When it comes to exploring the far reaches of our solar system, robots rule. Man may have left footprints on the moon but our probes have landed on or orbited every single other planet as well as countless asteroids and comets, beaming back valuable data and amazing images. The next few years will see greater milestones, with a rover scaling a mountain on Mars and a spacecraft getting the first close-up view of Pluto. But there’s always more to be done. Eager planetary scientists would love to get new information about each body in our solar system and numerous propositions have been made, including missions to fly through the clouds of Venus or drill down to the ocean on Europa. NASA’s latest budget hit the agency’s planetary science division particularly hard, making some of the more expensive missions nigh impossible. But there’s always room for hopes and dreams. Other countries’ space agencies are picking up the pace in their own robotic exploration — with occasionally disastrous results — while newcomers like India and China are laying down the foundation for future exploration. Here, we take a look at some of the coolest missions that we would love to see happen some day. Let us know in the comments which ones we missed out on or some ideas you’d like to see in the future. Above: Lunar Network Colonizing the moon is no longer in NASA’s cards, but planetary scientists are still eager to learn everything they can about Earth’s nearest celestial neighbor. Several years ago, a team of scientists proposed the International Lunar Network to NASA. The mission would plop four nuclear-powered science stations on the moon — each lasting about six years — to listen for moonquakes caused by meteorites with seismometers 10 times more sensitive than those left by Apollo astronauts. The devices were also pegged to study lunar magnetism and monitor the surface’s heat flow, but cost estimates for that plan ran close to $1.2 billion. Instead of letting the idea of a lunar seismic network die out, other scientists dreamed up LUNETTE: a smaller network of three nodes with a cost of about $425 million to NASA. (International collaborators would have picked up the remaining tab of $375 million.) Each solar-powered node would weigh about 220 pounds and last two years. And to survive the blisteringly cold 14-day-long lunar nights, each node would conserve power and trickle it out during a hibernation mode. The scientists’ proposal was rejected in 2011 — NASA narrowed its running to three missions last year — but a similar proposal might reappear during a call for entries next year. Image: NASA

Mercury Lander Our solar system’s innermost planet is also one of the least explored. NASA currently has the MESSENGER spacecraft in orbit around Mercury, sending back snapshots of its surface. But in a few years, Mercury orbit may get a little more crowded. Due to launch in 2015, BepiColombo – a spacecraft developed by the European Space Agency (ESA) and Japanese Space Agency (JAXA) – will fly two separate orbiters around the little planet, simultaneously mapping its surface and exploring its strange magnetic field. The mission could have been even more exciting and achieved a valuable first: landing a probe on the surface of Mercury. A proposed lander would have used retrorockets to descend to a polar region, where permanent shade could have provided respite from the glaring sun. The probe would have deployed a mole to tunnel a few feet under the planet’s surface and a micro-rover that could have explored a small area away from the lander, perhaps stumbling upon the ice deposits thought to exist at the poles. Unfortunately, budget cuts scaled back the ambitions of BepiColombo’s engineers. But getting to see pictures from the surface of our solar system’s smallest planet would have been one of the most exciting developments possible. Images: 1) The BepiColombo spacecraft arriving at Mercury, where it will separate into two orbiters. ESA 2) The proposed BepiColombo lander, with mole and micro-rover. ESA

Nuclear Venus Mission The last probes to reach Venus’ hellish surface were Russia’s Vega 1 and Vega 2 landers in 1985. Each spacecraft lasted about an hour before croaking in the 840-degree-Fahrenheit heat and crushing pressure. Planetary scientists consider Venus to be an Earth gone wrong and have been itching to go back and study its surface and atmosphere ever since, but NASA has poured its reserves into studying Mars. In 2006, however, Geoffrey Landis of NASA Glenn Research Center imagined a multibillion-dollar mission to Venus involving a nuclear-powered rover, multiple atmosphere-skimming airplanes and an orbiting mothership (above). The plan has since been scaled back to just a nuclear-powered lander, and NASA is finally entertaining the idea of a Cythrerean adventure by building an Extreme Environment Test Chamber– a thick-walled device that’s uniquely suited to simulate the punishing conditions on the planet’s surface, among other environments in space. Image: Venus probe descending to the surface. NASA

Martian Flyers While Mars is probably the most heavily explored planet after our own, scientists are always looking for the next great way to see the sights on the Red Planet. One oft-dreamed-about concept would be a Mars sample return mission, which would fly a chunk of the surface back to Earth for closer study. But the much cooler concepts involve nice little tours of Mars. While rovers have extended researchers’ range of view, an ideal situation would be a vehicle that can fly hundreds of miles to examine multiple locations over the course of a mission. Over the years, many have proposed such “hopper” missions, including engineer Robert Zubrin’s Gashopper (.pdf), which would use readily available carbon dioxide from the Martian atmosphere to create a simple rocket propellant, taking the vehicle for 20-mile jaunts. Add some wings to the mix, and the vehicle could fly to spy on distant terrain. Another awesome concept is the Mars Cannon Assisted Flying Exploration (.pdf), or CAFE mission, which would fire small aircraft over specific regions such as a canyon. And one of the coolest ideas is the small flying robots seen seen in the video above, the brainchild of engineer Robert Michelson of Georgia Tech. Such a mission would deploy tiny insect-like “entomopters” from a mobile base on Mars. Rapidly flapping wings would allow the probes to fly with ease in the lighter Martian gravity, taking pictures of the landscape and potentially grabbing small samples for study. Video: NASA Institute for Advanced Concepts (NIAC) Image: J.D. Denhar et al, Mars Cannon Assisted Flying Exploration (CAFE), Concepts and Approaches for Mars Exploration (2012)

Exploring Jupiter’s Moons Though NASA launched the Juno mission to explore Jupiter’s formation and properties, the giant planet’s moons remain one of the most interesting targets in the solar system. A long-standing concept is the international Europa Jupiter System Mission/Laplace. The mission, developed together by NASA and ESA with possible additions from both the Japanese and Russian space agencies, would have sent spacecraft to orbit two of Jupiter’s moons: Europa and Ganymede. Both are known to contain water ice, with an enormous ocean suspected of sitting below the surface of Europa’s frigid crust. The potential for life on either of these worlds makes a compelling case for further exploration. The NASA component of this mission was estimated at $4.7 billion last year, a price that is too far out of range for even the most optimistic NASA budget scenarios, and the mission has been put on ice. But the concept lives on with the European component, now called the Jupiter Icy Moon Explorer (JUICE). Recently selected for a 2022 launch date, this mission will visit the Jovian system, focusing on the previous two moons and one other, Callisto. Perhaps one day in the far future, NASA or another space agency will get to work on one of the most ambitious proposals that periodically pops up: a lander and drill to explore the ocean on Europa. Using a heated head, a probe could slip down through the Europan ice and deposit a remote submersible in the water to explore and search for any life forms. While the technical challenges of such a mission would be great, the payoff could potentially be spectacular. Images: 1) The Europa Jupiter System Mission/Laplace mission. Michael Carroll/NASA. 2) A cryobot that could explore the ocean of Europa. NASA

Titan Target One major target for exploration in the solar system has always been Saturn’s moon Titan. With its thick atmosphere, liquid ethane lakes, and subterranean water ocean, Titan really seems like a good place to find life. In 2005, NASA and ESA landed the Huygens probe on the edge of what seemed to be a dry lakebed on the moon but more detailed exploration remains high on the to-do list. The ambitious Titan Saturn System Mission (TSSM) would have been a joint NASA/ESA mission to send a spacecraft to orbit the enigmatic moon. In addition, it would have dropped a large balloon to drift through Titan’s atmosphere, taking important wind and weather data, and floated a boat-like lander on a lake to get a better understanding of the liquid forces shaping Titan’s surface. The mission remains on the drawing board, though one component of it may still see the light of day. The Titan Mare Explorer (TiME) would be a simpler undertaking, simply sending the boat-like probe to explore Ligeia Mare, one of Titan’s largest lakes. TiME is currently under investigation, competing with two other possible missions, including the Comet Hopper, for a 2016 launch slot. Images: 1) Titan Saturn System Mission. NASA/European Space Agency. 2) TiME. NASA

Ice Giant Exploration Since Voyager 2, no spacecraft has gotten up close and personal with our solar system’s ice giants, Uranus and Neptune. Mission proposals to explore these planets and their moon systems — similar to the Galileo spacecraft’s tour of Jupiter or Cassini’s flights around Saturn — have come up from time to time. During the most recent planetary science decadal survey, officials selected a Uranus probe as their number three most-wanted mission. But the $2.7 billion estimated price tag means that such a mission is unlikely to happen any time soon. British scientists have also proposed the much cheaper $600 million Uranus Pathfinder mission to the European Space Agency, though officials have yet to make a decision. Why no love for Neptune? Its moon, Triton, is known to be geologically active, with icy volcanoes spewing nitrogen from its surface. Unfortunately, Neptune’s great distance and the fact that closer Uranus remains mostly unexplored mean that the farthest planet in our solar system sits a little lower on the list of potential exploration. Perhaps with enough time and money, Neptune may one day get its turn. Images: 1) Uranus and its moons. NASA Ice Giants Decadal Study 2) Possible probe for a Uranus or Neptune mission. NASA Ice Giants Decadal Study