Setting the Record Straight: The Galileo Probe Mission Was a Success!

Misperceptions regarding the operational success and science accomplishments of the Galileo Jupiter atmospheric entry probe have acquired an inertia that threatens the perceived value, risk, and consideration of future giant planet entry probe missions. These misperceptions largely arise from the entry of the Galileo probe into a 5-micron hotspot, an uncharacteristically dry and cloud-free location near Jupiter’s equator. While local hotspot conditions prevented determination of weather-dependent global properties such as the abundance of water, the probe's measurements of parameters which are not weather dependent including noble gas abundances and isotope ratios have fundamentally altered our understanding of Jupiter's formation, evolution, and internal structure. In addition to its scientific success, the operational success of the Galileo probe represented a resounding technological achievement. The Galileo probe arrived at Jupiter at the location of a particularly sunny afternoon. and it is understood that in this cloud-free region the oxygen abundance as determined from the measurement of water is not representative of the planet as a whole. Furthermore, recent results from the Juno mission have shown that the mechanisms by which large-scale and local weather conditions can alter the abundance of condensable species are not well understood. This limitation does not, however, detract from abundance measurements of species not significantly influenced by local weather conditions. For example, the Galileo probe's determination that the He/H2 ratio is significantly smaller than inferred from remote sensing observations and lower than the solar ratio indicate that helium rainout is occurring on Jupiter. The helium rainout provides an additional heat source for the interior thereby affecting the refinement of Jupiter interior models. It is inevitable that future entry probe mission concepts will be compared to the highly successful Galileo probe. The science value and overall success or failure of an atmospheric entry probe must be evaluated based on the unique and valuable science achievable from single point in-situ measurements, and should not be judged based on expectations for global in-situ science results that would require a large number of entry probes distributed around the planet. In this paper we highlight Galileo probe lessons learned relevant to the design, concept of operation, instrumentation, technologies, and scientific objectives of future giant planet entry probe missions.