No spacecraft may ever surpass Cassini in its extensive exploration of Saturn, its rings, its moons, and discoveries, and it may be hard to top Voyager’s Grand Tour of Jupiter, Saturn, Uranus, and Neptune, or seeing Pluto up close for the first time, but there are still exciting vistas to encounter. The only obstacle is the vast distance to the outer worlds of the Solar System, which makes it expensive and will takes years, if not more than a decade, to get there.
Jupiter: This huge planet will get most of the attention beyond the asteroid belt with one active mission in progress, two approved and under construction, and another being proposed. Juno is currently orbiting Jupiter collecting data and images of its stormy clouds up close and figuring out how its weather works. It is a solar powered mission that orbits Jupiter in such a way that it avoids most of the hazardous portion of Jupiter’s powerful radiation belt. Juno is expected to burn up in Jupiter’s atmosphere next February. NASA’s Europa Multiple-Flyby Mission formerly known as Europa Clipper may launch as soon as June 2022. It is expected to use the newly developed Space Launch System, which would enable it to reach Jupiter in three years instead of six. It is a solar powered spacecraft that will go into orbit around Jupiter and make up to 45 close approaches to Europa to study its surface and suspected geysers. There may also be a small lander that would sample the ice directly, but it will not be able to drill through the ice into the ocean below. The orbiter will orbit Jupiter in such a way to avoid as much radiation as possible like Juno did and hopefully gather enough information for a future lander and confirm if geysers do exist. Possibly launching at the same time will be the JUpiter ICy moons Explorer (JUICE) that will arrive at Jupiter in 2030. It will make a few close passes of Europa and Callisto, but not Io where the radiation is lethal. It will fly past Ganymede several times before finally going into orbit around it in 2033. Ganymede is slightly larger than Mercury with a diversified terrain of icy and rocky blocks, craters, and frosty polar caps. It also has a weak magnetic field, possible weak aurora, and a thin atmosphere of oxygen. Ganymede is like a planet with a lot of similar characteristics, which JUICE will study intensely from orbit and accurately determine the extent and characteristics of the ocean far beneath its crust. It is also a solar powered spacecraft and its mission will end in 2034 when it crashes into Ganymede. Often pondered and actually drafted is a dedicated Io mission where a spacecraft would go into orbit around Jupiter and make several close flybys of Io. Known as the Io Volcano Observer, it would attempt to fly through the upper portion of the volcanic plumes in hopes of directly measuring the particle size, composition, and heat flow. Powerful radiation would keep the mission short and is the main reason this mission has yet to fly, but interest is increasing. If approved it could launch in May 2021 and arrive at Jupiter in February 2026.
Saturn: The end is near for the greatest spacecraft that may have ever flown as Cassini is running out of fuel and will burn up in Saturn’s atmosphere this September 15. It will continue to fly between Saturn’s cloud tops and rings like threading a needle for these remaining final months. Since its arrival on June 30, 2004 Cassini has made several historic discoveries including methane seas on Titan and geysers on Enceladus along with awe-inspiring views of the rings, plus the historic landing on Titan of the Huygens probe on January 14, 2005 with the first and only view from the surface. With so much to explore it would seem that another spacecraft would be ready to launch, but sadly no. The Titan Mare Explorer (TiME) was a beautiful opportunity to have a spacecraft splash down onto one of Titan’s methane seas and float along like a boat but it shockingly lost out to the Mars InSight lander in 2012. There may not be another chance until the 2040s when the sun returns to the north polar region where the seas are located after a long, dark polar winter, but plans to return are under way again with new proposals as 2017 began including possibly flying drones. Better prospects may be in store for Enceladus with a mission known as the Enceladus Life Finder (ELF) that would orbit Saturn but focus on Enceladus with several close flybys and with instruments that could sample the geyser plumes and detect any signature of life, past or present. It would collect samples from the plumes using aerogel and return it to Earth. ELF could launch in December 2021 if approved and the entire mission would last about 15 years.
Uranus: There may finally be an orbiter mission to this nearly forgotten planet as the European Space Agency (ESA) has plans to launch the Uranus Pathfinder mission in 2025, if approved, with arrival in 2037. It would orbit Uranus, drop a probe in its atmosphere, and make several close flybys of its moons. There are no plans for NASA to send an orbiter soon as its priority is with Europa. Hopefully the orbiter will be named Herschel after the discoverer of Uranus, Sir William Herschel.
Neptune: Triton is the big draw with its geysers, but it is surprising that no follow-up mission has flown since Voyager 2 discovered them in August 1989. A Neptune orbiter mission had been a higher priority than Uranus due to the geysers, but the greater distance has hindered mission plans. A Cassini-class orbiter was in the works in the early 1990s that would have launched in 2002 and arrived at Neptune in 2021, but ended up too expensive. A future orbiter mission will drop a probe into Neptune’s atmosphere, make several close approaches of Triton, and maybe deploy a small lander near one of its geysers. Nothing is expected to fly until the 2030s or 2040s.
Pluto: New Horizons will be a tough act to follow, and unfortunately, nothing will follow. A golden opportunity to launch another New Horizons-class spacecraft comes during December 2028, which would make for a flyby in January 2039, but nothing is in the works. It looks like the images we have of Pluto will remain the best for decades to come.
Comets: There are no active missions and none planned. A comet hopper mission where a lander would hop across the surface of a comet lost out to the Mars InSight lander in 2012.
Kuiperoids: New Horizons will fly within a few thousand miles of 2014 MU69 on January 1, 2019. No other dedicated missions are planned for the Kuiper Belt.
Is the Golden Age of Planetary Exploration ending, or is it entering a new phase? Missions to the planets will be more complex as the simpler flybys have been done and complex orbiters, landers, and rovers will be needed to broaden and deepen the exploration. It seems that there is a focus on Mars, but a focus on the Ocean Worlds may be dawning with concentration on Europa, Enceladus, and Titan. The Solar System beckons. It is an exciting time to be alive!