SPACE, they say, is as silent as the grave. But they are only half right. Space is full of sounds 鈥� it鈥檚 just that our ears are not tuned to pick them up.
Compressing a medium creates sound. On Earth, we squash the atmosphere when we sing or shout. Much of space also has an atmosphere of sorts, although thousands of times thinner than Earth鈥檚. Because the wavelength of any sound must exceed the distance between the particles in the medium, any noise created in space is at a frequency many times lower than anything we can hear.
What exactly is making the music out there? There are many sources: exploding stars, the plasma wind that blows off the sun, even the Earth produces a 鈥渉um鈥�, and there is plenty of white noise hanging around from he big bang.
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Then there are black holes, the operatic supremos of the cosmos. Do black holes sing? Late last year astronomers at the University of Cambridge detected a note 57 octaves below the middle C of a piano coming from a supermassive black hole at the centre of the Perseus galaxy cluster. It is produced by bubbles of plasma escaping from the black hole鈥檚 host galaxy out into the hot intergalactic gas. This compression happens only once every 10 million years, and the resulting note 鈥� the researchers claim it is a B flat 鈥� is a trillion times below our hearing range.
What of other planets? In 2008 the NetLander mission, organised by the French space agency, is due to land four probes on Mars, each carrying two microphones. We should get to hear the rush of winds on the Red Planet and the crackle of dust particles in the dry air 鈥� and all of it in stereo.