The excitement about space will pick up again in a week as the European Space Agencys Rosetta mission prepares to send down a lander to the comet 67P/Churyumov-Gerasimenko. Rosetta went into unstable orbit around the comet on August 6, just over a decade after it left earth on an Ariane launcher, and now it is ready to make physical contact. Comets have stirred the scientific and religious imagination since the dawn of human consciousness. Scientists have positedand many have disagreedthat they delivered water and the chemical basis of life to the primordial earth. Priests, oracles and poets have leveraged comet sightings to cause shock and awethe Sibylline prophecies and the Epic of Gilgamesh speak of fire raining down from the sky in the wake of comets. More usefully, the depiction of Halleys comet in the Bayeux Tapestry visually dates the Norman Conquest of Britain to 1066.
Halleys, one of six comets which spacecraft have made contact with so far, was studied up close when it swung by in 1986 (the debris it left in near space fell to earth last week as the Orionid showers, an annual October event). Three years later, Nasa launched Stardust to bring back physical materials from space. In 2006, its re-entry vehicle brought back samples and data from the comet 81P/Wild 2 and the asteroid 5535 Annefrank, and some cosmic dust too. The year before, in 2005, Nasas Deep Impact probe had slung an impactor into the comet 9P/Tempel 1, basically to see what was inside it, as revealed by the vapours and dust thrown up. The last, dramatic moments of the controlled crash are on film online.
But Rosetta is much more exciting. It is the first attempt to land instruments on a small space object, instead of crashing into it. Like Curiosity, the lander, named Philae (after the Isis temple in the Nile, which is not too far from Rashid in Egypt, whose Europeanised name is Rosetta) will drill into the surface. Churyumov-Gerasimenko has water ice and its surface appears to be rich in carbon, and could hold answers to intriguing questions about comets. One, was the water on earthand the moon and other planets, toodelivered by comets And two, did they also bring to earth the complex organic molecules that are the precursors of life
The question of water is relatively easily addressed. Earth has regular water and heavy water (popularly associated with nuclear reactors), which has a higher proportion of the hydrogen isotope deuterium. It is a heavier molecule because while the hydrogen nucleus consists of one proton, deuterium also tucks in a neutron. Now, if the proportion of deuterium in the ice on comets is similar to that in earths water (about one out of 3,200 molecules), it may be proposed that water vapour entered the atmosphere when comets passed byor crashed and burned, like the object that exploded over Russias Tunguska river in 1908, generating a centurys worth of conspiracy theories.
Where there is water, life is theoretically possible. This is the basis of abiogenesis, the notion that life developed from non-life, using water for a medium. In 1924, Alexander Oparin and JBS Haldane proposed a primordial soup in which life could come to a boil. Robert Shapiro broke down the idea into its components: a reducing atmosphere drawing energy from sunlight and lightning to form monomers, which could develop into long-chain molecules, including genetic material. In 1953, Stanley Miller modelled it to produce complex molecules.
However, the building blocks of life could have originated in space, too. A 2011 study had suggested the presence of amino acids in meteorites, which are fallen asteroids and comets. Formaldehyde, ethane and methanol have been detected in comets. Besides, the heads of comets have a very low albedothey reflect almost no lightsuggesting that they are made of dark, organic material. Rosettas Philae will shed new light on these questions on November 12, when it is expected to make landfall on 67P/Churyumov-Gerasimenko. It will be a delicate landing, though, since comets live in very low gravity and spew jets of dust and vapours unexpectedly, which could ruin a landing or damage the lander. But if it works, Rosetta will have something to write home about.