Huygens’s Revolutionary Titan Landing

Ten years ago, in our solar system far, far away, a spacecraft orbiting Saturn dispatched a small probe to an unexplored world—the moon Titan—an achievement that set off a space exploration revolution. The European Space Agency’s Huygens probe, carried by the National Aeronautics & Space Administration’s Cassini spacecraft, proceeded to land on Titan, which is Saturn’s largest moon and the only moon in our solar system with an atmosphere.

Though Huygens’s lived only a couple of hours on Titan, the dusty orange images it radioed home unveiled a geomorphology that is Earth-like yet utterly alien: pebbles made of water ice, not rock; ridges covered with sticky hydrocarbon tholins, not dirt; and channels carved by rivers of methane, not water.

Huygens’s success was particularly sweet because Titan’s thick nitrogen and methane atmosphere had previously thwarted astronomers’ attempts to look at the moon’s surface. The feat is still unmatched as the most distant landing on an astral body in our solar system.

In the years following the Huygens landing, numerous missions to our outer solar system that were in the planning stages or had recently been launched as of 2005 are now bearing fruit as well. The world’s space agencies have teamed up to make and dispatch orbiters, landers, impactors, and sample-return missions to explore comets, asteroids, and the distant dwarf planet Pluto.

Besides the Huygens mission, two made their mark in 2005. NASA’s Deep Impact spacecraft dropped a 363-kg copper chunk onto the surface of comet Tempel 1 and analyzed the debris that was kicked up. And Hayabusa, a craft launched by the Japan Aerospace Exploration Agency, JAXA, grabbed and returned particles of a near-Earth asteroid. Then in 2006, NASA’s Stardust returned particles of the comet Wild 2 to Earth.

More recently, in 2014, ESA’s Rosetta spacecraft reached the comet 67P/Churyumov-Gerasimenko and dispatched a tiny probe, Philae, to the comet’s surface. Although Philae experienced landing troubles, it marked the first controlled landing on a comet. In July of this year, NASA’s New Horizonsflew past Pluto and returned images of water ice mountains and flat plains (see page 29). And earlier this month, NASA’s Dawn spacecraft sent back new details about Ceres, a dwarf planet in the asteroid belt between Mars and Jupiter that shows signs of having both asteroidlike and cometlike properties.

After dropping off Huygens, Cassini continued to perform and has lasted long past its expected expiration date. In October, it dove through a water plume spewing from the surface of Saturn’s moon Enceladus. One outcome of the recent, numerous explorations of the outer solar system is that we now understand that some giant-planet moons, such as Enceladus and Jupiter’s Europa and Ganymede, are likely homes to subsurface liquid-water oceans. They may even rival Mars as promising locations to look for evidence of extraterrestrial life.

Next year, both NASA and ESA plan to launch Mars orbiters and landers. Also in the works are more asteroid missions, including an ESA sample-return mission. In addition, NASA will send another spacecraft to Jupiter, and ESA is planning a mission to explore Jupiter’s moons.

The quality of the Huygens images from 10 years ago may now pale in comparison to New Horizons’ recent detailed snapshots of Pluto’s surface. But 10 years into the future, wherever space probes are deployed, humanity will likely regard them as equally revolutionary as Huygens.