Tempel-1: iceberg or galactic rubber ball?

Deep Impact team ponders comet's composition

April Fool special NASA's Deep Impact probe launched successfully on 12 January, on a mission to more accurately understand the properties of comets.

Its target is Tempel-1 - chosen because although it might be rock and ice like most other comets, scientists have reason to suspect that much of its structure is highly elastic frozen cross-linked hydrocarbons (very much like Kevlar or stiff rubber).

Ice can be a notoriously tough material, being resistant to both heat and shock, but scientists still don't know if the average comet has enough ice to be as tough as an iceberg or if it is as weak as the famous Shumacher-Levy comet which broke up under the influence of Jupiter's gravity. If, however, Tempel-1 is indeed comprised of rubber-like hydrocarbons, as mission planners suspect, then the results of the Deep Impact strike will be very interesting.

The plan is to hit Tempel-1 with a 370kg (~820lb) impactor (49 per cent copper) at 10km per second, releasing 19GJ of energy (around 4.8 tonnes of TNT) with a view to determining the comet's coefficient of elasticity by bouncing the impactor back into space and measuring its trajectory carefully with the following "mothership" and various Earth-based telescopes. Some scientists, though, are betting that the comet will simply shatter.

So much the better, say many experts. With our growing awareness of the risks of the Earth being struck by a rogue comet or meteor, understanding the physical properties of the comet has become a matter of the very survival of humanity. Just how hard must we hit a comet before it breaks up? Which technologies will work? Will we need to gently land on a shatter-proof comet and use rockets to nudge it onto a different course, or will nuclear weapons achieve the desired result?

Multiple interdisciplinary teams (geologists, physicists, computer programmers and statisticians) have been trying to determine the optimal conditions to strike the comet to achieve the best possible science.

To add a little spice to the mission, the teams are also running a competition to see who can most accurately predict - in the event of cometal disintegration - the paths of any pieces during the 1000 solar orbits after impact. Remnants over 50 metres in diameter which are captured by a planet (regarded as "potting" a comet) score 100 points each. If they succeed in getting the comet to spin rapidly and thus "screw back" using the solar wind to crash into Jupiter then that is worth 1000 points. Hitting the mothership with the projectile ("snookered") gains 5000 points.

There is, of course, the remote possibility that a substantial piece of Tempel-1 will eventually impact with Earth - a scenario chillingly described by scientists as "Pot Black". Said one cometologist: "We don't really want to think about that too much. We're just hoping that everything goes on cue..." ®

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