NASA has formally asked planetary scientists to submit proposals for the next New Frontiers mission, a program that will provide approximately $1 billion for a spacecraft and science program to answer a fundamental question about the Solar System.

The "announcement of opportunity" posted on a government procurement site seeks proposals for six different mission concepts desired by the planetary science community. They are ambitious, ranging from bringing samples back from a comet or the Moon's south pole to visiting ocean worlds and a Venus lander that would perform a suite of experiments on that mysterious world's surface:

• Comet Surface Sample Return

• Lunar South Pole-Aitken Basin Sample Return

• Ocean Worlds (Titan and/or Enceladus)

• Saturn Probe

• Trojan Tour and Rendezvous

• Venus In Situ Explorer

These are all big-thinking, potentially amazing missions (the Planetary Society has more details about the individual missions). Unfortunately, NASA has said it can eventually pick only one. The space agency plans to narrow down select to a handful of proposals in November 2017. After reviewing more detailed concept studies, it will make a final decision around July, 2019. The agency has set a launch readiness date of "no later than" December 31, 2025.

New Frontiers missions are a step below NASA's Flagship mission program, multibillion probes such as the two Voyager, Galileo, and Cassini spacecraft, as well as the Mars Curiosity rover. Those are more ambitious in scope than NASA's Discovery class missions, however, such as the Dawn spacecraft now orbiting the dwarf planet Ceres.

Whatever mission NASA ultimately selects will have an impressive legacy to uphold. The three New Frontiers missions to date are New Horizons, the Juno spacecraft now orbiting the Jupiter system, and the recently launched OSIRIS-REx mission to return a sample from the asteroid Bennu.