It was supposed to be the comet of the century. Early last year, astronomers became increasingly excited about a ball of ice and dust that had travelled from beyond the reaches of the Solar System and was heading towards the Sun.

They calculated that this object, Comet Ison, would pass close to the Sun, sweep round behind it and then head towards Earth where it would put on the greatest show of heavenly fireworks in living memory.

But as Ison drifted behind the Sun, something entirely different happened instead. Nothing!

As astronomers watched, Comet Ison brightened dramatically, disappeared behind the Sun and was never seen or heard from again. The comet of the century simply vanished.

So what happened to Comet Ison? Today we get an answer thanks to the work of Zdenek Sekanina at the Jet Propulsion Laboratory in Pasadena and Rainer Kracht. Using unique images taken by a number of NASA spacecraft that monitor the Sun millions of kilometres from Earth, these guys have pieced together the most detailed picture yet of the comet’s dying days.

Their conclusion is that Comet Ison died in full view of the watching astronomers who witnessed the death without realising what they were seeing.

The new data comes from three spacecraft that have been continually photographing the Sun for some years. The first is the Solar and Heliospheric Observatory or SOHO spacecraft, a NASA and European Space Agency craft that was launched in 1995 to the L1 point some 1.5 million kilometres from Earth in the direction of the Sun, where the gravitational fields of both bodies cancel out.

The other two spacecraft are a pair called STEREO A and STEREO B which orbit the Sun ahead of, and behind, the Earth and so provide 3D images of solar storms coming our way.

The vantage points of these three spacecraft give them a unique view of Ison’s demise.

A comet’s point of closest approach to the Sun is known as its perihelion. This is an important point in its life. Before perihelion, it is approaching the Sun, while getting hotter and brighter. After the perihelion, it is travelling away from the Sun, usually on a journey to the distant reaches of the Solar System.

It is during the approach to perihelion that a comet undergoes it greatest changes. As its temperature rises, the ice it contains begins to boil and explode from its surface. The gases this produces reflect sunlight making the comet brighter. This gas and dust is blown away from the Sun by the solar wind producing the famous comet tail.

As it gets hotter still, particles of dust begin to burn producing a bright light due to the sodium they contain. At this point, a comet is at its brightest.

And that’s more or less what happened to Ison but with an unusual, devastating twist.

Sekanina and Kracht say that Ison’s brightness increased dramatically on 14 November last year, just 14 days before perihelion. This was accompanied by a sudden increase in the amount of water gas around the nucleus.

They say this was the result of a sudden release of water from inside the comet, probably as the result of fragmentation of the nucleus. In hindsight, this was the first indication that Ison was beginning to fall apart.

This kind brightening is common in comets and usually dims once the batch of water has escaped. Instead, Ison got even brighter and began to flare repeatedly.

This was the result of the comet breaking into many pieces and releasing all the water it contained inside. “This was a case of cataclysmic fragmentation that shattered the mass of the nucleus into an estimated hundreds of thousands of fragments,” say Sekanina and Kracht.

The water and dust released in this process contributed to a dramatic brightening. And although nobody realised it at the time, Sekanina and Kracht say this was the death of the comet. As astronomers watched excitedly from Earth, Ison died in full view.

Once the water had gone and the dust had been burnt, there was nothing left. Some 3 days before perihelion, the production of gas from Comet Ison dropped by a factor of 20. At this point, some 20 million kilometres from the Sun, the comet’s reserves of ices were exhausted and it ceased to exist as a comet as astronomers define it.

Nevertheless, it was still bright thanks to the sublimation of sodium from the boulders and pebbles that remained. But as the comet approached the Sun, even these vapourised, leaving nothing. On 28 November, some 3.5 hours before perihelion, the comet and its remains disappeared forever.

At this point, Ison was behind the Sun, as seen from Earth and astronomers all over the world, were waiting for it to re-appear in all its glory. Various radio and TV stations even broadcast live coverage of the event.

The result was something of an anticlimax. After its failure to appear, the consensus was that the comet had somehow broken up behind the Sun, perhaps due to the enormous tidal forces caused by the Sun’s gravity.

But Sekanina and Kracht say there is no evidence that tidal forces played a role in the comet’s break up. Indeed, the images show that the comet broke up at distances of at least 5 solar diameters, far too distant for tidal forces to have played a role.

Instead, they say that thermal forces probably tore the comet apart as it approached.

And there endeth the story of Comet Ison, the most spectacular flop in recent cometary history but also one of the hid its best secret till the end—it died in full view without the onlooking astronomers even noticing.