Argo (NASA spacecraft)

Argo was a NASA spacecraft concept from 2009 designed to explore the outer planets and beyond. It planned flybys of Jupiter, Saturn, Neptune, and a Kuiper belt object, with special attention to Neptune and its moon Triton. This focus aimed to address questions raised by Voyager 2’s 1989 flyby and offer insights into how ice giants form and evolve.

The mission was intended to compete for NASA’s New Frontiers 4 program, which had a budget of about $650 million. It was never formally proposed, partly due to a shortage of plutonium-238 needed for its radioisotope thermoelectric generator. A favorable launch window existed from 2015 through 2019, so any future version would require redesign for different planetary alignments.

Although Argo offered a Neptune mission within New Frontiers’ budget, it would be a flyby only, limiting time at Neptune and Triton compared to an orbiter. However, a gravity assist at Neptune would allow access to a wide range of Kuiper belt objects. With flybys of Jupiter and Saturn as well, the Planetary Society compared the mission to Voyager 2.

During its giant planet flybys, Argo could have studied well over 100 moons. Beyond Neptune, it could have visited Kuiper belt objects.