Cassini/CIRS experiment that flew by in 2000, we've never obtained thermal infrared observations of Jupiter sufficient to properly get to grips with Jovian meteorology. That's something we hope to correct with future missions.
Nevertheless, Galileo provided excellent reconnaissance of Jupiter's churning atmosphere and its diverse satellite system, whetting our appetite for a future return to the gas giant. Chief among its achievements was the in situ probe, which penetrated deep into the Jovian cloud layers to measure the composition of the planet, providing us with clues to how our solar system formed and evolved from the original protosolar nebula. Ground based observations by my colleague Glenn Orton (JPL) using the NASA Infrared Telescope Facility showed that the probe had entered a region of unique meteorology known as a hotspot, with powerful down-draughts clearing the air of its main volatile species, ammonia and water.