Dirty Snowballs from Space: How to ruin your eyesight for Science!

The 29th of November, 2013 is a special day. It’s the International Day of Women Human Rights Defenders, the first flight over the South Pole happened in 1929, in 1945 Yugoslavia became a republic, in 2004 Godzilla was honoured with a star on the Hollywood Walk of Fame and it’s also Bill Weasley’s birthday.

Who knew right?

This year, on the 29th of November, we find out if ISON  survives its harrowing trip around the Sun.

ISON (named for the International Scientific Optical Network) is a comet, a giant ball of stardust and ice, that will pass within 1.2 million km of the Sun… not super awesome if your major building block traditionally melts if exposed to high temperatures.

There are a number of different types of comets, as I have recently discovered while trawling through blogs and infographics (I’ll list them below), and many different ways of classifying them into groups. Short-period comets are the ones I think most of us non-astronomers would be familiar with. These guys get caught in the Sun’s gravitational field and take about 200 years or so to make a full orbit. The most famous example would be Halley’s comet which actually appears in a forgotten corner of the Bayeux Tapestry made in about 1080 AD. Edmond Halley noticed, after Isaac Newton calculated a parabolic orbit for another comet, that the comets recorded in 1531, 1607 and 1682 were actually the same object and predicted it would come back in 1758. When it did, it was named after him.

Not all comets come around again with such regularity, or if they do, they take their sweet time to do it (I’m talking 30 million years!). Long-period comets come from about 100000 astronomical units (1 au is the distance between the Earth and the Sun) away, from a place on the outskirts of the solar system known as the Oort cloud (it should be noted that all comets come from here originally – the short-period comets are just encouraged to hand around).

This is where ISON fits in. It was discovered early this year by a couple of amateur astronomers from Belarus and Russia and has subsequently been tracked by NASA and other observatories around the world. When its trajectory was calculated, scientists realised it was going to pass within 0.4 astronomical units of Earth and we all got a bit excited… until we remembered it had to go around the sun.

Coronograph of ISON passing close to the sun (Circle in the middle). (NASA - SOHO Observatory)
Coronograph of ISON passing close to the sun (Circle in the middle). (NASA – SOHO Observatory)

It’s here that we need to introduce another group of comets; the Sun-grazers. These comets pass within 1.5 million km of the Sun and will either survive, with a bit of melting, break into a million pieces after losing all structural integrity, or crash into the sun… which for the sun is kind of like swatting a microscopic fly.

On an interesting side note, there’s apparently a group of short-period, sun-grazing comets following a path around the sun called the Kreutz Group. Scientists think there was a giant comet that broke apart hundreds or thousands of years ago and the tiny comets left are just the remnants. Cool huh?

Anyway, I’d imagine the mood at the SOHO facility (one of the observatories that’s tracking ISON – they have this special instrument that blocks the Sun and images what’s around it called a Coronograph) is fairly mixed right now. On the one hand, there’s a long-period, rare comet sling-shotting around the sun that could break apart and on the other hand… well there’s a long-period, rare comet sling-shotting around the sun that could break apart. The closest we’ve ever got to a comet previously has been a mission called “Stardust” that got within 236 km and sampled Comet Wild 2 in January 2004. We don’t really know much about what comets are made of, which is a shame because they’re thought to be formed out of material from the beginning of the solar system. Stardust collected some of the interstellar dust surrounding the comet and bought it back to earth. They found a whole bunch of minerals and, get this, glycine. An amino acid essential for life… in a comet… This mission was extremely successful, but we still don’t know what’s at the centre, the nucleus of a comet.

So if ISON breaks apart, there will be a whole bunch of sad amateur astronomers and scientists keen to observe this rarity, but I imagine there will be an equally large bunch of astro-biologists and chemists rubbing their hands together in gleeful anticipation. We may get a better idea of what comets are actually made of.


Halley's comet
Halley’s comet

In the meantime, Southern Hemisphere people can apparently go outside right now, block the sun and see ISON with the naked eye. But be careful – looking directly into the sun is traditionally not awesome. You can also see it on the horizon pre-dawn which I’m seriously considering doing tomorrow having already attempted the former idea with limited success…

…by which I mean “Damn the sun is bright.”

Further reading

Follow the hashtag #willitbreakup on twitter for regular updates

Brief info from NASA on comets

Brilliant ISON infographic

Sydney Observatory

NASA Asteroid and Comet Watch


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