Image via WikipediaWhen it comes to space probes, there are three basic levels of design difficulty: Impactors, orbiters, and landers. Impactors are literally designed to crash into other celestial objects, recording until they smash into another spacerock, which saves flight engineers the trouble of targeting anything beyond a nearby gravity well. Orbiters are designed to enter -- and in some cases slingshot through -- a nearby gravity well, which requires far more navigational calculus. Landers, as you might imagine, require not only reaching another gravity well, but delicately succumbing it to it in such a fashion as to not crush or incinerate the space probe on its way to reaching the target surface.
When it comes to space probe targets, there are also three basic levels of difficulty: Our own moon, other local planets, and other local non-planets besides our own moon. The moon is biggest local gravity well besides the Earth itself. We've been crashing...er, impacting probes with the moon for fifty years, since the Soviet Luna 2 "flyby" in 1959. The moon is in Earth orbit, so hitting the moon is basic crashing one object in Earth gravity well into another. Hitting another planet requires leaving Earth's gravity well and successfully falling into another one -- which isn't so easy with the sun out there trying to turn everything into another Sol-trapped pseudo-comet. Hitting a non-planet besides the moon? That's the really hard trick, as you not only leave Earth's gravity well, but you have to hit a target usually trapped within the gravity well of another planetary body. That's bit like riding a waterfall and choosing which rock to land on at the bottom -- without ending up in the pool of water underneath.
It should comes as no surprise, then, that we've only in recent years been able to land a space probe on a non-planet besides our own moon, and we've only pulled it off a bare handful of times.
Besides our own moon, how many non-planets have humans successfully landed a spaceprobe on?
Humans have successfully landed probes on three non-planets besides our own moon: Asteroid Eros, Asteroid 25143 Itokawa, and the Saturnian moon Titan. The landings were performed by the probes NEAR Shoemaker, Hayabusa, and the Huygens probe, respectively.
(Before you space probe nerds get uppity, NASA's Stardust probe didn't land on comets Wild 2 or Tempel 1, it just collected dust thrown off in the comets' tails. Deep Impact was designed to crash into comet Tempel 1; it was an impactor, not a lander.)
Hayabusa gets extra points for returning material samples from Itokawa (we think), taking it into territory only Stardust and the Apollo lunar surface missions (also landers) have gone before. There really is a fourth kind of space probe after impactor, orbiter and lander -- return lander, a craft that can touch down and then take off again from a target celestial object. That's the sort of mission and design profile generally reserved for manned spacecraft, but the robot astronauts are getting in on the "To space...and back!" business. That's not just an extraordinary technical achievement, it a scientifically significant citation of the truly trivial.