Comet ISON: has this lonely traveller reached the end of its journey? | WPN World People News

"A good traveller has no fixed plans, and is not intent on arriving.” - Lao Tzu

Comet ISON has just experienced a highlight of a journey that has taken millions of years. A relic from the formation of the solar system, ISON spent 4.6 billion years in the Oort cloud, a giant swarm of icy objects between 5000 and 100,000 AU from the Sun. Nudged out of the cloud by the gravitational pull of a passing star, ISON has been on a lonely journey through the solar system, destined to become a sungrazer, a comet that comes so close to the Sun that it passes through the corona.

An image of Comet ISON, captured by the Hubble Telescope

The fate of a sungrazer comet depends on its size - larger comets have a better chance of survival. In 2011, Comet Lovejoy was pulled apart when it passed close to the Sun. Comet Encke, in 2007, had its tail blasted away by a solar-ejected cloud known as a coronal mass ejection.The icy nucleus of comet ISON was estimated to be 2 km across, right on the limit for survival, making it too close to call.

Initial reports on 28th November 2013 suggested that ISON had not survived, but the picture this morning looks a little more hopeful. It’s unclear whether ISON will still be visible to the naked eye in the pre-dawn skies for much of December. Opinions vary, and there’s a lot we don’t know about comets. They are balls of ice (frozen gases) and dust, and as they heat up this material boils off and streams behind them as a tail. Their behaviour is unpredictable, and changeable.

Although we hope that ISON will put on a good show for us here on Earth, scientists are hoping it will shed some light on some big questions. ISON was formed when the solar system was formed, and has been frozen in the Oort cloud ever since. We believe that it formed in the same place as Neptune, nearer to the Sun than Neptune is now. Looking at the gases in ISON’s tail could tell us the temperature at which ISON and Neptune formed, and hence how far from the Sun they were at the time. In ISON’s tail we may find an explanation of why the inner planets are rocky, but the outer planets are gas giants. One theory is that they formed at different temperatures: that the inner planets formed where it was hot enough to melt metals, and the gas giants formed further out, where it was cold enough to condense gases.

ISON could also help us to decide where Earth’s water came from. The key to this question is the ratio between ordinary water (H2O) and heavy water (D2O, made with deuterium) in Earth’s oceans. Comets are balls of ice, much of which is water ice, and one theory is that Earth’s water was delivered by comets. If the ordinary water/ heavy water ratio in comets is the same as that in the oceans, then that’s evidence that the theory is correct. However, results so far are ambiguous and we need data from more comets, such as ISON.

Elsewhere, scientists have conducted experiments to show that amino acids – the building blocks of life as we know it – can form in comets, and in the impacts when comets hit planets. What we need now is to take samples from real-life comets, to see whether amino acids are present. We didn’t have enough time to do that with ISON , but a mission is already underway to take a sample from Comet 67P/Churyumov-Gerasimenko. Rosetta was launched in 2004, and has also been on a long journey. Rosetta is currently hibernating, to conserve energy, but the spacecraft will wake up in January 2014 in preparation for a rendezvous with the comet later in the year. Rosetta is an exciting mission, with plans to fix a lander to the surface of the comet’s nucleus. Watch out for more details next year.

So whether or not ISON has survived its close encounter with the Sun, and whether or not it puts on a show for us, there’s some really exciting science going on with comets. These lonely travellers offer us an opportunity to find out more about the origins of Earth’s water, of the solar system, and maybe even of life itself.

Source: Science and Technology Facilities Council