Tuesday, 28 September 2010

How close could an average spaceship get to the Sun before melting?

I must start by apologising for being so lax in posting articles and podcast episodes over the last month. We've recently bought a house and moved in, and this process has taken up nearly all my time, given me multiple headaches and left me without the Internet at home.

A while back we put a call out for your burning science questions, and plenty of great questions came in on this site, via email, on twitter (@westius) and over at facebook. I apologise for my delays in publishing these questions and their answers - you can follow the questions that have already been answered in the podcast or on the blog using the Science Week tag.

One interesting question that came in was How close could an 'average' spaceship get to the Sun before melting? Here is an answer from a much more intelligent person than I, Physics PhD holder and all round good bloke, David Bofinger.

An old-fashioned spaceship would probably be made of aluminium, a more modern one might be made of a mixture of aluminium, graphite fibre and polycyanate. Assuming you want the spaceship to actually melt, rather than just fall to bits because a few bits melted, then you probably want to raise it to the melting point of aluminium, which is 933 Kelvin. Of course it will take a lot less than that to kill any crew and cook any electronics on the ship. But melt you asked for and melt we shall give.

We'll assume for the moment that the spaceship is a simple sphere and that we haven't done anything clever to keep the spaceship cool. It will heat up to a temperature such that it's radiating away as fast as it's absorbing heat from the sun. The closer it gets to the Sun the more it absorbs, the more it needs to radiate so the higher its temperature will get.

If we put it in orbit around the Earth, then it's about 150 million kilometres from the Sun and the temperature it reaches is 279 Kelvin, i.e. about 6 degrees Centigrade. (Earth is mostly warmer than this because it has greenhouse gases in its atmosphere.) To melt the aluminium in the spaceship we need to take it into 13 million kilometres, about a twelfth of the distance from Earth and four times closer than Mercury.

Of course there's all sorts of tricks we can play to get closer. We can make the spaceship silvery on the side facing the Sun and black on the side facing space. That will make it absorb less and radiate more. If we made it as white as snow on the Sun side and black as coal on the space side then we could get in as close as 6 million kilometres, about eight times closer than Mercury and twenty-five times closer than Earth. If we made the spaceship long and thin and pointed it toward the sun we could maximise our ability to dump heat compared with how much we absorbed. That might get us in a little close yet. If we pull out all the stops we might do as well as NASA's planned solar probe, which intends approaching within 6.6 million kilometres of the sun while staying cool enough to have functional electronics and cameras.

The moral is that if you want to go close to the sun you don't want an average spaceship, but something built to take the heat.

If you have an alternate opinion, I'd love to hear it.


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