Comet ISON Fizzles ... But There's a Sting in the Tail

If there is one thing we know about comets, it is that their behaviour is really hard to predict and that they will always surprise us – and sometimes disappoint.

Unfortunately it looks like comet ISON, or most of it, did not survive its terrifying encounter with the Sun yesterday when it made a close approach at just 1.2 million kms from our closest star’s fiery surface. This distance may seem large but it is close enough to have subjected the comet to temperatures of around 2,700°C. To survive such a close shave with the Sun may sound unlikely. But a few other sungrazing comets have been known to survive closer passes so there was immense hope it would perform a death-defying stunt and emerge intact.

ISON did not leave us without a final serving of mystery though. Soon after reaching its nearest point to the Sun (known as perihelion), Twitter world and news agencies were alight, lamenting its loss following no signs of it emerging. It was assumed to have disintegrated – RIP ISON.

But then, moments later, new images emerged showing a hint of something appearing on the other side of the Sun. Was this a hint of something ISON or nothing more than a ghostly version of its former self? As we go to press we are left with more questions than answers and it is still a little unclear if any ISON material survived or if what we see is a dispersed tail of dust with no comet nucleus.

What we do know is that ISON no longer has a large nucleus of material so its future journey is unlikely to be spectacular as there is limited material to produce a tail. Still, comets are hard to predict so there could be some surprises in store.

Comet of the century

Comet ISON had been dubbed the Comet of the Century. Ever since it was first spotted in September 2012 scientists believed it had the potential to shine brighter than the Moon. Although ISON hasn’t quite lived up to these predictions, it’s certainly put on a good show on its perilous journey from the Oort cloud, a region of icy comets lying nearly a light year from the Sun, where it lived for 4.6 billion years.

In recent weeks ISON has been entertaining us with a show of varying levels of dust and ice streaming from its nucleus. The dust production of ISON has, at times, seemed to completely shut down. This led some worried scientists to suspect the comet had died very early, its shining light distinguished.

But then ISON surprised us all by turning on again and the last sightings before perihelion suggested that it had not yet run out of juice.

All is not lost

Bright comets from the Oort cloud passing the Earth are extremely rare. Many astronomers set their telescopes on this icy object to learn more about the mysterious visitors from the outer Solar System. ISON holds secrets 4.6 billion years old, including potential information about the earliest formed gases and dust in our Solar System. Astronomers have also acquired reams of new data about the comet on its journey to the Sun that will help them understand, and hopefully better predict, the behaviour of future cometary visitors.

The rarity of this event explains why scientists have been unsure how to predict what the comet was going to do as they don’t have much to compare it to. Passing the Sun is certainly no easy manoeuvre: the extremes of gravity and heat acting to rip apart and explode the comet nucleus, possibly vaporising it in a split second.

So, although we have probably been denied the opportunity to observe a beautiful comet blazing through our night skies in December, it’s not all bad news. ISON’s curtailed life allowed us to learn more about the behaviour of these ancient icy dustballs. And, while scientists pore over new comet data, we can all anticipate the next one to be spotted hurtling toward the Sun. It could be a good few years, but there’s one thing we can be assured; comets will always take us on a thrilling ride.

By Natalie Starkey, who received funding from The Science and Technology Facilities Council. She is affiliated with Planetary and Space Sciences at The Open University.