IF ALL goes well, the next decade will prove to be a golden age of cometary exploration. Four missions should rendezvous with comets. One will return the first sample of cometary dust to earth; another will become the first to orbit and land on a comet; a third will blast a hole in its cometary target's surface. And on July 1st, NASA, America's space agency, plans to launch a mission that will, in a rather gentler way, fly past at least two comets over the next five years.
This mission, the Comet Nucleus Tour (CONTOUR), will carry out the closest flypast of a comet so far. It will head first for comet Encke, coming within 100km of it on November 12th 2003. Next, it will meet comet Schwassmann-Wachmann 3, on June 19th 2006. With luck, it will then go on to visit a third comet—though which one has yet to be decided.
CONTOUR's two definite targets are very different. Encke is a “toughened, old comet”, as Donald Yeomans, a researcher at NASA's Jet Propulsion Laboratory, in Pasadena, puts it. It goes around the sun every 3.3 years, and has probably been doing so for a long time. Schwassmann-Wachmann 3, by contrast, is a “young, fragile object”. So fragile, indeed, that it split into three bits in 1995. That means that CONTOUR will be able to explore the interior of this comet, as well as its surface. The probe will also make chemical analyses of the gas and dust that surround the comets.
CONTOUR is only the start. Next January, the European Space Agency (ESA) plans to launch its Rosetta mission. Rosetta is due to rendezvous with the enticingly named comet 46 P/Wirtanen in 2011. If all goes well, it will even land on it. Also early next year, NASA will launch Deep Impact. On July 4th 2005, America's celebration of its successful rebellion against British rule should be enlivened by the firing from this probe of a 370kg missile. The result will be a football-field-sized, seven-storey-deep crater in comet Tempel 1. That should reveal what goes on in the core of a comet.
Last but not least, NASA's Stardust mission will, in 2004, meet comet Wild 2, and attempt to collect some dust from its vicinity. Stardust (which was launched in 1999) should become the first probe to retrieve material from beyond the orbit of the moon, when a re-entry capsule parachutes the samples back to earth in January 2006.
The question “why”?
One of the reasons for all this interest is that comets are primitive, cosmic leftovers. When the solar system was forming, around 4.6 billion years ago, they were among the bits that did not become planets. Because they were never subject to the kinds of thermal and chemical change that happen on planets, their cores of ice and rock are, in a sense, time capsules that can be used to find out more about conditions in the early solar system. For example, many astronomers believe, on the basis of chemical evidence found in meteorites, that a supernova explosion occurred near the nebula from which the sun formed, just before the process of formation started. Indeed, it may have triggered the nebula's collapse into a star. Analysing the composition of comets could provide further proof of this theory.
Besides providing a better understanding of the solar system as a whole, comets may be able to answer specific questions about the early history of the earth. One is whether much of the water in the earth's oceans, and the original gases in its atmosphere before life got to work on them, were “plastered” on to the planet by collisions with comets, rather than emerging from its interior through volcanic vents.
Again, this is a question that will be tackled by looking for tell-tale chemicals. If the water found on comets contains an earth-like ratio of deuterium (a form of hydrogen that has a heavy nucleus) to ordinary, light hydrogen, that would favour the plastering theory. Previous measurements by ESA's Giotto mission to comet Halley suggested that the cometary ratio is skewed more heavily towards deuterium than that on earth, and thus that the plasterers are wrong. But both Encke and Schwassmann-Wachmann 3 belong to a different class of comet from Halley, so the answer they provide may be different.
A more intriguing area that these missions are meant to cast light on is the origin of life—or, rather, the chemical precursors of life. Comets contain amino acids, which are the building-blocks of proteins. If comets ever did contribute to the earth's oceans, they may have brought part of the complex carbon-based chemistry of life with them. Paul Mahaffy, a CONTOUR scientist at NASA's Goddard Space Flight Centre, says that the big question is exactly how complex the molecules on comets actually are. The instruments on CONTOUR, and other missions, will analyse dust for carbon-based molecules, and should thus be able to provide an answer.
There may, however, be a more prosaic reason for the sudden interest in comets: most other bodies in the solar system have been explored already. All the planets save Pluto have been visited by at least one spacecraft. So have many of the bigger moons, and several asteroids. With the arguable exception of studies of Mars, deepening knowledge about the solar system is not as glamorous an activity as widening it. Hence the search for new targets, even if they are mere leftovers from the kitchen that created the main cosmic banquet.