16 January, 2006

Stardust doesn't crash

The capsule from the Stardust probe, carrying particles collected from the halo of Comet Wild-2, has landed intact in the the Utah desert. This must have come as a bit of a relief for the scientists involved after what happened to the similar Genesis mission, which returned solar wind particles to Earth in 2004 with a much bigger bump than expected, because the deceleration sensors which were meant to trigger the parachutes were put in backwards (it seems, incidentally, that not all has been lost from that mission).

So why the obsession with chasing dust? It's all to do with trying to work out how the solar system formed 4.6 billion years ago, a process which is still poorly understood. We have a general theory: a cloud of interstellar gas began collapsing under its own gravity into a solar nebula, triggered by a nearby supernova or other disturbance. This collapse heated up the centre of the nebula enough the trigger nuclear fusion, forming the Sun. Meanwhile, material in the cooler outer parts of the nebula condensed and coalesced into small protoplanets, which through numerous collisions eventually formed the larger planets we see today. Knowing the end result - small rocky planets closer to the sun, icy gas giants further out - provides important constraints on how this process occurred, but not enough to really understand what happened. You can produce several quite different models which give similar end results, mainly because we have to guess our starting conditions - the initial composition of the solar nebula is poorly known, and which model works best in producing a familiar solar system depends on which initial composition you assume.

This is where Comet Wild-2 (pronounced "Vilt-2", which I personally think makes it sound far less exciting) comes in. Comets are left over bits of material from the outer parts of the solar nebula, which were too far away from each other to coalesce into anything bigger; their composition is therefore likely to be very close to that of the original nebula itself, because only in large bodies can mixing and differentiation of elements (such as in the Earth, where heavy elements such as iron have been concentrated in the core) occur. Wild-2 has spent most of it's life in the outer reaches, meaning that has been kept in pristine condition; only recently, when a close encounter with Jupiter in 1974 slung it into the inner solar system, has the sun been close enough to start boiling off its volatile materials. This change in orbit also placed it within much easier reach of Earth, making a mission to return samples of material unaltered since the birth of the solar system possible (for the curious, in the case of Genesis, the outer layers of the sun from which the solar wind originates are also thought to contain material which is as yet unaltered by nuclear fusion from its initial state).

As well as constraining our models of planetary formation, knowing the composition of the early solar nebula can also answer other important questions. One thing scientists want to know is, is our sort of solar system commom, or rare? Almost all of the extra-solar planets we know about are gas giants orbiting their stars closer than Mercury does. Is this due to present limitations with detection techniques (after all, it was less than 10 years ago that no-one could say for sure that there were any extra-solar planets), or is our home system a weird aberration? By comparing our own solar nebula to other planet-forming nebula (for example, in the Orion Nebula), we might get some clues. Additionally, people will be looking for the presence (or not) of organic molecules, which may indicate that collisions with comets billions of years ago may have provided the raw materials for the emergence of life on Earth.

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