NASA has chosen four potential missions that may end up being deployed as part of its Discovery Program, which invites scientists and engineers to design and develop concept studies for new missions. These exciting projects focus on objects in our Solar System currently not being explored by NASA, and while they may not move forward to official missions, they broaden NASA's possible plans and targets.

Two of the proposals plan on investigating Venus, one wants to visit Io, one of the most peculiar moons of Jupiter, and the last one wants to go back to Triton, the largest moon of Neptune. Each mission was awarded $3 million for a 9-month study of the mission's requirements, which will conclude in a Concept Study Report. Up to two missions will continue to develop towards flight after the reports are reviewed.

“These selected missions have the potential to transform our understanding of some of the Solar System’s most active and complex worlds,” Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate, said in a statement. “Exploring any one of these celestial bodies will help unlock the secrets of how it, and others like it, came to be in the cosmos.”

The two Venus missions might have the same target but they would focus on very different aspects of Earth’s evil twin. First, there is the DAVINCI+ (Deep Atmosphere Venus Investigation of Noble gases, Chemistry, and Imaging Plus) mission, whose goal will be to gain a better picture of the dense and toxic Venusian atmosphere, and even work out if the planet ever had an ocean.

The VERITAS (Venus Emissivity, Radio Science, InSAR, Topography, and Spectroscopy) mission will create detailed maps of the planet's surface and its geology. The objective is to work out why the Earth and Venus of today are so different despite being similar sizes.

The third mission that has moved to the next development stage is the Io Volcano Observer, or IVO. The backronym might not be as strong in this mission but its focus is as tantalizing as learning more about Venus. Io is the most volcanically active world in the Solar System. The heat that powers these volcanoes comes from Jupiter's incredible gravity, but so much is unknown about this active moon. The mission will perform close flybys of Io studying how the volcanism began and continues, as well as assessing the presence of a magma ocean beneath the crust.

Last but certainly not least is TRIDENT, a mission that would go back to the edge of the Solar System, to the largest moon of Neptune: Triton. Researchers believe that this world used to be a Kuiper Belt object like Pluto and by getting too close to Neptune ended up being captured. Everything we know about this moon so far comes from the Voyager 2 probe, which is now in interstellar space.

It is one of the coldest natural places in the Solar System but surprisingly has geysers and the second youngest surface of any other world. What’s it secret? Complex geology? A subsurface ocean? TRIDENT hopes to find out.

So, we have nine months to find out which possible two will go forward to the next stage of development and become a reality as a future NASA mission.