Artist's impression of Rosetta as it flew by asteroid Steins on Sept. 5, 2008

Image by C. Carreau, courtesy of ESA

Lurking in our solar system's remote hinterlands are two collections of icy bodies, the frozen remnants of our solar system's formative years. One, the Kuiper Belt, rings the sun just beyond Neptune's orbit. The other, the Oort cloud, surrounds local space somewhere between 5,000 and 100,000 astronomical units away from the sun (1 AU equals the average Earth-sun distance, roughly 93 million miles, or 150 million kilometers). When a frosty denizen of either frigid community departs to seek adventure in the inner solar system, we call it a comet.

The ancient Greeks mistrusted these "long-haired" hippy "stars" as erratic portents of ill fortune, but modern astronomers value comets for the glimpses they offer into the solar system's past. As frozen, primitive objects covered in volatile substances, they act as cold storage for the building blocks of our solar system. As repositories of the carbon, hydrogen, oxygen and nitrogen that make up nucleic and amino acids, they might also help explain how life arose on our planet [source: ESA].

Our knowledge of comets has taken off in recent decades, fueled by a succession of spacecraft flying to, rendezvousing with and even ramming the dirty ice balls [source: ESA]:

  • In 2001, NASA's Deep Space 1 mission to the 9969 Braille asteroid later observed the Borrelly comet.
  • The agency's Stardust mission, launched February 1999, gathered dust from comet Wild-2 and returned it to Earth in 2006.
  • NASA's two-vehicle Deep Impact mission, launched January 2005, rammed an impactor into comet Tempel-1 to see what it was made of.

The closer we can get the better: A comet's brightness pales next to the brilliance of its starry background, so it defies easy observation from land-based or orbital observatories. It obligingly brightens from outgassing, jettisoning material as it swings sunward, but by then a surrounding cloud of gas and dust, or coma, obscures views of its nucleus.

With the International Rosetta Mission, we landed a spacecraft on comet 67P/Churyumov-Gerasimenko in November 2014 with plans to ride it around the sun.

The craft had to be as nimble as a shuttle pilot and nearly as self-sufficient as a crew of oil-drilling roughnecks, for its approach had to steer clear of whatever the comet threw off and its radio connection to mission control approached a 50-minute lag [source: ESA]. Now deployed, the duo of orbiter and lander will attempt to address some of the many unanswered questions surrounding comets and the formation of our solar system.