Apparently, comets go crashing into the sun more frequently than I previously believed, with one grazing our stellar companion about every three days. The Large Angle and Spectrometric Coronagraph (LASCO), a piece of equipment aboard the Solar and Heliospheric Observatory (SOHO), has been in operation for 15 years, and it has watched more than 2000 comets approach the Sun. Only the largest of these comets, those with diameters of up to 100 m, have traversed their perihelion (the closest approach to the Sun) and survived to see another orbit. Comet C/2011 N3 (SOHO) was not one of the survivors. Fortunately, the Atmospheric Imaging Assembly aboard the Solar Dyanmics Observatory captured the last moments of this comet as it slammed into the Sun's inner corona, and the results have appeared in Science.

Comet C/2011 N3 (SOHO) was first seen on July 5, 2011 at 23:46 UTC when it was about 0.2 solar radii away from the Sun. Astronomers watched the comet's descent in five different wavebands in the extreme ultraviolet spectrum (EUV) for approximately 20 minutes before it faded into the Sun's atmosphere. During this final dive, it was traveling at approximately 600 km/s, which led to some blurring in the images and movies. In association with the paper, Science is hosting a pair of videos of the comet's last moments.

Using the EUV observations, the authors of the Science paper calculated the orbit of the comet and determined that it would have had a orbital perihelion of 97,200 km above the solar surface. This value is in good agreement with orbital parameters calculated using SOHO/LASCO and STEREO/SECCHI (Sun Earth Connection Coronal and Heliospheric Investigation) coronagraphs. This agreement suggests that the nucleus of the comet didn't slow down as it flew into the solar corona.

There was a feature visible in the absorption spectrum, just ahead of the comet's tail. That turned out to be the comet's coma—the dense region that surrounds the nucleus. It was seen about 7 arcseconds ahead of the tail when it crossed the edge of the sun (called the limb); at the distance of the sun, this meant that the coma was about 5000 km ahead of the bright part of the tail.

Using various absorption spectra features, the authors were able to determine that the radius of this dense leading region is 1400±600 km. However, due to the motion blurring and shutter speed of the camera, the authors consider this an upper bound. By making some assumptions about the opacity of the coma, they were able to narrow down the actual radius to somewhere between 50 and 700 km.

Using the time-lapse photos, it was possible to see a series of arches behind the comet's nucleus. These were interpreted as tracks of material that the comet shed as it plowed through the Sun's atmosphere. It was possible to estimate the rate of mass loss that produced these arches—the comet's orbital trajectory and speed were known, along with the time it took to decelerate in the corona and the corona's plasma density. For the 20 minutes the comet was seen falling into the star, it was losing between 106 and 108 g/s, corresponding to a total mass loss of between 6x108 and 6x1010g.

By doing some basic heat transfer analyses, the authors were able to determine that a single solid block of comet couldn't lose this amount of mass in this time window. Instead, the core must have been made up of numerous fragments. (A solid block would not have had enough surface area to possibly shed this amount of mass.) To make the heat transfer numbers work, the authors estimate that there were around one to two dozen fragments that were larger than 10 m in diameter.

This entire exercise proved to be a valuable new glimpse into the interior of comets. Previous estimates involved derivations based on changes in a comet's light curves as it orbited, and involved assumptions about its albedo and mass density. Direct observations made during a spacecraft flyby gave more information, but have been very rare. Given that our current observatories see a "Sun-grazing" comet on average once every three days, that should equate to several comets per year that survive long enough to plow into the solar corona. So, we'll hopefully be seeing more data of this sort in the future.

Science, 2012. DOI: 10.1126/science.1211688 (about DOIs)