Damian Peach

Near the banks of the Potomac River, in an office cluttered with craft-beer coasters and a Doctor Who mug, Karl Battams keeps watch for daredevil comets that skim just above the surface of the Sun.

A decade ago, when the astrophysicist joined the US Naval Research Laboratory in Washington DC, he had no deep interest in comets. But he was pressed into service because the lab operates instruments on two solar-physics missions that can spot objects passing very close to the Sun. They have detected some 2,600 such 'sun-grazing' comets so far, and it is part of Battams' job to catalogue those discoveries. He is the only dedicated sun-grazing-comet tracker in the world. Hopefully, I'll be getting a summer student, he says one overcast January morning. But pretty much it's just me.

All of the Solar System's comets travel around the Sun, but sun-grazers are those that fly within about three solar radii of the star's centre (some 1.4 million kilometres above its surface). Battams rose to fame last autumn as the public face of a research group tracking the most famous sun-grazer of all, Comet ISON. As ISON sailed into the inner Solar System, expectations grew quickly among astronomers and amateur skywatchers. Many hoped that it might survive its close passage to become a dramatic sight in the night sky and continued fodder for scientific study. Instead, the comet disintegrated spectacularly in November, just hours before it was set to sweep past the Sun.

Scientists are left wondering why ISON suffered the fate it did. Early results suggest that it may have been just too small and too volatile to survive the Sun's searing heat (M. M. Knight and K. Battams Preprint at http://arxiv.org/abs/1401.7028; 2014). ISON was a tiny, gassy comet making its first ever trip to the inner Solar System a combination that may have doomed it from the beginning.

Yet its death could mark a renaissance for the study of sun-grazing comets. ISON was spotted quite far out in the Solar System, and its unusual trajectory allowed spacecraft orbiting Earth, Mars and Mercury to photograph it from many vantage points. That made ISON the most studied sun-grazer yet. What researchers have learned so far suggests that sun-grazers have a lot to reveal about the diversity of comets, and how hard it is to predict what they might do. Even as they wring findings out of the ISON event, astronomers are gearing up for the next close cometary encounter, later this year.

The sheer amount of observational firepower involved in studying ISON set a new standard for coordinating a flotilla of spacecraft and ground-based telescopes. It was about bringing all of it together, says Battams. That's never been done before.

For centuries, skywatchers have recognized objects that disappear into the Sun and re-emerge on the other side. In 1687, Isaac Newton published the first calculations of a sun-grazer's orbit, showing that the great comet of 1680 moved according to his laws of gravitation. But it was not until the era of satellites that people could watch sun-grazers up close.

Amateur astronomers discover most sun-grazers, just days before they pass through the Sun's atmosphere, by trawling through images taken by the Solar and Heliospheric Observatory (SOHO) spacecraft. Launched in 1995, SOHO stares at the Sun with a set of three US Navy-built coronagraphs that block out the central disk of the Sun, allowing astronomers to see details in and around its blazing outer atmosphere. Once they have found a candidate comet, the amateurs alert Battams.

Satellite Locations: NASA/STEREO. Photographs, Left to Right: Vitali Nevski/Artyom Novichonok; NASA/ESA/J.-Y. Li (Plan. Sci. Inst.)/Hubble Comet ISON Imaging Sci. Team; NASA/STEREO; ESA/NASA/SOHO

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Astronomy: Death of a comet