In what could be the most spectacular death of a popular science metaphor yet, the European Space Agency’s comet lander Philae will take seven hours this Wednesday night to drift with a gentle crunch down onto the dark surface of the comet Churyumov-Gerasimenko.

But will Philae’s slow-motion view of jutting ridges and rock piles amid plains of dust and sharp-edged boulders really finish off the “comets are dirty snowballs” catch phrase that ignorant reporters and lazy astronomers keep trundling out in stories about comet missions?

Barring some unforeseen mishap, Philae, having been detached from the comet orbiting mother ship Rosetta, isn’t touching down on anything either “snowy” or “pristine”.

The touchdown, the lowest-velocity arrival on another cosmic body in history, involves the dishwasher-sized device descending over a very uneven, slowly rotating body until contact is made in almost zero-G conditions.

Philae is then programmed to harpoon itself into the comet’s surface, using a tiny thruster jet on top to force the lander to remain bonded to the comet’s surface for long enough that it can screw its landing legs into place.

The irregular double-lobed comet is more than 4000 metres long in one of its many directions. If you were on it you could probably pole vault yourself into orbit around it.

This should be achieved around 2.35am on Thursday morning Australian Eastern Standard Time. The comet at the moment is 28 light minutes from Earth, but slowly falling ever closer towards the sun on its 6.75-year periodical orbit, which takes it closer to the sun than Mars, and farther from it than Jupiter.

The Rosetta mission aims to fly with the comet during the next year or more as it awakens under the increased heat of the sun, and Philae, provided it doesn’t land on a gas or dust jet, could perform detailed science on Churyumov-Gerasimenko’s surface for as long as six weeks.

This is not a “showy” comet in terms of throwing off spectacular tails of dusts and gases. It has been churning its way from fire to ice in its short periodical orbit for too long to have much left by way of volatile substances.

Ever since another European probe, Giotto, sped through the fuzzy coma of Halley’s Comet in 1986, capturing images and some chemical insights, comets have been known to be incredibly dark objects, coated with something akin to crude oil or tar and reflecting less light than bitumen.

They show signs of containing more unidentified complex chemicals than your typical rave party,  including amino acids. Contrary to popular opinion they are not pristine, primordial remnants of matter from which the planets coalesced more than 4 billion years ago. Comets are very old but they are anything but pristine as a result of their travel and other high-energy events they must have been subjected to, including nearby supernovas and collisions with other bodies, some solid, some nebulous.

In 2010, another fly-by mission of Comet Hartley 2 put paid to the “icy” myth once and for all, finding crystalline particles that could only be made in temperatures of more than 1000C. Another nail in the snowball, so to speak.

But that is considered all too hard to canvas by many science commentators, who favour  punch lines and overly simplistic summaries to promote their projects and hoped-for research grants. Churyumov-Gerasimenko is a mouthful that doesn’t lend itself to glib reporting or oversimplification.

Peter Fray

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