To understand the strategy the VOX team is following, it’s useful to look at the history of the Juno Jupiter orbiter mission. Every ten years, the planetary scientific community establishes its priorities for the exploration of the solar system in process known as the Decadal Survey. The 2003 Decadal Survey report called for a Jupiter Polar Orbiter with Probes mission. The orbiter was to study global patterns and the magnetosphere while three atmospheric probes were to examine the composition and deep structure of the atmosphere. Unfortunately, since the Galileo mission with its shallow atmospheric probe, the United States has lost the technology to build probes capable of entering Jupiter’s atmosphere. The team that proposed the Juno mission focused on the desired scientific goals and demonstrated that they could be met with just an orbiter with the right instruments. Based on that well-presented argument, the mission was selected and is now orbiting Jupiter.

The goals for a New Frontiers mission for Venus have assumed an atmospheric probe that would also conduct surface measurements once it landed. The scientific requirements NASA’s managers established for a mission appear tailored for that kind of a mission based on the goals set out by the more recent 2013 Decadal Survey report. For example, the ratio of key gases and their isotopes can only be made by a probe that enters and directly samples the atmosphere. With those measurements, other questions such as past hydrologic cycles and the existence of now lost oceans can be addressed. During its descent the probe could measure the properties of the atmosphere, its clouds, and its winds at different altitudes. Measurements of the atmosphere at the surface could address the weathering environment of the crust. Chemical analysis of the soils and rocks at the landing site could provide insight into the physics of and chemistry of the crust.

Two of the proposed New Frontiers missions would fly either one or two entry probe-landers to address these questions. (You can see summaries of these at this blog post.)

The VOX proposal takes an entirely different approach. As the abstract states, “At the time of the Decadal Survey the ability to map mineralogy from orbit and present-day radar techniques to detect active [surface] deformation were not fully appreciated. VOX leverages these methods and in-situ noble gases to answer [the key] New Frontiers science objectives.”

The VOX mission would deploy a small, simple atmospheric probe to make the measurements of the key gases and isotopes. The rest of the measurements would be made from instruments on an orbiter, much as the Juno mission uses orbital measurements to replace measurements from atmospheric probes. (The composition measurements for Jupiter that could only be made from within an atmosphere had already been made by the Galileo mission’s atmospheric probe.)

The key elements of the VOX proposal closely resemble the VERITAS mission that was a finalist for the last Discovery program mission but not selected (losing out to the Lucy and Psyche asteroid missions). The principal investigator for VERITAS and VOX are the same, Suzanne Smrekar at the Jet Propulsion laboratory. The core of the VERITAS proposal was an orbiter that would carry a modern radar instrument to remap Venus and a spectrometer to measure the surface composition and study key aspects of the atmosphere. If selected, the VERITAS mission team was considering proposing a small atmospheric probe call Cupid’s Arrow to toe-dip into the atmosphere to make key composition measurements. A version of that probe now called the Atmospheric Sample Vehicle (ASV) would be included in the VOX mission.

The VOX orbiter would use three instruments to globally study Venus:

A radar instrument would image the surface at 15- to 30-meter resolution, map elevations to refine topography measurements, and search for minute changes in surface elevations to look for evidence of current volcanic or tectonic activity. The improvements in resolution for each of these would be one to two orders of magnitude better than those made by the Magellan orbiter in the 1990s.

The orbiter’s radio signal would be used to map the gravity field at much higher resolution than the Magellan mission to study the interior structure of the planet

The Venus Emissivity Mapper (VEM) would use spectral bands in five near-infrared wavelengths where the atmosphere is transparent to map the surface composition. Measurements in additional bands would study cloud structure and the presence of water vapor in the lowermost atmosphere.