Planetary exploration is an expensive undertaking with a single spacecraft often costing $1 billion or more. Such a steep price tag nearly ended hopes of exploring the Solar System decades ago, but a new way of creating a budget was devised so that planetary exploration could continue at a steady pace. Missions are now categorized into three classes: Flagship, with a cost over $1 billion; New Frontiers, which is capped at $850 million; and Discovery with a cap of $450 million. These caps do not include the cost of the launch vehicle and mission operations. Flagship mission examples include Curiosity, Cassini, the future Mars Perseverance Rover and the Europa Clipper. New Frontiers missions include New Horizons, Juno, OSIRIS-Rex, and the future Dragonfly Titan drone. Discovery missions include MESSENGER, Dawn, and the future Lucy and Psyche missions. The time has arrived for the selection of another one or two Discovery missions as final selections occur roughly every four years. Lucy and Psyche were selected in January 2017; the next Discovery mission should be selected for development and launch in early 2021 and, if the budget allows, perhaps two missions will be selected.

Four missions from a proposal of thirteen were selected for further review on February 13 with each being awarded $3 million for further development. These include two missions to Venus and one each to Io and Triton. All are very exciting with ambitious goals:

1) DAVINCI+ (Deep Atmosphere of Venus Investigation of Noble gases, Chemistry, and Imaging, PLUS). This mission involves a carrier/flyby/orbiter probe and a deep atmosphere chemistry probe bristling with instruments that have already been flight-proven on Curiosity, Juno, and OSIRIS-Rex. The primary goals are to determine Venus’ volcanic activity, the history of its water inventory, and the potential that it may have been habitable. These goals will address key questions as to the origin of Venus’ atmosphere, how it evolved, and why it is different from Earth’s and Mars’? Was there an early ocean, and if so, where did it go? What is the rate of volcanic activity? What are the tesserae (tile-like) highlands, and what is their origin? A short-lived probe would descend through the atmosphere and sample the various gases along with temperature, pressure, and wind as it descends. It would also take several high-resolution images of the surface below as it descends, which can be constructed in 3-D. It would land on a tesserae region thought to be ancient terrain, and study the interaction of the atmosphere with the surface. The mission also includes an orbiter with a multiband imaging system that would study Venus in ultraviolet and near-infrared. Night-side infrared imaging would allow imaging of the intense heat from the surface with the possibility of detecting flowing lava from active volcanoes. DAVINCI+ is a mission that is sorely needed if there is any hope of assessing climate change on Earth and the workings of Venus-like exoplanets.

2) VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) is a proposed mission to map the surface of Venus at a higher resolution than Magellan. It would produce the first maps of global deformation and surface composition, thermal emissivity, and gravity field. It would attempt to determine if Venus had an ocean, if volcanoes are active, and if they are tightly clustered or widely scattered. VERITIS would use Interferometric Synthetic Aperture Radar (InSAR) coupled with a multispectral near-infrared emissivity mapping capability to map the surface in detail as fine as 100 feet across, and measure height to within 20 feet. The goal is to understand Venus’ geologic evolution, determine what geologic processes are currently operating, and find evidence for past and present water. VERITAS would orbit Venus for at least two years and would provide a high resolution global map of the surface temperature. There is also an option to add a nanosat probe, which would be deployed into the atmosphere with the sole goal of sampling the gases and determining their isotopes.

3) Io Volcano Observer (IVO) is a low-cost, outer planet mission to explore Jupiter’s moon, Io, to understand how/when tidal heat is generated inside Io, how it is transported to the surface, and how Io is evolving. The study of Io aids in the understanding of the thermal history of Europa and Ganymede, which can then be applied to Saturn’s moons, Titan and Enceladus, and Neptune’s moon, Triton. IVO would launch during 2026-2028 and use Mars and Earth for a gravity assist boost to Jupiter. IVO would fly past Io before going into orbit around Jupiter in 2031, and then fly past Io nine times over four years getting as close as 120 miles from the surface. IVO’s orbit around Jupiter could be optimized to study the magnetic and gravity fields surrounding Io. Erupting volcanoes would be observed in both sunlight and darkness permitting analysis of the thin atmosphere. IVO would be capable of detecting a magma ocean beneath the frozen crust if it exists. It may be possible to fly IVO through one or more of the erupting plumes for direct measurements, but great care must be taken with timing and the location of the flyby to avoid serious damage or loss of the spacecraft. The plume encounters would occur late in the mission after the primary mission concludes. This mission is the most exciting of the four as each flyby will show changes in Io visually and in infrared. Time lapse videos of the erupting plumes could show them in motion! This technique was demonstrated when New Horizons flew past Jupiter in 2007 on its way to Pluto.

4) Trident, named after Neptune’s 3-pronged spear, would be a flyby mission past Neptune and Triton much like the Voyager 2 flyby during August 1989, but with state of the art technology. The spacecraft would take advantage of a gravity assist boost from Jupiter in 2032 by launching in 2026 and would fly past Neptune and Triton on June 28, 2038. Trident would use mostly proven technology from current spacecraft already in operation such as cameras identical to New Horizons. It would fly within 200 miles of the surface, study the geysers in fine detail, analyze the composition of the plumes and the tenuous atmosphere. Trident would also carry a magnetometer to detect the possible existence of an underground ocean. This mission may be the last chance to explore Neptune and Triton for a long time until a fully dedicated, Flagship-class orbiter is launched.

These are rewarding missions at a price that fits the budget. Early next year the decision will be made on which one, or hopefully two, will finally fly. The Io Volcano Observer is this author’s favorite of the four and would be first pick. DAVINCI+ would be the second favorite as Venus is in need of serious exploration. Venus, Io, and Triton all have one thing in common, active geologic processes in the form of volcanoes/geysers. By studying them it will further our understanding of how Earth has evolved. It is truly a wonderful time to be alive to experience these worlds close up, to discover, and to wonder.