“The sword of Fergus, the sword of Letè from Faery: Whenever he desired to strike with it, it became the size of a rainbow in the air. Thereupon Fergus turned his hand slantwise … so that he smote the three tops of the three hills, so that they are still visible on the moor, and these are the three Maels of Meath.”
– Táin Bó Cuailgne (The Cattle Raid of Cooley)
ON 30 JUNE, 1908, another sword the size of a rainbow appeared in the air approximately 5km above the ground and exploded, sending a shockwave across hills in the region that flattened the trees on their tops over an area of about 2,000 square kilometres. In 2013 another object burst high up in the air that shattered windows and damaged building for kilometres around.
What happened over Tunguska in Siberia in the early 20th century was a strike from a fragment of comet, and the Chelyabinsk event last year was an impact from a near-Earth asteroid. It’s very possible that the Cattle Raid of Cooley – and, indeed, countless other tales from around the world – refers to a similar event whereby our planet was struck by an object from space causing some manner of destruction. Not only that, but it’s also thought that water on our planet may have originally been delivered to it by a comet strike, where the object’s icy surface melted on impact, flooding an otherwise desolate, young planet.
Considering our natural and cultural affinity with comets, it should come as no surprise that we are currently undertaking a mission at Comet Churyumov–Gerasimenko: earlier this month the Rosetta spacecraft entered orbit around the almost peanut-shaped icy body, and in November it will release a probe that will, for the first time ever, land on a comet.
Pic: Comet 67P/C-G, via European Space Agency
Famous comets
The most famous comet is probably Halley’s Comet, though most people might remember Hale-Bopp in the mid-1990s that was a most beautiful and wondrous visitor to our evening skies. Like Caladbolg – the Sword of Fergus – some comets can stretch across the sky, not unlike a vast night-time rainbow. The tail of the comet is made of dust, water vapour, and electrical particles, released by the comet as it warms up on its approach to the sun. Scientists have measured one comet tail to be about 570m km long – almost four times the distance between the sun and the Earth. The cloud of vapour that makes up the head of the comet can grow to be bigger than the sun (which is a star about 1.4m km across).
That said, even though they sometimes appear as vast bright objects, comets are mostly nothing: they’re a tenuous cloud of gas and dust that melts off an icy body maybe only a few tens of km across itself. Comet Churyumov–Gerasimenko – also (thankfully) known as Comet 67P – measures about 5km across its widest part. Such a small body has very low gravity: a person standing on it could jump and send themselves out into deep space. As such, it can be quite a feat for a spacecraft like Rosetta to orbit a comet like 67P.
Pic: Hale-Bopp, Philipp Salzgeber
First time such a landing has ever been undertaken
Rosetta was launched in 2004 and spent ten years travelling through space before arriving at Comet 67P this month. It will make lots of observations of the comet over the coming months, but it also carries a probe called Philae which is expected to land on the comet in November. It will detach from the main orbiting spacecraft and slowly descend to the cold surface of the comet at a rate of about 1m per second. Any faster and it would risk bouncing off again and getting lost in space. When it gets close to the surface, it will fire harpoons to latch itself to the comet.
This will be the first time such a landing has ever been undertaken, and Philae will then study the comet for evidence of organic molecules, amino acids, and nucleic acids. These are the building blocks of DNA and RNA: if comets brought water to Earth, then could they have also delivered the precursors of life?
The Rosetta mission is a European venture partially funded by Ireland, so each one of us has a direct stake in what’s going to be one of the most exciting space missions of the decade, if not the century. Space Technology Ireland Ltd, based in Maynooth, is the company that developed and built the data communication subsystem that will transmit information coming from the science experiments on Philae back to the orbiting Rosetta. In turn, this data will then be transmitted back to Earth for analysis by astronomers and planetary scientists.
These are exciting times in astronomy; not just for astronomers, but also for everyday people around the world who, thanks to the internet and modern technology, can interact with and learn about what will be some of the most important missions of their kind to ever have been undertaken by humans.
Conor Farrell is an avid science enthusiast and studied physics with astronomy at Dublin City University. He has worked at Astronomy Ireland and Dunsink Observatory in the past, and loves to promote all things space-related to a wider audience. In his spare time he writes about science and current affairs, and can be followed on Twitter at @conorsthoughts.
Read more articles by Conor Farrell.
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