Astrophile: The pride and fall of warrior comet Lovejoy

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: Particularly hardy sungrazer comet
Origins: Long lineage of sun-divers

After passing through the edge of the sun, Lovejoy the warrior was hailed as the great survivor. Amid warnings that such a perilous quest would almost certainly be fatal, Lovejoy dived into the sun's flaming corona and, to everyone's surprise, emerged on the other side, battered but not broken. Fellow warriors had come charging at the fiery beast before but almost all were consumed. The world rejoiced as the hero's bright banner outshone every star in the sky.

That, at least, is how it appeared back in December 2011, when that feat of derring-do occurred. Now it seems that Lovejoy ? a comet ? was not as hardy as we thought. A new analysis finds that the comet lost its head and disintegrated two days after its impressive feat, thanks to a hidden build-up of thermal stress.

Thankfully, the tale isn't without hope. Zdenek Sekanina and Paul Chodas, of NASA's Jet Propulsion Laboratory in California, who carried out the new study, also suggest that Lovejoy was the first of a whole cluster of bright comets that should approach the sun in the 2040s.

Lovejoy, more technically known as comet C/2011 W3, was first spotted by its namesake ? Australian amateur astronomer Terry Lovejoy ? a few weeks before it ploughed into the sun's corona. Scientists calculated its orbital path, which revealed that the comet would reach perihelion, its closest approach to the sun, in mid-December.

Vanishing head

The comet was part of the Kreutz family of sungrazers. Orbiting telescopes such as NASA's Solar and Heliospheric Observatory had witnessed the demise of hundreds of sungrazers before, and all expectations were that Lovejoy would also perish.

On 16 December, a host of space-based telescopes and ground-based enthusiasts watched as Lovejoy flew into the solar corona, where temperatures reach 1 million Kelvin. Amazingly, the object reappeared after almost an hour. In the coming days, sky-watchers in the southern hemisphere saw the comet and its bright tail near the pre-dawn horizon as it retreated from the sun.

Lovejoy was hailed as an incredible find by professional and hobbyist astronomers alike. And it was. But the story is not as simple as it seems.

Sekanina and Chodas have reanalysed pictures taken on 20 December, showing that Lovejoy's nucleus, the cometary equivalent of a head, had suddenly vanished. The headless tail boasted only a long, bright "spine", which Sekanina and Chodas now think was the denser dust of the broken nucleus.

Dry body

Using data on the likely structure and composition of comets from past missions, the team assumed that Lovejoy's nucleus was a roughly spherical, rocky conglomeration of consolidated dust, covered with a layer of water ice. They then used near-infrared studies of heat emissions from other comets to calculate the rate at which heat would move through Lovejoy's nucleus.

According to the researchers, the comet's trek into the corona rapidly vaporised all its surface ice, leaving only the dusty core. Shortly after the ordeal, this dry body started to crack and flake away in the extreme heat. "Very close to perihelion, already 10 metres or more of the comet has completely disintegrated," says Sekanina.

Lovejoy was just the right size to survive the dive itself. Starting at around 500 metres wide, the comet emerged at less than 200 metres across.

But Sekanina and Chodas now show that even once it was moving away from the sun, heat continued to penetrate deeper into the comet. "The surface of the comet as it recedes gets cooler, but the interior still gets warmer, because to transfer heat takes time," says Sekanina.

Crumpled core

That was enough to seal its fate, the pair conclude. When enough thermal stress had built up in its core, the comet crumpled ? which in some ways vindicates those who said it would be destroyed. "The guys who said the comet won't survive perihelion were off by 1.6 days," says Sekanina.

But Sekanina and Chodas were also interested in the famous comet's origins, and what this might say about future sungrazers. Kreutz sungrazers all follow the same orbit, so are thought to be pieces of a larger comet that broke apart millennia ago. These fragments then repeatedly broke up, spawning further children and grandchildren.

By combining pictures of the retreating tail with images of Lovejoy's approach, the pair traced the comet's orbital period, the time it takes to complete a turn around the sun. Their calculations suggest that Lovejoy has an orbital period of about 698 years.

Sun-grazing flurry

There's no perfect match for a sungrazing comet appearing 698 years ago in historical accounts, but the team estimates that Lovejoy was most likely born from a Kreutz descendant that neared the sun in the 14th century and then spilt into further fragments, a cluster of which may visit the sun in a few decades.

Similar bright clusters were seen in the 1880s and 1960s. Due to gravitational perturbations with the planets, some comets in these clusters are known to alter course and appear roughly 20 to 30 years ahead of the pack. Sekanina and Chodas calculate that Lovejoy fits the profile of such a front runner, which means we should be in for another flurry of sungrazers in the 2040s.

"We have concluded that the 1960s is not the end of the whole show. There will be another cluster coming," says Sekanina.

Will one of these be as hardy as Lovejoy and survive a plunge through the sun's corona? Who knows. Perhaps one will emerge that is even tougher and will live long after its visit.

Journal reference: Astrophysical Journal, doi.org/jck

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