The VERITAS Venus orbiter mission, seen above in a version proposed in the prior round of the Discovery program, is one of the four finalists for the latest round of NASA's program of low-cost planetary science missions. Credit: NASA/JPL-Caltech

WASHINGTON — NASA is considering missions to Venus and two outer solar system moons as the next in its Discovery line of planetary science missions.

NASA announced Feb. 13 it selected four finalists in the next round of the Discovery program from an unspecified number of proposals submitted last summer. Each of the mission proposals will receive $3 million for what are known as Phase A concept studies to be completed in nine months. NASA will select up to two of the missions for development in 2021.

“These selected missions have the potential to transform our understanding of some of the solar system’s most active and complex worlds,” said Thomas Zurbuchen, associate administrator for science at NASA, in an agency statement about the selections.

Two of the finalists would go to Venus, a planet last visited by NASA with a dedicated mission by the Magellan orbiter in the early 1990s. The Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus, or DAVINCI+, mission includes an orbiter and a probe that would descend through the planet’s dense atmosphere to measure its composition. Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy, or VERITAS, is a Venus orbiter that would map the surface using a synthetic aperture radar and also link infrared emissions from the surface to geological features.

The other two proposed missions seek to study moons in the outer solar system. Io Volcano Observer (IVO) would perform a series of close flybys of Io, the innermost of Jupiter’s four large moons and the most volcanically active body in the solar system, to monitor that volcanic activity. Trident would make a single close flyby of Triton, Neptune’s largest moon, which has plumes erupting from its surface that could be linked to a subsurface ocean.

Discovery is NASA’s line of relatively low-cost planetary science missions, less expensive than New Frontiers or flagship-class spacecraft. Missions selected in this round of the Discovery program would have a cost cap, excluding launch and operations, of $500 million. Those missions would launch in one of two windows, one from January 2025 through December 2026 and the other from July 2028 through December 2029.

Earlier in the Discovery program, NASA selected one mission at a time, but in the previous round of the program that concluded in early 2017 decided to select two. NASA spaced out the competitions in part to address the time and expense scientists incur developing proposals, while selecting multiple missions at a time to maintain an average of one mission every 24 months.

In that last Discovery competition, versions of both DAVINCI+ (then known as DAVINCI) and VERITAS were two of five finalists. NASA, though, selected two asteroid missions, Lucy and Psyche, for development, while providing additional funding for a third, NEOCam, to support development of its instrument to search for near Earth objects (NEOs). Lucy and Psyche remain on schedule for launches in October 2021 and 2022 respectively, while NEOCam has evolved to the NEO Surveillance Mission, a directed mission that will not compete with other science missions.

Suzanne Smrekar of NASA’s Jet Propulsion Laboratory, who was the principal investigator on the earlier VERITAS mission proposal, is again leading the new VERITAS proposal. DAVINCI+ has Jim Garvin of NASA’s Goddard Space Flight Center as its principal investigator. Lori Glaze, who led the earlier DAVINCI mission proposal, is now the director of NASA’s planetary science division.

Jeff Foust writes about space policy, commercial space, and related topics for SpaceNews. He earned a Ph.D. in planetary sciences from the Massachusetts Institute of Technology and a bachelor’s degree with honors in geophysics and planetary science...