Hunting for clues: missions to a comet
By getting up close and personal with these dirty snowballs, we’ve learned more about their role
1 Stardust
This audacious NASA mission was sent to return a sample of Comet Wild 2 to Earth. Launched in February 1999 and rendezvousing with its quarry in 2004, it successfully returned the sample home in 2006. Initial analysis found it contained the amino acid glycine, although some researchers had suggested the analysis might have contaminated the sample, even though the clean room used for the experiment was 100-times cleaner than a hospital operating room.
3 Rosetta and Philae
In 2014 the ESA successfully landed the washing machine-sized Philae lander on Comet 67P. The orbiting Rosetta spacecraft found many examples of molecules never seen before on a comet, including the first undisputed discovery of glycine – the simplest amino acid. However, its findings also cemented the idea that the water content on comets is significantly different to that found on Earth.
2 Giotto
This ESA mission was sent to study the famous Halley’s Comet up close in 1986. The comet orbits the Sun roughly once every 76 years, and so it was a once-in-a-lifetime opportunity. One of the key findings was a higher proportion of so-called heavy water on Halley than is found in Earth’s oceans. This made the case that comets like Halley were not the primary source of Earth’s water as some astronomers had suggested.
4 Herschel Space Observatory
A telescope that saw in infrared, Herschel was used to peer more closely at Comet Hartley 2. It found that there was a good match between the proportion of heavy water present on the comet and in Earth’s oceans. The comet is thought to have formed much further out in the Solar System than some of the other comets studied, and that may indicate that very longperiod comets were the source of our water.