A close-up view of Mercury's cratered surface taken by the MESSENGER spacecraftNASA's MESSENGER is still snapping pictures of Mercury's surface as it spirals ever-closer for its mission-ending collision, scheduled for April 30. The smallest craters visible in this image are about 15 meters wide--the same size as the crater MESSENGER will carve out with its impact. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Carnegie Institution of Washington

On April 30, if all goes well, after running out of fuel to fight off orbital decay NASA’s long-running MESSENGER spacecraft will end its mission to Mercury by crashing into the planet’s surface at nearly 4 kilometers per second. Over the past four years, MESSENGER radically changed our view of the closest planet to the sun, revealing it to be a very different place from the inert, volatile-depleted world scientists had expected.

MESSENGER’s maps of the planet’s surface composition showed unpredicted enrichments of delicate heat-sensitive elements such as sulphur and chlorine, suggesting Mercury somehow formed more gently than previously thought. In permanently shadowed craters at Mercury’s north and south poles, the probe found substantial deposits of water ice, as well as dark, organic-rich material likely delivered by infalling comets or asteroids from the outer solar system. Strangest of all, MESSENGER spied a wholly new type of landform apparently unique to Mercury–mysterious “hollows,” flat-bottomed pits that almost look as if they were acid-etched into the surface. The hollows are bright and unblemished by craters, and thus must be geologically young. They may still be forming even today, but no one knows for sure, in part because Mercury lacks any other precision-dated freshly formed features with which to make comparisons. When MESSENGER slams into Mercury, it will carve out a bright, young, 15-meter-wide pit of its own near the much larger Shakespeare crater, providing a small-but-useful reference mark against which future missions can compare the hollows and other youthful features of Mercury’s surprisingly active surface.

MESSENGER is only the latest entry in a lengthy list of spacecraft crashed into celestial bodies for the sake of science. Here, on the occasion of MESSENGER’s swan-song plunge, are five of my favorites:


NASA's LCROSS spacecraft approaching the moonCredit: NASA

On October 9th, 2009, NASA sent a Centaur upper rocket stage and small, trailing spacecraft hurtling at 2.5 kilometers per second into the south polar lunar crater Cabeus, lofting debris clouds that revealed the presence of water ice on the shadowy crater floor. This Lunar Crater Observation and Sensing Satellite (LCROSS) mission wasn’t the first time space scientists had pummeled the moon’s south pole. In 1999, another NASA mission, Lunar Prospector, was deorbited into the crater Shoemaker, and in 2008, India’s space agency crashed its Moon Impact Probe into Shackleton crater. Both crashes confirmed ice in abundance, enough to be used for drinking water or manufacturing rocket fuel, making the lunar south pole an ideal site for possible future landings and moon bases.

2. Deep Impact

Comet Tempel 1 releases a plume after colliding with a component of NASA's Deep Impact missionCredit: NASA/JPL-Caltech/UMD

On July 4th, 2005, NASA’s Deep Impact mission celebrated America’s independence day with some extraterrestrial fireworks, smashing a 370-kilogram “Smart Impactor” at more than ten kilometers per second into the comet Tempel 1. The impactor consisted of a camera and trajectory-tweaking rockets built around a 100-kilogram copper slug, and it returned imagery up until a few seconds before impact. It hit the comet with a force equivalent to about 5 tons of exploding dynamite, ejecting millions of kilograms’ worth of Tempel 1′s innards in a vast plume and leaving behind a 150-meter-wide crater. Analysis of the plume revealed the comet to be dustier and more porous than expected, with a density similar to that of a hard-packed snow bank.

3. Deep Space 2

A penetrator from NASA's Deep Space 2 missionCredit: NASA

Deep Space 2 was a pair of tiny 2.5-kilogram spacecraft included in NASA’s ill-fated Mars Polar Lander (MPL), which attempted to land on the Red Planet in late 1999. MPL released Deep Space 2 as it began its descent toward its own landing site, and they fell freely without parachutes through the thin Martian atmosphere to a hard “lithobraking” landing on the surface. They were designed to break open upon impact, punching probes up to a half-meter into the soil. Radio transmitters in the aft sections of the probes would then beam the gathered data homeward. The entire concept was elegant and advanced–and it was a colossal failure. No signals were ever detected from the little twin spacecraft, which may have simply landed on rock too hard for their probes to penetrate. The MPL mothership suffered an equally ignominious fate when its landing rockets cut out too early, destroying it in another, unplanned lithobraking. The loss of MPL and Deep Space 2 were major setbacks, and contributed to NASA rethinking its entire program of Mars exploration.

4. Pioneer Venus Multiprobe

NASA's Pioneer Venus Multiprobe approaches VenusCredit: NASA

NASA’s Pioneer Venus Multiprobe arrived at Earth’s sister planet in late 1978, and to date remains one of the most unique interplanetary missions ever undertaken. The spacecraft carried one large 1.5-meter-wide probe and three smaller probes of about half the size. All four probes, along with the spacecraft itself, plunged into the Venusian atmosphere carrying a diverse suite of instruments to sniff and study the alien air and weather. One small probe was targeted toward Venus’s sunlit “day” side, and another was sent to the night side, while the third was sent to Venus’s northern latitudes, providing a multi-dimensional view of the planet as a whole. None of the probes were expected to survive impact with the surface, and only the large probe had a parachute, but surprisingly the small “day” probe continued sending back signals for nearly an hour after landing. A companion spacecraft, the separately-launched Pioneer Venus Orbiter, arrived at Venus around the same time and continued circling the planet until August 1992, when it was deorbited to break apart and burn during atmospheric entry.

5. The Galileo Probe

NASA's Galileo Probe enters Jupiter's atmosphereCredit: NASA/Ken Hodges

Most space buffs remember NASA’s Galileo orbiter, which arrived at Jupiter in December of 1995 and spent nearly eight years exploring the gas-giant planet and its accompanying icy moons. Among the orbiter’s many discoveries, perhaps the most important was that the icy moon Europa harbors a large subsurface ocean. But the most impressive feat of the whole mission actually fell to another often-overlooked component, Galileo’s atmospheric probe, which managed to survive entry into the Jovian atmosphere at a speed of some 47 kilometers per second–the most extreme atmospheric entry ever attempted. To endure the intense frictional heat, which exceeded 15,000 degrees, the probe carried a 150-kilogram ablative shield made of carbon phenolic material which shed more than half its mass during the fiery entry. Within minutes of entering Jupiter’s atmosphere, the probe’s shield and parachute had slowed its velocity to only a tenth of a kilometer per second. During its descent it detected steady winds of more than 500 kilometers per hour, as well as less helium and water vapor than researchers had expected, leading to substantial revisions in theoretical models of Jupiter’s cloud layers and formation. The Galileo probe continued transmitting for nearly an hour as it fell more than 150 kilometers into Jupiter’s murk, before finally being crushed and melted by the steadily increasing heat and pressure.