Comets pass near enough to Earth to feature regularly in our skies. The famous Halley’s Comet, for example, passes us roughly every 75 years and is next expected to be visible from our planet in 2061.
Comets are generally known to be made of ice and frozen dust but, in the pursuit of discovering exactly what the composition of a comet was, NASA decided to analyse a chunk of one. This was no mean feat and a dedicated mission called Deep Impact was set up to send a probe into space to investigate the cosmic projectile. The probe shot a small impactor into the core of Jupiter family comet Tempel 1, which threw up debris that allowed both the probe and various telescopes to study its makeup using infrared spectrometry.
By breaking apart the light reflected from this debris, the Spitzer Space Telescope was able to determine the chemical signatures of a huge variety of compounds. This included forms of iron, clays, carbonates, silicates and – perhaps most curiously – the mineral which makes up the gemstone spinel (one of which appears in the British Crown Jewels). The vapour trail was determined to be comprised of water vapour and carbon dioxide gas.