The speed of a crash landing on its surface is typically between 70 and 100% of the escape velocity of the target moon, and thus this is the total velocity which must be shed from the target moon’s gravitational attraction for a soft landing to occur. For Earth’s Moon, the escape velocity is 2. 38 kilometres per second (1. 48 mi/s). The change in velocity (referred to as a delta-v) is usually provided by a landing rocket, which must be carried into space by the original launch vehicle as part of the overall spacecraft. An exception is the soft moon landing on Titan carried out by the Huygens probe in 2005. As the moon with the thickest atmosphere, landings on Titan may be accomplished by using atmospheric entry techniques that are generally lighter in weight than a rocket with equivalent capability.
The original goal of Cassini-Huygens was to study Saturn and its large, misty, tortured, moon Titan. Titan, the second-largest moon in our Solar System, after Ganymede of Jupiter, is a world long-shrouded in mystery, hiding behind a thick orange veil, and slashed with hydrocarbon lakes and seas. However, there are other enticing moons known to circle the ringed planet. Saturn's mid-sized icy moons (Mimas, Enceladus, Tethys, Dione, Rhea, Hyperion, Iapetus, and Phoebe) are enchanting worlds. Each one of these frozen little moons reveals an interesting and unique geology. So far, Saturn is known to sport 62 icy moons! Launched as GRAIL A and GRAIL B in September 2011, the probes were renamed Ebb and Flow by schoolchildren in Montana. The probes operated in almost circular orbit near the lunar poles at an altitude of approximately 34 miles, until their mission came to an end in December 2012. The distance between the twin probes altered a bit as they soared over areas of lesser and greater gravity that were caused by visible topological features on the Moon's surface, such as impact craters and mountains--as well as by masses that were secreted beneath the lunar surface. There are over 100 moons dancing around the eight major planets of our Sun's family. Most of them are small, frozen, icy objects, harboring only a relatively scanty amount of rocky material, that circle around the quartet of giant gaseous planets that dwell in the outer, frigid realm of our Solar System--far from the comforting warmth and brilliance of our Star. The quartet of majestic, giant denizens of our outer Solar System--Jupiter, Saturn, Uranus, and Neptune--are enveloped with gaseous atmospheres, and orbited by a multitude of dancing, sparkling moons and moonlets. In marked contrast, the inner region of our Solar System--where our Earth dwells--is almost bereft of moons. Of the quartet of relatively petite, rocky "terrestrial" planets--Mercury, Venus, Earth, and Mars--only Earth is circled by a large Moon. Mercury and Venus are moonless, and Mars is orbited by a duo of tiny, lumpy, potato-shaped moons, Phobos and Deimos. Phobos and Deimos are probably escaped asteroids, born in the Main Asteroid Belt between Mars and Jupiter, that were captured by the gravitational embrace of the Red Planet long ago.