Which Alien World Should We Land on Next?

​Ten years ago today, the ESA's Huygens probe successfully landed on Saturn's moon Titan after a bumpy 2.5 hour​ descent through thick cloud cover. It was the first time a probe had ever touched down on an object in the outer solar system, and it remains the most distant landing​ ever achieved in spaceflight.

On top of all that, Huygens survived long enough to transmit loads of information about its alien surroundings, before contact was lost a little over an hour after landing. Its dutiful reporting included a number of evocative images of its final resting place near Titan's mysterio​us Xanadu region, including this sepia snapshot of the moon's icy, rock-filled landscape.

It's an incredibly humbling thing to vicariously experience an alien landscape through the eyes of our explorers, be they human or robotic. Indeed, the feeling was revived two months ago, when ESA made another historic landing with the Ro​setta/Philae spacecraft: the first mission ever to land on a comet. Naturally, it raises the question: what other, untouched worlds should be receive a lander from Earth next?

That question provokes a lot of debate, because everyone has their own favorite destination in mind when it comes to exploring the solar system. So I'll kick off by outlining the missions that are already in progress, and are likely to deliver the next big landing win.

First up, JAXA's Hayabusa2 spacecraft, which launched last month, is slated to send multiple rovers, landers, and even a small bomb to asteroid 1999 JU3 comet 2018. Though it is shaping up to be an exciting mission to an unexplored asteroid, we have already landed robots on other asteroids twice before, so it probably won't be revered on the same level as Rosetta. Still, blowing up asteroids is undeniably awesome, and Hayabusa2 is well on its way to doing it.

Asteroids and comets are great and all, but what's the next roughly spherical world on the landing list? For a while, it looked like Mercury would be the lucky winner. After all, scientists have sent several landers to Mars and Venus, but have never shown the Mercurian surface any love (though the planet does sport a cool o​rbiter called MESSENGER).

Things started to look up when ESA's BepiColombo mission, scheduled to reach Mercury in 2024 with a 2016 launch, announced a ground component. But BepiColombo's Mercury Surface Element was axed wit​h a budget cut in 2003, so we won't be landing on the solar system's innermost planet anytime soon.

No other landing missions to new worlds have been greenlit at​ this time, excepting Phobos-Grunt's failed attempt to land on the Martian moon Phobos (which is basically an asteroid masquera​ding as a moon anyway). But scientists have been actively advocating a return to the outer solar system, and in particular, to the moons of the gas giants. Huygens proved these missions are possible, but the question is: where to go first?

For most, the obvious answer is Europa, a potentially life-bearing ice moon in the Jovian system. Ever since its unusually smooth surface was first imaged by Pioneer 10, scientists have been clamboring to put together a landing mission to the moon. Several subsequent flyby missions have confirmed that the hype is justified: the moon has more water than​ Earth locked away in its subterranean ocean, and is graced with curiously disappearing ​geysers. It is simply begging for a lander.

NASA, ESA, and Roscosmos have all proposed concept missions for landing or impacting with Europa, but so far nothing has made it past the planning stage. Last year, however, NASA invested 15 million dollars into "pre-formulation work" for a Europa mission, so we may yet see a lander—or better yet, a submersible—in the near future.

Europa may be the most intriguing of the Jovian moons, but it's not the only one worth landing on. The Russian space agency Roscosmos has be​en eyeing Ganymede too, and for good reason. Ganymede is the largest moon in the solar system, beating out both Mercury and Pl​uto in diameter. Two times more massive than our own moon, Ganymede would be considered a fully fledged planet if it was orbiting the Sun.

Plus, like its sister satellite Europa, the giant moon appears to su​pport a subsurface ocean. Russia proposed bundling a Ganymede lander into an upcoming European mission to Jupiter called JUICE (JUpiter IC​y moons Explorer), scheduled to launch in 2022, but nothing has been confirmed yet.

Saturn's moon Enceladus also boasts a subsurface ​ocean, but odds are, either Europa and Ganymede will give humans their first taste of alien seas. Indeed, concept missions for landing on Saturnian moons are sometim​es bandied about, but they are nowhere near as popular as Jovian moon landers. The only exception is a return mission to Titan, this time with a boat for sailing its liquid methane seas. Sure, Huygens already bagged the "first lander" milestone, but Titan is definitely a weird enough world to warrant sending a mini-space-yacht there.

Beyond these half-dozen candidates, the interest in landing missions tapers off dramatically. Nobody cares about going to Uranus, even for a flyby. The first and only time anyone bothered to get a close look at the planet was in 1986 with​ Voyager 2, and we've pretty much left it alone ever since. So, a landing on the Uranian moons, with their Shakespearean naming scheme, is unlikely. Sorry Miran​da, but it seems we just don't think of you as a brave, new world.

Meanwhile, landing missions to even more distant worlds in our solar system, like Neptune's badass retrogr​ade moon Triton, or dwarf planets like Pluto o​r Ceres, are occasionally indulged as possibilities. But for the next two decades, exploration of these worlds will likely be left to flybys l​ike the New Horizons probe.

Even the likeliest of these lander missions can seem like quixotic fantasies, given how many false starts we've experienced along the way. For decades, scientists have coveted Europa, Mercury, Ganymede, and other intriguing worlds that share our Sun, and it can sometimes seem as if we're no closer to them than we were in the 1970s.

But the Huygens probe's tenth anniversary is a reminder that these worlds are completely within reach. Huygens, after all, was launched in 1997, and was bas​ed on technologies of the 1980s. Since it landed on Titan on January 14, 2005, we have extensively explored Mars, expanded the role of private space industry, and landed on a comet.

Pushing the frontier back to these distant worlds will be expensive, risky, and time-consuming. But Huygens will always be there to remind us that it is far from impossible. If we could successfully launch a probe to Titan while pogs were still a fad, imagine what might be kickstarted in the age of ion thrusters, nanotech, and unparalleled global cooperation.

As Bill Watterson's Calvin would say, let's go exploring.